195 research outputs found
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
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
Probing Planck's law at home
International audienceWe report on physics around an incandescent lamp. Using a consumer grade digital camera, we combine electrical and optical measurements to explore Planck's law of the black body radiation. This simple teaching experiment is successfully used to measure both Stefan's and Planck's constants. Our measurements lead to a strikingly accurate value for Planck's constant: h = 6.7±0.4×10 −34 kg.m 2 .s −1. A digital camera is thus a sufficiently good equipment to measure a constant directly related to quantum mechanics
Cryogenic Calibration Setup for Broadband Complex Impedance Measurements
Reflection measurements give access to the complex impedance of a material on
a wide frequency range. This is of interest to study the dynamical properties
of various materials, for instance disordered superconductors. However
reflection measurements made at cryogenic temperature suffer from the
difficulty to reliably subtract the circuit contribution. Here we report on the
design and first tests of a setup able to precisely calibrate in situ the
sample reflection, at 4.2 K and up to 2 GHz, by switching and measuring, during
the same cool down, the sample and three calibration standards.Comment: (6 pages, 6 figures
Experimental determination of the statistics of photons emitted by a tunnel junction
We report on a microwave Hanbury-Brown Twiss experiment probing the
statistics of GHz photons emitted by a tunnel junction in the shot noise regime
at low temperature. By measuring the crosscorrelated fluctuations of the
occupation numbers of the photon modes of both detection branches we show that,
while the statistics of electrons is Poissonian, the photons obey chaotic
statistics. This is observed even for low photon occupation number when the
voltage across the junction is close to .Comment: Submitted to Phys.Rev.Let
ARTICLE Photon-assisted tunnelling with nonclassical light
International audienceAmong the most exciting recent advances in the field of superconducting quantum circuits is the ability to coherently couple microwave photons in low-loss cavities to quantum electronic conductors. These hybrid quantum systems hold great promise for quantum information-processing applications; even more strikingly, they enable exploration of new physical regimes. Here we study theoretically the new physics emerging when a quantum electronic conductor is exposed to nonclassical microwaves (for example, squeezed states, Fock states). We study this interplay in the experimentally relevant situation where a superconducting microwave cavity is coupled to a conductor in the tunnelling regime. We find that the conductor acts as a nontrivial probe of the microwave state: the emission and absorption of photons by the conductor is characterized by a nonpositive definite quasi-probability distribution, which is related to the Glauber-Sudarshan P-function of quantum optics. These negative quasi-probabilities have a direct influence on the conductance of the conductor
Noninvasiveness and time symmetry of weak measurements
Measurements in classical and quantum physics are described in fundamentally
different ways. Nevertheless, one can formally define similar measurement
procedures with respect to the disturbance they cause. Obviously, strong
measurements, both classical and quantum, are invasive -- they disturb the
measured system. We show that it is possible to define general weak
measurements, which are noninvasive: the disturbance becomes negligible as the
measurement strength goes to zero. Classical intuition suggests that
noninvasive measurements should be time symmetric (if the system dynamics is
reversible) and we confirm that correlations are time-reversal symmetric in the
classical case. However, quantum weak measurements -- defined analogously to
their classical counterparts -- can be noninvasive but not time symmetric. We
present a simple example of measurements on a two-level system which violates
time symmetry and propose an experiment with quantum dots to measure the
time-symmetry violation in a third-order current correlation function.Comment: 19 pages, 5 figures, more information at
http://www.fuw.edu.pl/~abednorz/tasym
High kinetic inductance microwave resonators made by He-Beam assisted deposition of tungsten nanowires
We evaluate the performance of hybrid microwave resonators made by combining sputtered Nb thin films with Tungsten nanowires grown with a He-beam induced deposition technique. Depending on growth conditions, the nanowires have a typical width w [35 - 75] nm and thickness t [5 - 40] nm. We observe a high normal state resistance R [65 - 150] Ω / which together with a critical temperature T c [4 - 6] K ensures a high kinetic inductance making the resonator strongly nonlinear. Both lumped and coplanar waveguide resonators were fabricated and measured at low temperature exhibiting internal quality factors up to 3990 at 4.5 GHz in the few photon regime. Analyzing the wire length, temperature, and microwave power dependence, we extracted a kinetic inductance for the W nanowire of L K 15 pH / which is 250 times higher than the geometrical inductance, and a Kerr non-linearity as high as K W, He / 2 π = 200 ± 120 Hz / photon at 4.5 GHz. The nanowires made with the helium focused ion beam are thus versatile objects to engineer compact, high impedance, superconducting environments with a mask and resist free direct write process
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