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arXiv:2105.04274 · IEEE International Symposium on Information Theory (ISIT)

Compound Channel Capacities under Energy Constraints and Application

Andrea Cacioppo, Janis Nötzel, Matteo Rosati

Published Jul 12, 2021• 7 citations
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Abstract

Compound channel models offer a simple and straightforward way of analyzing the stability of decoder design under model variations. With this work we provide a coding theorem for a large class of practically relevant compound channel models. We give explicit formulas for the cases of the Gaussian classical-quantum compound channels with unknown noise, unknown phase and unknown attenuation. We show analytically how the classical compound channel capacity formula motivates nontrivial choices of the displacement parameter of the Kennedy receiver. Our work demonstrates the value of the compound channel model as a method for the design of receivers in quantum communication.

DOI
10.1109/isit45174.2021.9518144

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Compound channel models offer a simple and straightforward way of analyzing the stability of decoder design under model variations. With this work we provide a coding theorem for a large class of practically relevant compound channel models. We give explicit formulas for the cases of the Gaussian classical-quantum compound channels with unknown noise, unknown phase and unknown attenuation. We show analytically how the classical compound channel capacity formula motivates nontrivial choices of the displacement parameter of the Kennedy receiver. Our work demonstrates the value of the compound channel model as a method for the design of receivers in quantum communication.
Compound channels model situations where a sender and receiver both have only partial information about the stationary channel they are transmitting over. They can be used to analyse the robustness of a communication system. With this work we provide a coding theorem for a class of practically relevant quantum compound channel models, where we consider the system uncertainty as due to a stationary but unknown jamming signal. Namely, we give an explicit formula for the case of the lossy Bosonic channel in the presence of a semi-classical attack utilizing the injection of coherent states into the transmission line. Mathematically, this is modelled by letting transmitter and jammer access the two different ports of a beam-splitter, and the receiver observe one of the output ports. While the transmitter can modulate their input, the jammer must transmit a constant (but unknown) sequence of states. We conjecture a new quantum entropy power inequality and show its relation to the capacity under study.
Recent efforts in the design of communication systems have in addition to data rates embraced a focus on other metrics, such as round-trip latency. Data transmission over a channel is typically preceded by so-called pilot symbols, which let the receiver learn the channel conditions. Together with appropriate feedback, this allows for transmission at high data rates. Trading data rates for communication latency, a communication link may however avoid feedback and instead operate under channel uncertainty, in which case the accurate model is the compound channel. Further reductions of latency may be harnessed from quantum receiver design. In this work we focus in particular on the application of the sequential decoding approach to the bosonic compound channel with unknown phase rotations, loss parameters and thermal noise values. We show that sequential decoding is optimal.
The classical capacity of a noisy optical channel is the maximum rate at which information can be transmitted reliably, given a constraint on the mean input energy. For the ubiquitous bosonic Gaussian channels, which include standard models of loss and noise in fibre and free-space optical links, determining this rate requires addressing whether entanglement between channel uses or non-Gaussian signals can outperform Gaussian encodings. More precisely, the question is whether the energy-constrained Holevo information is additive across arbitrarily many channel uses and coincides with its Gaussian counterpart. We answer these fundamental questions in the affirmative for every single-mode bosonic Gaussian channel at every input energy. Namely, we prove that independently prepared Gaussian states with Gaussian-distributed random displacements achieve the capacity, including in the low-energy regime where squeezing the signals competes with encoding information in their displacement. Our proof connects an entanglement inequality to a sharp, additive lower bound on output entropy. We then characterise communication above capacity: under a strict total photon-number cutoff, the probability of correct decoding decreases exponentially with the number of channel uses, and we derive explicit bounds on the decay exponent. When only the mean photon number is constrained, a simple energy-rescaling formula determines exactly the entire asymptotic rate--error trade-off: the capacity at mean photon number NNN and allowed average decoding error εεε equals the vanishing-error capacity at mean photon number N/(1−ε)N/(1-ε)N/(1−ε). Together, these results identify the ultimate limits of classical communication for some of the most fundamental models of optical communication.
If a sender and a receiver lack precise knowledge about the communication line that connects them, designing a scheme to reliably transmit information becomes more challenging. This has been studied in classical and quantum information theory in the context of compound channel models and arbitrarily varying channel models. However, a fully quantum version of system uncertainty allows for an even more challenging coding scenario with entangled channel uses. This type of model has previously been investigated for classical and quantum capacity. Here, we address the problem of entanglement-assisted capacity in the presence of such system uncertainty. We find that, under the assumption of a finite environment dimension, it is equal to a corresponding compound capacity. Intriguingly, our results imply that in certain fully quantum arbitrarily varying channel models, the entanglement-assisted capacity can be positive while the classical capacity is equal to zero, a phenomenon that does not occur in regular single-channel coding.
Quantum channels with no quantum capacity can be activated by specific kinds of channels. This activation effect might be useful to overcome noise of channels by attaching other channels, which can enhance the capacity of a given channel. We show that such an activation is possible by specific positive-partial-transpose channels and Gaussian lossy channels whose quantum capacities are known. We also test the more general case involving a Gaussian thermal attenuator whose exact value of quantum capacity has been unknown so far. By employing a recently suggested narrow upper bound on the quantum capacity of the thermal attenuator, we confirm the fact that an activation of quantum capacity occurs as well. This result is applicable for realistic situations in which Gaussian channels describe the noises of communication systems.
In this thesis, we are interested in the limits of quantum communication with and without entanglement, and with and without noise assumptions on the communication setup. When a sender and a receiver are connected by a communication line that is governed by noise which is modelled by a quantum channel, they hope to design a coding scheme, i.e. messages and message decoders, in such a way that they are robust to this noise. The amount of message bits per channel use is called the achievable rate of the coding scheme, and the maximal achievable rate for a given quantum channel is called capacity of the quantum channel. Here, we are interested in coding schemes and capacities under various assumptions, in particular in the case where the sender and the receiver share quantum entanglement, which turns out to be the most natural generalization of the classical communication analogue that lies at the basis of many of our modern technologies.