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NATO science for peace and security series. D, Information and communication security

Superadditivity of multiple access gaussian channels

Łukasz Czekaj, Korbicz Jarek K., Chhajlany Ravindra W., Horodecki Pawe lstrok

Published Jan 1, 2010• 0 citations
Publisher

Abstract

We study classical capacity regions of quantum Gaussian multiple access channels (MAC). In classical variants of such channels, whilst some capacity superadditivity-type effects such as the so called water filling effect may be achieved, a fundamental classical additivity law can still be identified, viz. adding resources to one sender is never advantageous to other senders in sending their respective information to the receiver. Here, we show that quantum resources allow violation of this law, by providing an illustrative scheme of an experimentally feasible Gaussian MAC.

DOI
10.3233/978-1-60750-547-1-171

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Superadditivity effects of communication capacities are known in the case of discrete variable quantum channels. We describe the continuous variable analog of one of these effects in the framework of Gaussian multiple access channels (MACs). Classically, superadditivity-type effects are strongly restricted: For example, adding resources to one sender is never advantageous to other senders in sending their respective information to the receiver. We show that this rule can be surpassed using quantum resources, giving rise to a type of truly quantum superadditivity. This is illustrated here for two examples of experimentally feasible Gaussian MACs.
We study subadditivity of the minimum output entropy (Hmin{H}_{\text{min}}Hmin​) of quantum multiple access channels (MACs). We provide an example of violation of the additivity theorem for Hmin{H}_{\text{min}}Hmin​ known in classical information theory. Our result is based on a fundamental property of MACs\text{MACs}MACs, i.e., independence of each sender. The channels used in the example can be constructed explicitly. On the basis of subadditivity of Hmin{H}_{\text{min}}Hmin​ we also provide an example of extremal superadditivity (superactivation) of the classical capacity region of MACs.
Superadditivity effects in the classical capacity of discrete multiaccess channels and continuous variable (CV) Gaussian MACs are analyzed. Several examples of the manifestation of superadditivity in the discrete case are provided, including, in particular, a channel which is fully symmetric with respect to all senders. Furthermore, we consider a class of channels for which input entanglement across more than two copies of the channels is necessary to saturate the asymptotic rate of transmission from one of the senders to the receiver. The five-input entanglement of Shor error correction codewords surpass the capacity attainable by using arbitrary two-input entanglement for these channels. In the CV case, we consider the properties of the two channels (a beam-splitter channel and a ``nondemolition'' quadratures coupling sum (XP) gate channel) analyzed in Czekaj et al. [Phys. Rev. A 82, 020302(R) (2010)] in greater detail and also consider the sensitivity of capacity superadditivity effects to thermal noise. We observe that the estimates of the amount of two-mode squeezing required to achieve capacity superadditivity are more optimistic than previously reported.
We consider quantum channels with two senders and one receiver. For an arbitrary such channel, we give multi-letter characterizations of two different two-dimensional capacity regions. The first region characterizes the rates at which it is possible for one sender to send classical information while the other sends quantum information. The second region gives the rates at which each sender can send quantum information. We give an example of a channel for which each region has a single-letter description, concluding with a characterization of the rates at which each user can simultaneously send classical and quantum information
We consider quantum channels with two senders and one receiver. For an arbitrary such channel, we give multiletter characterizations of two different two-dimensional capacity regions. The first region comprises the rates at which it is possible for one sender to send classical information, while the other sends quantum information. The second region consists of the rates at which each sender can send quantum information. For each region, we give an example of a channel for which the corresponding region has a single-letter description. One of our examples relies on a new result proved here, perhaps of independent interest, stating that the coherent information over any degradable channel is concave in the input density operator. We conclude with connections to other work and a discussion on generalizations where each user simultaneously sends classical and quantum information.