V. M. Bastidas

Nonequilibrium quantum phase transitions in the Dicke model.

We establish a set of nonequilibrium quantum phase transitions in the Dicke model by considering a monochromatic nonadiabatic modulation of the atom-field coupling. For weak driving the system exhibits a set of sidebands which allow the circumvention of the no-go theorem which otherwise forbids the occurrence of superradiant phase transitions. At strong driving we show that the system exhibits a rich multistable structure and exhibits both first- and second-order nonequilibrium quantum phase transitions.

Quantum signatures of chimera states.

Chimera states are complex spatiotemporal patterns in networks of identical oscillators, characterized by the coexistence of synchronized and desynchronized dynamics. Here we propose to extend the phenomenon of chimera states to the quantum regime, and uncover intriguing quantum signatures of these states. We calculate the quantum fluctuations about semiclassical trajectories and demonstrate that chimera states in the quantum regime can be characterized by bosonic squeezing, weighted quantum correlations, and measures of mutual information. Our findings reveal the relation of chimera states to quantum information theory, and give promising directions for experimental realization of chimera states in quantum systems.

Floquet engineering of long-range p -wave superconductivity

M.B., A.G.-L., and G.P. acknowledge the Spanish Ministry of Economy and Competitiveness through Grant No. MAT2011-24331 and the associated FPI scholarship (M.B.). V.M.B. and T.B. wish to acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) via GRK 1558, Grants No. BRA 1528/7, No. BRA 1528/8, and No. SFB 910.

Nonequilibrium quantum phase transitions in the Ising model

We establish a set of nonequilibrium quantum phase transitions in the Ising model driven under monochromatic nonadiabatic modulation of the transverse field. We show that besides the Ising-like critical behavior, the system exhibits an anisotropic transition which is absent in equilibrium. The nonequilibrium quantum phases correspond to states which are synchronized with the external control in the long-time dynamics.

Quantum criticality and dynamical instability in the kicked-top model.

We investigate precursors of critical behavior in the quasienergy spectrum due to the dynamical instability in the kicked top. Using a semiclassical approach, we analytically obtain a logarithmic divergence in the density of states, which is analogous to a continuous excited state quantum phase transition in undriven systems. We propose a protocol to observe the cusp behavior of the magnetization close to the critical quasienergy.

Driven Open Quantum Systems and Floquet Stroboscopic Dynamics.

We provide an analytic solution to the problem of system-bath dynamics under the effect of high-frequency driving that has applications in a large class of settings, such as driven-dissipative many-body systems. Our method relies on discrete symmetries of the system-bath Hamiltonian and provides the time evolution operator of the full system, including bath degrees of freedom, without weak-coupling or Markovian assumptions. An interpretation of the solution in terms of the stroboscopic evolution of a family of observables under the influence of an effective static Hamiltonian is proposed, which constitutes a flexible simulation procedure of nontrivial Hamiltonians. We instantiate the result with the study of the spin-boson model with time-dependent tunneling amplitude. We analyze the class of Hamiltonians that may be stroboscopically accessed for this example and illustrate the dynamics of system and bath degrees of freedom.

Semiclassical excited-state signatures of quantum phase transitions in spin chains with variable-range interactions

We study the excitation spectrum of a family of transverse-field spin chain models with variable interaction range and arbitrary spin

Excited-state quantum phase transitions and periodic dynamics

S

ac-Driven quantum phase transition in the Lipkin-Meshkov-Glick model.

, which in the case of

Publisher’s Note: Driven Open Quantum Systems and Floquet Stroboscopic Dynamics [Phys. Rev. Lett. 117 , 250401 (2016)]

S=1/2