140 research outputs found
Theory of cavity-assisted microwave cooling of polar molecules
We analyze cavity-assisted cooling schemes for polar molecules in the
microwave domain, where molecules are excited on a rotational transition and
energy is dissipated via strong interactions with a lossy stripline cavity, as
recently proposed by A. Andre et al., Nature Physics 2, 636 (2006). We identify
the dominant cooling and heating mechanisms in this setup and study cooling
rates and final temperatures in various parameter regimes. In particular we
analyze the effects of a finite environment temperature on the cooling
efficiency, and find minimal temperature and optimized cooling rate in the
strong drive regime. Further we discuss the trade-off between efficiency of
cavity cooling and robustness with respect to ubiquitous imperfections in a
realistic experimental setup, such as anharmonicity of the trapping potential
Nonequilibrium magnetic phases in spin lattices with gain and loss
We study the magnetic phases of a nonequilibrium spin chain, where coherent interactions between neighboring lattice sites compete with alternating gain and loss processes. This competition between coherent and incoherent dynamics induces transitions between magnetically aligned and highly mixed phases, across which the system changes from a low to an infinite temperature state. We show that the origin of these transitions can be traced back to the dynamical effect of parity–time-reversal symmetry breaking, which has no counterpart in the theory of equilibrium phase transitions. This mechanism also results in very atypical features and we find first-order transitions without phase coexistence and mixed-order transitions which do not break the underlying U(1) symmetry, even in the appropriate thermodynamic limit. Thus, despite its simplicity, the current model considerably extends the phenomenology of nonequilibrium phase transitions beyond that commonly assumed for driven-dissipative spins and related systems
Ultrastrong coupling phenomena beyond the Dicke model
We study effective light-matter interactions in a circuit QED system
consisting of a single resonator, which is coupled symmetrically to
multiple superconducting qubits. Starting from a minimal circuit model, we
demonstrate that in addition to the usual collective qubit-photon coupling the
resulting Hamiltonian contains direct qubit-qubit interactions, which have a
drastic effect on the ground and excited state properties of such circuits in
the ultrastrong coupling regime. In contrast to a superradiant phase transition
expected from the standard Dicke model, we find an opposite mechanism, which at
very strong interactions completely decouples the photon mode and projects the
qubits into a highly entangled ground state. These findings resolve previous
controversies over the existence of superradiant phases in circuit QED, but
they more generally show that the physics of two- or multi-atom cavity QED
settings can differ significantly from what is commonly assumed.Comment: 11 pages, 8 figure
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