1,721 research outputs found
Decoherence processes in a current biased dc SQUID
A current bias dc SQUID behaves as an anharmonic quantum oscillator
controlled by a bias current and an applied magnetic flux. We consider here its
two level limit consisting of the two lower energy states | 0 \right> and |
1 \right>. We have measured energy relaxation times and microwave absorption
for different bias currents and fluxes in the low microwave power limit.
Decoherence times are extracted. The low frequency flux and current noise have
been measured independently by analyzing the probability of current switching
from the superconducting to the finite voltage state, as a function of applied
flux. The high frequency part of the current noise is derived from the
electromagnetic environment of the circuit. The decoherence of this quantum
circuit can be fully accounted by these current and flux noise sources.Comment: 4 pages, 4 figure
Variable electrostatic transformer: controllable coupling of two charge qubits
We propose and investigate a novel method for the controlled coupling of two
Josephson charge qubits by means of a variable electrostatic transformer. The
value of the coupling capacitance is given by the discretized curvature of the
lowest energy band of a Josephson junction, which can be positive, negative, or
zero. We calculate the charging diagram of the two-qubit system that reflects
the transition from positive to negative through vanishing coupling. We also
discuss how to construct a phase gate making use of the controllable coupling.Comment: final version, to appear in Phys. Rev. Let
Scalable superconducting qubit circuits using dressed states
We study a coupling/decoupling method between a superconducting qubit and a
data bus that uses a controllable time-dependent electromagnetic field (TDEF).
As in recent experiments, the data bus can be either an LC circuit or a cavity
field. When the qubit and the data bus are initially fabricated, their detuning
should be made far larger than their coupling constant, so these can be treated
as two independent subsystems. However, if a TDEF is applied to the qubit, then
a "dressed qubit" (i.e., qubit plus the electromagnetic field) can be formed.
By choosing appropriate parameters for the TDEF, the dressed qubit can be
coupled to the data bus and, thus, the qubit and the data bus can exchange
information with the assistance of the TDEF. This mechanism allows the
scalability of the circuit to many qubits. With the help of the TDEF, any two
qubits can be selectively coupled to (and decoupled from) a common data bus.
Therefore, quantum information can be transferred from one qubit to another.Comment: 10 pages, 5 figure
Heisenberg Uncertainty Principle as Probe of Entanglement Entropy: Application to Superradiant Quantum Phase Transitions
Quantum phase transitions are often embodied by the critical behavior of
purely quantum quantities such as entanglement or quantum fluctuations. In
critical regions, we underline a general scaling relation between the
entanglement entropy and one of the most fundamental and simplest measure of
the quantum fluctuations, the Heisenberg uncertainty principle. Then, we show
that the latter represents a sensitive probe of superradiant quantum phase
transitions in standard models of photons such as the Dicke Hamiltonian, which
embodies an ensemble of two-level systems interacting with one quadrature of a
single and uniform bosonic field. We derive exact results in the thermodynamic
limit and for a finite number N of two-level systems: as a reminiscence of the
entanglement properties between light and the two-level systems, the product
diverges at the quantum critical point as . We
generalize our results to the double quadrature Dicke model where the two
quadratures of the bosonic field are now coupled to two independent sets of two
level systems. Our findings, which show that the entanglement properties
between light and matter can be accessed through the Heisenberg uncertainty
principle, can be tested using Bose-Einstein condensates in optical cavities
and circuit quantum electrodynamicsComment: 7 pages, 3 figures. Published Versio
Intermittent origin of the large violations of the fluctuation dissipation relations in an aging polymer glass
The fluctuation-dissipation relation (FDR) is measured on the dielectric
properties of a polymer glass (polycarbonate)in the range . It
is found that after a quench below the glass transition temperature the
fluctuation dissipation theorem is strongly violated. The amplitude and the
persistence time of this violation are decreasing functions of frequency. At
frequencies larger than 1Hz it persists for about . The origin of this
violation is a highly intermittent dynamics characterized by large
fluctuations. The relevance of these results for recent models of aging
dynamics are discussed.Comment: to be published in Europhysics Letter
Observation of transition from escape dynamics to underdamped phase diffusion in a Josephson junction
We have investigated the dynamics of underdamped Josephson junctions. In
addition to the usual crossover between macroscopic quantum tunnelling and
thermally activated (TA) behaviour we observe in our samples with relatively
small Josephson coupling E_J, for the first time, the transition from TA
behaviour to underdamped phase diffusion. Above the crossover temperature the
threshold for switching into the finite voltage state becomes extremely sharp.
We propose a (T,E_J) phase-diagram with various regimes and show that for a
proper description of it dissipation and level quantization in a metastable
well are crucial.Comment: 4 pages, 3 figure
Intermittent quakes and record dynamics in the thermoremanent magnetization of a spin-glass
A novel method for analyzing the intermittent behavior of linear response
data in aging systems is presented and applied to spin-glass thermoremanent
magnetization (TRM) (Rodriguez et al. Phys. Rev. Lett. 91, 037203, 2003).
The probability density function (PDF) of magnetic fluctuations is shown to
have an asymmetric exponential tail, demonstrating that the demagnetization
process is carried by intermittent, significant, spin rearrangements or
\emph{quakes}. These quakes are most pronounced shortly after the field
removal, and in the non-equilibrium aging regime .
For a broad temperature range, we study the dependence of the TRM decay rate on
, the time since the initial quench and on , the time at which the
magnetic field is cut. The and dependence of the rate is extracted
numerically from the data and described analytically using the assumption that
the linear response is subordinated to the intermittent process which
spasmodically release the initial imbalances created by the quench.Comment: 8 pages, 9 figures. The paper has been expanded and restructured, the
figures have been enlarged and improved. Final version, to appear in Phy.
Rev.
Out-of-equilibrium dynamics in a gaussian trap model
The violations of the fluctuation-dissipation theorem are analyzed for a trap
model with a gausssian density of states. In this model, the system reaches
thermal equilibrium for long times after a quench to any finite temperature and
therefore all aging effect are of a transient nature. For not too long times
after the quench it is found that the so-called fluctuation-dissipation ratio
tends to a non-trivial limit, thus inicating the possibility for the definition
of a time scale dependent effective temperature. However, different definitions
of the effective temperature yield distinct results. In particular plots of the
integrated response versus the correlation function strongly depend on the way
they are constructed. Also the definition of effective temperatures in the
frequency domain is not unique for the model considered. This may have some
implications for the interpretation of results from computer simulations and
experimental determinations of effective temperatures.Comment: Proceedings of the workshop on non-equilibrium phenomena in
supercooled fluids, glasses and amorphous materials (17-22 September, Pisa
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