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1. Belief propagation with quantum messages for quantum-enhanced classical communications

   https://doi.org/10.1038/s41534-021-00422-1  

   Rengaswamy, Narayanan ; Seshadreesan, Kaushik P. ; Guha, Saikat ; Pfister, Henry D. ( June 2021 , npj Quantum Information)

   For space-based laser communications, when the mean photon number per received optical pulse is much smaller than one, there is a large gap between communications capacity achievable with a receiver that performs individual pulse-by-pulse detection, and the quantum-optimal “joint-detection receiver” that acts collectively on long codeword-blocks of modulated pulses; an effect often termed “superadditive capacity”. In this paper, we consider the simplest scenario where a large superadditive capacity is known: a pure-loss channel with a coherent-state binary phase-shift keyed (BPSK) modulation. The two BPSK states can be mapped conceptually to two non-orthogonal states of a qubit, described by an inner product that is a function of the mean photon number per pulse. Using this map, we derive an explicit construction of the quantum circuit of a joint-detection receiver based on a recent idea of “belief-propagation with quantum messages” (BPQM). We quantify its performance improvement over the Dolinar receiver that performs optimal pulse-by-pulse detection, which represents the best “classical” approach. We analyze the scheme rigorously and show that it achieves the quantum limit of minimum average error probability in discriminating 8 (BPSK) codewords of a length-5 binary linear code with a tree factor graph. Our result suggests that a BPQMmore »receiver might attain the Holevo capacity of this BPSK-modulated pure-loss channel. Moreover, our receiver circuit provides an alternative proposal for a quantum supremacy experiment, targeted at a specific application that can potentially be implemented on a small, special-purpose, photonic quantum computer capable of performing cat-basis universal qubit logic.

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2. Energy and bandwidth efficiency optimization of quantum-enabled optical communication channels

   https://doi.org/10.1038/s41534-022-00573-9  

   Jabir, M. V. ; Annafianto, N. Fajar R. ; Burenkov, I. A. ; Battou, A. ; Polyakov, S. V. ( May 2022 , npj Quantum Information)

   We present a systematic study of quantum receivers and modulation methods enabling resource efficient quantum-enhanced optical communication. We introduce quantum-inspired modulation schemes that theoretically yield a better resource efficiency than legacy protocols. Experimentally, we demonstrate below the shot-noise limit symbol error rates forM ≤ 16 legacy and quantum-inspired communication alphabets using software-configurable optical communication time-resolving quantum receiver testbed. Further, we experimentally verify that our quantum-inspired modulation schemes boost the accuracy of practical quantum measurements and significantly optimize the combined use of energy and bandwidth for communication alphabets that are longer thanM = 4 symbols.
3. A Novel Architecture for Ultra-High Signal-to-Interference-Noise-Ratio Reception in Visible Light Communication

   https://doi.org/10.1109/COMSNETS.2019.8711377  

   Rahman, MD Rashed ; Islam, S M ; Ashok, Ashwin ( January 2019 , 11th International Conference on Communication Systems & Networks (COMSNETS))

   Theoretical models estimate visible light communication (VLC) data capacity to be of the order of Tera-bits-per-second (Tbps). However, practical limitations in receiver designs have limited state-of-the-art VLC prototypes to (multiple) orders of magnitude lower data rates. This paper explores a new architecture to realize ultra-high data rates in visible light communication systems by dramatically improving the Signal-to-Interference-Noise-Ratio (SINR) at the receiver. The key idea is to leverage the fast sampling rates of photodiode receivers and integrate a shutter mechanism that filters noise and interference thus creating a high-speed imaging receiver effect. Through adaptive selection of the exact receiver area over which the transmitted light is detected, the SINR can be dramatically increased yet not compromising the high sampling rate achievable using state-of-the-art photoreceptors. In addition to introducing the new hybrid architecture for high SINR reception, in this paper, we study the feasibility of noise and interference reduction through a proof-of-concept experimentation.
4. Reconstruction of Sparsely Sampled Seismic Data via Residual U-Net

   https://doi.org/10.1109/LGRS.2020.3035835  

   Tang, Shuhang ; Ding, Yinshuai ; Zhou, Hua-Wei ; Zhou, Heng ( November 2020 , IEEE Geoscience and Remote Sensing Letters)

   Reconstruction of sparsely sampled seismic data is critical for maintaining the quality of seismic images when significant numbers of shots and receivers are missing.We present a reconstruction method in the shot-receiver-time (SRT) domain based on a residual U-Net machine learning architecture, for seismic data acquired in a sparse 2-D acquisition and name it SRT2D-ResU-Net. The SRT domain retains a high level of seismic signal connectivity, which is likely the main data feature that the reconstructing algorithms rely on. We develop an “in situ training and prediction” workflow by dividing the acquisition area into two nonoverlapping subareas: a training subarea for establishing the network model using regularly sampled data and a testing subarea for reconstructing the sparsely sampled data using the trained model. To establish a reference base for analyzing the changes in data features over the study area, and quantifying the reconstructed seismic data, we devise a baseline reference using a tiny portion of the field data. The baselines are properly spaced and excluded from the training and reconstruction processes. The results on a field marine data set show that the SRT2D-ResU-Net can effectively learn the features of seismic data in the training process, and the average correlation between themore »reconstructed missing traces and the true answers is over 85%.« less
5. Coherence in logical quantum channels

   https://doi.org/10.1088/1367-2630/ab8e5c  

   Iverson, Joseph K. ; Preskill, John ( July 2020 , New Journal of Physics)

   We study the effectiveness of quantum error correction against coherent noise. Coherent errors (for example, unitary noise) can interfere constructively, so that in some cases the average infidelity of a quantum circuit subjected to coherent errors may increase quadratically with the circuit size; in contrast, when errors are incoherent (for example, depolarizing noise), the average infidelity increases at worst linearly with circuit size. We consider the performance of quantum stabilizer codes against a noise model in which a unitary rotation is applied to each qubit, where the axes and angles of rotation are nearly the same for all qubits. In particular, we show that for the toric code subject to such independent coherent noise, and for minimal-weight decoding, the logical channel after error correction becomes increasingly incoherent as the length of the code increases, provided the noise strength decays inversely with the code distance. A similar conclusion holds for weakly correlated coherent noise. Our methods can also be used for analyzing the performance of other codes and fault-tolerant protocols against coherent noise. However, our result does not show that the coherence of the logical channel is suppressed in the more physically relevant case where the noise strength is heldmore »constant as the code block grows, and we recount the difficulties that prevented us from extending the result to that case. Nevertheless our work supports the idea that fault-tolerant quantum computing schemes will work effectively against coherent noise, providing encouraging news for quantum hardware builders who worry about the damaging effects of control errors and coherent interactions with the environment.

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