790 research outputs found
Nonlinear absorption of surface acoustic waves by composite fermions
Absorption of surface acoustic waves by a two-dimensional electron gas in a
perpendicular magnetic field is considered. The structure of such system at the
filling factor close to 1/2 can be understood as a gas of {\em composite
fermions}. It is shown that the absorption at can be strongly
nonlinear, while small deviation form 1/2 will restore the linear absorption.
Study of nonlinear absorption allows one to determine the force acting upon the
composite fermions from the acoustic wave at turning points of their
trajectories.Comment: 7 pages, 1 figure, submitted to Europhysics letter
Looping on the Bloch sphere: Oscillatory effects in dephasing of qubits subject to broad-spectrum noise
For many implementations of quantum computing, 1/f and other types of
broad-spectrum noise are an important source of decoherence. An important step
forward would be the ability to back out the characteristics of this noise from
qubit measurements and to see if it leads to new physical effects. For certain
types of qubits, the working point of the qubit can be varied. Using a new
mathematical method that is suited to treat all working points, we present
theoretical results that show how this degree of freedom can be used to extract
noise parameters and to predict a new effect: noise-induced looping on the
Bloch sphere. We analyze data on superconducting qubits to show that they are
very near the parameter regime where this looping should be observed.Comment: 4 pages, 3 figure
Nonlinear acoustic and microwave absorption in disordered semiconductors
Nonlinear hopping absorption of ultrasound and electromagnetic waves in
amorphous and doped semiconductors is considered. It is shown that even at low
amplitudes of the electric (or acoustic) field the nonlinear corrections to the
relaxational absorption appear anomalously large. The physical reason for such
behavior is that the nonlinear contribution is dominated by a small group of
close impurity pairs having one electron per pair. Since the group is small, it
is strongly influenced by the field. An external magnetic field strongly
influences the absorption by changing the overlap between the pair components'
wave functions. It is important that the influence is substantially different
for the linear and nonlinear contributions. This property provides an
additional tool to extract nonlinear effects.Comment: correction : misspelled name in references correcte
Ray optics in flux avalanche propagation in superconducting films
Experimental evidence of wave properties of dendritic flux avalanches in
superconducting films is reported. Using magneto-optical imaging the
propagation of dendrites across boundaries between a bare NbN film and areas
coated by a Cu-layer was visualized, and it was found that the propagation is
refracted in full quantitative agreement with Snell's law. For the studied film
of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was
close to n=1.4. The origin of the refraction is believed to be caused by the
dendrites propagating as an electromagnetic shock wave, similar to damped modes
considered previously for normal metals. The analogy is justified by the large
dissipation during the avalanches raising the local temperature significantly.
Additional time-resolved measurements of voltage pulses generated by segments
of the dendrites traversing an electrode confirm the consistency of the adapted
physical picture.Comment: 4 pages, 4 figure
Low-frequency noise as a source of dephasing of a qubit
With the growing efforts in isolating solid-state qubits from external
decoherence sources, the material-inherent sources of noise start to play
crucial role. One representative example is electron traps in the device
material or substrate. Electrons can tunnel or hop between a charged and an
empty trap, or between a trap and a gate electrode. A single trap typically
produces telegraph noise and can hence be modeled as a bistable fluctuator.
