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1. 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)

   Abstract 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. 

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2. Accuracy Self-Estimation in an 8-Dimensional Quantum State Identification at a Telecom Wavelength

   Annafianto, N. F.; Jabir, M.; Burenkov, I. A.; Battou, A.; Polyakov, S. V. (May 2023, Conference on Lasers and Electro-Optics (CLEO))

   We experimentally obtain single-shot confidences of a state estimation measurement with M=8 arbitrary sets of non-orthogonal states. The 8-dimensional vector of confidences represents the best knowledge of the input state for each individual quantum measurement. 

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3. Single-Shot Non-Gaussian Measurements for Optical Phase Estimation 125 (12), 120505 (2020).

   https://doi.org/10.1103/PhysRevLett.125.120505  

   DiMario, M T; Becerra, F E (September 2020, Physical review letters)

   null (Ed.)

   Estimation of the properties of a physical system with minimal uncertainty is a central task in quantum metrology. Optical phase estimation is at the center of many metrological tasks where the value of a physical parameter is mapped to the phase of an electromagnetic field and single-shot measurements of this phase are necessary. While there are measurements able to estimate the phase of light in a single shot with small uncertainties, demonstrations of near-optimal single-shot measurements for an unknown phase of a coherent state remain elusive. Here, we propose and demonstrate strategies for single-shot measurements for ab initio phase estimation of coherent states that surpass the sensitivity limit of heterodyne measurement and approach the Cramer-Rao lower bound for coherent states. These single-shot estimation strategies are based on real-time optimization of coherent displacement operations, single photon counting with photon number resolution, and fast feedback. We show that our demonstration of these optimized estimation strategies surpasses the heterodyne limit for a wide range of optical powers without correcting for detection efficiency with a moderate number of adaptive measurement steps. This is, to our knowledge, the most sensitive single-shot measurement of an unknown phase encoded in optical coherent states. 

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4. On Fault Tolerant Single-Shot Logical State Preparation and Robust Long-Range Entanglement

   https://doi.org/10.4230/lipics.itcs.2025.16  

   Bergamaschi, Thiago; Liu, Yunchao (January 2025, Schloss Dagstuhl – Leibniz-Zentrum für Informatik)

   Meka, Raghu (Ed.)

   Preparing encoded logical states is the first step in a fault-tolerant quantum computation. Standard approaches based on concatenation or repeated measurement incur a significant time overhead. The Raussendorf-Bravyi-Harrington cluster state [Raussendorf et al., 2005] offers an alternative: a single-shot preparation of encoded states of the surface code, by means of a constant depth quantum circuit, followed by a single round of measurement and classical feedforward [Bravyi et al., 2020]. In this work we generalize this approach and prove that single-shot logical state preparation can be achieved for arbitrary quantum LDPC codes. Our proof relies on a minimum-weight decoder and is based on a generalization of Gottesman’s clustering-of-errors argument [Gottesman, 2014]. As an application, we also prove single-shot preparation of the encoded GHZ state in arbitrary quantum LDPC codes. This shows that adaptive noisy constant depth quantum circuits are capable of generating generic robust long-range entanglement. 

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5. Demonstration of optimal non-projective measurement of binary coherent states with photon counting

   https://doi.org/10.1038/s41534-022-00595-3  

   DiMario, M. T.; Becerra, F. E. (July 2022, npj Quantum Information)

   Abstract Quantum state discrimination is a central problem in quantum measurement theory, with applications spanning from quantum communication to computation. Typical measurement paradigms for state discrimination involve a minimum probability of error or unambiguous discrimination with a minimum probability of inconclusive results. Alternatively, an optimal inconclusive measurement, a non-projective measurement, achieves minimal error for a given inconclusive probability. This more general measurement encompasses the standard measurement paradigms for state discrimination and provides a much more powerful tool for quantum information and communication. Here, we experimentally demonstrate the optimal inconclusive measurement for the discrimination of binary coherent states using linear optics and single-photon detection. Our demonstration uses coherent displacement operations based on interference, single-photon detection, and fast feedback to prepare the optimal feedback policy for the optimal non-projective quantum measurement with high fidelity. This generalized measurement allows us to transition among standard measurement paradigms in an optimal way from minimum error to unambiguous measurements for binary coherent states. As a particular case, we use this general measurement to implement the optimal minimum error measurement for phase-coherent states, which is the optimal modulation for communications under the average power constraint. Moreover, we propose a hybrid measurement that leverages the binary optimal inconclusive measurement in conjunction with sequential, unambiguous state elimination to realize higher dimensional inconclusive measurements of coherent states. 

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