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Optica Quantum

Joint-detection learning for optical communication at the quantum limit

Matteo Rosati, Albert Solana

Published May 30, 2024• 11 citations
Publisher

Abstract

Optical communication technology can be enhanced by using quantum signals to transfer classical bits. This requires the message-carrying signals to interact coherently at the decoder via a joint-detection receiver (JDR). To date, the realization of a JDR using optical technologies has remained elusive: the only explicit design, called a Green–Hadamard receiver (GHR), increases distinguishability at the cost of reducing the code size. We introduce a supervised-learning framework for the systematic discovery of optical JDR designs based on parametrized photonic integrated circuits. We find JDR designs with higher decoding success probability than any single-symbol receiver, including homodyne, Kennedy, and Dolinar. Furthermore, our new receiver families surpass the GHR receiver for mean photon number >0.1, both in terms of code size and decoding probability, paving the way for practical applications of JDR in optical fiber networks and free-space.

DOI
10.1364/opticaq.521637

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Optical communication technology can be enhanced by using quantum signals to transfer classical bits. This requires the message-carrying signals to interact coherently at the decoder via a joint-detection receiver (JDR). To date, the realization of a JDR using optical technologies has remained elusive: the only explicit design, called a Green–Hadamard receiver (GHR), increases distinguishability at the cost of reducing the code size. We introduce a supervised-learning framework for the systematic discovery of optical JDR designs based on parametrized photonic integrated circuits. We find JDR designs with higher decoding success probability than any single-symbol receiver, including homodyne, Kennedy, and Dolinar. Furthermore, our new receiver families surpass the GHR receiver for mean photon number >0.1, both in terms of code size and decoding probability, paving the way for practical applications of JDR in optical fiber networks and free-space.
Quantum information theory predicts that communication technology can be enhanced by using quantum signals to transfer classical bits. In order to fulfill this promise, the message-carrying signals must interact coherently at the decoding stage via a joint-detection receiver (JDR), whose realization with optical technologies remains an outstanding open problem to date. We introduce a supervised-learning framework for the systematic discovery of new JDR designs based on parametrized photonic integrated circuits. Our framework relies on the synthesis of a training set comprising quantum codewords and the corresponding classical message label; the codewords are processed by the JDR circuit and, after photo-detection, produce a guess for the label. The circuit parameters are then updated by minimizing a suitable loss function, reaching an optimal JDR design for that specific architecture. We showcase our method with coherent-state codes for the pure-loss bosonic channel, modelling optical-fiber and space communication, with a circuit architecture comprising linear optics, squeezing and threshold photo-detectors. We train JDR circuits for several code families, varying energy and code-size. We discover optical JDR circuit setups for maximum-size codes and small message-length that offer up to a 333-fold enhancement in the bit decoding rate with respect to the optimal single-symbol receiver, and less than 7%7\%7%-away from the theoretically optimal decoder, for which an explicit design is missing to date. Furthermore, the discovered receivers surpass previous JDR designs both in terms of bit decoding and bit transmission rate. Finally, we observe that the best-performing codes are those which can be mapped, via the JDR's optical processing, to modulations with different energy levels on different symbols, making the message symbols more distinguishabile via photo-detection.
Abstract Bit-transmission can be enhanced by the use of quantum detection techniques, realizing a joint-detection receiver (JDR) that is able to decode transmitted signals via a collective operation and achieve the Holevo channel capacity. Explicit JDR designs proposed so far employ the Hadamard or Fourier transform to perform a phase-to-intensity translation of the information encoding, effectively falling in the class of on-off-keying (OOK) modulation techniques; they improve over classical decoders but fall short of the Holevo capacity, particularly at large signal mean photon number n ≳ 1 . Here we introduce new families of decoders based on multi-pulse and multi-level codes. We compute the rate of these codes exactly, and provide a comprehensive study of their performance. We show that multi-pulse codes can approach the rate of OOK closely, providing a simplified design for quantum-enhanced communication in the photon-starved regime; furthermore, multi-level codes can approach generalized-OOK strategies with multiple pulse types, thus they can be employed in the larger photon-number regime.
