A quantum thermodynamic cycle with a chiral multiferroic working substance such as LiCu2O2 is presented. Shortcuts to adiabaticity are employed to achieve an efficient, finite time quantum thermodynamic cycle which is found to depend on the spin ordering. The emergent electric polarization associated with the chiral spin order, i.e. the magnetoelectric coupling, renders possible steering of the spin order by an external electric field and hence renders possible an electric-field control of the cycle. Due to the intrinsic coupling between of the spin and the electric polarization, the cycle performs an electro-magnetic work. We determine this work's mean square fluctuations, the irreversible work, and the output power of the cycle. We observe that the work mean square fluctuations are increased with the duration of the adiabatic strokes while the irreversible work and the output power of the cycle show a non-monotonic behavior. In particular the irreversible work vanishes at the end of the quantum adiabatic strokes. This fact confirms that the cycle is reversible. Our theoretical findings evidence the existence of a system inherent maximal output power. By implementing a Lindblad master equation we quantify the role of thermal relaxations on the cycle efficiency. We also discuss the role of entanglement encoded in the non-collinear spin order as a resource to affect the quantum thermodynamic cycle.

Many-body localization (MBL) is an emergent phase in correlated quantum systems with promising applications, particularly in quantum information. Here, we unveil the existence and analyze this phase in a chiral multiferroic model system. Conventionally, MBL occurrence is traced via level statistics by implementing a standard finite-size scaling procedure. Here, we present an approach based on the full distribution of the ratio of adjacent energy spacings. We find a strong broadening of the histograms of counts of these level spacings directly at the transition point from MBL to the ergodic phase. The broadening signals reliably the transition point without relying on an averaging procedure. The fast convergence of the histograms even for relatively small systems allows monitoring the MBL dynamics with much less computational effort. Numerical results are presented for a chiral spin chain with a dynamical DzyaloshinskiiMoriya interaction, an established model to describe the spin excitations in a single-phase spin-driven multiferroic system. The multiferroic MBL phase is uncovered and it is shown how to steer it via electric fields.

No abstract

In this paper we study the time evolution of the entanglement between two remote NV Centers (nitrogen vacancy in diamond) connected by a dual-mode nanomechanical resonator with magnetic tips on both sides. Calculating the negativity as a measure for the entanglement, we find that the entanglement between two spins oscillates with time and can be manipulated by varying the parameters of the system. We observed the phenomenon of a sudden death and the periodic revivals of entanglement in time. For the study of quantum decoherence, we implement a Lindblad master equation. In spite of its complexity, the model is analytically solvable under fairly reasonable assumptions, and shows that the decoherence influences the entanglement, the sudden death, and the revivals in time.

Staring from the kicked rotator as a paradigm for a system exhibiting classical chaos, we discuss the role of quantum coherence resulting in dynamical localization in the kicked quantum rotator. In this context, the disorder-induced Anderson localization is also discussed. Localization in interacting, quantum many-body systems (many-body localization) may also occur in the absence of disorder, and a practical way to identify its occurrence is demonstrated for an interacting spin chain. I. HAMILTONIAN CHAOS VS. RANDOM IMPURITIESAs put by Edward Lorenz, classical deterministic chaos is 1 : "when the present determines the future, but the approximate present does not approximately determine the future". The instability of phase trajectories is quantified by the Lyapunov exponent 2 : In phase space, the state of the N dimensional dynamical system with 2N degrees of freedom x ≡ (p n , q n ), n = 1, ..N is mapped to a single dot and the evolution of the system is described by a phase trajectory circumscribed by the system's state. Let us track the time evolution of two different phase trajectories see Fig.(1) x(t) + δ(t), x(t) emanated from slightly different initial conditions x 0 , x 0 + δ 0 . During the time evolution the distance between phase trajectories δ(t) = δ(0) exp λt increases provided that the Lyapunov exponent is positive λ > 0. Therefore, arbitrary small uncertainties in the initial conditions accumulate over a long period to the substantial error, calling thus for a statistical description when evaluating observable quantities.x(t) (0)|| exp(λt) → → → → → → FIG. 1. Time evolution of two different phase trajectories. A. Model of the minimal D=3/2 Hamiltonian chaosThe kicked rotator is a prototype of systems exhibiting chaos. The Hamiltonian of the one dimensional classical periodically kicked rotator readsHere (p, θ) is the canonical pair of momentum-angle variables and n specifies the number of kicks. Solution of the classical kicked rotator problem is described by the Chirikov's map0 1 2 π π 3 2 π 2π 0 1 2 π π 3 2 π 2π p θ FIG. 2. The Chirikov's map with k = 1.0. The graph was calculated using Julia programming language 37 with the Dy-namicalSystems.jl package.The phase space of the Chirikov's map consists of regions of regular and chaotic motions. If the strength of the kick exceeds the specific value k > 0.97, regular islands disappear, and the chaotic sea covers the whole phase space see Fig.(2) leading to a diffusive growth of the momentum in time (in the number of kicks) p n − p 0 2 = k 2 2 n. The kicked rotator is realized in a number of different physical setting. For example, in Ref. 3 the model was employed to investigate spin systems. Contrary to the classical case, the quantum coun-arXiv:1904.00091v1 [cond-mat.stat-mech]