Search

Landauer’s principle provides a fundamental lower bound for energy dissipation occurring with information erasure in the quantum regime. While most studies have related the entropy reduction incorporated with the erasure to the lower bound (entropic bound), recent efforts have also provided another lower bound associated with the thermal fluctuation of the dissipated energy (thermodynamic bound). The coexistence of the two bounds has stimulated comparative studies of their properties; however, these studies were performed for systems where the time-evolution of diagonal (population) and off-diagonal (coherence) elements of the density matrix are decoupled. In this paper, we aimed to broaden the comparative study to include the influence of quantum coherence induced by the tilted system–reservoir interaction direction. By examining their dependence on the initial state of the information-bearing system, we find that the following properties of the bounds are generically held regardless of whether the influence of the coherence is present or not: the entropic bound serves as the tighter bound for a sufficiently mixed initial state, while the thermodynamic bound is tighter when the purity of the initial state is sufficiently high. The exception is the case where the system dynamics involve only phase relaxation; in this case, the two bounds coincide when the initial coherence is zero; otherwise, the thermodynamic bound serves the tighter bound. We also find the quantum information erasure inevitably accompanies constant energy dissipation caused by the creation of system–reservoir correlation, which may cause an additional source of energetic cost for the erasure.

The ability to control quanta shown by quantum pumping has been intensively studied, aiming to further develop nano fabrication. In accordance with the fast progress of the experimental techniques, the focus on quantum pumping extends to include the quicker transport. For this purpose, it is necessary to remove the &ldquo, adiabatic&rdquo, or &ldquo, slow&rdquo, condition, which has been the central concept of quantum pumping since its first proposal for a closed system. In this article, we review the studies which go beyond the conventional adiabatic approximation for open quantum systems to transfer energy quanta and electron spins with using the full counting statistics. We also discuss the recent developments of the nonadiabatic treatments of quantum pumping.

Non-Markovian dynamical maps are derived by using the projection operator method in Hilbert–Schmidt space. The eigenvalues and eigenvectors of the obtained maps describe the relaxation processes. We found that a dynamical map for a spin-bath model periodically becomes positive but not completely positive by the non-Markovian effect of the system–reservoir interaction.

A highly sensitive heterodyne-detected photon echo enabled us to observe the signal from one layer of neutral self-assembled InP quantum dots under an electric field. The photon echo signal dramatically increased when charged InP quantum dots were neutralized with the application of negative electric bias. This fact suggests the hypothesis that charged quantum dots do not produce the photon echo. With the further increase of negative electric bias, photoluminescence was quenched and the photon echo decayed faster. The photon echo showed tunneling-induced dephasing and decayed nonexponentially, reflecting the non-Markovian nature of the tunneling process.

The formation of the atomic coherence via a photon field has been considered to play an essential role in the laser action. The importance of the formation has been also recognized from the viewpoint of cavity QED. This paper analyzes quantum mechanical dynamics of a self-organized formation of coherence between atoms and photon field. We solved an equation of a reduced density operator for a quantum laser model with plural atoms by using coherent states and orthogonal polynomials, and obtained a solution in a continued fraction form. We numerically evaluate time evolution of averaged physical quantities and quasi-probability density of photon field. Under an appropriate condition for laser oscillation, time evolution of an average of induced atomic dipole moments shows a co-operative phenomenon. Numerical evaluation of the quasi-probability density of a photon field reveals that a coherently more stabilized and stronger photon field emerges as the number of atoms increases. To our knowledge, this is the ...

We formulate a quantum statistical mechanical theory of relaxation for a spin 1/2 system interacting with a quantum environment. Our theory has the advantage compared to previous ones in simultaneously treating both (i) arbitrary modulation speed (and/or any strength of modulation) of the fluctuating field and (ii) any temperature region of the environment.

A systematic damping theoretical formalism is presented to give basic equations of statistical physics. Our formalism naturally yields the so-called time convolution (TC) and time convolutionless (TCL) equations both for a density matrix and for dynamical variables. The method is explicitly used to obtain equations in the Heisenberg picture (equations of motion for dynamical variables). In contrast to the existing derivation of equations of motion, the present formalism gives a systematic method for deriving these equations without specifying the projection operator. Moreover, the formalism is valid even for time-dependent Hamiltonian systems. A simple example is given to illustrate our formalism.

A mathematical correspondence is established between a set of equations due to Mori formalism and those of generalized tri-diagonal recurrence relations. Thus, once a set of coupled equations describing physical phenomena are given, the corresponding Langevin equations can be obtained directly. This enables us to construct a hierarchical structure of physical variables and time scales

An extended version of a quantum statistical mechanical laser model is solved rigorously. With the use of coherent states and orthogonal polynomials, the original equation is solved to give a continued fraction. The average of the induced atomic dipole moment is evaluated to give the cooperative rotational motion.

A microscopic laser equation is solved rigorously. In order to transform the original equation of the density matrix into a more tractable partial differential equation, spin and boson coherent states are used. The resulting equation for the probability density is solved to give a continued fraction with the use of orthogonal polynomials. As a preliminary result, dynamics of certain physical quantities are calculated.

A rigorous solution is found for a basic equation which determines the time evolution of the nonlinear spin relaxation process. Spin coherent representation is used to transform an original operator equation into a Fokker-Planck-type c -number equation for the probability density which is expanded in terms of spherical harmonics. Then the expansion coefficients are completely determined in the form of a continued fraction. Relevant averages such as moments and cumulants are calculated.

It is known that a quantum system with finite degrees of freedom can simulate a composite of a system and an environment if the state of the hypothetical environment is randomized by external manipulation. We show theoretically that any phase decoherence phenomena of a single qubit can be simulated with a two-qubit system and demonstrate experimentally two examples: one is phase decoherence of a single qubit in a transmission line, and the other is that in a quantum memory. We perform NMR experiments employing a two-spin molecule and clearly measure decoherence for both cases. We also prove experimentally that the bang-bang control efficiently suppresses decoherence., Comment: 25 pages, 7 figures; added references