Counterdiabatic (CD) driving presents a way of generating adiabatic dynamics at arbitrary pace, where excitations due to non-adiabaticity are exactly compensated by adding an auxiliary driving term to the Hamiltonian. While this CD term is theoretically known and given by the adiabatic gauge potential, obtaining and implementing this potential in many-body systems is a formidable task, requiring knowledge of the spectral properties of the instantaneous Hamiltonians and control of highly nonlocal multibody interactions. We show how an approximate gauge potential can be systematically built up as a series of nested commutators, remaining well-defined in the thermodynamic limit. Furthermore, the resulting CD driving protocols can be realized up to arbitrary order without leaving the available control space using tools from periodically-driven (Floquet) systems. This is illustrated on few-and many-body quantum systems, where the resulting Floquet protocols significantly suppress dissipation and provide a drastic increase in fidelity.

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Adiabatic evolution is a common strategy for manipulating quantum states and has been employed in diverse fields such as quantum simulation, computation and annealing. However, adiabatic evolution is inherently slow and therefore susceptible to decoherence. Existing methods for speeding up adiabatic evolution require complex many-body operators or are difficult to construct for multi-level systems. Using the tools of Floquet engineering, we design a scheme for high-fidelity quantum state manipulation, utilizing only the interactions available in the original Hamiltonian. We apply this approach to a qubit and experimentally demonstrate its performance with the electronic spin of a Nitrogen-vacancy center in diamond. Our Floquet-engineered protocol achieves state preparation fidelity of 0.994 ± 0.004, on the same level as the conventional fast-forward protocol, but is more robust to external noise acting on the qubit. Floquet engineering provides a powerful platform for high-fidelity quantum state manipulation in complex and noisy quantum systems.

We study the localization properties of weakly interacting Bose gas in a quasiperiodic potential. The Hamiltonian of the non-interacting system reduces to the well known 'Aubry-André model', which shows the localization transition at a critical strength of the potential. In the presence of repulsive interaction we observe multi-site localization and obtain a phase diagram of the dilute Bose gas by computing the superfluid fraction and the inverse participation ratio. We construct a low-dimensional classical Hamiltonian map and show that the onset of localization is manifested by the chaotic phase space dynamics. The level spacing statistics also identify the transition to localized states resembling a Poisson distribution that are ubiquitous for both non-interacting and interacting systems. We also study the quantum fluctuations within the Bogoliubov approximation and compute the quasiparticle energy spectrum. Enhanced quantum fluctuation and multi-site localization phenomenon of noncondensate density are observed above the critical coupling of the potential. We briefly discuss the effect of the trapping potential on the localization of matter wave.(SFF). The effect of nonlinearity due to interactions on the dynamical map is studied. We compute the Bogoliubov quasiparticle energies and amplitudes to investigate disorder induced quantum fluctuations and to study the localization of quasi-particle amplitudes and non-condensate densities. We also investigate the localization of Bogoliubov excitations from spectral statistics. The crossover to disorder induced localization is studied for Bose gas with weak attractive interaction. The effect of the trapping potential on localization and effect of disorder on the center of mass (COM) motion are presented in section 4. Finally we summarize our results in section 5.

Long-lived dark states, in which an experimentally accessible qubit is not in thermal equilibrium with a surrounding spin bath, are pervasive in solid-state systems. We explain the ubiquity of dark states in a large class of inhomogeneous central spin models using the proximity to integrable lines with exact dark eigenstates. At numerically accessible sizes, dark states persist as eigenstates at large deviations from integrability, and the qubit retains memory of its initial polarization at long times. Although the eigenstates of the system are chaotic, exhibiting exponential sensitivity to small perturbations, they do not satisfy the eigenstate thermalization hypothesis. Rather, we predict long relaxation times that increase exponentially with system size. We propose that this intermediate chaotic but non-ergodic regime characterizes mesoscopic quantum dot and diamond defect systems, as we see no numerical tendency towards conventional thermalization with a finite relaxation time.

Fluid structure interaction (FSI) problems are becoming highly complex as FSI has vast applications, which help to model many real life problems. Here, a oneway coupling strategy has been proposed for the FSI problem, where fluid has been modelled in computational fluid dynamics (CFD) and its interface with the solid structure has been modelled with a no-slip no-penetration boundary condition. Using Abaqus CFD simulations, the calculated interface pressure has been applied on the POD modes of the solid structure domain to obtain the displacement field. Here, we assumed that the motion of the flexible solid structure has no effects on the fluid flow as it is a one-way coupling. This assumption is well governed for the less flexible structure. However, it has been noted that this shows good predictions for flexible structures too. The benefit of this approach is that for the same interface boundary of the FSI domain, we can change the structural material properties and predict the results without going for the complete FSI simulation problem. Here, we solved a case study problem example for the fluid flow over a flexible beam and results are validated using the Abaqus FSI Cosimulation.