77 research outputs found
High Frequency Dynamics and Third Cumulant of Quantum Noise
The existence of the third cumulant of voltage fluctuations has
demonstrated the non-Gaussian aspect of shot noise in electronic transport.
Until now, measurements have been performed at low frequency, \textit{i.e.} in
the classical regime where voltage fluctuations arise
from charge transfer process. We report here the first measurement of at
high frequency, in the quantum regime . In this
regime, experiment cannot be seen as a charge counting statistics problem
anymore. It raises central questions of the statistics of quantum noise: 1) the
electromagnetic environment of the sample has been proven to strongly influence
the measurement, through the possible modulation of the noise of the sample.
What happens to this mechanism in the quantum regime? 2) For , the noise is due to zero point fluctuations and keeps its equilibrium
value: with the conductance of the sample. Therefore,
is independent of the bias voltage and no photon is emitted by the
conductor. Is it possible, as suggested by some theories, that in
this regime? With regard to these questions, we give theoretical and
experimental answers to the environmental effects showing that they involve
dynamics of the quantum noise. Using these results, we investigate the question
of the third cumulant of quantum noise in the a tunnel junction
Dynamics of Quantum Noise in a Tunnel Junction under ac Excitation
We report the first measurement of the \emph{dynamical response} of shot
noise (measured at frequency ) of a tunnel junction to an ac excitation
at frequency . The experiment is performed in the quantum regime,
at very low temperature T=35mK and high
frequency GHz. We observe that the noise responds in phase
with the excitation, but not adiabatically. The results are in very good
agreement with a prediction based on a new current-current correlator.Comment: Theory removed. More experimental details. One extra figur
Measurement of non-Gaussian shot noise: influence of the environment
We present the first measurements of the third moment of the voltage
fluctuations in a conductor. This technique can provide new and complementary
information on the electronic transport in conducting systems. The measurement
was performed on non-superconducting tunnel junctions as a function of voltage
bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate
the significant effect of the electromagnetic environment of the sample.Comment: 13 pages, for the SPIE International Symposium on Fluctuations and
Noise, Maspalomas, Gran Canaria, Spain (May 2004
Evidence for Two Time Scales in Long SNS Junctions
We use microwave excitation to elucidate the dynamics of long superconductor
/ normal metal / superconductor Josephson junctions. By varying the excitation
frequency in the range 10 MHz - 40 GHz, we observe that the critical and
retrapping currents, deduced from the dc voltage vs. dc current characteristics
of the junction, are set by two different time scales. The critical current
increases when the ac frequency is larger than the inverse diffusion time in
the normal metal, whereas the retrapping current is strongly modified when the
excitation frequency is above the electron-phonon rate in the normal metal.
Therefore the critical and retrapping currents are associated with elastic and
inelastic scattering, respectively
Environmental effects in the third moment of voltage fluctuations in a tunnel junction
We present the first measurements of the third moment of the voltage
fluctuations in a conductor. This technique can provide new and complementary
information on the electronic transport in conducting systems. The measurement
was performed on non-superconducting tunnel junctions as a function of voltage
bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate
the significant effect of the electromagnetic environment of the sample.Comment: Major revision. More experimental results. New interpretation. 4
pages, 3 figure
Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots
We have measured the electromagnetic response of micron-size isolated
mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them
to an electromagnetic micro-resonator. Both dissipative and non dissipative
responses exhibit a large magnetic field dependent quantum correction, with a
characteristic flux scale which corresponds to a flux quantum in a dot. The
real (dissipative) magnetoconductance changes sign as a function of frequency
for low enough density of electrons. The signal observed at frequency below the
mean level spacing corresponds to a negative magnetoconductance, which is
opposite to the weak localization seen in connected systems, and becomes
positive at higher frequency. We propose an interpretation of this phenomenon
in relation to fundamental properties of energy level spacing statistics in the
dots.Comment: 4 pages, 4 eps figure
Dynamic response of isolated Aharonov-Bohm rings coupled to an electromagnetic resonator
We have measured the flux dependence of both real and imaginary conductance
of isolated mesoscopic rings at 310 MHz. The rings are coupled to
a highly sensitive electromagnetic superconducting micro-resonator and lead to
a perturbation of the resonance frequency and quality factor. This experiment
provides a new tool for the investigation of the conductance of mesoscopic
systems without any connection to invasive probes. It can be compared with
recent theoretical predictions emphasizing the differences between isolated and
connected geometries and the relation between ac conductance and persistent
currents. We observe periodic oscillations on both components of the
magnetoconductance. The oscillations of the imaginary conductance whose sign
corresponds to diamagnetism in zero field, are 3 times larger than the Drude
conductance . The real part of the periodic magnetoconductance is of the
order of and is apparently negative in low field. It is thus notably
different from the weak localisation oscillations observed in connected rings,
which are much smaller and opposite in sign.Comment: 4 pages, revtex, epsf, 4 Postscript file
Quasiprobabilistic Interpretation of Weak measurements in Mesoscopic Junctions
The impossibility of measuring noncommuting quantum mechanical observables is
one of the most fascinating consequences of the quantum mechanical postulates.
Hence, to date the investigation of quantum measurement and projection is a
fundamentally interesting topic. We propose to test the concept of weak
measurement of noncommuting observables in mesoscopic transport experiments,
using a quasiprobablistic description. We derive an inequality for current
correlators, which is satisfied by every classical probability but violated by
high-frequency fourth-order cumulants in the quantum regime for experimentally
feasible parameters.Comment: 4 pages, published versio
Persistent currents with long-range hopping in 1D single-isolated-diffusive rings
We show from exact calculations that a simple tight-binding Hamiltonian with
diagonal disorder and long-range hopping integrals, falling off as a power
of the inter-site separation, correctly describes the experimentally
observed amplitude (close to the value of an ordered ring) and flux-periodicity
() of persistent currents in single-isolated-diffusive normal metal rings
of mesoscopic size. Long-range hopping integrals tend to delocalize the
electrons even in the presence of disorder resulting orders of magnitude
enhancement of persistent current relative to earliar predictions.Comment: 4 pages, 3 figure
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