Saikat Guha
- Professor, Optical Sciences
- Member of the Graduate Faculty
- Professor, Electrical and Computer Engineering
Contact
- (520) 621-6997
- Meinel Optical Sciences, Rm. 702
- Tucson, AZ 85721
- saikat@arizona.edu
Bio
No activities entered.
Interests
No activities entered.
Courses
2024-25 Courses
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Dissertation
OPTI 920 (Fall 2024)
2023-24 Courses
-
Directed Graduate Research
OPTI 792 (Spring 2024) -
Dissertation
OPTI 920 (Spring 2024) -
Dissertation
PHYS 920 (Spring 2024) -
Directed Graduate Research
OPTI 792 (Fall 2023) -
Dissertation
OPTI 920 (Fall 2023) -
Dissertation
PHYS 920 (Fall 2023) -
Master's Report
OPTI 909 (Fall 2023) -
Thesis
OPTI 910 (Fall 2023)
2022-23 Courses
-
Dissertation
OPTI 920 (Summer I 2023) -
Independent Study
OPTI 599 (Summer I 2023) -
Directed Graduate Research
OPTI 792 (Spring 2023) -
Dissertation
OPTI 920 (Spring 2023) -
Dissertation
PHYS 920 (Spring 2023) -
Independent Study
OPTI 599 (Spring 2023) -
Thesis
OPTI 910 (Spring 2023) -
Directed Graduate Research
OPTI 792 (Fall 2022) -
Dissertation
OPTI 920 (Fall 2022) -
Dissertation
PHYS 920 (Fall 2022)
2021-22 Courses
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Research
OPTI 900 (Summer I 2022) -
Directed Graduate Research
OPTI 792 (Spring 2022) -
Dissertation
OPTI 920 (Spring 2022) -
Dissertation
PHYS 920 (Spring 2022) -
Independent Study
OPTI 599 (Spring 2022) -
Information in a Photon
OPTI 595B (Spring 2022) -
Thesis
OPTI 910 (Spring 2022) -
Directed Graduate Research
OPTI 792 (Fall 2021) -
Dissertation
OPTI 920 (Fall 2021) -
Dissertation
PHYS 920 (Fall 2021)
2020-21 Courses
-
Dissertation
OPTI 920 (Spring 2021) -
Dissertation
PHYS 920 (Spring 2021) -
Independent Study
OPTI 599 (Spring 2021) -
Information in a Photon
OPTI 495B (Spring 2021) -
Information in a Photon
OPTI 595B (Spring 2021) -
Dissertation
OPTI 920 (Fall 2020) -
Independent Study
OPTI 599 (Fall 2020) -
Independent Study
PHYS 599 (Fall 2020)
2019-20 Courses
-
Directed Graduate Research
OPTI 792 (Spring 2020) -
Dissertation
OPTI 920 (Spring 2020) -
Independent Study
PHYS 599 (Spring 2020) -
Information in a Photon
OPTI 495B (Spring 2020) -
Information in a Photon
OPTI 595B (Spring 2020) -
Directed Graduate Research
OPTI 792 (Fall 2019) -
Independent Study
PHYS 599 (Fall 2019) -
Photonic Quantum Info Process
OPTI 647 (Fall 2019)
2018-19 Courses
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Directed Research
OPTI 492 (Summer I 2019) -
Directed Graduate Research
OPTI 792 (Spring 2019) -
Independent Study
OPTI 599 (Spring 2019) -
Information in a Photon
OPTI 495B (Spring 2019) -
Information in a Photon
OPTI 595B (Spring 2019) -
Photonic Quantum Info Process
OPTI 647 (Fall 2018)
2017-18 Courses
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Directed Research
OPTI 492 (Summer I 2018) -
Directed Graduate Research
OPTI 792 (Spring 2018) -
Independent Study
OPTI 599 (Spring 2018) -
Directed Graduate Research
OPTI 792 (Fall 2017)
Scholarly Contributions
Journals/Publications
- Dhara, P., & Guha, S. (2024). Phonon-Induced Decoherence in Color-Center Qubits. Phys. Rev. Res..More infoElectron spin states of solid-state defects such as Nitrogen- andSilicon-vacancy {\em color centers} in diamond are a leading quantum-memorycandidate for quantum communications and computing. Via open-quantum-systemsmodeling of spin-phonon coupling -- the major contributor of decoherence -- ata given temperature, we derive the time dynamics of the density operator of anelectron-spin qubit. We use our model to corroborate experimentally-measureddecoherence rates. We further derive the temporal decay of distillableentanglement in spin-spin entangled states heralded via photonic Bell-statemeasurements. Extensions of our model to include other decoherence mechanisms,e.g., undesired hyperfine couplings to the neighboring nuclear-spinenvironment, will pave the way to a rigorous predictive model for engineeringartificial-atom qubits with desirable properties.[Journal_ref: Phys. Rev. Res. 6 (1), 013055 (2024)]
- Forney, G. D., & Guha, S. (2024). Simple Rate-1/3 Convolutional and Tail-Biting Quantum Error-Correcting Codes.More infoSimple rate-1/3 single-error-correcting unrestricted and CSS-type quantumconvolutional codes are constructed from classical self-orthogonal$\F_4$-linear and $\F_2$-linear convolutional codes, respectively. Thesequantum convolutional codes have higher rate than comparable quantum blockcodes or previous quantum convolutional codes, and are simple to decode. Ablock single-error-correcting [9, 3, 3] tail-biting code is derived from theunrestricted convolutional code, and similarly a [15, 5, 3] CSS-type block codefrom the CSS-type convolutional code.[Journal_ref: ]
- Guha, S. (2024). Receiver Design to Harness Quantum Illumination Advantage.More infoAn optical transmitter that uses entangled light generated by spontaneousparametric downconversion (SPDC), in conjunction with an optimalquantum-optical receiver (whose implementation is not yet known) is inprinciple capable of obtaining up to a 6 dB gain in the error-probabilityexponent over the optimum-reception un-entangled coherent-state lidar to detectthe presence of a far-away target subject to entanglement-breaking loss andnoise in the free-space link [Lloyd'08, Tan'08]. We present an explicit designof a structured quantum-illumination receiver, which in conjunction with theSPDC transmitter is shown to achieve up to a 3 dB error-exponent advantage overthe classical sensor. Apart from being fairly feasible for a proof-of-principledemonstration, this is to our knowledge the first structured design of aquantum-optical sensor for target detection that outperforms the comparablebest classical lidar sensor appreciably in a low-brightness, lossy and noisyoperating regime.[Journal_ref: ]
- Guha, S. (2024). Structured optical receivers to attain superadditive capacity and the Holevo limit. Phys. Rev. Lett..More infoWhen classical information is sent over a quantum channel, attaining theultimate limit to channel capacity requires the receiver to make jointmeasurements over long codeword blocks. For a pure-state channel, we constructa receiver that can attain the ultimate capacity by applying a single-shotunitary transformation on the received quantum codeword followed bysimultaneous (but separable) projective measurements on thesingle-modulation-symbol state spaces. We study the ultimate limits ofphoton-information-efficient communications on a lossy bosonic channel. Basedon our general results for the pure-state quantum channel, we show some of thefirst concrete examples of codes and structured joint-detection opticalreceivers that can achieve fundamentally higher (superadditive) channelcapacity than conventional receivers that detect each modulation symbolindividually.[Journal_ref: Phys. Rev. Lett. 106, 240502 (2011)]
- Guha, S., & Erkmen, B. I. (2024). Gaussian-state quantum-illumination receivers for target detection. Phys. Rev. A.More infoThe signal half of an entangled twin-beam, generated using spontaneousparametric downconversion, interrogates a region of space that is suspected ofcontaining a target, and has high loss and high (entanglement-breaking)background noise. A joint measurement is performed on the returned light andthe idler beam that was retained at the transmitter. An optimal quantumreceiver, whose implementation is not yet known, was shown to achieve 6 dB gainin the error-probability exponent relative to that achieved with a singlecoherent-state (classical) laser transmitter and the optimum receiver. Wepresent two structured optical receivers that achieve up to 3 dB gain in theerror exponent over that attained with the classical sensor. These are to ourknowledge the first designs of quantum-optical sensors for target detection,which can be readily implemented in a proof-of-concept experiment, thatappreciably outperform the best classical sensor in the low-signal-brightness,high-loss and high-noise operating regime.[Journal_ref: Phys. Rev. A 80, 052310 (2009)]
- Guha, S., & Shapiro, J. H. (2024). Capacity of optical reading, Part 1: Reading boundless error-free bits using a single photon. Phys. Rev. A.More infoWe show that nature imposes no fundamental upper limit to the number ofinformation bits per expended photon that can, in principle, be read reliablywhen classical data is encoded in a medium that can only passively modulate theamplitude and phase of the probe light. We show that with a coherent-state(laser) source, an on-off (amplitude-modulation) pixel encoding, andshot-noise-limited direct detection (an overly-optimistic model for commercialCD/DVD drives), the highest photon information efficiency achievable inprinciple is about 0.5 bit per transmitted photon. We then show that acoherent-state probe can read unlimited bits per photon when the receiver isallowed to make joint (inseparable) measurements on the reflected light from alarge block of phase-modulated memory pixels. Finally, we show an example of aspatially-entangled non-classical light probe and a receiverdesign---constructable using a single-photon source, beam splitters, andsingle-photon detectors---that can in principle read any number of error-freebits of information. The probe is a single photon prepared in a uniformcoherent superposition of multiple orthogonal spatial modes, i.e., a W-state.The code, target, and joint-detection receiver complexity required by acoherent-state transmitter to achieve comparable photon efficiency performanceis shown to be much higher in comparison to that required by the W-statetransceiver.[Journal_ref: Phys. Rev. A 87, 062306 (2013)]
