We consider all-optical networks from a quantum perspective. We show that optimal quantum receivers allow a$\sim 57$% decrease in energy consumption of all-optical amplifiers. Furthermore, we prove that quantum receivers allow for a logarithmic scaling of the system capacity with the number of pulses per second, while standard Shannon-type systems are limited by the transmit power. Based on our findings we argue for a new approach to optical communication network design, wherein in-line amplifiers are operated at very low gains and in conjunction with high-spectral-bandwidth fiber and a quantum receiver, enhancing data transmission in a practically relevant quantum limit.
We consider all-optical networks from a quantum perspective. We show that optimal quantum receivers allow a∼57% decrease in energy consumption of all-optical amplifiers. Furthermore, we prove that quantum receivers allow for a logarithmic scaling of the system capacity with the number of pulses per second, while standard Shannon-type systems are limited by the transmit power. Based on our findings we argue for a new approach to optical communication network design, wherein in-line amplifiers are operated at very low gains and in conjunction with high-spectral-bandwidth fiber and a quantum receiver, enhancing data transmission in a practically relevant quantum limit.
The search for schemes that minimize the energy associated with the transmission of information is a longstanding fundamental issue in communication theory. In this paper we extend fundamental limits to the energy consumption per unit of information, as given by Shannon's theory, to the quantum domain. Unlike previous studies, we address a scenario where the signal may be manipulated in an arbitrary way while propagating from the transmitter to the receiver. This situation characterizes many realistic scenarios, such as multi-span fiber-optic communication systems. We obtain the ultimate quantum limit on the energy consumption in this scenario and propose a simple binary energy modulation scheme that approaches this limit within one order of magnitude for practically relevant values of spectral efficiency. Under the same conditions, the quantum energy consumption limit of the standard optically amplified coherent communication scheme is three orders of magnitude above the ultimate identified limit. Throughout the paper we consider transmission of classical information over a quantum channel.
Born about a century ago, Quantum Mechanics has revolutionized the description and the interpretation of Physics at sub-microscopic level. In the last decades, due to the influence of mathematical and engineering research fields, Quantum Mechanics has given birth to related research areas like Quantum Computation, Quantum Information and Quantum Communication. With the discovery of the laser, and later the development of fiber optics and satellite networks, Quantum Communication and Quantum Optics seems to have a natural field of application in Communication Systems. Despite this, the interest in this technology and studies for communication purpose has been overshadowed by the great results in communication networks achieved in the last decades with classical paradigms. However, due to the increasing demand of communication data rate, system designers are now looking at Quantum Mechanics for new and more performanting solutions in communication purposes. Early theoretical studies on Quantum Discrimination Theory and Quantum Information predict better performance for Communication Systems that take advantage of the quantum laws. In addition, Quantum Mechanics provides the deepest description of the physical phenomena, and there are scenarios where a quantum model fits best, as in in deep space communications, where the received signal is really weak, or in a satellite networks, where we are interested in strongly reducing the power of transmitted signals, possibly without sacrificing performance significantly. However, if on one side Quantum Communication Theory promises great gains in the performance of communication systems, on the other hand it fails to describe how to implement physical devices that reach these ultimate limits. So far, only a few architectures achieving these performances are known, and only for simple modulation formats. We are interested in the scenario of optical communications, where the message transmitted is encoded in a sequence of coherent states. Transmitter devices for coherent modulation are well known and consist in laser pulse generators. Instead, receiver implementations working at the quantum limit performance limit are yet to be found. In this Thesis I deal with different topics in the quantum transmission scenario. First, I review existing classical (suboptimal) and quantum (suboptimal and optimal) receiver schemes for the binary coherent modulation. I present a new formulation of the optimal scheme known as Dolinar Receiver with the multiple copies problem, focusing on the information gained during the measurement. Second, I analyze the binary communication in a noisy environment, studying the error probability and the capacity of the binary channel induced. Given the description of the quantum channel, I optimize both the transmitted quantum states and the measurement operators employed in the communication. Third, I consider the Pulse Position Modulation, that is particularly suitable for space and satellite communication due to its simplicity of implementation and high capacity. I review some known suboptimal receivers, and I propose a receiver scheme which approaches the limit performance predicted with quantum theory outperforming the existing schemes. To sum up the results of this Thesis, in order to approach the limit performance predicted by Quantum Mechanics, an optimization is always necessary to exceed the classical effects and trigger the quantum phenomena. In particular, the information gained during the measurement plays an important role, for example in the definition of adaptive receivers. In this Thesis both these aspects have been deeply investigated.
This tutorial reviews the Holevo capacity limit as a universal tool to analyze the ultimate transmission rates in a variety of optical communication scenarios, ranging from conventional optically amplified fiber links to free-space communication with power-limited optical signals. The canonical additive white Gaussian noise model is used to describe the propagation of the optical signal. The Holevo limit exceeds substantially the standard Shannon limit when the power spectral density of noise acquired in the course of propagation is small compared to the energy of a single photon at the carrier frequency per unit time-bandwidth area. General results are illustrated with a discussion of efficient communication strategies in the photon-starved regime.
Non-linear effects are believed to reduce the communication rate in fiber networks, leading to the introduction of the non-linear Shannon (NLS) limit. Here, we analyze, for the first time to our knowledge, the effect of quantum detection techniques in amplified optical fibers affected by non-linearity, by computing their Holevo channel capacity. First, we report that the Holevo capacity under lumped Raman amplification (RA) can surpass the NLS capacity of a coherent-detection system with distributed Raman amplification (DRA) for a moderate number of spans. Then, we observe that the Holevo RA system offers a reduction of amplification power costs at a fixed communication rate, with respect to the DRA coherent-detection system. Our results rely on the ability of a quantum joint-detection receiver (JDR) to better extract information from low-power, low-noise signals, determining the type of amplifier that is best suited to work in conjunction with the JDR.
This paper surveys a new way for research of quantum cryptography. The conventional cipher is designed by a mathematical algorithm and its security is evaluated by the complexity of the algorithm for the cryptanalysis and ability of computers. This kind of cipher cannot exceed the Shannon limit of cryptography, and it can be decrypted with probability one in principle by trying all the possible keys against the data length equal to the secret key length. A cipher based on quantum effect so called physical cipher may exceed the Shannon limit of the cryptography. The quantum stream cipher by �/� or Yuen-2000 protocol (Y- 00) which operates at Gbit/sec is a typical example of such a cipher, and may exceed the Shannon limit. In this paper, we show an existence of cipher which exceeds the Shannon limit, and discuss practical realization methods for them. First, the conditions to exceed the Shannon limit are summarized. Then we discuss the generalized secret capacity of the wiretap channel model supported by a secret key to discuss an existence of cipher exceeding the Shannon limit. The generalized secret capacities for space communication and fiber communication are given. In gen- eral, one needs quantum optimum receiver to realize such capacity. But when bandwidth limitation of users is deleted or the eavesdropper has a device limit, a cipher scheme exceeding the Shannon limit may be realized merely by the conventional optical communication devices which are affected by quantum noise at optical signal measurement.
The information capacity of an optical channel under power constraints is ultimately limited by the quantum nature of transmitted signals. We discuss currently available and emerging photonic technologies whose combination can be shown theoretically to enable nearly quantum-limited operation of a noisy optical communication link in the photon-starved regime, with the information rate scaling linearly in the detected signal power. The key ingredients are quantum pulse gating to facilitate mode selectivity, photon-number-resolved direct detection, and a photon-efficient high-order modulation format such as pulse position modulation, frequency shift keying, or binary phase shift keyed Hadamard words decoded optically using structured receivers.