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Physical Review A

Interaction time of Schrödinger cat states with a periodically driven quantum system: Symplectic covariance approach

D. Woźniak, Bartłomiej J. Spisak, Maciej Kalka, M. Wołoszyn, M. Wleklińska, Piotr Pigoń, Damian Kołaczek

Published Aug 12, 2024• 1 citation
Publisher

Abstract

The symplectic invariance property of the Wigner-R\'enyi entropies family is used to estimate the interaction time for the propagating Schr\"odinger cat state that interacts with the periodically changing potential. With the help of this property, the hierarchy of the interaction times is specified for the quantum dynamics of the periodically driven system. It is also demonstrated that the lower and upper bounds of the interaction time are determined by the Wigner-R\'enyi entropies corresponding to increasing R\'enyi index values, respectively. The interpretation of the presented results is supported by calculations of the dynamical transmission and capture coefficients, which give a clear insight into the scattering of the non-Gaussian state on the periodically changing potential, which switches between the Gaussian potential barrier and the well.

DOI
10.1103/physreva.110.022416

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The symplectic invariance property of the Wigner-R'enyi entropies family is used to estimate the interaction time for the propagating Schr"odinger cat state that interacts with the periodically changing potential. With the help of this property, the hierarchy of the interaction times is specified for the quantum dynamics of the periodically driven system. It is also demonstrated that the lower and upper bounds of the interaction time are determined by the Wigner-R'enyi entropies corresponding to increasing R'enyi index values, respectively. The interpretation of the presented results is supported by calculations of the dynamical transmission and capture coefficients, which give a clear insight into the scattering of the non-Gaussian state on the periodically changing potential, which switches between the Gaussian potential barrier and the well.
The asymptotic state of a periodically driven many-body quantum system in contact with an environment is investigated. The combined action of the driving and the environment steers the system towards a state being characterized by a time-periodic density operator. To compute this asymptotic non-equilibrium state at stroboscopic instants of time, we introduce the dissipative Floquet map, find the stroboscopic density operator as its eigen-operator and demonstrate how particle interactions affect properties of the density operator. We illustrate the idea with a periodically rocked open Bose-Hubbard dimer and discuss the relations between the interaction-induced bifurcation transitions in a mean-field dynamics and changes in the characteristics of the quantum many-body state. We argue that Floquet maps can provide insight into the system relaxation towards its asymptotic state and may help to identify the stroboscopic time-independent generator mimicking the action of the original time-dependent one.
Finding the equilibration time scale is an important open question in studying the equilibration of quantum systems. There are many kinds of systems that are unable to achieve equilibration, such as Anderson insulator, many-body localization systems and some integrable systems. For those systems that can reach equilibration, it was proved that there exists a general equilibration time scale. But these upper bounds are unrealistically long, and the lower bounds are also unrealistically short. How to get an accurate and general equilibration time scale is still unclear. In this paper, we study local equilibration time scales in quantum lattice systems. With the relation between equilibration and entanglement entropy, we define a new criterion for equilibration. This criterion is based on Rényi entropy, which is simpler for calculation. Moreover, the production of Rényi entropy is highly dependent on the spreading of information, and hence the tools developed in quantum information theory can help a lot. For the upper bound of equilibration time, we use the normal criterion of the time average ofthe fluctuation of the observation. This equilibration criterion is assessed with concrete observable operator, hence it is more accurate than the criterion of Rényi entropy. But this criterion is more complicated for calculation. With an appropriate assumption about the initial state, we present a new upper bound of equilibration time. Since the results are not constrained by the Hamiltonian of the system, this bound can be applied to various situations. If we are concerned with the local equilibration, we can limit the observation to a small region. If the whole system is big enough, the local observation would always find that the rest part is staying at the canonical ensemble, so that the local equilibration time will not increase with the size of whole system. When the local region is small enough, the upper bound of equilibration time can be much shorter. The local Rényi entropy has close relation to the propagation of information. The limitation of the speed of information propagation in a quantum lattice system can be given by the Lieb-Robinson bound, with which we evaluate the production rate of local 2-Rényi entropy. With these, we give a new lower bound of equilibration time for systems whose interactions are respectively short-range, exponentially-decaying and long-range. In earlier works, the lower bound of equilibration time is equal to the Lieb-Robinson time of the local system. In our rigorous proof, however, the lower bound is related to the strength of hopping, which also decides the Lieb-Robinson velocity. This means the equilibration time is related to the Lieb-Robinson time indeed. But the strictly equal relation may not hold. These new bounds are important to understanding the process of quantum equilibration.
The concept of the symplectic covariance property of the Wigner distribution function and the symplectic invariance of the Wigner-Rényi entropies has been leveraged to estimate the interaction time of the moving quantum state in the presence of an absolutely integrable time-dependent potential. For this study, the considered scattering centre is represented initially by the Gaussian barrier. Two modifications of this potential energy are considered: a sudden change from barrier to barrier and from barrier to well. The scattering state is prepared in the form of a Schrödinger cat, and the Moyal equation governs its further time evolution. The whole analysis of the considered scattering problem is conducted in the above-barrier regime using the phase-space representation of quantum theory. The presented concept shifts the focus from the dynamics of the quantum state to a symplectically invariant state functions in the form of the Wigner-Rényi entropies of order one and one-half. These quantities serve as indicators of the beginning and end of the interaction of the non-Gaussian state with the sufficiently fast decaying potential representing the scattering centre. The presented approach is significant because it provides a new way to estimate the interaction time of moving quantum states with the dynamical scattering centre. Moreover, it allows for studying scattering processes in various physical systems, including atoms, molecules, and condensed matter systems.
Under a periodically driven external field or a temporally oscillating boundary, quantum few-body systems whose corresponding classical systems show chaos are modeled by the random matrices with an oscillating parameter. In such systems, time evolution of the system from the ground state shows the quantum diffusion. However, due to periodic oscillation of the parameter, quantum interference effect prevents the energy of the system from diverging and the energy saturates after some transient time. We show an example where the energy eventually fluctuates around a constant saturated energy. Since the value of this saturated energy depends on the frequency of the driving, we also investigate the dependence of the saturated energy on the frequency in the small frequency region.
Fermionic non-Gaussianity is a resource for universal quantum computation that can be generated by interactions in quantum many-body systems. Using the magic Rényi entropy (MRE) as a measure of non-Gaussianity, we derive its universal upper bound and Haar mean, proving that typical Haar-random states attain the maximal MRE density of ln⁡(4/3)\ln(4/3)ln(4/3) per Majorana in the large-system limit. We also show that the MRE can differ extensively between states with identical covariance matrices. To investigate how non-Gaussianity grows toward this maximal density, we evolve a Gaussian state in imaginary and real time under the Sachdev-Ye-Kitaev Hamiltonian. By varying the imaginary- and real-time durations, we identify a first-order transition marked by a kink in the MRE density and spontaneous breaking of the permutation symmetry among the four copies used to evaluate the MRE. This transition represents a qualitative change in the non-Gaussianity of the state that is not reflected in the thermal free energy.