- Guha, S., & Shapiro, J. H. (2024). Classical Information Capacity of the Bosonic Broadcast Channel.More infoWe show that when coherent-state encoding is employed in conjunction withcoherent detection, the Bosonic broadcast channel is equivalent to a classicaldegraded Gaussian broadcast channel whose capacity region is dual to that ofthe classical Gaussian multiple-access channel. We further show that if aminimum output-entropy conjecture holds true, then the ultimate classicalinformation capacity of the Bosonic broadcast channel can be achieved by acoherent-state encoding. We provide some evidence in support of the conjecture.[Journal_ref: ]
- Guha, S., & Shapiro, J. H. (2024). Enhanced standoff sensing resolution using quantum illumination.More infoLoss and noise quickly destroy quantum entanglement. Nevertheless, recentwork has shown that a quadrature-entangled light source can reap a substantialperformance advantage over all classical-state sources of the same averagetransmitter power in scenarios whose loss and noise makes them entanglementbreaking, standoff target-detection being an example. In this paper, we make afirst step in extending this quantum illumination paradigm to the opticalimaging domain, viz., to obtain better spatial resolution for standoff opticalsensing. Our canonical imaging scenario---restricted, for simplicity, to onetransverse dimension---is taken to be that of resolving one versus twoclosely-spaced in-phase specular point targets. We show that an entangled-statetransmitter, which uses continuous-wave-pumped spontaneous parametricdownconversion (SPDC), achieves an error-probability exponent that exceeds thatof all classical-state transmitters of the same average power. Using theseerror-exponent results, we find the ultimate spatial-resolution limits forcoherent-state and SPDC imaging systems that use their respectivequantum-optimal receivers, thus quantifying the latter's spatial-resolutionadvantage over the former. We also propose a structured optical receiver thatis ideally capable of harnessing 3 dB (of the full 6 dB) gain in theerror-probability exponent achievable by the SPDC transmitter and itsquantum-optimal receiver.[Journal_ref: ]
- Guha, S., Dutton, Z., & Shapiro, J. H. (2024). On quantum limit of optical communications: concatenated codes and joint-detection receivers.More infoWhen classical information is sent over a channel with quantum-statemodulation alphabet, such as the free-space optical (FSO) channel, attainingthe ultimate (Holevo) limit to channel capacity requires the receiver to makejoint measurements over long codeword blocks. In recent work, we showed areceiver for a pure-state channel that can attain the ultimate capacity byapplying a single-shot optical (unitary) transformation on the receivedcodeword state followed by simultaneous (but separable) projective measurementson the single-modulation-symbol state spaces. In this paper, we study theultimate tradeoff between photon efficiency and spectral efficiency for the FSOchannel. Based on our general results for the pure-state quantum channel, weshow some of the first concrete examples of codes and laboratory-realizablejoint-detection optical receivers that can achieve fundamentally higher(superadditive) channel capacity than receivers that physically detect eachmodulation symbol one at a time, as is done by all conventional (coherent ordirect-detection) optical receivers.[Journal_ref: ]
- Kaur, E., & Guha, S. (2024). Entanglement distribution in two-dimensional square grid network.More infoWe study entanglement generation in a quantum network where repeater nodescan perform $n$-qubit Greenberger-Horne-Zeilinger(GHZ) swaps, i.e., projectivemeasurements, to fuse $n$ imperfect-Fidelity entangled-state fragments. We showthat the distance-independent entanglement distribution rate found previouslyfor this protocol, assuming perfectly-entangled states at the link level, doesnot survive. This is true also in two modified protocols we study: one thatincorporates $l \to 1$ link-level distillation and another that spatiallyconstrains the repeater nodes involved in the swaps. We obtain analyticalformulas for a GHZ swap of multiple Werner states, which might be ofindependent interest. Whether the distance-independent entanglement rate mightre-emerge with a spatio-temporally-optimized scheduling of GHZ swaps andmulti-site block-distillation codes remains open.[Journal_ref: ]
- Patil, A., & Guha, S. (2024). Clifford Manipulations of Stabilizer States: A graphical rule book for Clifford unitaries and measurements on cluster states, and application to photonic quantum computing.More infoStabilizer states along with Clifford manipulations (unitary transformationsand measurements) thereof -- despite being efficiently simulable on a classicalcomputer -- are an important tool in quantum information processing, withapplications to quantum computing, error correction and networking. Clusterstates, defined on a graph, are a special class of stabilizer states that arecentral to measurement based quantum computing, all-photonic quantum repeaters,distributed quantum computing, and entanglement distribution in a network. Allcluster states are local-Clifford equivalent to a stabilizer state. In thispaper, we review the stabilizer framework, and extend it, by: incorporatinggeneral stabilizer measurements such as multi-qubit fusions, and providing anexplicit procedure -- using Karnaugh maps from Boolean algebra -- forconverting arbitrary stabilizer gates into tableau operations of the CHPformalism for efficient stabilizer manipulations. Using these tools, we developa graphical rule-book and a MATLAB simulator with a graphical user interfacefor arbitrary stabilizer manipulations of cluster states, a user of which,e.g., for research in quantum networks, will not require any background inquantum information or the stabilizer framework. We extend our graphicalrule-book to include dual-rail photonic-qubit cluster state manipulations withprobabilistically-heralded linear-optical circuits for various rotated Bellmeasurements, i.e., fusions (including new `Type-I' fusions we propose, whereonly one of the two fused qubits is destructively measured), by incorporatinggraphical rules for their success and failure modes. Finally, we show howstabilizer descriptions of multi-qubit fusions can be mapped to linear opticalcircuits.[Journal_ref: ]
- Sajjad, A., Grace, M. R., & Guha, S. (2024). Quantum limits of parameter estimation in long-baseline imaging.More infoDistributed aperture telescopes are a well-established approach for boostingresolution in astronomical imaging. However, theoretical limits on quantitativeimaging precision, and the fundamentally best possible beam-combining anddetection schemes to use with such arrays, remain largely unexplored. Usingmathematical tools of the quantum and classical Cramer-Rao bounds, we performanalyses showing the fundamental origins of the enhancement provided bydistributed imaging systems, over and above a single monolithic telescope, andconsider the precision with which one can estimate any desired parameterembedded in a scene's incoherent radiation with a multi-aperture imagingsystem. We show how quantum-optimal measurements can be realized viabeam-combination strategies of two classes: (1) multi-axial: where light fromdifferent apertures is directed to a common focal plane, e.g., of asegmented-aperture telescope; and (2) co-axial: where light collected at eachaperture, e.g., telescope sites of a long-baseline array, is routed to anoptical interferometer. As an example, we show an explicit calculation of thequantum Fisher information (QFI) for estimating the angular separation betweentwo-point emitters using two identical apertures separated by a baselinedistance. We show that this QFI splits instructively into additivecontributions from the single apertures and from the baseline. We quantify therelative benefits of intra-telescope (e.g., spatial-mode) optical processingand inter-telescope beam combination. We show how both receiver designs cancapture both sources of information and discuss how similar methods could beextended to more general imaging tasks. We discuss translating QFI-attainingmeasurements to explicit receiver designs, and the use of pre-sharedentanglement to achieve the QFI when it is impractical to co-locate and combinelight collected by the apertures.[Journal_ref: ]
