High Frequency Dynamics and Third Cumulant of Quantum Noise

The existence of the third cumulant $S_{3}$ 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 $\hbar \omega < eV, k_BT$ where voltage fluctuations arise from charge transfer process. We report here the first measurement of $S_3$ at high frequency, in the quantum regime $\hbar \omega > eV, k_BT$. 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 $\hbar \omega > eV$, the noise is due to zero point fluctuations and keeps its equilibrium value: $S_2= G \hbar \omega$ with $G$ the conductance of the sample. Therefore, $S_2$ is independent of the bias voltage and no photon is emitted by the conductor. Is it possible, as suggested by some theories, that $S_3 \neq 0$ 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 $\omega$) of a tunnel junction to an ac excitation at frequency $\omega_0$. The experiment is performed in the quantum regime, $\hbar\omega\sim\hbar\omega_0\gg k_BT$ at very low temperature T=35mK and high frequency $\omega_0/2\pi=6.2$ 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

Measurements of flux dependent screening in Aharonov-Bohm rings

In order to investigate the effect of electronic phase coherence on screening we have measured the flux dependent polarizability of isolated mesoscopic rings at 350 MHz. At low temperature (below 100 mK) both non-dissipative and dissipative parts of the polarizability exhibit flux oscillations with a period of half a flux quantum in a ring. The sign and amplitude of the effect are in good agreement with recent theoretical predictions. The observed positive magneto-polarizability corresponds to an enhancement of screening when time reversal symmetry is broken. The effect of electronic density and temperature are also measured.Comment: 4 pages, revtex, 4 figures, to appear in Phys. Rev. Let

Noise Thermal Impedance of a Diffusive Wire

The current noise density S of a conductor in equilibrium, the Johnson noise, is determined by its temperature T: S=4kTG with G the conductance. The sample's noise temperature Tn=S/(4kG) generalizes T for a system out of equilibrium. We introduce the "noise thermal impedance" of a sample as the amplitude of the oscillation of Tn when heated by an oscillating power. For a macroscopic sample, it is the usual thermal impedance. We show for a diffusive wire how this (complex) frequency-dependent quantity gives access to the electron-phonon interaction time in a long wire and to the diffusion time in a shorter one, and how its real part may also give access to the electron-electron inelastic time. These times are not simply accessible from the frequency dependence of S itself.Comment: 4 pages, 2 figure

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

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

Very low shot noise in carbon nanotubes

We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for this strong shot noise reduction: i) Transport within a rope takes place through a few nearly ballistic tubes within a rope and possibly involves non integer effective charges. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50).Comment: Submitted to Eur. Phys. J. B (Jan. 2002) Higher resolution pictures are posted on http://www.lps.u-psud.fr/Collectif/gr_07/publications.htm