Information must take up space, must weigh, and its flux must be limited. Quantum limits on communication and information storage leading to these conclusions are described here. Quantum channel capacity theory is reviewed for both steady state and burst communication. An analytic approximation is given for the maximum signal information possible with occupation number signal states as a function of mean signal energy. A theorem guaranteeing that these states are optimal for communication is proved. A heuristic "proof" of the linear bound on communication is given, followed by rigorous proofs for signals with specified mean energy, and for signals with given energy budget. And systems of many parallel quantum channels are shown to obey the linear bound for a natural channel architecture. The time-energy uncertainty principle is reformulated in information language by means of the linear bound. The quantum bound on information storage capacity of quantum mechanical and quantum field devices is reviewed. A simplified version of the analytic proof for the bound is given for the latter case. Solitons as information caches are discussed, as is information storage in one-dimensional systems. The influence of signal self-gravitation on communication is considered. Finally, it is shown that acceleration of a receiver acts to block information transfer.
Information must take up space, must weigh, and its flux must be limited. Quantum limits on communication and information storage leading to these conclusions are described here. Quantum channel capacity theory is reviewed for both steady state and burst communication. An analytic approximation is given for the maximum signal information possible with occupation number signal states as a function of mean signal energy. A theorem guaranteeing that these states are optimal for communication is proved. A heuristic "proof" of the linear bound on communication is given, followed by rigorous proofs for signals with specified mean energy, and for signals with given energy budget. And systems of many parallel quantum channels are shown to obey the linear bound for a natural channel architecture. The time-energy uncertainty principle is reformulated in information language by means of the linear bound. The quantum bound on information storage capacity of quantum mechanical and quantum field devices is reviewed. A simplified version of the analytic proof for the bound is given for the latter case. Solitons as information caches are discussed, as is information storage in one-dimensional systems. The influence of signal self-gravitation on communication is considered. Finally, it is shown that acceleration of a receiver acts to block information transfer.
Information must take up space, must weigh, and its flux must be limited. Quantum limits on communication and information storage leading to these conclusions are described here. Quantum channel capacity theory is reviewed for both steady state and burst communication. An analytic approximation is given for the maximum signal information possible with occupation number signal states as a function of mean signal energy. A theorem guaranteeing that these states are optimal for communication is proved. A heuristic "proof" of the linear bound on communication is given, followed by rigorous proofs for signals with specified mean energy, and for signals with given energy budget. And systems of many parallel quantum channels are shown to obey the linear bound for a natural channel architecture. The time-energy uncertainty principle is reformulated in information language by means of the linear bound. The quantum bound on information storage capacity of quantum mechanical and quantum field devices is reviewed. A simplified version of the analytic proof for the bound is given for the latter case. Solitons as information caches are discussed, as is information storage in one-dimensional systems. The influence of signal self-gravitation on communication is considered. Finally, it is shown that acceleration of a receiver acts to block information transfer.
The capacity C of a communication channel is the maximum rate at which information can be transmitted without error from the channel's input to its output. The authors review quantum limits on the capacity that can be achieved with linear bosonic communication channels that have input power P. The limits arise ultimately from the Einstein relation that a field quantum at frequency f has energy E=hf. A single linear bosonic channel corresponds to a single transverse mode of the bosonic field---i.e., to a particular spatial dependence in the plane orthogonal to the propagation direction and to a particular spin state or polarization. For a single channel the maximum communication rate is ${C}_{\mathrm{WB}}=(\frac{\ensuremath{\pi}}{\mathrm{ln}2})\sqrt{\frac{2P}{3h}}$ bits/s. This maximum rate can be achieved by a "number-state channel," in which information is encoded in the number of quanta in the bosonic field and in which this information is recovered at the output by counting quanta. Derivations of the optimum capacity CWB are reviewed. Until quite recently all derivations assumed, explicitly or implicitly, a number-state channel. They thus left open the possibility that other techniques for encoding information on the bosonic field, together with other ways of detecting the field at the output, might lead to a greater communication rate. The authors present their own general derivation of the single-channel capacity upper bound, which applies to any physically realizable technique for encoding information on the bosonic field and to any physically realizable detection scheme at the output. They also review the capacities of coherent communication channels that encode information in coherent states and in quadrature-squeezed states. A three-dimensional bosonic channel can employ many transverse modes as parallel single channels. An upper bound on the information flux that can be transferred down parallel bosonic channels is derived.