- Sureka, M., & Guha, S. (2024). Gaussian Boson Sampling to Accelerate NP-Complete Vertex-Minor Graph Classification.More infoGaussian Boson Sampling (GBS) generate random samples of photon-clickpatterns from a class of probability distributions that are hard for aclassical computer to sample from. Despite heroic demonstrations for quantumsupremacy using GBS, Boson Sampling, and instantaneous quantum polynomial (IQP)algorithms, systematic evaluations of the power of these quantum-enhancedrandom samples when applied to provably hard problems, and performancecomparisons with best-known classical algorithms have been lacking. We proposea hybrid quantum-classical algorithm using the GBS for the NP-complete problemof determining if two graphs are vertex minor of one another. The graphs areencoded in GBS and the generated random samples serve as feature vectors in thesupport vector machine (SVM) classifier. We find a graph embedding that allowstrading between the one-shot classification accuracy and the amount of inputsqueezing, a hard-to-produce quantum resource, followed by repeated trials andmajority vote to reach an overall desired accuracy. We introduce a newclassical algorithm based on graph spectra, which we show outperforms variouswell-known graph-similarity algorithms. We compare the performance of ouralgorithm with this classical algorithm and analyze their time vs problem-sizescaling, to yield a desired classification accuracy. Our simulation suggeststhat with a near-term realizable GBS device- $5$ dB pulsed squeezer, $12$-modeunitary, and reasonable assumptions on coupling efficiency, on-chip losses anddetection efficiency of photon number resolving detectors-we can solve a$12$-node vertex minor instances with about $10^3$ fold lower time compared toa powerful desktop computer.[Journal_ref: ]
- Takeoka, M., & Guha, S. (2024). Capacity of optical communication in loss and noise with general Gaussian receivers. Phys. Rev. A.More infoLaser-light (coherent-state) modulation is sufficient to achieve the ultimate(Holevo) capacity of classical communication over a lossy and noisy opticalchannel, but requires a receiver that jointly detects long modulated codewordswith highly nonlinear quantum operations, which are near-impossible to realizeusing current technology. We analyze the capacity of the lossy-noisy opticalchannel when the transmitter uses coherent state modulation but the receiver isrestricted to a general quantum-limited Gaussian receiver, i.e., one that mayinvolve arbitrary combinations of Gaussian operations (passive linear optics:beamsplitters and phase-shifters, second order nonlinear optics (or activelinear optics): squeezers, along with homodyne or heterodyne detectionmeasurements) and any amount of classical feedforward within the receiver.Under these assumptions, we show that the Gaussian receiver that attains themaximum mutual information is either homodyne detection, heterodyne detection,or time sharing between the two, depending upon the received power level. Inother words, our result shows that to exceed the theoretical limit ofconventional coherent optical communications, one has to incorporatenon-Gaussian, i.e., third or higher-order nonlinear operations in the receiver.Finally we compare our Gaussian receiver limit with experimentally feasiblenon-Gaussian receivers and show that in the regime of low received photon flux,it is possible to overcome the Gaussian receiver limit by relatively simplenon-Gaussian receivers based on photon counting.[Journal_ref: Phys. Rev. A 89, 042309 (2014)]
- Wadood, S. A., Sethuraj, K. R., Liang, K., Grace, M. R., La Rue, G., Guha, S., & Vamivakas, A. N. (2024). Experimental demonstration of quantum-inspired optical symmetric hypothesis testing. Optics letters, 49(3), 750-753.More infoWe use a phase-sensitive measurement to perform a binary hypothesis testing, i.e., distinguish between one on-axis and two symmetrically displaced Gaussian point spread functions. In the sub-Rayleigh regime, we measure a total error rate lower than allowed by direct imaging. Our results experimentally demonstrate that linear-optical spatial mode transformations can provide useful advantages for object detection compared with conventional measurements, even in the presence of realistic experimental cross talk, paving the way for meaningful improvements in identifying, detecting, and monitoring real-world, diffraction-limited scenes.
- Srivastava, P., Ganu, T., & Guha, S. (2023). Towards Zero-Shot and Few-Shot Table Question Answering using GPT-3.More infoWe present very early results on using GPT-3 to perform question answering ontabular data. We find that stock pre-trained GPT-3 is able to zero-shot learnthe table structure from a serialized JSON array-of-arrays representation, andable to answer lookup queries and simple comparison questions in naturallanguage without any fine-tuning. We further find that simple promptengineering to include few-shot static Q&A examples significantly improvesaccuracy. Lastly, we find that intermixing passage text improves accuracy evenfurther on heterogeneous data. We apply our approach on a novel dataset ofsimple tables in newspaper infographics with promising results. Overall, wefind much cause for optimism in this basic approach.[Journal_ref: ]
- Cui, C., Horrocks, W., Hao, S., Guha, S., Peyghambarian, N., Zhuang, Q., & Zhang, Z. (2022). Quantum receiver enhanced by adaptive learning. Light, science & applications, 11(1), 344.More infoQuantum receivers aim to effectively navigate the vast quantum-state space to endow quantum information processing capabilities unmatched by classical receivers. To date, only a handful of quantum receivers have been constructed to tackle the problem of discriminating coherent states. Quantum receivers designed by analytical approaches, however, are incapable of effectively adapting to diverse environmental conditions, resulting in their quickly diminishing performance as the operational complexities increase. Here, we present a general architecture, dubbed the quantum receiver enhanced by adaptive learning, to adapt quantum receiver structures to diverse operational conditions. The adaptively learned quantum receiver is experimentally implemented in a hardware platform with record-high efficiency. Combining the architecture and the experimental advances, the error rate is reduced up to 40% over the standard quantum limit in two coherent-state encoding schemes.
- Grace, M. R., & Guha, S. (2022). Identifying Objects at the Quantum Limit for Superresolution Imaging. Physical review letters, 129(18), 180502.More infoWe consider passive imaging tasks involving discrimination between known candidate objects and investigate the best possible accuracy with which the correct object can be identified. We analytically compute quantum-limited error bounds for hypothesis tests on any library of incoherent, quasimonochromatic objects when the imaging system is dominated by optical diffraction. We further show that object-independent linear-optical spatial processing of the collected light exactly achieves these ultimate error rates, exhibiting scaling superior to spatially resolved direct imaging as the scene becomes more severely diffraction limited. We apply our results to example imaging scenarios and find conditions under which superresolution object discrimination can be physically realized.
- Hao, S., Shi, H., Gagatsos, C. N., Mishra, M., Bash, B., Djordjevic, I., Guha, S., Zhuang, Q., & Zhang, Z. (2022). Demonstration of Entanglement-Enhanced Covert Sensing. Physical review letters, 129(1), 010501.More infoThe laws of quantum physics endow superior performance and security for information processing: quantum sensing harnesses nonclassical resources to enable measurement precision unmatched by classical sensing, whereas quantum cryptography aims to unconditionally protect the secrecy of the processed information. Here, we present the theory and experiment for entanglement-enhanced covert sensing, a paradigm that simultaneously offers high measurement precision and data integrity by concealing the probe signal in an ambient noise background so that the execution of the protocol is undetectable with a high probability. We show that entanglement offers a performance boost in estimating the imparted phase by a probed object, as compared to a classical protocol at the same covertness level. The implemented entanglement-enhanced covert sensing protocol operates close to the fundamental quantum limit by virtue of its near-optimum entanglement source and quantum receiver. Our work is expected to create ample opportunities for quantum information processing at unprecedented security and performance levels.