Abstract Optical communication is the standard for high-bandwidth information transfer in today’s digital age. The increasing demand for bandwidth has led to the maturation of coherent transceivers that use phase- and amplitude-modulated optical signals to encode more bits of information per transmitted pulse. Such encoding schemes achieve higher information density, but also require more complicated receivers to discriminate the signaling states. In fact, achieving the ultimate limit of optical communication capacity, especially in the low light regime, requires coherent joint detection of multiple pulses. Despite their superiority, such joint detection receivers are not in widespread use because of the difficulty of constructing them in the optical domain. In this work we describe how optomechanical transduction of phase information from coherent optical pulses to superconducting qubit states followed by the execution of trained short-depth variational quantum circuits can perform joint detection of communication codewords with error probabilities that surpass all classical, individual pulse detection receivers. Importantly, we utilize a model of optomechanical transduction that captures non-idealities such as thermal noise and loss in order to understand the transduction performance necessary to achieve a quantum advantage with such a scheme. We also execute the trained variational circuits on an IBM-Q device with the modeled transduced states as input to demonstrate that a quantum advantage is possible even with current levels of quantum computing hardware noise.
We report a joint-detection (JD) experiment between two reconfigurable photonic processors in which Hadamard codewords are generated on a transmitter processor and a Hadamard transform is performed on a receiver (decoder) processor. Crucially, the decoder executes multi-input phase stabilization concurrently with the optical Hadamard combining, ensuring the phase alignment required for coherent joint detection. We demonstrate JD using 2×2 and 4×4 Hadamard transformations and verify stable temporal outputs under active on-chip feedback control (FBC). The approach leverages our multi-input FBC scheme to suppress low-frequency inter-input phase fluctuations and to realize coherent combining consistent with Green-Machine style architectures. These results indicate a practical path to scalable, codeword-level optical joint detection on programmable photonic hardware and are compatible with larger-mode processors and higher-order code families.
We describe a general quantum receiver protocol that maps laser-light-modulated classical communications signals into quantum processors for decoding with quantum logic gates. The quantum gates enable joint quantum measurements over a code word to achieve the quantum limit of communication capacity. Our receiver design requires qubit resources that only logarithmically increase with the size of the code word and accommodates practically relevant coherent-state modulation containing multiple photons per pulse. Focusing on classical-quantum polar codes, we outline the necessary quality of quantum operations and code-word lengths to demonstrate a quantum-processing-enhanced communication rate surpassing that of any known classical optical receiver-decoder pair. Specifically, we show that a small quantum receiver of four qubits with operational errors of approximately 0.2 % can already provide a 5% gain in the communication rate in the weak-signal limit. Additionally, we outline a possible hardware implementation of the receiver where efficient spin-photon interfaces such as cavity-coupled diamond color centers or atomic qubits are used to input the received photonic signal to a small-scale quantum processor for decoding. Our results outline a promising route for potential quantum advantage in classical communication with near-term, small-scale quantum computers.
When classical information is sent over a channel with quantum-state modulation alphabet, such as the free-space optical (FSO) channel, attaining the ultimate (Holevo) limit to channel capacity requires the receiver to make joint measurements over long codeword blocks. In recent work, we showed a receiver for a pure-state channel that can attain the ultimate capacity by applying a single-shot optical (unitary) transformation on the received codeword state followed by simultaneous (but separable) projective measurements on the single-modulation-symbol state spaces. In this paper, we study the ultimate tradeoff between photon efficiency and spectral efficiency for the FSO channel. Based on our general results for the pure-state quantum channel, we show some of the first concrete examples of codes and laboratory-realizable joint-detection optical receivers that can achieve fundamentally higher (superadditive) channel capacity than receivers that physically detect each modulation symbol one at a time, as is done by all conventional (coherent or direct-detection) optical receivers.