The quantum limitations on the maximum communication rate (capacity) possible through a single noiseless channel with signals of finite duration are investigated. They may be summarized in the characteristic information function defined here. In the absence of dispersion this description is a Lorentz-invariant one, and applies also in the presence of exterior gravitational fields. For long duration the characteristic information function corresponds to the standard quantum-channel-capacity formula of Gordon, of Lebedev and Levitin, and of Pendry. For finite signal duration it proves useful to distinguish between heralded and self-heralding signals, according to whether their arrival is anticipated or not. The two types have different characteristic functions. These are calculated here for occupation-number signal states in channels where the carrier quantum field may be represented by independent modes. The Gordon-Lebedev-Levitin-Pendry formula provides an upper bound to the exact results for both types of signals. The linear bound on communication rate of Bremermann and the present author bounds the capacity only for self-heralding signals. However, it is the better estimator of capacity for self-heralding signals with modest information content.
In this paper, we give two independent and rigorous derivations for the quantum bound on the information transmission rate proposed independently by Bekenstein [Phys. Rev. Lett. 46, 623 (1981)] and Bremermann .ul2 [Proceedings of the Fifth Berkeley Symposium on Mathematical Statistics and Probability, edited by L. M. LeCam and J. Neyman (University of California Press, Berkeley, 1967], preceded by a heuristic argument showing why such a bound must hold. In both approaches, information carriers are quanta for some field. The first method resembles the microcanonical approach to statistical mechanics, where the strategy of overestimating the real number of states by relaxing the indistinguishability of quanta was adopted. The second is based entirely upon maximum-entropy methods. Amazingly enough, the results obtained by these physically unrelated premises turn out to be identical, namely, that the single (noiseless) channel capacity is I${\mathrm{\ifmmode \dot{}\else \.{}\fi{}}}_{\mathrm{max}}$=E/2\ensuremath{\pi}\ensuremath{\Elzxh} bits ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. It is further shown that, in a finite time \ensuremath{\tau}, no information can ever be conveyed unless the energy threshold 2\ensuremath{\pi}\ensuremath{\Elzxh}/\ensuremath{\tau} is reached, allowing the reinterpretation of the time-energy uncertainty in informational-theoretic language.
Quantum channels are functions that map density matrices to density matrices. They can be used to describe a vast array of system environment interactions for quantum mechanical systems. The focus in quantum information theory lies on the study of the information content -- the quantum entropy -- of input and output system. The property of how much information a channel can convey is called capacity. When two quantum channels are combined they act on a bigger, possibly entangled state in parallel. The effects of entanglement on the capacity of a pair of channels have been widely studied. Still, recent work showed unexpected behavior. Hastings found that the capacities of two channels to transmit classical information does not simply add, but can exceed the sum of the individual quantum capacities, a property called superadditivity. We study Hastings' work and find estimates for how large the dimensions of such channels would have to be. The question of capacity relates to the behavior of quantum channels in extremal cases, when the output entropy is minimized. It is natural to wonder about the "standard" behavior of quantum channels and consider their average output entropy. We have studied this behavior in particular in cases where many relatively simple quantum channels act in parallel. When combining quantum channels in parallel, the parts of the system evolve independently of each other -- up to entanglement -- and only interact with the environment. Expanding on this idea one can think of a system where the parts not only interact with the environment but also directly with each other, a quantum channel network. Interesting cases of such systems arise naturally in the study of large biological molecules. In this case, information is often transmitted via energy pulses. Thus, the study of entropy and the study of energy overlap. Of particular interest to us are antennas in photo-active systems, they absorb light energy and transports it to a chemical reaction site. Recently, it has been discovered in the FMO molecule that this process evolves coherently, i.e. shows quantum oscillatory behavior. We study such quantum channel networks and how they can be approximated by kinetic networks. When considering the transport efficiency, this helps separating possible coherent effects from incoherent hopping.
I derive a universal upper bound on the capacity of any communication channel between two distant systems. The Holevo quantity, and hence the mutual information, is at most of order EΔt/ℏ, where E is the average energy of the signal, and Δt is the amount of time for which detectors operate. The bound does not depend on the size or mass of the emitting and receiving systems, nor on the nature of the signal. No restrictions on preparing and processing the signal are imposed. As an example, I consider the encoding of information in the transverse or angular position of a signal emitted and received by systems of arbitrarily large cross section. In the limit of a large message space, quantum effects become important even if individual signals are classical, and the bound is upheld.