- , A. M., , M. P., , D. T., , S. G., & , A. S. (2021). Percolation Thresholds for Robust Network Connectivity.More infoCommunication networks, power grids, and transportation networks are allexamples of networks whose performance depends on reliable connectivity oftheir underlying network components even in the presence of usual networkdynamics due to mobility, node or edge failures, and varying traffic loads.Percolation theory quantifies the threshold value of a local control parametersuch as a node occupation (resp., deletion) probability or an edge activation(resp., removal) probability above (resp., below) which there exists a giantconnected component (GCC), a connected component comprising of a number ofoccupied nodes and active edges whose size is proportional to the size of thenetwork itself. Any pair of occupied nodes in the GCC is connected via at leastone path comprised of active edges and occupied nodes. The mere existence ofthe GCC itself does not guarantee that the long-range connectivity would berobust, e.g., to random link or node failures due to network dynamics. In thispaper, we explore new percolation thresholds that guarantee not only spanningnetwork connectivity, but also robustness. We define and analyze four measuresof robust network connectivity, explore their interrelationships, andnumerically evaluate the respective robust percolation thresholds for the 2Dsquare lattice.[Journal_ref: ]
- , B. J., , P. N., , D. T., & , S. G. (2021). On a Class of Stochastic Multilayer Networks.More infoIn this paper, we introduce a new class of stochastic multilayer networks. Astochastic multilayer network is the aggregation of $M$ networks (one perlayer) where each is a subgraph of a foundational network $G$. Each layernetwork is the result of probabilistically removing links and nodes from $G$.The resulting network includes any link that appears in at least $K$ layers.This model is an instance of a non-standard site-bond percolation model. Twosets of results are obtained: first, we derive the probability distributionthat the $M$-layer network is in a given configuration for some particulargraph structures (explicit results are provided for a line and an algorithm isprovided for a tree), where a configuration is the collective state of alllinks (each either active or inactive). Next, we show that for appropriatescalings of the node and link selection processes in a layer, links areasymptotically independent as the number of layers goes to infinity, and followPoisson distributions. Numerical results are provided to highlight the impactof having several layers on some metrics of interest (including expected sizeof the cluster a node belongs to in the case of the line). This model findsapplications in wireless communication networks with multichannel radios,multiple social networks with overlapping memberships, transportation networks,and, more generally, in any scenario where a common set of nodes can be linkedvia co-existing means of connectivity.[Journal_ref: ]
- , C. G., & , S. G. (2021). Efficient representation of Gaussian states for multi-mode non-Gaussian quantum state engineering via subtraction of arbitrary number of photons. Phys. Rev. A.More infoWe introduce a complete description of a multi-mode bosonic quantum state inthe coherent-state basis (which in this work is denoted as "$K$" function ),which---up to a phase---is the square root of the well-known Husimi "$Q$"representation. We express the $K$ function of any $N$-mode Gaussian state as afunction of its covariance matrix and displacement vector, and also that of ageneral continuous-variable cluster state in terms of the modal squeezing andgraph topology of the cluster. This formalism lets us characterize the nonGaussian state left over when one measures a subset of modes of a Gaussianstate using photon number resolving detection, the fidelity of the obtainednon-Gaussian state with any target state, and the associated heraldingprobability, all analytically. We show that this probability can be expressedas a Hafnian, re-interpreting the output state of a circuit claimed todemonstrate quantum supremacy termed Gaussian boson sampling. As anexample-application of our formalism, we propose a method to prepare a two-modecoherent-cat-basis Bell state with fidelity close to unity and successprobability that is fundamentally higher than that of a well-known scheme thatsplits an approximate single-mode cat state---obtained by photon numbersubtraction on a squeezed vacuum mode---on a balanced beam splitter. Thisformalism could enable exploration of efficient generation of cat-basisentangled states, which are known to be useful for quantum error correctionagainst photon loss.[Journal_ref: Phys. Rev. A 99, 053816 (2019)]
- , C. N., & , S. G. (2021). Multiple photon subtraction from Gaussian states cannot produce arbitrary non-Gaussian quantum states of light.More infoGaussian states and measurements collectively are not powerful-enoughresources for quantum computing, as any Gaussian dynamics can be simulatedefficiently, classically. Photon subtraction from squeezed vacuum---asingle-mode Gaussian state in quantum optics---can produce an approximate catstate, a macroscopic superposition of two coherent states. Furthermore, it isknown that any one non-Gaussian Hamiltonian, along with Gaussian unitaries,makes for universal quantum resources. Photon subtraction, a readily-realizablenon-Gaussian operation, therefore, has been a popular tool to try and engineernon-Gaussian states for universal quantum processing. In this paper, we give aformula to calculate the fidelity between a non-Gaussian target state and theheralded state resulting from an $N$-mode Gaussian state subjected tomulti-mode, multi-photon subtraction. We also derive an easy-to-calculate upperbound of said fidelity. Then, we consider an example of an $N$-mode coherentcat-basis cluster state (CCCS), a resource sufficient for universal quantumcomputing---and prove that the fidelity between the state produced by photonsubtraction on any Gaussian state and the CCCS cannot be more than $1/2^N$.Further, we prove that photon subtraction can be used to prepare states whosefidelity with coherent GHZ states is very close to one.[Journal_ref: ]
- , E. K., , S. G., & , M. M. (2021). Asymptotic security of discrete-modulation protocols for continuous-variable quantum key distribution. Phys. Rev. A.More infoWe consider discrete-modulation protocols for continuous-variable quantum keydistribution (CV-QKD) that employ a modulation constellation consisting of afinite number of coherent states and that use a homodyne or aheterodyne-detection receiver. We establish a security proof for collectiveattacks in the asymptotic regime, and we provide a formula for an achievablesecret-key rate. Previous works established security proofs fordiscrete-modulation CV-QKD protocols that use two or three coherent states. Themain constituents of our approach include approximating a complex, isotropicGaussian probability distribution by a finite-size Gauss-Hermite constellation,applying entropic continuity bounds, and leveraging previous security proofsfor Gaussian-modulation protocols. As an application of our method, wecalculate secret-key rates achievable over a lossy thermal bosonic channel. Weshow that the rates for discrete-modulation protocols approach the ratesachieved by a Gaussian-modulation protocol as the constellation size isincreased. For pure-loss channels, our results indicate that in the high-lossregime and for sufficiently large constellation size, the achievable key ratesscale optimally, i.e., proportional to the channel's transmissivity.[Journal_ref: Phys. Rev. A 103, 012412 (2021)]
- , G. V., , S. G., , P. N., & , D. T. (2021). On the Capacity Region of Bipartite and Tripartite Entanglement Switching.More infoWe study a quantum switch serving a set of users. The function of the switchis to create bi- or tripartite entangled state among users at the highestpossible rates at a fixed ratio. We model a set of randomized switchingpolicies. Discovering that some are better than others, we present analyticalresults for the case where the switch stores one qubit per user, and find thatthe best policies outperform a time division multiplexing (TDM) policy forsharing the switch between bipartite and tripartite state generation. Thisperformance improvement decreases as the number of users grows. The model iseasily augmented to study the capacity region in the presence of qubitdecoherence, obtaining similar results. Moreover, decoherence appears to havelittle effect on capacity. We also study a smaller class of policies when theswitch stores two qubits per user.[Journal_ref: ]
- , G. V., , S. G., , P. N., & , D. T. (2021). On the Stochastic Analysis of a Quantum Entanglement Switch.More infoWe study a quantum entanglement switch that serves $k$ users in a startopology. We model variants of the system using Markov chains and standardqueueing theory and obtain expressions for switch capacity and the expectednumber of qubits stored in memory at the switch. While it is more accurate touse a discrete-time Markov chain (DTMC) to model such systems, we quicklyencounter practical constraints of using this technique and switch to usingcontinuous-time Markov chains (CTMCs). Using CTMCs allows us to obtain a numberof analytic results for systems in which the links are homogeneous orheterogeneous and for switches that have infinite or finite buffer sizes. Inaddition, we can model the effects of decoherence of quantum states fairlyeasily using CTMCs. We also compare the results we obtain from the DTMC againstthe CTMC in the case of homogeneous links and infinite buffer, and learn thatthe CTMC is a reasonable approximation of the DTMC. From numericalobservations, we discover that decoherence has little effect on capacity andexpected number of stored qubits for homogeneous systems. For heterogeneoussystems, especially those operating close to stability constraints, buffer sizeand decoherence can have significant effects on performance metrics. We alsolearn that in general, increasing the buffer size from one to two qubits perlink is advantageous to most systems, while increasing the buffer size furtheryields diminishing returns.[Journal_ref: ]
- , H. Q., , K. B., , C. W., & , S. G. (2021). Quantum precision of beam pointing.More infoWe consider estimating a small transverse displacement of an optical beamover a line-of-sight propagation path: a problem that has numerous importantapplications ranging from establishing a lasercom link, single-moleculetracking, guided munition, to atomic force microscopy. We establish theultimate quantum limit of the accuracy of sensing a beam displacement, andquantify the classical-quantum gap. Further, using normal-mode decomposition ofthe Fresnel propagation kernel, and insights from recent work onentanglement-assisted sensing, we find a near-term realizablemulti-spatio-temporal-mode continuous-variable entangled-state probe and areceiver design, which attains the quantum precision limit. We find aHeisenberg-limited sensitivity enhancement in terms of the number of entangledtemporal modes, and a curious super-Heisenberg quantum enhanced scaling interms of the number of entangled spatial modes permitted by thediffraction-limited beam propagation geometry.[Journal_ref: ]
