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ABSTRACT: We present a theoretical study of a hybrid circuit-QED system composed of two
semiconducting charge-qubits confined in a microwave resonator. The qubits are
defined in terms of the charge states of two spatially separated double quantum
dots (DQDs) which are coupled to the same photon mode in the microwave
resonator. We analyze a transport setup where each DQD is attached to
electronic reservoirs and biased out-of-equilibrium by a large voltage, and
study how electron transport across each DQD is modified by the coupling to the
common resonator. In particular, we show that the inelastic current through
each DQD reflects an indirect qubit-qubit interaction mediated by off-resonant
photons in the microwave resonator. As a result of this interaction, both
charge qubits stay entangled in the steady (dissipative) state. Finite shot
noise cross-correlations between currents across distant DQDs are another
manifestation of this nontrivial steady-state entanglement.
04/2013;
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ABSTRACT: Our aim in this work is to study the nonequilibrium behavior of the
topological quantum phase transition in the transverse Wen-plaquette model. We
show that under the effect of a nonadiabatic driving the system exhibits a new
topological phase and a rich phase diagram. We define generalized topological
order parameters by considering cycle-averaged expectation values of string
operators in a Floquet state
02/2013;
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ABSTRACT: We establish a set of nonequilibrium quantum phase transitions in the
Lipkin-Meshkov-Glick model under monochromatic modulation of the inter-particle
interaction. We show that the external driving induces a rich phase diagram
that characterizes the multistability in the system. Interestingly, the number
of stable configurations can be tuned by increasing the amplitude of the
driving field. Furthermore, by studying the quantum evolution, we demonstrate
that the system exhibits a set of quantum phases that correspond to dynamically
stabilized states.
11/2012;
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ABSTRACT: We propose an all-electronic technique to manipulate and control interacting
quantum systems by unitary single-jump feedback conditioned on the outcome of a
capacitively coupled electrometer and in particular a single-electron
transistor. We provide a general scheme to stabilize pure states in the quantum
system and employ an effective Hamiltonian method for the quantum master
equation to elaborate on the nature of stabilizable states and the conditions
under which state purification can be achieved. The state engineering within
the quantum feedback scheme is shown to be linked with the solution of an
inverse eigenvalue problem. Two applications of the feedback scheme are
presented in detail: (i) stabilization of delocalized pure states in a single
charge qubit and (ii) entanglement stabilization in two coupled charge qubits.
In the latter example we demonstrate the stabilization of a maximally entangled
Bell state for certain detector positions and local feedback operations.
09/2012;
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ABSTRACT: 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.
07/2012;
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ABSTRACT: We consider the Dicke model in the ultrastrong-coupling limit to investigate thermal phase transitions and their precursors at finite particle numbers N for bosonic and fermionic systems. We derive partition functions with degeneracy factors that account for the number of configurations and derive explicit expressions for the Landau free energy. This allows us to discuss the difference between the original Dicke (fermionic) and the bosonic case. We find a crossover between these two cases that shows up, for example, in the specific heat.
Phys. Rev. E. 07/2012; 86(1).
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ABSTRACT: 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.
Physical Review Letters 01/2012; 108(4):043003. · 7.37 Impact Factor
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ABSTRACT: We present a theory of finite-frequency noise in non-equilibrium conductors.
It is shown that Non-Markovian correlations are essential to describe the
physics of quantum noise. In particular, we show the importance of a correct
treatment of the initial system-bath correlations, and how these can be
calculated using the formalism of quantum master equations. Our method is
particularly important in interacting systems, and when the measured
frequencies are larger that the temperature and applied voltage. In this
regime, quantum-noise steps are expected in the power spectrum due to vacuum
fluctuations. This is illustrated in the current noise spectrum of single
resonant level model and of a double quantum dot --charge qubit-- attached to
electronic reservoirs. Furthermore, the method allows for the calculation of
the single-time counting statistics in quantum dots, measured in recent
experiments.
06/2010;
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ABSTRACT: We present a theory of frequency-dependent counting statistics of electron transport through nanostructures within the framework of Markovian quantum master equations. Our method allows to calculate finite-frequency current cumulants of arbitrary orders, as we explicitly show for the second and third order cumulants. Our formulae generalize previous zero-frequency expressions in the literature and can be viewed as an extension of MacDonald's formula beyond shot noise. When combined with an appropriate treatment of tunneling, using, e. g. Liouvillian perturbation theory in Laplace space, our method can deal with arbitrary bias voltages and frequencies, as we illustrate with the paradigmatic example of transport through a single resonant level model. We discuss various interesting limits, including the recovering of the fluctuation-dissipation theorem near linear response, as well as some drawbacks of our treatment due to the lack of quantum fluctuations inherent to the Markovian description.
04/2010;
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ABSTRACT: We study the relationship between entanglement and parametric resonance in a
system of two coupled time-dependent oscillators. As a measure of bipartite
entanglement, we calculate the linear entropy for the reduced density operator,
from which we study the entanglement dynamics. In particular, we find that the
bipartite entanglement increases in time up to a maximal mixing scenario, when
the set of auxiliary dynamical parameters are under parametric resonance.
Moreover, we obtain a closed relationship between the correlations in the
ground state, the localisation of the Wigner function in phase space, and the
localisation of the wave function of the total system.
