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arXiv:1201.4747 · Physical Review A

Capacities of linear quantum optical systems

Cosmo Lupo, Vittorio Giovannetti, Stefano Pirandola, Stefano Mancini, Seth Lloyd

Published Jun 19, 2012• 10 citations
arXivHTMLPDF

Abstract

A wide variety of communication channels employ the quantized electromagnetic field to convey information. Their communication capacity crucially depends on losses associated to spatial characteristics of the channel such as diffraction and antenna design. Here we focus on the communication via a finite pupil, showing that diffraction is formally described as a memory channel. By exploiting this equivalence we then compute the communication capacity of an optical refocusing system, modeled as a converging lens. Even though loss of information originates from the finite pupil of the lens, we show that the presence of the refocusing system can substantially enhance the communication capacity. We mainly concentrate on communication of classical information, the extension to quantum information being straightforward.

DOI
10.1103/physreva.85.062314

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A wide variety of communication channels employ the quantized electromagnetic field to convey information. Their communication capacity crucially depends on losses associated to spatial characteristics of the channel such as diffraction and antenna design. Here we focus on the communication via a finite pupil, showing that diffraction is formally described as a memory channel. By exploiting this equivalence we then compute the communication capacity of an optical refocusing system, modeled as a converging lens. Even though loss of information originates from the finite pupil of the lens, we show that the presence of the refocusing system can substantially enhance the communication capacity. We mainly concentrate on communication of classical information, the extension to quantum information being straightforward.
We consider the problem of quantum communication mediated by a passive optical refocusing system. The model captures the basic features of all those situations in which a signal is either refocused by a repeater for long-distance communication, or it is focused on a detector prior to the information decoding process. Introducing a general method for linear passive optical systems, we determine the conditions under which optical refocusing implies information transmission gain. Although the finite aperture of the repeater may cause loss of information, we show that the presence of the refocusing system can substantially enhance the rate of reliable communication with respect to the free-space propagation. We explicitly address the transferring of classical messages over the quantum channel, but the results can be easily extended to include the case of transferring quantum messages as well.
The classical information capacity of channels that are subject to quantum Gaussian noise is studied. Recent work has established the capacity of the pure-loss channel, as well as bounds on and a conjecture for the capacity of the lossy channel with isotropic-Gaussian excess noise. This work is applied to the pure-loss free-space channel that uses multiple Hermite-Gaussian (HG) or Laguerre-Gaussian (LG) spatial modes to communicate between soft-aperture transmit and receive pupils, and to the lossy channel with anisotropic (colored) Gaussian noise.
We introduce a general model describing correlated noise effects in quantum optical communication via attenuating media. The memory effects account for the environment finite relaxation times, which are unavoidable in any realistic model. The use of a proper set of collective field variables allows us to unravel the memory, showing that the n-fold concatenation of the memory channel is unitarily equivalent to the direct product of n single-mode lossy bosonic channels. We then compute the ultimate (classical and quantum) transmission rates, showing their enhancement with respect to the memoryless case and proving that coherent state encoding is optimal.
Contemporary tendencies in developing computing and communication systems are, more than ever, pointing out the necessity of transfer from computer and information systems to the optic and even quantum ones [1–4]. Saturation of microelectronics potencies can be seen in the simple and obvious facts, i.e. slowdown of effectiveness increase rates for the classical Turing computing systems and transfer to multicore parallel computations in electronics, as well as greater attention is paid to the optical communication channels, memory and data transmission systems, which is due to high density of information pack in the optical spectrum of electromagnetic waves [5–7].Taking into account the classical Shannon’s understanding of the information quantity and specific characteristics of an optical communication channel, in this work, we are trying to draw a parallel between the classical communication channel and the quantum one, which enables us, in some simple cases, to consider the approximate dependence of maximum information transmission rate on the key characteristics of laser emitter, including the average transmission power, bandwidth, and polarization.
Quantum information processing exploits the quantum nature of information. It offers fundamentally new solutions in the field of computer science and extends the possibilities to a level that cannot be imagined in classical communication systems. For quantum communication channels, many new capacity definitions were developed in comparison to classical counterparts. A quantum channel can be used to realize classical information transmission or to deliver quantum information, such as quantum entanglement. Here we review the properties of the quantum communication channel, the various capacity measures and the fundamental differences between the classical and quantum channels.
Quantum information processing exploits the quantum nature of information. It offers fundamentally new solutions in the field of computer science and extends the possibilities to a level that cannot be imagined in classical communication systems. For quantum communication channels, many new capacity definitions were developed in comparison to classical counterparts. A quantum channel can be used to realize classical information transmission or to deliver quantum information, such as quantum entanglement. Here we review the properties of the quantum communication channel, the various capacity measures and the fundamental differences between the classical and quantum channels.