- , K. P., , H. K., & , S. G. (2021). Continuous-variable entanglement distillation over a pure loss channel with multiple quantum scissors. Phys. Rev. A.More infoEntanglement distillation is a key primitive for distributing high-qualityentanglement between remote locations. Probabilistic noiseless linearamplification based on the quantum scissors is a candidate for entanglementdistillation from noisy continuous-variable (CV) entangled states. Being anon-Gaussian operation, quantum scissors is challenging to analyze. We presenta derivation of the non-Gaussian state heralded by multiple quantum scissors ina pure loss channel with two-mode squeezed vacuum input. We choose the reversecoherent information (RCI)---a proven lower bound on the distillableentanglement of a quantum state under one-way local operations and classicalcommunication (LOCC), as our figure of merit. We evaluate a Gaussian lowerbound on the RCI of the heralded state. We show that it can exceed theunlimited two-way LOCCassisted direct transmission entanglement distillationcapacity of the pure loss channel. The optimal heralded Gaussian RCI with twoquantum scissors is found to be significantly more than that with a singlequantum scissors, albeit at the cost of decreased success probability. Ourresults fortify the possibility of a quantum repeater scheme for CV quantumstates using the quantum scissors.[Journal_ref: Phys. Rev. A 100, 022315 (2019)]
- , K. P., , H. K., & , S. G. (2021). Continuous-variable quantum repeater based on quantum scissors and mode multiplexing. Phys. Rev. Research.More infoQuantum repeaters are indispensable for high-rate, long-distance quantumcommunications. The vision of a future quantum internet strongly hinges onrealizing quantum repeaters in practice. Numerous repeaters have been proposedfor discrete-variable (DV) single-photon-based quantum communications.Continuous variable (CV) encodings over the quadrature degrees of freedom ofthe electromagnetic field mode offer an attractive alternative. For example, CVtransmission systems are easier to integrate with existing optical telecomsystems compared to their DV counterparts. Yet, repeaters for CV have remainedelusive. We present a novel quantum repeater scheme for CV entanglementdistribution over a lossy bosonic channel that beats the direct transmissionexponential rate-loss tradeoff. The scheme involves repeater nodes consistingof a) two-mode squeezed vacuum (TMSV) CV entanglement sources, b) the quantumscissors operation to perform nondeterministic noiseless linear amplificationof lossy TMSV states, c) a layer of switched, mode multiplexing inspired bysecond-generation DV repeaters, which is the key ingredient apart fromprobabilistic entanglement purification that makes DV repeaters work, and d) anon-Gaussian entanglement swap operation. We report our exact results on therate-loss envelope achieved by the scheme.[Journal_ref: Phys. Rev. Research 2, 013310 (2020)]
- , M. R., & , S. G. (2021). Perturbation Theory for Quantum Information.More infoWe report lowest-order series expansions for primary matrix functions ofquantum states based on a perturbation theory for functions of linearoperators. Our theory enables efficient computation of functions of perturbedquantum states that assume only knowledge of the eigenspectrum of the zerothorder state and the density matrix elements of a zero-trace, Hermitianperturbation operator, not requiring analysis of the full state or theperturbation term. We develop theories for two classes of quantum stateperturbations, perturbations that preserve the vector support of the originalstate and perturbations that extend the support beyond the support of theoriginal state. We highlight relevant features of the two situations, inparticular the fact that functions and measures of perturbed quantum stateswith preserved support can be elegantly and efficiently represented usingFr\'echet derivatives. We apply our perturbation theories to find simpleexpressions for four of the most important quantities in quantum informationtheory that are commonly computed from density matrices: the Von Neumannentropy, the quantum relative entropy, the quantum Chernoff bound, and thequantum fidelity.[Journal_ref: ]
- , M. R., & , S. G. (2021). Quantum-Optimal Object Discrimination in Sub-Diffraction Incoherent Imaging.More infoWe consider imaging tasks involving discrimination between known objects andinvestigate the best possible accuracy with which the correct object can beidentified. Using the quantum Chernoff bound, we analytically find the ultimateachievable asymptotic error rate for symmetric hypothesis tests between\emph{any} two incoherent 2D objects when the imaging system is dominated byoptical diffraction. Furthermore, we show that linear-optical demultiplexing ofthe spatial modes of the collected light exactly saturates this ultimateperformance limit, enabling a quadratic improvement over the asymptotic errorrate achieved by direct imaging as the objects become more severelydiffraction-limited. We extend our results to identify the quantum limit andoptimal measurement for discrimination between an arbitrary number of candidateobjects. Our work constitutes a complete theoretical treatment of the ultimatequantitative limits on passive, sub-diffraction, incoherent objectdiscrimination and is readily applicable to a multitude of real-worldapplications.[Journal_ref: ]
- , P. D., , A. P., , H. K., & , S. G. (2021). Sub-exponential rate versus distance with time multiplexed quantum repeaters.More infoQuantum communications capacity using direct transmission over length-$L$optical fiber scales as $R \sim e^{-\alpha L}$, where $\alpha$ is the fiber'sloss coefficient. The rate achieved using a linear chain of quantum repeatersequipped with quantum memories, probabilistic Bell state measurements (BSMs)and switches used for spatial multiplexing, but no quantum error correction,was shown to surpass the direct-transmission capacity. However, this rate stilldecays exponentially with the end-to-end distance, viz., $R \sim e^{-s{\alphaL}}$, with $s < 1$. We show that the introduction of temporal multiplexing -i.e., the ability to perform BSMs among qubits at a repeater node that weresuccessfully entangled with qubits at distinct neighboring nodes at {\emdifferent} time steps - leads to a sub-exponential rate-vs.-distance scaling,i.e., $R \sim e^{-t\sqrt{\alpha L}}$, which is not attainable with just spatialor spectral multiplexing. We evaluate analytical upper and lower bounds to thisrate, and obtain the exact rate by numerically optimizing the time-multiplexingblock length and the number of repeater nodes. We further demonstrate thatincorporating losses in the optical switches used to implement timemultiplexing degrades the rate-vs.-distance performance, eventually fallingback to exponential scaling for very lossy switches. We also examine models forquantum memory decoherence and describe optimal regimes of operation topreserve the desired boost from temporal multiplexing. QM decoherence is seento be more detrimental to the repeater's performance over switching losses.[Journal_ref: ]
- , R. K., , S. G., & , A. A. (2021). Fundamental limit of resolving two point sources limited by an arbitrary point spread function.More infoEstimating the angular separation between two incoherently radiatingmonochromatic point sources is a canonical toy problem to quantify spatialresolution in imaging. In recent work, Tsang {\em et al.} showed, using aFisher Information analysis, that Rayleigh's resolution limit is just anartifact of the conventional wisdom of intensity measurement in the imageplane. They showed that the optimal sensitivity of estimating the angle is onlya function of the total photons collected during the camera's integration timebut entirely independent of the angular separation itself no matter how smallit is, and found the information-optimal mode basis, intensity detection inwhich achieves the aforesaid performance. We extend the above analysis, whichwas done for a Gaussian point spread function (PSF) to a hard-aperture pupilproving the information optimality of image-plane sinc-Bessel modes, andgeneralize the result further to an arbitrary PSF. We obtain newcounterintuitive insights on energy vs. information content in spatial modes,and extend the Fisher Information analysis to exact calculations of minimummean squared error, both for Gaussian and hard aperture pupils.[Journal_ref: ]
- , R. S., , S. G., & , O. P. (2021). Fock state interferometry for quantum enhanced phase discrimination.More infoWe study Fock state interferometry, consisting of a Mach-ZehnderInterferometer with two Fock state inputs and photon-number-resolved detectionat the two outputs. We show that it allows discrimination of a discrete numberof apriori-known optical phase shifts with an error probability lower than whatis feasible with classical techniques under a mean photon number constraint. Wecompare its performance with the optimal quantum probe for M-ary phasediscrimination, which unlike our probe, is difficult to prepare. Our techniquefurther allows discriminating a null phase shift from an increasingly small oneat zero probability of error under ideal conditions, a feature impossible toattain using classical probe light. Finally, we describe one application toquantum reading with binary phase-encoded memory pixels.[Journal_ref: ]
- , S. G., , Q. Z., & , B. B. (2021). Infinite-fold enhancement in communications capacity using pre-shared entanglement.More infoPre-shared entanglement can significantly boost communication rates in theregime of high thermal noise, and a low-brightness transmitter. In this regime,the ratio between the entanglement-assisted capacity and the Holevo capacity,the maximum reliable-communication rate permitted by quantum mechanics withoutany pre-shared entanglement as a resource, is known to scale as $\log(1/N_S)$,where $N_S \ll 1$ is the mean transmitted photon number per mode. This isespecially promising in enabling a large boost to radio-frequencycommunications in the weak-transmit-power regime, by exploiting pre-sharedoptical-frequency entanglement, e.g., distributed by the quantum internet. Inthis paper, we propose a structured design of a quantum transmitter andreceiver that leverages continuous-variable pre-shared entanglement from adownconversion source, which can harness this purported infinite-fold capacityenhancement---a problem open for over a decade. Finally, the implication ofthis result to the breaking of the well-known {\em square-root law} for covertcommunications, with pre-shared entanglement assistance, is discussed.[Journal_ref: ]
