Information Flow, Non-Markovianity and Geometric Phases

Geometric phases and information flows of a two-level system coupled to its environment are calculated and analyzed. The information flow is defined as a cumulant of changes in trace distance between two quantum states, which is similar to the measure for non-Markovianity given by Breuer. We obtain an analytic relation between the geometric phase and the information flow for pure initial states, and a numerical result for mixed initial states. The geometric phase behaves differently depending on whether there are information flows back to the two-level system from its environment.Comment: 12 pages, 11 figure

Adiabatic Decoherence-Free Subspaces and its Shortcuts

The adiabatic theorem and "shortcuts to adiabaticity" for the adiabatic dynamics of time-dependent decoherence-free subspaces are explored in this paper. Starting from the definition of the dynamical stable decoherence-free subspaces, we show that, under a compact adiabatic condition, the quantum state follows time-dependent decoherence-free subspaces (the adiabatic decoherence free subspaces) into the target subspace with extremely high purity, even though the dynamics of the quantum system may be non-adiabatic. The adiabatic condition mentioned in the adiabatic theorem is very similar with the adiabatic condition for closed quantum systems, except that the operators required to be "slowness" is on the Lindblad operators. We also show that the adiabatic decoherence-free subspaces program depends on the existence of instantaneous decoherence-free subspaces, which requires that the Hamiltonian of open quantum systems has to be engineered according to the incoherent control program. Besides, "the shortcuts to adiabaticity" for the adiabatic decoherence-free subspaces program is also presented based on the transitionless quantum driving method. Finally, we provide an example of physical systems that support our programs. Our approach employs Markovian master equations and applies primarily to finite-dimensional quantum systems.Comment: 17 pages,5 figure

The Dynamical Invariant of Open Quantum System

The dynamical invariant, whose expectation value is constant, is generalized to open quantum system. The evolution equation of dynamical invariant (the dynamical invariant condition) is presented for Markovian dynamics. Different with the dynamical invariant for the closed quantum system, the evolution of the dynamical invariant for the open quantum system is no longer unitary, and the eigenvalues of it are time-dependent. Since any hermitian operator fulfilling dynamical invariant condition is a dynamical invariant, we propose a sort of special dynamical invariant (decoherence free dynamical invariant) in which a part of eigenvalues are still constant. The dynamical invariant in the subspace spanned by the corresponding eigenstates evolves unitarily. Via the dynamical invariant condition, the results demonstrate that this dynamical invariant exists under the circumstances of emergence of decoherence free subspaces

Multilevel quantum Otto heat engines with identical particles

A quantum Otto heat engine is studied with multilevel identical particles trapped in one-dimensional box potential as working substance. The symmetrical wave function for Bosons and the anti-symmetrical wave function for Fermions are considered. In two-particle case, we focus on the ratios of $W^i$ ($i=B,F$) to $W_s$, where $W^B$ and $W^F$ are the work done by two Bosons and Fermions respectively, and $W_s$ is the work output of a single particle under the same conditions. Due to the symmetric of the wave functions, the ratios are not equal to $2$. Three different regimes, low temperature regime, high temperature regime, and intermediate temperature regime, are analyzed, and the effects of energy level number and the differences between the two baths are calculated. In the multiparticle case, we calculate the ratios of $W^i_M/M$ to $W_s$, where $W^i_M/M$ can be seen as the average work done by a single particle in multiparticle heat engine. For other working substances whose energy spectrum have the form of $E_n\sim n^2$, the results are similar. For the case $E_n\sim n$, two different conclusions are obtained

Berry Phase and Hannay's Angle in a Quantum-Classical Hybrid System

Berry phase, which had been discovered for more than two decades, provides us a very deep insight on the geometric structure of quantum mechanics. Its classical counterpart--Hannay's angle is defined if closed curves of action variables return to the same curves in phase space after a time evolution. In this paper, we study the Berry phase and Hannay's angle in a quantum-classical hybrid system under the Born-Oppenheimer approximation. By quantum-classical hybrid system, we denote a composite system consists of a quantum subsystem and a classical subsystem. The effects of subsystem-subsystem couplings on the Berry phase and Hannay's angle are explored. The results show that the Berry phase has been changed sharply by the couplings, whereas the couplings have small effect on the Hannay's angle.Comment: 8 pages, 2 figure

Electron tunneling through a single magnetic barrier in HgTe topological insulator

