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arXiv:1802.08287 · Optica

Quantum receiver for large alphabet communication

Ivan A. Burenkov, Olga V. Tikhonova, Sergey V. Polyakov

Published Feb 16, 2018• 29 citations
arXivHTMLPDF

Abstract

Quantum mechanics allows measurements that surpass the fundamental sensitivity limits of classical methods. To benefit from the quantum advantage in a practical setting, the receiver should use communication channels resources optimally; this can be done employing large communication alphabets. Here we show the fundamental sensitivity potential of a quantum receiver for coherent communication with frequency shift keying. We introduce an adaptive quantum protocol for this receiver, show that its sensitivity outperforms other receivers for alphabet sizes above 4 and scales favorably, whereas quantum receivers explored to date suffer from degraded sensitivity with the alphabet size. In addition, we show that the quantum measurement advantage allows the much better use of the frequency space in comparison to classical frequency keying protocols and orthogonal frequency division multiplexing.

DOI
10.1364/optica.5.000227

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Quantum mechanics allows measurements that surpass the fundamental sensitivity limits of classical methods. To benefit from the quantum advantage in a practical setting, the receiver should use communication channels resources optimally; this can be done employing large communication alphabets. Here we show the fundamental sensitivity potential of a quantum receiver for coherent communication with frequency shift keying. We introduce an adaptive quantum protocol for this receiver, show that its sensitivity outperforms other receivers for alphabet sizes above 4 and scales favorably, whereas quantum receivers explored to date suffer from degraded sensitivity with the alphabet size. In addition, we show that the quantum measurement advantage allows the much better use of the frequency space in comparison to classical frequency keying protocols and orthogonal frequency division multiplexing.
The efficient use of communication channels motivates extensive research in novel communication protocols. Modern communication protocols use large alphabets contain up to a few thousand symbols, thus optimizing the use of power and available frequency space. To date, quantum receivers that discriminate up to approximately 20 symbols were theoretically investigated and receivers with as many as 4 nonorthogonal coherent states have been experimentally demonstrated. However, all heretofore explored quantum receivers suffer from the sensitivity degradation with the alphabet size. Particularly, their Helstrom Bound (HB) nearly reaches the classical standard quantum limit (SQL). Here we introduce an M-ary quantum receiver based on a coherent frequency shift keying (CFSK) protocol with a record power sensitivity, free of the above deficiency. The CFSK not only provides better accuracy for longer alphabets but also allows discrimination with a practically attainable symbol error rate (SER) situated much below the HBs of other encodings for large alphabets. Our receiver operates with a classical transmitter, and with any communication channel, including the existing global fiber network. It can be used to increase the amplification-free range in a network and/or reduce power requirements on the transmitter by more than 1000 times. In addition, the quantum measurement advantage can significantly optimize the use of the frequency space in comparison to classical frequency keying protocols. This advantage can be used in deep-space telecom links to enhance the satellite power budget. In existing fiber network links, quantum CFSK receivers can improve the amplification-free range by approximately the factor of 2.
In classical digital communications two main families of “M-ary” modulation schemes are generally distinguished: bandwidth limited and power limited. The canonical comparison of these modulation methods is based on normalized data rate (R/W) (bits per second per hertz of bandwidth) and the signal to noise rate per bit required to achieve a given error probability for different M. In a classical picture, the two families reside in two separate semi-plains R/W < 1 and R/W > 1, i.e. energy efficiency and bandwidth efficiency cannot be optimized at the same time. However, we find an alphabet family that can be paired with a quantum receiver to simultaneously optimize bandwidth and power efficiency of a communication channel. Particularly we found that coherent frequency shift keying (CFSK) gives rise to a family of communication protocols that are bandwidth limited in nature, but whose bandwidth usage can be optimized so that R/W>1 for a range of alphabet lengths M, while power sensitivity beats that of power-limited protocols. We will report our theoretical findings and experimental progress towards implementation of this protocol family.
We evaluate the performance of coherent frequency-shift keying (CFSK) [1], [2] alphabets for communication on quantum channels. We show that, contrarily to what previously thought, the square-root-measurement (SRM) is sub-optimal for discriminating CFSK states. Furthermore, we compute the maximum information transmission rate of the CFSK alphabet, observing that it employs at least as many frequency modes as the signal states, and compare it with standard phase-shift-keying. Finally, we introduce a discretized CFSK alphabet with higher mode-efficiency, exhibiting comparable error-probability performance with respect to CFSK and better rate performance. Our results suggest the existence of a tradeoff between the CFSK reduced error-probability and its mode efficiency.
We report on the first experimental implementation of a scalable M-ary quantum receiver with time-resolved photon detection and real-time adaptive displacement using off-the-shelf components. We implemented M=4, 6, 8, 12, and 16 coherent frequency shift keying communication links with optimal parameters for maximal energy sensitivity. Then, we compared the measured sensitivity with the best sensitivity that can be achieved with a perfect classical measurement (shot noise limit). We observe unconditional, below the shot-noise limit sensitivity and a favorable sensitivity scaling with the alphabet length M.
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.
We study the problem of decoding classical information encoded on quantum states at the output of a quantum channel, with particular focus on increasing the communication rates towards the maximum allowed by Quantum Mechanics. After a brief introduction to the main theoretical formalism employed in the rest of the thesis, i.e., continuous-variable Quantum Information Theory and Quantum Communication Theory, we consider several decoding schemes. First, we treat the problem from an abstract perspective, presenting a method to decompose any quantum measurement into a sequence of easier nested measurements through a binary-tree search. Furthermore we show that this decomposition can be used to build a capacity-achieving decoding protocol for classical communication on quantum channels and to solve the optimal discrimination of some sets of quantum states. These results clarify the structure of optimal quantum measurements, showing that it can be recast in a more operational and experimentally-oriented fashion. Second, we consider a more practical approach and describe three receiver structures for coherent states of the electromagnetic field with applications to single-mode state discrimination and multi-mode decoding at the output of a quantum channel. We treat the problem bearing in mind the technological limitations faced nowadays in the field of optical communications: we evaluate the performance of general decoding schemes based on such technology and report increased performance of two schemes, the first one employing a non-Gaussian transformation and the second one employing a code tailored to be read out easily by the most common detectors. Eventually we characterize a large class of multi-mode adaptive receivers based on common technological resources, obtaining a no-go theorem for their capacity.