- , T. K., & , S. G. (2021). Boosting Linear-Optical Bell Measurement Success Probability with Pre-Detection Squeezing and Imperfect Photon-Number-Resolving Detectors. Phys. Rev. A.More infoLinear optical realizations of Bell state measurement (BSM) on twosingle-photon qubits succeed with probability $p_s$ no higher than $0.5$.However pre-detection quadrature squeezing, i.e., quantum noise limited phasesensitive amplification, in the usual linear-optical BSM circuit, can yield${p_s \approx 0.643}$. The ability to achieve $p_s > 0.5$ has been found to becritical in resource-efficient realizations of linear optical quantum computingand all-photonic quantum repeaters. Yet, the aforesaid value of $p_s > 0.5$ isnot known to be the maximum achievable using squeezing, thereby leaving it openwhether close-to-$100\%$ efficient BSM might be achievable using squeezing as aresource. In this paper, we report new insights on why squeezing-enhanced BSMachieves $p_s > 0.5$. Using this, we show that the previously-reported ${p_s\approx 0.643}$ at single-mode squeezing strength $r=0.6585$---for unambiguousstate discrimination (USD) of all four Bell states---is an experimentallyunachievable point result, which drops to $p_s \approx 0.59$ with the slightestchange in $r$. We however show that squeezing-induced boosting of $p_s$ withUSD operation is still possible over a continuous range of $r$, with anexperimentally achievable maximum occurring at $r=0.5774$, achieving ${p_s\approx 0.596}$. Finally, deviating from USD operation, we explore atrade-space between $p_s$, the probability with which the BSM circuit declaresa "success", versus the probability of error $p_e$, the probability of an inputBell state being erroneously identified given the circuit declares a success.Since quantum error correction could correct for some $p_e > 0$, this tradeoffmay enable better quantum repeater designs by potentially increasing theentanglement generation rates with $p_s$ exceeding what is possible withtraditionally-studied USD operation of BSMs.[Journal_ref: Phys. Rev. A 99, 032302 (2019)]
- , Z. D., , R. K., , A. A., & , S. G. (2021). Attaining the quantum limit of super resolution in imaging an object's length via pre-detection spatial mode sorting. Phys. Rev. A.More infoRecent work considered the ultimate (quantum) limit of the precision ofestimating the distance between two point objects. It was shown that theperformance gap between the quantum limit and that of ideal continuumimage-plane direct detection is the largest for highly sub-Rayleigh separationof the objects, and that a pre-detection mode sorting could attain the quantumlimit. Here we extend this to a more general problem of estimating the lengthof an incoherently radiating extended (line) object. We find, as expected bythe Rayleigh criterion, the Fisher information (FI) per integrated photonvanishes in the limit of small length for ideal image plane direct detection.Conversely, for a Hermite-Gaussian (HG) pre-detection mode sorter, thisnormalized FI does not decrease with decreasing object length, similar to thetwo point object case. However, unlike in the two-object problem, the FI perphoton of both detection strategies gradually decreases as the object lengthgreatly exceeds the Rayleigh limit, due to the relative inefficiency ofinformation provided by photons emanating from near the center of the objectabout its length. We evaluate the quantum Fisher information per unitintegrated photons and find that the HG mode sorter exactly achieves this limitat all values of the object length. Further, a simple binary mode sortermaintains the advantage of the full mode sorter at highly sub-Rayleigh length.In addition to this FI analysis, we quantify improvement in terms of the actualmean squared error of the length estimate. Finally, we consider the effect ofimperfect mode sorting, and show that the performance improvement over directdetection is robust over a range of sub-Rayleigh lengths.[Journal_ref: Phys. Rev. A 99, 033847 (2019)]
- Habif, J. L., Jagannathan, A., Gartenstein, S., Amory, P., & Guha, S. (2021). Quantum-limited discrimination of laser light and thermal light. Optics express, 29(5), 7418-7427.More infoUnderstanding the fundamental sensitivity limit of an optical sensor requires a full quantum mechanical description of the sensing task. In this work, we calculate the fundamental (quantum) limit for discriminating between pure laser light and thermal noise in a photon-starved regime. The Helstrom bound for discrimination error probability for single mode measurement is computed along with error probability bounds for direct detection, coherent homodyne detection and the Kennedy receiver. A generalized Kennedy (GK) receiver is shown to closely approach the Helstrom limit. We present an experimental demonstration of this sensing task and demonstrate a 15.4 dB improvement in discrimination sensitivity over direct detection using a GK receiver and an improvement of 19.4% in error probability over coherent detection.
- He, W., Guha, S., Shapiro, J. H., & Bash, B. A. (2021). Performance analysis of free-space quantum key distribution using multiple spatial modes. Optics express, 29(13), 19305-19318.More infoIn the diffraction-limited near-field propagation regime, free-space optical quantum key distribution (QKD) systems can employ multiple spatial modes to improve their key rate. This improvement can be effected by means of high-dimensional QKD or by spatial-mode multiplexing of independent QKD channels, with the latter, in general, offering higher key rates. Here, we theoretically analyze spatial-mode-multiplexed, decoy-state BB84 whose transmitter mode set is either a collection of phase-tilted, flat-top focused beams (FBs) or the Laguerre-Gaussian (LG) modes. Although for vacuum propagation the FBs suffer a QKD rate penalty relative to the LG modes, their potential ease of implementation make them an attractive alternative. Moreover, in the presence of turbulence, the FB modes may outperform the LG modes.
- Cui, C., Seshadreesan, K. P., Guha, S., & Fan, L. (2020). High-Dimensional Frequency-Encoded Quantum Information Processing with Passive Photonics and Time-Resolving Detection. Physical review letters, 124(19), 190502.More infoIn this Letter, we propose a new approach to process high-dimensional quantum information encoded in a photon frequency domain. In contrast to previous approaches based on nonlinear optical processes, no active control of photon energy is required. Arbitrary unitary transformation and projection measurement can be realized with passive photonic circuits and time-resolving detection. A systematic circuit design for a quantum frequency comb with arbitrary size has been given. The criteria to verify quantum frequency correlation has been derived. By considering the practical condition of the detector's finite response time, we show that high-fidelity operation can be readily realized with current device performance. This work will pave the way towards scalable and high-fidelity quantum information processing based on high-dimensional frequency encoding.
- Grace, M. R., Dutton, Z., Ashok, A., & Guha, S. (2020). Approaching quantum-limited imaging resolution without prior knowledge of the object location. Journal of the Optical Society of America. A, Optics, image science, and vision, 37(8), 1288-1299.More infoPassive imaging receivers that demultiplex an incoherent optical field into a set of orthogonal spatial modes prior to detection can surpass canonical diffraction limits on spatial resolution. However, these mode-sorting receivers exhibit sensitivity to contextual nuisance parameters (e.g., the centroid of a clustered or extended object), raising questions on their viability in realistic scenarios where prior information about the scene is limited. We propose a multistage detection strategy that segments the total recording time between different physical measurements to build up the required prior information for near quantum-optimal imaging performance at sub-Rayleigh length scales. We show, via Monte Carlo simulations, that an adaptive two-stage scheme that dynamically allocates recording time between a conventional direct detection measurement and a binary mode sorter outperforms idealized direct detection alone when no prior knowledge of the object centroid is available, achieving one to two orders of magnitude improvement in mean squared error for simple estimation tasks. Our scheme can be generalized for more sophisticated tasks involving multiple parameters and/or minimal prior information.
- , C. G., & , S. G. (2019). Efficient representation of Gaussian states for multi-mode non-Gaussian quantum state engineering via subtraction of arbitrary number of photons.More infoWe introduce a complete description of a multi-mode bosonic quantum state inthe coherent-state basis (which in this work is denoted as $K$ function ),which - up to a phase - is the square root of the well-known Husimi $Q$representation. We express the $K$ function of any $N$-mode Gaussian state as afunction of its covariance matrix and displacement vector, and also that of ageneral continuous-variable cluster state in terms of the modal squeezing andgraph topology of the cluster. This formalism lets us characterize the nonGaussian state left over when one measures a subset of modes of a Gaussianstate using photon number resolving detection, the fidelity of the obtainednon-Gaussian state with any target state, and the associated heraldingprobability, all analytically. We show that this probability can be expressedas a Hafnian, re-interpreting a recent proposal for quantum supremacy termedGaussian boson sampling. As an example-application of our formalism, we proposea method to prepare a two-mode coherent-cat-basis Bell state with fidelityclose to unity and success probability that is fundamentally higher than thatof a well-known scheme that splits an approximate single-mode catstate---obtained by photon number subtraction on a squeezed vacuum mode---on abalanced beam splitter. This formalism could enable exploration of efficientgeneration of cat-basis entangled states, which are known to be useful forquantum error correction against photon loss.[Journal_ref: ]
- , K. P., , H. K., & , S. G. (2019). A continuous-variable quantum repeater with quantum scissors.More infoThe quantum scissors operation lends itself as a tool for continuous variableentanglement distillation over lossy communication channels. We show that aquantum scissors can distill a near-perfect ebit of entanglement from atwo-mode squeezed vacuum state whose one share has undergone arbitrary (pure)loss, with a success probability that scales proportional to the channeltransmissivity. This is akin to entanglement distillation in single-photondiscrete variable entanglement distribution protocols. Invoking amultiplexing-based design for a quantum repeater scheme that was proposed fordiscrete variable encodings, we show a repeater scheme for CV quantumcommunication using quantum scissors that beats the direct-transmissionrate-loss tradeoff at large distances.[Journal_ref: ]