Electron tunneling through a single magnetic barrier in a HgTe topological insulator has been theoretically investigated. We find that the perpendicular magnetic field would not lead to spin-flip of the edge states due to the conservation of the angular moment. By tuning the magnetic field and Fermi energy, the edge channels can be transited from switch-on states to switch-off states and the current can be transmitted from unpolarized states to totally spin polarized states. These features offer us and efficient way to control the topological edge state transport, and pave a way to construct the nanoelectronic devices utilizing the topological edge states.Comment: 4 pages, 5 figure

Shortcuts to adiabaticity in non-Hermitian quantum systems without rotating-wave approximation

The technique of shortcuts to adiabaticity (STA) has attracted broad attention due to their possible applications in quantum information processing and quantum control. However, most studies published so far have been only focused on Hermitian systems under the rotating-wave approximation (RWA). In this paper, we propose a modified STA technique to realize population transfer for a non-Hermitian system without RWA. We work out an exact expression for the control function and present examples consisting of two- and three-level systems with decay to show the theory. The results suggest that the STA technique presented here is robust for fast passages. We also find that the decay has small effect on the population transfer in the three-level system. To shed more light on the physics behind this result, we reduce the quantum three-level system to an effective two-level one with large detunings. The STA technique of effective two-level system is studied. Thereby the high-fidelity population transfer can be implemented in non-Hermitian systems by our method, and it works even without RWA.Comment: 15 pages, 5 figure

Bound states of gain-guided solitons in a passively mode-locked fiber laser

We report on the observation of bound states of gain-guided solitons (GGSs) in a dispersion-managed erbium-doped fiber laser operating in the normal net cavity dispersion regime. Despite of the fact that the GGS is a chirped soliton and there is strong pulse stretching and compression along the cavity in the laser, the bound solitons observed have a fixed pulse separation, which is invariant to the pump strength change. Numerical simulation confirmed the experimental observations

The gravitational deflection of light in MOND

The deflection angle $\Delta\phi$ of light rays by the gravitational field of a spherical system $M(r)$ is calculated using the MOdified Newtonian Dynamics (MOND). It is shown that $\Delta\phi$ with an impact parameter $r_0$ can be expressed by the measured rotation velocity $v(r)$ as $\Delta\phi(r_0)=2\int_{r_0}^{\infty} \frac{v^2(r)}{c^2}\frac{r_0dr}{r\sqrt{r^2-r_0^2}}$, where v(r)=\left\{\left(Ga_0M(r)\right)^{1/4}, & r_0>r_c;\left(\frac{GM(r)}{r}\right)^{1/2},& r_0\leq r_c,\right, and $r_c$ is the critical radius that is determined by the critical acceleration $a_0$. In the Newtonian limit of the gravitational acceleration $a\gg a_0$, $\Delta\phi$ approaches $\Delta\phi=2Gm(r_0)/c^2r_0$ with the projected surface mass $m(r_0)$. Whilst the asymptotic value of $\Delta\phi$ reaches a constant $\pi(v_{\infty}/c)^2$ in the low-acceleration limit of $a\ll a_0$. Taking the empirical correction of a factor of 2 from the theory of general relativity into account and utilizing the relation between rotation velocity $v$ and velocity dispersion $\sigma$, MOND results naturally in a constant deflection angle, $4\pi(\sigma/c)^2$, which has been widely used in the present-day study of gravitational lensing by galaxies and clusters of galaxies, implying that without introducing the massive halos acting as $r^{-2}$ for dark matter MOND has no difficulty in reproducing the known cases of gravitational lensing associated with galaxies and clusters of galaxies.Comment: 6 pages plus 3 figures, A&A LATEX styl

Large magnetic anisotropy in FexTaS2Fe_xTaS_2 single crystals

In intercalated transition metal dichalcogenide $Fe_xTaS_2$ (0.2 $\leq$ x $\leq$ 0.4) single crystals, large magnetic anisotropy is observed. Transport property measurements indicate that heavy Fe-doping leads to a large anisotropy of resistivity ($\rho$$_{c}$/$\rho$$_{ab}$). A sharp M-H hysteresis curve is observed with magnetic field along c-axis, while a linear magnetization appears with magnetic field applied in the ab-plane. The angular dependent magnetic susceptibility from in-plane to out-of-plane indicates that magnetic moments are strongly pinned along the c-axis in an unconventional manner and the coercive field reaches as large as 6 T at T = 5 K. First-principles calculation clearly suggests that the strong spin-orbital coupling give rise to such a large anisotropy of magnetism. The strong pinning effect of magnetic moments along c-axis makes this material a very promising candidate for the development of spin-aligner in spintronics devices.Comment: 5 pages, 4 figure