We study two anisotropically interacting spins coupled to optical phonons; we restrict our analysis to the regime of strong coupling to the environment, to the antiadiabatic region, and to the subspace with zero value for S z T (the z-component of the total spin). In the case where each spin is coupled to a different phonon bath, we assume that the system and the environment are initially uncorrelated (and form a simply separable state) in the polaronic frame of reference. By analyzing the polaron dynamics through non-Markovian quantum master equation, we find that the system manifests a small amount of decoherence that decreases both with increasing non-adiabaticity and with enhancing strength of coupling; whereas, under the Markovian approximation, the polaronic system exhibits a decoherence free behavior. For the situation where both spins are coupled to the same phonon bath, we also show that the system is decoherence free in the subspace where S z T is fixed. To suppress decoherence through quantum control, we employ a train of pi pulses and demonstrate that unitary evolution of the system can be retained.We propose realization of a weakly decohering charge qubit from an electron in an oxide-based (tunnel-coupled) double quantum dot system.

We study the process of nonlinear stimulated Raman adiabatic passage within a classical meanfield framework. Depending on the sign of interaction, the breakdown of adiabaticity in the interacting non-integrable system, is not related to bifurcations in the energy landscape, but rather to the emergence of quasi-stochastic motion that drains the followed quasi-stationary state. Consequently, faster sweep rate, rather than quasi-static variation of parameters, is better for adiabaticity.The analysis of quasi-static adiabatic processes is a central theme in quantum thermodynamics, coherent control, quantum state engineering, nonlinear and quantum optics, and nanotechnology. The adiabatic paradigm extends from microscopic systems with few degrees of freedom, through mesoscopic nano-machinary, to macroscopic steam engines. Throughout this vast range of applications, common wisdom has it that "slow is better", i.e. that excitations from the followed adiabatic state can be avoided by slower variation of the system's control parameters. Here, we show that when chaotic stages are encountered during an adiabatic scenario, slow variation can in fact damage its efficiency.The effect is demonstrated using a minimal example: a stimulated Raman adiabatic passage (STIRAP) [1,2] in the presence of interactions. Advances in Bose-Einstein condensation (BEC), nonlinear optics, and the control of light in coupled waveguides [3,4], have triggered great interest in the application of adiabatic passage to interacting systems. The effects of interactions on two-mode adiabatic schemes were studied using various Bose-Hubbard dimer Hamiltonians, . The common denominator for all these studies is the quest for energetic stability. The dynamics follows a stationary point (SP) of the instantaneous Hamiltonian H(x), where x = x(t) is a control parameter. This SP, that has some x-dependent energy E[SP], is required to be a local minimum (or a local maximum) of the energy landscape. Nonlinear instability is attributed to the emergence of a separatrix in the energy landscape due to a bifurcation of such a SP.

Understanding coherent dynamics of excitons, spins, or hard-core bosons (HCBs) has tremendous scientific and technological implications for light harvesting and quantum computation. Here, we study decay of excited-state population and decoherence in two models for HCBs, namely: an infinite-range HCB model governed by Markovian dynamics and a two-site HCB model with sitedependent strong potentials and subject to non-Markovian dynamics. Both models are investigated in the regimes of antiadiabaticity and strong HCB-phonon coupling with each site providing a different local optical phonon environment; furthermore, the HCB systems in both models are taken to be initially uncorrelated with the environment in the polaronic frame of reference. For the infinite-range model, we derive an effective many-body Hamiltonian that commutes with the long-range system Hamiltonian and thus has the same set of eigenstates; consequently, a quantummaster-equation approach shows that the quantum states of the system do not decohere. In the case of the two-site HCB model, we show clearly that the degree of decoherence and decay of excited state are enhanced by the proximity of the site-energy difference to the eigenenergy of phonons and are most pronounced when the site-energy difference is at resonance with twice the polaronic energy; additionally, the decoherence and the decay effects are reduced when the strength of HCB-phonon coupling is increased. Even for a multimode situation, the degree of decoherence and decay are again dictated by the nearness of the energy difference to the allowed phonon mode eigenenergies. PACS numbers: 71.38.-k, 03.65.Yz, 85.35.Be, 87.10.HkWe choose the basis {|10 , |01 } for our analysis and obtain the following useful expressions: e −iH L s t |10 = [p(t) * |10 − iκq(t)|01 ]e i J 4 t , (48) and

We study the many-body dynamics of stimulated Raman adiabatic passage in the presence of onsite interactions. In the classical mean-field limit, explored in Phys. Rev. Lett. 121, 250405 (2018), interaction-induced chaos leads to the breakdown of adiabaticity under the quasi-static variation of the parameters, thus producing low sweep rate boundaries on efficient population transfer. We show that for the corresponding many-body system, alternative quantum pathways from the initial to the target state, open up at even slower sweep rates. These quantum detours avoid the chaotic classical path and hence allow a robust and efficient population transfer.

We develop optimal protocols for efficient photon transfer in a cavity-QED network. This is executed through stimulated Raman adiabatic passage scheme where time-varying inductive or capacitive couplings (with carefully chosen sweep rate) play a key role. We work in a regime where the semiclassical limit is valid, and we investigate the dynamical chaos caused by the light-matter coupling. We show that this plays a crucial role in estimating the lower bound on the sweep rate for ensuring efficient photon transfer. We present Hermitian as well as an open quantum system extension of the model. Without loss of generality, we study the three cavity and four cavity cases and our results can be adapted to larger networks. Our analysis is also significant in designing transport protocols aimed for nonlinear open quantum systems, in general.