- , K. P., , H. K., & , S. G. (2019). Continuous-variable entanglement distillation over a pure loss channel with quantum scissors.More infoEntanglement distillation, the process of distilling from several copies of anoisy entangled state fewer copies of a more entangled state, is a key elementof a quantum repeater scheme for entanglement distribution. Quantum scissors,which can effect probabilistic noiseless linear amplification (NLA) on low meanphoton number states, has been proposed as a candidate for entanglementdistillation from noisy continuous variable (CV) quantum states distributedacross a communication channel. Being a non-Gaussian operation though, thequantum scissors is challenging to analyze, especially when more than one isdeployed. We present a derivation of the non-Gaussian state heralded bymultiple quantum scissors on a pure loss channel. We choose the reversecoherent information (RCI) of the heralded state as our figure of merit. Aprerequisite for any entanglement distillation scheme to be useful in a quantumrepeater scheme for entanglement distribution over the channel is that the RCIit heralds exceeds the direct transmission repeater-less entanglementdistribution capacity. We evaluate a Gaussian lower bound on the RCI for oneand two quantum scissors deployed in a pure loss channel and show that thereexist parameter regimes where this condition is met. We show that the optimalheralded RCI with two quantum scissors is significantly more than with a singlequantum scissors, albeit at the cost of decreased success probability. Ouranalysis fortifies the possibility of a quantum repeater scheme for continuousvariable quantum states based on quantum scissors.[Journal_ref: ]
- , M. P., , D. T., , D. E., & , S. G. (2019). Percolation thresholds for photonic quantum computing.More infoAny quantum algorithm can be implemented by an adaptive sequence of singlenode measurements on an entangled cluster of qubits in a square latticetopology. Photons are a promising candidate for encoding qubits but assemblinga photonic entangled cluster with linear optical elements relies onprobabilistic operations. Given a supply of $n$-photon-entangled microclusters,using a linear optical circuit and photon detectors, one can assemble a randomentangled state of photons that can be subsequently "renormalized" into alogical cluster for universal quantum computing. In this paper, we prove thatthere is a fundamental tradeoff between $n$ and the minimum success probability$\lambda_c^{(n)}$ that each two-photon linear-optical fusion operation musthave, in order to guarantee that the resulting state can be renormalized:$\lambda_c^{(n)} \ge 1/(n-1)$. We present a new way of formulating this problemwhere $\lambda_c^{(n)}$ is the bond percolation threshold of a logical graphand provide explicit constructions to produce a percolated cluster using $n=3$photon microclusters (GHZ states) as the initial resource. We settle aheretofore open question by showing that a renormalizable cluster can becreated with $3$-photon microclusters over a 2D graph without feedforward,which makes the scheme extremely attractive for an integrated-photonicrealization. We also provide lattice constructions, which show that $0.5 \le\lambda_c^{(3)} \le 0.5898$, improving on a recent result of $\lambda_c^{(3)}\le 0.625$. Finally, we discuss how losses affect the bounds on the threshold,using loss models inspired by a recently-proposed method to produce photonicmicroclusters using quantum dot emitters.[Journal_ref: ]
- , M. P., , H. K., , D. T., , L. T., , L. J., , P. B., , D. E., & , S. G. (2019). Routing entanglement in the quantum internet.More infoRemote quantum entanglement can enable numerous applications includingdistributed quantum computation, secure communication, and precision sensing.In this paper, we consider how a quantum network-nodes equipped with limitedquantum processing capabilities connected via lossy optical links-candistribute high-rate entanglement simultaneously between multiple pairs ofusers (multiple flows). We develop protocols for such quantum "repeater" nodes,which enable a pair of users to achieve large gains in entanglement rates overusing a linear chain of quantum repeaters, by exploiting the diversity ofmultiple paths in the network. Additionally, we develop repeater protocols thatenable multiple user pairs to generate entanglement simultaneously at ratesthat can far exceed what is possible with repeaters time sharing amongassisting individual entanglement flows. Our results suggest that theearly-stage development of quantum memories with short coherence times andimplementations of probabilistic Bell-state measurements can have a much moreprofound impact on quantum networks than may be apparent from analyzing linearrepeater chains. This framework should spur the development of a generalquantum network theory, bringing together quantum memory physics, quantuminformation theory, and computer network theory.[Journal_ref: ]
- , R. S., , B. B., , D. G., , S. G., & , D. T. (2019). Covert Single-hop Communication in a Wireless Network with Distributed Artificial Noise Generation.More infoCovert communication, also known as low probability of detection (LPD)communication, prevents the adversary from knowing that a communication istaking place. Recent work has demonstrated that, in a three-party scenario witha transmitter (Alice), intended recipient (Bob), and adversary (Warden Willie),the maximum number of bits that can be transmitted reliably from Alice to Bobwithout detection by Willie, when additive white Gaussian noise (AWGN) channelsexist between all parties, is on the order of the square root of the number ofchannel uses. In this paper, we begin consideration of network scenarios bystudying the case where there are additional "friendly" nodes present in theenvironment that can produce artificial noise to aid in hiding thecommunication. We establish achievability results by considering constructionswhere the system node closest to the warden produces artificial noise anddemonstrate a significant improvement in the throughput achieved covertly,without requiring close coordination between Alice and the noise-generatingnode. Conversely, under mild restrictions on the communication strategy, wedemonstrate no higher covert throughput is possible. Extensions to theconsideration of the achievable covert throughput when multiple wardensrandomly located in the environment collaborate to attempt detection of thetransmitter are also considered.[Journal_ref: ]
- Babaeian, M., Nguyen, D. T., Demir, V., Akbulut, M., Blanche, P. A., Kaneda, Y., Guha, S., Neifeld, M. A., & Peyghambarian, N. (2019). A single shot coherent Ising machine based on a network of injection-locked multicore fiber lasers. Nature communications, 10(1), 3516.More infoCombinatorial optimization problems over large and complex systems have many applications in social networks, image processing, artificial intelligence, computational biology and a variety of other areas. Finding the optimized solution for such problems in general are usually in non-deterministic polynomial time (NP)-hard complexity class. Some NP-hard problems can be easily mapped to minimizing an Ising energy function. Here, we present an analog all-optical implementation of a coherent Ising machine (CIM) based on a network of injection-locked multicore fiber (MCF) lasers. The Zeeman terms and the mutual couplings appearing in the Ising Hamiltonians are implemented using spatial light modulators (SLMs). As a proof-of-principle, we demonstrate the use of optics to solve several Ising Hamiltonians for up to thirteen nodes. Overall, the average accuracy of the CIM to find the ground state energy was ~90% for 120 trials. The fundamental bottlenecks for the scalability and programmability of the presented CIM are discussed as well.
- Pant, M., Towsley, D., Englund, D., & Guha, S. (2019). Percolation thresholds for photonic quantum computing. Nature communications, 10(1), 1070.More infoDespite linear-optical fusion (Bell measurement) being probabilistic, photonic cluster states for universal quantum computation can be prepared without feed-forward by fusing small n-photon entangled clusters, if the success probability of each fusion attempt is above a threshold, [Formula: see text]. We prove a general bound [Formula: see text], and develop a conceptual method to construct long-range-connected clusters where [Formula: see text] becomes the bond percolation threshold of a logical graph. This mapping lets us find constructions that require lower fusion success probabilities than currently known, and settle a heretofore open question by showing that a universal cluster state can be created by fusing 3-photon clusters over a 2D lattice with a fusion success probability that is achievable with linear optics and single photons, making this attractive for integrated-photonic realizations.
- , D. D., & , S. G. (2018). Noisy Feedback and Loss Unlimited Private Communication. IEEE International Symposium on Information Theory (ISIT), Vail, CO,, 586-590.More infoAlice is transmitting a private message to Bob across a bosonic wiretapchannel with the help of a public feedback channel to which all parties,including the fully-quantum equipped Eve, have completely noiseless access. Wefind that by altering the model such that Eve's copy of the initial round offeedback is corrupted by an iota of noise, one step towards physical relevance,the capacity can be increased dramatically. It is known that the privatecapacity with respect to the original model for a pure-loss bosonic channel isat most $- \log(1-\eta)$ bits per mode, where $\eta$ is the transmissivity, inthe limit of infinite input photon number. This is a very pessimistic result asthere is a finite rate limit even with an arbitrarily large number of inputphotons. We refer to this as a loss limited rate. However, in our altered modelwe find that we can achieve a rate of $(1/2) \log(1 + 4 \eta N_S)$ bits permode, where $N_S$ is the input photon number. This rate diverges with $N_S$, insharp contrast to the result for the original model. This suggests thatphysical considerations behind the eavesdropping model should be taken moreseriously, as they can create strong dependencies of the achievable rates onthe model. For by a seemingly inconsequential weakening of Eve, we obtain aloss-unlimited rate. Our protocol also works verbatim for arbitrary i.i.d.noise (not even necessarily Gaussian) injected by Eve in every round, and evenif Eve is given access to copies of the initial transmission and noise. Theerror probability of the protocol decays super-exponentially with theblocklength.[Journal_ref: 2018 IEEE International Symposium on Information Theory (ISIT), Vail, CO, 2018, pp. 586-590]
- Lu, X., Krovi, H., Nair, R., Guha, S., & Shapiro, J. H. (2018). Quantum-optimal detection of one-versus-two incoherent optical sources with arbitrary separation. NPJ QUANTUM INFORMATION, 4.
- Soltani, R., Goeckel, D., Towsley, D., Bash, B. A., & Guha, S. (2018). Covert Wireless Communication With Artificial Noise Generation. IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, 17(11), 7252-7267.
