Many-body localized systems exhibit a unique characteristic of avoiding thermalization, primarily attributed to the presence of a local disorder potential in the Hamiltonian. In recent years there has been an interest in simulating these systems on quantum devices. However, actual quantum devices are subject to unavoidable decoherence that can be modeled as coupling to a bath or continuous measurements. The quantum Zeno effect is also known to inhibit thermalization in a quantum system, where repeated measurements suppress transport. In this work we study the interplay of many-body localization and the many-body quantum Zeno effect. In a prethermal regime, we find that signatures of many-body localization are enhanced when the system is coupled to a bath that contains measurements of local fermion population, subject to the appropriate choice of system and bath parameters.
Strong disorder inhibits thermalization in isolated quantum systems and may lead to many-body localization (MBL). In realistic situations, however, the observation of MBL is hindered by residual couplings of the system to an environment, which acts as a bath and pushes the system to thermal equilibrium. This paper is concerned with the transient dynamics prior to thermalization and studies how the relaxation of a disordered system is altered under the influence of external driving and dissipation. We consider a scenario where a disordered quantum spin chain is placed into a strong magnetic field that polarizes the system. By suddenly removing the external field, a nonequilibrium situation is induced and the decay of magnetization probes the degree of localization. We show that by driving the system with light, one can distinguish between different dynamical regimes as the spins are more or less susceptible to the drive depending on the strength of the disorder. We provide evidence that some of these signatures remain observable at intermediate time scales even when the spin chain is subject to noise due to coupling to an environment. From a numerical point of view, we demonstrate that the open-system dynamics starting from a class of experimentally relevant mixed initial states can be efficiently simulated by combining dynamical quantum typicality with stochastic unraveling of Lindblad master equations.
We study the role of dipolar interactions in the standard protocol used to achieve dynamic nuclear polarization (DNP). We point out that a critical strength of interactions is required to obtain significant nuclear hyperpolarization. Otherwise, the electron spins do not thermalize among each other, due to the incipient many-body localization transition in the electron spin system. Only when the interactions are sufficiently strong, in the so-called spin-temperature regime, they establish an effective thermodynamic behavior in the out-of-equilibrium stationary state. The highest polarization is reached at a point where the spin temperature is just not able to establish itself anymore. We provide numerical predictions for the level of nuclear hyperpolarization and present an analytical technique to estimate the spin temperature as a function of interaction strength and quenched disorder. We show that, at sufficiently strong coupling, nuclear spins perfectly equilibrate to the spin temperature that establishes among the spins of radicals.
We theoretically investigate thermalization and spin diffusion driven by a quantum spin bath for a realistic solid-state NMR experiment. We consider polycrystalline L-alanine, and investigate how the spin polarization spreads among several
13C nuclear spins, which interact via dipole-dipole coupling with the bath of strongly dipolar-coupled
1H nuclear (proton) spins. We do this by using direct numerical simulation of the many-spin time-dependent Schrödinger equation. We find that, although the proton spins located near the carbon sites interact most strongly with the
13C spins, this interaction alone is not enough to drive spin diffusion and thermalize the
13C nuclear spins. We demonstrate that the thermalization within the
13C subsystem is driven by the collective many-body dynamics of the proton spin bath, and specifically, that the onset of thermalization among the
13C spins is directly related to the onset of chaotic behavior in the proton spin bath. Therefore, thermalization and spin diffusion within the
13C subsystem is controlled by the proton spins located far from the C sites. In spite of their weak coupling to the
13C spins, these far-away protons help produce a network of strongly coupled proton spins with collective dynamics, that drives thermalization.
Dynamic nuclear polarization (DNP) is to date the most effective technique to increase the nuclear polarization opening disruptive perspectives for medical applications. In a DNP setting, the interacting spin system is quasi-isolated and brought out of equilibrium by microwave irradiation. Here we show that the resulting stationary state strongly depends on the ergodicity properties of the spin many-body eigenstates. In particular, the dipolar interactions compete with the disorder induced by local magnetic fields resulting in two distinct dynamical phases: while for weak interaction, only a small enhancement of polarization is observed, for strong interactions the spins collectively equilibrate to an extremely low effective temperature that boosts DNP efficiency. We argue that these two phases are intimately related to the problem of thermalization in closed quantum systems where a many-body localization transition can occur varying the strength of the interactions.
We examine what happens when a strongly many-body localized system is coupled to a weak heat bath, with both system and bath containing similar numbers of degrees of freedom. Previous investigations of localized systems coupled to baths operated in regimes where the back action of the system on the bath is negligible and concluded that the bath generically thermalizes the system. In this paper we show that when the system is strongly localized and the bath is only weakly ergodic, the system can instead localize the bath. We demonstrate this both in the limit of weak coupling between system and bath and in the limit of strong coupling and also for two different types of ``weak'' bath---baths which are close to an atomic limit and baths which are close to a noninteracting limit. The existence of this ``many-body localization proximity effect'' indicates that many-body localization is more robust than previously appreciated, and it can not only survive coupling to a (weak) heat bath but can even destroy the bath.