Coherent information

About: Coherent information is a research topic. Over the lifetime, 1225 publications have been published within this topic receiving 46672 citations.

Positivity and nonadditivity of quantum capacities using generalized erasure channels

Abstract: The positivity and nonadditivity of the one-letter quantum capacity (maximum coherent information) $Q^{(1)}$ is studied for two simple examples of complementary quantum channel pairs $(B,C)$. They are produced by a process, we call it gluing, for combining two or more channels to form a composite. (We discuss various other forms of gluing, some of which may be of interest for applications outside those considered in this paper.) An amplitude-damping qubit channel with damping probability $0\leq p \leq 1$ glued to a perfect channel is an example of what we call a generalized erasure channel characterized by an erasure probability $\lambda$ along with $p$. A second example, using a phase-damping rather than amplitude-damping qubit channel, results in the dephrasure channel of Ledtizky et al. [Phys. Rev. Lett. 121, 160501 (2018)]. In both cases we find the global maximum and minimum of the entropy bias or coherent information, which determine $Q^{(1)}(B_g)$ and $Q^{(1)}(C_g)$, respectively, and the ranges in the $(p,\lambda)$ parameter space where these capacities are positive or zero, confirming previous results for the dephrasure channel. The nonadditivity of $Q^{(1)}(B_g)$ for two channels in parallel occurs in a well defined region of the $(p,\lambda)$ plane for the amplitude-damping case, whereas for the dephrasure case we extend previous results to additional values of $p$ and $\lambda$ at which nonadditivity occurs. For both cases, $Q^{(1)}(C_g)$ shows a peculiar behavior: When $p=0$, $C_g$ is an erasure channel with erasure probability $1-\lambda$, so $Q^{(1)}(C_g)$ is zero for $\lambda \leq 1/2$. However, for any $p>0$, no matter how small, $Q^{(1)}(C_g)$ is positive, though it may be extremely small, for all $\lambda >0$. Despite the simplicity of these models we still lack an intuitive understanding of the nonadditivity of $Q^{(1)}(B_g)$ and the positivity of $Q^{(1)}(C_g)$.

Fisher information and quantum–classical field theory: classical statistics similarity

Abstract: The classical statistics indication for the impossibility to derive quantum mechanics from classical mechanics is proved. The formalism of the statistical Fisher information is used. Next the Fisher information as a tool of the construction of a self-consistent field theory, which joins the quantum theory and classical field theory, is proposed.

Quantum data gathering

Abstract: Measurement of a quantum system – the process by which an observer gathers information about it – provides a link between the quantum and classical worlds. The nature of this process is the central issue for attempts to reconcile quantum and classical descriptions of physical processes. Here, we show that the conventional paradigm of quantum measurement is directly responsible for a well-known disparity between the resources required to extract information from quantum and classical systems. We introduce a simple form of quantum data gathering, “coherent measurement”, that eliminates this disparity and restores a pleasing symmetry between classical and quantum statistical inference. To illustrate the power of quantum data gathering, we demonstrate that coherent measurements are optimal and strictly more powerful than conventional one-at-a-time measurements for the task of discriminating quantum states, including certain entangled many-body states (e.g., matrix product states).

Gain of information in a quantum measurement

Abstract: The changes of entropy taking place in a quantum system during a measurement process are reviewed, with a view to clarifying the concepts of entropy, information and quantum measurement. It is shown that a non-negative amount of information is gained in spite of the loss of information (or entropy increase) caused by the reduction of the wavepacket.

Quantum information conversion between matter and light representations

Abstract: Structures and methods allow: transfer of quantum information represented using the states of light (160) to a representation using the states of matter systems (120); transfer of quantum information represented by the states of matter systems (120) to a representation using the states of light (134); and error resistant encoding of quantum information using entangled states of matter and light to minimize errors.