- , O. J., & , S. G. (2017). A de Bruijn identity for discrete random variables. Proceedings of the International Symposium on Information Theory,, 898-902.More infoWe discuss properties of the "beamsplitter addition" operation, whichprovides a non-standard scaled convolution of random variables supported on thenon-negative integers. We give a simple expression for the action ofbeamsplitter addition using generating functions. We use this to give aself-contained and purely classical proof of a heat equation and de Bruijnidentity, satisfied when one of the variables is geometric.[Journal_ref: Proceedings of the International Symposium on Information Theory, 2017, p898-902]
- Guha, S., Towsley, D., Nain, P., Çapar, ., Swami, A., & Basu, P. (2016). Spanning connectivity in a multilayer network and its relationship to site-bond percolation. Physical review. E, 93(6), 062310.More infoWe analyze the connectivity of an M-layer network over a common set of nodes that are active only in a fraction of the layers. Each layer is assumed to be a subgraph (of an underlying connectivity graph G) induced by each node being active in any given layer with probability q. The M-layer network is formed by aggregating the edges over all M layers. We show that when q exceeds a threshold q_{c}(M), a giant connected component appears in the M-layer network-thereby enabling far-away users to connect using "bridge" nodes that are active in multiple network layers-even though the individual layers may only have small disconnected islands of connectivity. We show that q_{c}(M)≲sqrt[-ln(1-p_{c})]/sqrt[M], where p_{c} is the bond percolation threshold of G, and q_{c}(1)≡q_{c} is its site-percolation threshold. We find q_{c}(M) exactly for when G is a large random network with an arbitrary node-degree distribution. We find q_{c}(M) numerically for various regular lattices and find an exact lower bound for the kagome lattice. Finally, we find an intriguingly close connection between this multilayer percolation model and the well-studied problem of site-bond percolation in the sense that both models provide a smooth transition between the traditional site- and bond-percolation models. Using this connection, we translate known analytical approximations of the site-bond critical region, which are functions only of p_{c} and q_{c} of the respective lattice, to excellent general approximations of the multilayer connectivity threshold q_{c}(M).
- Barzanjeh, S., Guha, S., Weedbrook, C., Vitali, D., Shapiro, J. H., & Pirandola, S. (2015). Microwave quantum illumination. Physical review letters, 114(8), 080503.More infoQuantum illumination is a quantum-optical sensing technique in which an entangled source is exploited to improve the detection of a low-reflectivity object that is immersed in a bright thermal background. Here, we describe and analyze a system for applying this technique at microwave frequencies, a more appropriate spectral region for target detection than the optical, due to the naturally occurring bright thermal background in the microwave regime. We use an electro-optomechanical converter to entangle microwave signal and optical idler fields, with the former being sent to probe the target region and the latter being retained at the source. The microwave radiation collected from the target region is then phase conjugated and upconverted into an optical field that is combined with the retained idler in a joint-detection quantum measurement. The error probability of this microwave quantum-illumination system, or quantum radar, is shown to be superior to that of any classical microwave radar of equal transmitted energy.
- Bash, B. A., Gheorghe, A. H., Patel, M., Habif, J. L., Goeckel, D., Towsley, D., & Guha, S. (2015). Quantum-secure covert communication on bosonic channels. Nature communications, 6, 8626.More infoComputational encryption, information-theoretic secrecy and quantum cryptography offer progressively stronger security against unauthorized decoding of messages contained in communication transmissions. However, these approaches do not ensure stealth--that the mere presence of message-bearing transmissions be undetectable. We characterize the ultimate limit of how much data can be reliably and covertly communicated over the lossy thermal-noise bosonic channel (which models various practical communication channels). We show that whenever there is some channel noise that cannot in principle be controlled by an otherwise arbitrarily powerful adversary--for example, thermal noise from blackbody radiation--the number of reliably transmissible covert bits is at most proportional to the square root of the number of orthogonal modes (the time-bandwidth product) available in the transmission interval. We demonstrate this in a proof-of-principle experiment. Our result paves the way to realizing communications that are kept covert from an all-powerful quantum adversary.
- Takeoka, M., Guha, S., & Wilde, M. M. (2014). Fundamental rate-loss tradeoff for optical quantum key distribution. Nature communications, 5, 5235.More infoSince 1984, various optical quantum key distribution (QKD) protocols have been proposed and examined. In all of them, the rate of secret key generation decays exponentially with distance. A natural and fundamental question is then whether there are yet-to-be discovered optical QKD protocols (without quantum repeaters) that could circumvent this rate-distance tradeoff. This paper provides a major step towards answering this question. Here we show that the secret key agreement capacity of a lossy and noisy optical channel assisted by unlimited two-way public classical communication is limited by an upper bound that is solely a function of the channel loss, regardless of how much optical power the protocol may use. Our result has major implications for understanding the secret key agreement capacity of optical channels-a long-standing open problem in optical quantum information theory-and strongly suggests a real need for quantum repeaters to perform QKD at high rates over long distances.
- Dutton, Z., Guha, S., & Wilde, M. M. (2012). Performance of polar codes for quantum and private classical communication. Proceedings of the, 572-579.More infoWe analyze the practical performance of quantum polar codes, by computingrigorous bounds on block error probability and by numerically simulating them.We evaluate our bounds for quantum erasure channels with coding block lengthsbetween 2^10 and 2^20, and we report the results of simulations for quantumerasure channels, quantum depolarizing channels, and "BB84" channels withcoding block lengths up to N = 1024. For quantum erasure channels, we observethat high quantum data rates can be achieved for block error rates less than10^(-4) and that somewhat lower quantum data rates can be achieved for quantumdepolarizing and BB84 channels. Our results here also serve as bounds for andsimulations of private classical data transmission over these channels,essentially due to Renes' duality bounds for privacy amplification andclassical data transmission of complementary observables. Future work might beable to improve upon our numerical results for quantum depolarizing and BB84channels by employing a polar coding rule other than the heuristic used here.[Journal_ref: Proceedings of the 50th Annual Allerton Conference on Communication, Control, and Computing, pages 572-579, October 2012]
- Wilde, M. M., Hayden, P., & Guha, S. (2012). Information trade-offs for optical quantum communication. Physical review letters, 108(14), 140501.More infoRecent work has precisely characterized the achievable trade-offs between three key information processing tasks-classical communication (generation or consumption), quantum communication (generation or consumption), and shared entanglement (distribution or consumption), measured in bits, qubits, and ebits per channel use, respectively. Slices and corner points of this three-dimensional region reduce to well-known protocols for quantum channels. A trade-off coding technique can attain any point in the region and can outperform time sharing between the best-known protocols for accomplishing each information processing task by itself. Previously, the benefits of trade-off coding that had been found were too small to be of practical value (viz., for the dephasing and the universal cloning machine channels). In this Letter, we demonstrate that the associated performance gains are in fact remarkably high for several physically relevant bosonic channels that model free-space or fiber-optic links, thermal-noise channels, and amplifiers. We show that significant performance gains from trade-off coding also apply when trading photon-number resources between transmitting public and private classical information simultaneously over secret-key-assisted bosonic channels.
- Guha, S. (2011). Structured optical receivers to attain superadditive capacity and the Holevo limit. Physical review letters, 106(24), 240502.More infoAttaining the ultimate (Holevo) limit to the classical capacity of a quantum channel requires the receiver to make joint measurements over long code-word blocks. For a pure-state channel, we show that the Holevo limit can be attained by a receiver that uses a multisymbol unitary transformation on the quantum code word followed by separable projective measurements. We show a concatenated coding and joint-detection architecture to approach the Holevo limit. We then construct some of the first concrete examples of codes and structured joint-detection receivers for the lossy bosonic channel, which can achieve fundamentally higher (superadditive) capacity than conventional receivers that detect each modulation symbol individually. We thereby pave the way for research into codes and structured receivers for reliable communication data rates approaching the Holevo limit.
- Guha, S., Habif, J. L., & Takeoka, M. (2011). PPM demodulation: On approaching fundamental limits of optical communications. Journal of Modern Optics, Volume.More infoWe consider the problem of demodulating M-ary optical PPM (pulse-positionmodulation) waveforms, and propose a structured receiver whose mean probabilityof symbol error is smaller than all known receivers, and approaches the quantumlimit. The receiver uses photodetection coupled with optimized phase-coherentoptical feedback control and a phase-sensitive parametric amplifier. We presenta general framework of optical receivers known as the conditional pulse nullingreceiver, and present new results on ultimate limits and achievable regions ofspectral versus photon efficiency tradeoffs for the single-spatial-modepure-loss optical communication channel.[Journal_ref: Journal of Modern Optics, Volume 58 Issue 3, 257, 2011]
- Tan, S. H., Erkmen, B. I., Giovannetti, V., Guha, S., Lloyd, S., Maccone, L., Pirandola, S., & Shapiro, J. H. (2008). Quantum illumination with Gaussian states. Physical review letters, 101(25), 253601.More infoAn optical transmitter irradiates a target region containing a bright thermal-noise bath in which a low-reflectivity object might be embedded. The light received from this region is used to decide whether the object is present or absent. The performance achieved using a coherent-state transmitter is compared with that of a quantum-illumination transmitter, i.e., one that employs the signal beam obtained from spontaneous parametric down-conversion. By making the optimum joint measurement on the light received from the target region together with the retained spontaneous parametric down-conversion idler beam, the quantum-illumination system realizes a 6 dB advantage in the error-probability exponent over the optimum reception coherent-state system. This advantage accrues despite there being no entanglement between the light collected from the target region and the retained idler beam.