413 research outputs found
Squeezing as the source of inefficiency in the quantum Otto cycle
The availability of controllable macroscopic devices, which maintain quantum
coherence over relatively long time intervals, for the first time allows an
experimental realization of many effects previously considered only as
Gedankenexperiments, such as the operation of quantum heat engines. The
theoretical efficiency \eta of quantum heat engines is restricted by the same
Carnot boundary \eta_C as for the classical ones: any deviations from
quasistatic evolution suppressing \eta below \eta_C. Here we investigate an
implementation of an analog of the Otto cycle in a tunable quantum coherent
circuit and show that the specific source of inefficiency is the quantum
squeezing of the thermal state due to the finite speed of compression/expansion
of the system.Comment: 17 pages, 5 figure
Entangling continuous variables with a qubit array
We show that an array of qubits embedded in a waveguide can emit entangled
pairs of microwave photon beams. The quadratures obtained from a homodyne
detection of these outputs beams form a pair of correlated continuous variables
similarly to the EPR experiment. The photon pairs are produced by the decay of
plasmon-like collective excitations in the qubit array. The maximum intensity
of the resulting beams is only bounded by the number of emitters. We calculate
the excitation decay rate both into a continuum of photon state and into a
one-mode cavity. We also determine the frequency of Rabi-like oscillations
resulting from a detuning.Comment: 5 pages, 3 figure
Time-dependent Real-space Renormalization-Group Approach: application to an adiabatic random quantum Ising model
We develop a time-dependent real-space renormalization-group approach which
can be applied to Hamiltonians with time-dependent random terms. To illustrate
the renormalization-group analysis, we focus on the quantum Ising Hamiltonian
with random site- and time-dependent (adiabatic) transverse-field and
nearest-neighbour exchange couplings. We demonstrate how the method works in
detail, by calculating the off-critical flows and recovering the ground state
properties of the Hamiltonian such as magnetization and correlation functions.
The adiabatic time allows us to traverse the parameter space, remaining near-to
the ground state which is broadened if the rate of change of the Hamiltonian is
finite. The quantum critical point, or points, depend on time through the
time-dependence of the parameters of the Hamiltonian. We, furthermore, make
connections with Kibble-Zurek dynamics and provide a scaling argument for the
density of defects as we adiabatically pass through the critical point of the
system
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