10/2009;
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ABSTRACT: Noise is a result of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain details that characterize the correlations within a given noise source and its interaction with the environment, but they are often difficult to measure. Here we report measurements of the transient cumulants n(m) of the number n of passed charges to very high orders (up to m = 15) for electron transport through a quantum dot. For large m, the cumulants display striking oscillations as functions of measurement time with magnitudes that grow factorially with m. Using mathematical properties of high-order derivatives in the complex plane we show that the oscillations of the cumulants in fact constitute a universal phenomenon, appearing as functions of almost any parameter, including time in the transient regime. These ubiquitous oscillations and the factorial growth are system-independent and our theory provides a unified interpretation of previous theoretical studies of high-order cumulants as well as our new experimental data.
Proceedings of the National Academy of Sciences 07/2009; 106(25):10116-9. · 9.68 Impact Factor
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ABSTRACT: Noise is a result of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain details that characterize the correlations within a given noise source and its interaction with the environment, but they are often difficult to measure. Here we report measurements of the transient cumulants ãããã of the number of passed charges to very high orders (up to = 15) for electron transport through a quantum dot. For large , the cumulants display striking oscillations as functions of measurement time with magnitudes that grow factorially with . Using mathematical properties of high-order derivatives in the complex plane we show that the oscillations of the cumulants in fact constitute a universal phenomenon, appearing as functions of almost any parameter, including time in the transient regime. These ubiquitous oscillations and the factorial growth are system-independent and our theory provides a unified interpretation of previous theoretical studies of high-order cumulants as well as our new experimental data.
Proceedings of the National Academy of Sciences. 06/2009; 106:10116--10119.
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ABSTRACT: We show that the intriguing observation of noise enhancement in the charge transport through two vertically coupled quantum dots can be explained by the interplay of quantum coherence and strong Coulomb blockade. We demonstrate that this novel mechanism for super-Poissonian charge transfer is very sensitive to decoherence caused by electron-phonon scattering as inferred from the measured temperature dependence.
Physical Review Letters 12/2007; 99(20):206602. · 7.37 Impact Factor
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ABSTRACT: We present a formalism to calculate finite-frequency current correlations in
interacting nanoscale conductors. We work within the n-resolved density matrix
approach and obtain a multi-time cumulant generating function that provides the
fluctuation statistics, solely from the spectral decomposition of the
Liouvillian. We apply the method to the frequency-dependent third cumulant of
the current through a single resonant level and through a double quantum dot.
Our results, which show that deviations from Poissonian behaviour strongly
depend on frequency, demonstrate the importance of finite-frequency
higher-order cumulants in fully characterizing interactions.
03/2007;
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ABSTRACT: We study charge entanglement in two Coulomb-coupled double quantum dots in thermal equilibrium and under stationary non-equilibrium transport conditions. In the transport regime, the entanglement exhibits a clear switching threshold and various limits due to suppression of tunneling by Quantum Zeno localisation or by an interaction induced energy gap. We also calculate quantum noise spectra and discuss the inter-dot current correlation as an indicator of the entanglement in transport experiments. Comment: 4 pages, 4 figures
02/2006;
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ABSTRACT: We present a formalism to calculate frequency dependent electron current noise for transport through two-level systems (such as coupled quantum dots or Cooper-pair boxes) in presence of dissipation. Perturbation theories in various regimes are formulated within a matrix scheme in Laplace scheme which we evaluate in detail both for weak and strong coupling to a bosonic environment.
Physics of Condensed Matter 07/2004; 40(4):357-363. · 1.53 Impact Factor
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ABSTRACT: We investigate the entanglement properties of an ensemble of atoms interacting with a single bosonic field mode via the Dicke (superradiance) Hamiltonian. The model exhibits a quantum phase transition and a well-understood thermodynamic limit, allowing the identification of both quantum and semi-classical many-body features in the behaviour of the entanglement. We consider the entanglement between the atoms and the field, an investigation initiated in [N. Lambert, C. Emary and T. Brandes, Phys. Rev. Lett. {\bf 92}, %073602 (2004)]. In the thermodynamic limit, we give exact results for all entanglement partitions and observe a logarithmic divergence of the atom-field entanglement, and discontinuities in the average linear entropy. Comment: 9 pages, 3 figures. Significantly Shortened. Removed concurrence section, multipartite section significantly changed. 14 pages, 4 figures, submitted to PRA
05/2004;
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ABSTRACT: We derive a Floquet-like formalism to calculate the stationary average current through an AC driven double quantum dot in presence of dissipation. The method allows us to take into account arbitrary coupling strengths both of a time-dependent field and a bosonic environment. We numerical evaluate a truncation scheme and compare with analytical, perturbative results such as the Tien-Gordon formula. Comment: 14 pages, 6 figures. To appear in Phys. Rev. B
04/2004;
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ABSTRACT: We investigate the transport and coherence properties of a double quantum dot coupled to a single damped boson mode. Our numerically results reveal how the properties of the boson distribution can be steered by altering parameters of the electronic system such as the energy difference between the dots. Quadrature amplitude variances and the Wigner function are employed to illustrate how the state of the boson mode can be controlled by a stationary electron current through the dots. Comment: 10 pages, 6 figures, to appear in Phys. Rev. B
02/2003;