Since the distribution of hopping rates is exponentially broad, many traps
produce flicker-noise with spectrum close to 1/f. Here we develop a theory of
decoherence of a qubit in the environment consisting of two-state fluctuators,
which experience transitions between their states induced by interaction with
thermal bath. Due to interaction with the qubit the fluctuators produce
1/f-noise in the qubit's eigenfrequency. We calculate the results of qubit
manipulations - free induction and echo signals - in such environment. The main
problem is that in many important cases the relevant random process is both
non-Markovian and non-Gaussian. Consequently the results in general cannot be
represented by pair correlation function of the qubit eigenfrequency
fluctuations. Our calculations are based on analysis of the density matrix of
the qubit using methods developed for stochastic differential equations. The
proper generating functional is then averaged over different fluctuators using
the so-called Holtsmark procedure. The analytical results are compared with
simulations allowing checking accuracy of the averaging procedure and
evaluating mesoscopic fluctuations. The results allow understanding some
observed features of the echo decay in Josephson qubits.Comment: 18 pages, 8 figures, Proc. of NATO/Euresco Conf. "Fundamental
Problems of Mesoscopic Physics: Interactions and Decoherence", Granada,
Spain, Sept.200
Exact solution of a model of qubit dephasing due to telegraph noise
We present a general and exact formalism for finding the evolution of a
quantum system subject to external telegraph noise. The various qubit
decoherence rates are determined by the eigenvalues of a transfer matrix. The
formalism can be applied to a qubit subject to an arbitrary combination of
dephasing and relaxational telegraph noise, in contrast to existing
non-perturbative methods that treat only one or the other of these limits. We
present 3 applications: 1) We obtain the full qubit dynamics on time scales
short compared with the enviromental correlation times. In the strong coupling
cases this reveals unexpected oscillations and induced magnetization
components; 2) We find in strong coupling case strong violations of the widely
used relation 1/T = 1/2T + 1/T, which is a result of
perturbation theory; 3) We discuss the effects of bang-bang and spin-echo
controls of the qubit dynamics in general settings of the telegraph noises.
%The result shows that these methods are not very effective in %reducing
decoherence arising from a single telegraph noise. Finally, we discuss the
extension of the method to the cases of many telegraph noise sources and
multiple qubits. The method still works when white noise is also present.Comment: 7 pages, 6 figures, revised and extende
Decoherence in qubits due to low-frequency noise
The efficiency of the future devices for quantum information processing is
limited mostly by the finite decoherence rates of the qubits. Recently a
substantial progress was achieved in enhancing the time, which a solid-state
qubit demonstrates a coherent dynamics. This progress is based mostly on a
successful isolation of the qubits from external decoherence sources. Under
these conditions the material-inherent sources of noise start to play a crucial
role. In most cases the noise that quantum device demonstrate has 1/f spectrum.
This suggests that the environment that destroys the phase coherence of the
qubit can be thought of as a system of two-state fluctuators, which experience
random hops between their states. In this short review we discuss the current
state of the theory of the decoherence due to the qubit interaction with the
fluctuators. We describe the effect of such an environment on different
protocols of the qubit manipulations - free induction and echo signal. It turns
out that in many important cases the noise produced by the fluctuators is
non-Gaussian. Consequently the results of the interaction of the qubit with the
fluctuators are not determined by the pair correlation function only.
We describe the effect of the fluctuators using so-called spin-fluctuator
model. Being quite realistic this model allows one to evaluate the qubit
dynamics in the presence of one fluctuator exactly. This solution is found, and
its features, including non-Gaussian effects are analyzed in details. We extend
this consideration for the systems of large number of fluctuators, which
interact with the qubit and lead to the 1/f noise. We discuss existing
experiments on the Josephson qubit manipulation and try to identify
non-Gaussian behavior.Comment: 25 pages, 7 figure
Interaction of Lamb modes with two-level systems in amorphous nanoscopic membranes
Using a generalized model of interaction between a two-level system (TLS) and
an arbitrary deformation of the material, we calculate the interaction of Lamb
modes with TLSs in amorphous nanoscopic membranes. We compare the mean free
paths of the Lamb modes with different symmetries and calculate the heat
conductivity . In the limit of an infinitely wide membrane, the heat
conductivity is divergent. Nevertheless, the finite size of the membrane
imposes a lower cut-off for the phonons frequencies, which leads to the
temperature dependence . This temperature dependence
is a hallmark of the TLS-limited heat conductance at low temperature.Comment: 9 pages, 2 figure
Exact solution for the dynamical decoupling of a qubit with telegraph noise
We study the dissipative dynamics of a qubit that is afflicted by classical
random telegraph noise and it is subject to dynamical decoupling. We derive
exact formulas for the qubit dynamics at arbitrary working points in the limit
of infinitely strong control pulses (bang-bang control) and we investigate in
great detail the efficiency of the dynamical decoupling techniques both for
Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal
point. We demonstrate that control sequences can be successfully implemented as
diagnostic tools to infer spectral proprieties of a few fluctuators interacting
with the qubit. The analysis is extended in order to include the effect of
noise in the pulses and we give upper bounds on the noise levels that can be
tolerated in the pulses while still achieving efficient dynamical decoupling
performance
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