283 research outputs found
Dephasing due to nonstationary 1/f noise
Motivated by recent experiments with Josephson qubits we propose a new
phenomenological model for 1/f noise due to collective excitations of
interacting defects in the qubit's environment. At very low temperatures the
effective dynamics of these collective modes are very slow leading to
pronounced non-Gaussian features and nonstationarity of the noise. We analyze
the influence of this noise on the dynamics of a qubit in various regimes and
at different operation points. Remarkable predictions are absolute time
dependences of a critical coupling and of dephasing in the strong coupling
regime.Comment: 4 pages, 2 figures, to be published in the proceedings of the Vth
Rencontres de Moriond in Mesoscopic Physic
Integer and fractional charge Lorentzian voltage pulses analyzed in the frame of Photon-assisted Shot Noise
The periodic injection of electrons in a quantum conductor using periodic
voltage pulses applied on a contact is studied in the energy and time-domain
using shot noise computation in order to make comparison with experiments. We
particularly consider the case of periodic Lorentzian voltage pulses. When
carrying integer charge, they are known to provide electronic states with a
minimal number of excitations, while other type of pulses are all accompanied
by an extra neutral cloud of electron and hole excitations. This paper focuses
on the low frequency shot noise which arises when the pulse excitations are
partitioned by a single scatterer in the framework of the Photo Assisted Shot
Noise (PASN) theory. As a unique tool to count the number of excitations
carried per pulse, shot noise reveals that pulses of arbitrary shape and
arbitrary charge show a marked minimum when the charge is integer. Shot noise
spectroscopy is also considered to perform energy-domain characterization of
the charge pulses. In particular it reveals the striking asymmetrical spectrum
of Lorentzian pulses. Finally, time-domain information is obtained from Hong Ou
Mandel like noise correlations when two trains of pulses generated on opposite
contacts collide on the scatterer. As a function of the time delay between
pulse trains, the noise is shown to measure the electron wavepacket
autocorrelation function for integer Lorentzian thanks to electron
antibunching. In order to make contact with recent experiments all the
calculations are made at zero and finite temperature
Fractionalization of minimal excitations in integer quantum Hall edge channels
A theoretical study of the single electron coherence properties of Lorentzian
and rectangular pulses is presented. By combining bosonization and the Floquet
scattering approach, the effect of interactions on a periodic source of voltage
pulses is computed exactly. When such excitations are injected into one of the
channels of a system of two copropagating quantum Hall edge channels, they
fractionalize into pulses whose charge and shape reflects the properties of
interactions. We show that the dependence of fractionalization induced
electron/hole pair production in the pulses amplitude contains clear signatures
of the fractionalization of the individual excitations. We propose an
experimental setup combining a source of Lorentzian pulses and an Hanbury Brown
and Twiss interferometer to measure interaction induced electron/hole pair
production and more generally to reconstruct single electron coherence of these
excitations before and after their fractionalization.Comment: 18 pages, 10 figures, 1 tabl
Diffeomorphism-invariant properties for quasi-linear elliptic operators
For quasi-linear elliptic equations we detect relevant properties which
remain invariant under the action of a suitable class of diffeomorphisms. This
yields a connection between existence theories for equations with degenerate
and non-degenerate coerciveness.Comment: 16 page
Real time decoherence of Landau and Levitov quasi-particles in quantum Hall edge channels
Quantum Hall edge channels at integer filling factor provide a unique
test-bench to understand decoherence and relaxation of single electronic
excitations in a ballistic quantum conductor. In this Letter, we obtain a full
visualization of the decoherence scenario of energy (Landau) and time (Levitov)
resolved single electron excitations at filling factor . We show that
the Landau excitation exhibits a fast relaxation followed by spin-charge
separation whereas the Levitov excitation only experiences spin-charge
separation. We finally suggest to use Hong-Ou-Mandel type experiments to probe
specific signatures of these different scenarios.Comment: 14 pages, 8 figure
Realization of a twin beam source based on four-wave mixing in Cesium
Four-wave mixing (4WM) is a known source of intense non-classical twin beams.
It can be generated when an intense laser beam (the pump) and a weak laser beam
(the seed) overlap in a medium (here cesium vapor), with
frequencies close to resonance with atomic transitions. The twin beams
generated by 4WM have frequencies naturally close to atomic transitions, and
can be intense (gain ) even in the CW pump regime, which is not the case
for PDC phenomenon in non-linear crystals. So, 4WM is well suited
for atom-light interaction and atom-based quantum protocols. Here we present
the first realization of a source of 4-wave mixing exploiting line of
Cesium atoms.Comment: 10 pages, 10 figure
High-power test results of a 3 GHz single-cell cavity
Compact, reliable and little consuming accelerators are required for the
treatment of tumours with ions. TERA proposes the "cyclinac", composed of a
high-frequency, fast-cycling linac which boosts the energy of the particles
previously accelerated in a cyclotron. The dimensions of the linac can be
reduced if high gradients are used. TERA initiated a high-gradient test program
to understand the operational limit of such structures. The program foresees
the design, prototyping and high-power test of several high-gradient structures
operating at 3 and 5.7 GHz. The high-power tests of the 3 GHz single-cell
cavity were completed in Winter 2012. The maximum BDR threshold measured for
Emax of 170 MV/m and RF pulses of 2.5 \mu s was 3 x 10-6 bpp/m
Dephasing by a nonstationary classical intermittent noise
We consider a new phenomenological model for a classical
intermittent noise and study its effects on the dephasing of a two-level
system. Within this model, the evolution of the relative phase between the
states is described as a continuous time random walk (CTRW). Using
renewal theory, we find exact expressions for the dephasing factor and identify
the physically relevant various regimes in terms of the coupling to the noise.
In particular, we point out the consequences of the non-stationarity and
pronounced non-Gaussian features of this noise, including some new anomalous
and aging dephasing scenarii.Comment: Submitted to Phys. Rev.
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