arXiv subjects

Bertrand Reulet

Publications and source records attributed to Bertrand Reulet.

52 records · Page 3Linked to original sources

arXiv2015

Experimental Violation Of Bell-like Inequalities By Electronic Shot Noise

Quantum electronic noise, the random current fluctuations generated by electrons crossing a quantum conductor, has been thoroughly studied from the electron point of view. Recent experiments have shown that such noise may have striking properties in terms of the quantum electromagnetic field it generates. Both the existence of vacuum squeezing \cite{PAN_squeezing} and generation of correlated photon pairs \cite{C4quantique} have been observed. Here we demonstrate that this electromagnetic field also violates Bell-like inequalities, thereby proving its true quantum nature. This is achieved by the observation of two-mode quadrature squeezing of vacuum: the electromagnetic field quadratures at two different frequencies can be so correlated that their difference goes below vacuum fluctuations. Thus, electronic noise contains entangled photon pairs, a key ingredient for quantum computing and communication.

cond-mat.mes-hall↗

arXiv2014

Pauli-Heisenberg Blockade of Electron Quantum Transport

Conduction of electrons in matter is ultimately described by quantum mechanics. Yet at low frequency or long time scales, low temperature quantum transport is perfectly described by this very simple idea: electrons are emitted by the contacts into the sample which they may cross with a finite probability. Combined with Fermi statistics, this partition of the electron flow accounts for the full statistics of electron transport. When it comes to short time scales, a key question must be clarified: are there correlations between successive attempts of the electrons to cross the sample? While there are theoretical predictions and several experimental indications for the existence of such correlations, no direct experimental evidence has ever been provided. Here we show a direct experimental proof of how temperature and voltage bias control the electron flow: while temperature $T$ leads to a jitter which tends to decorrelate electron transport after a time $\hbar/k_BT$, the bias voltage $V$ induces strong correlations/anticorrelations which oscillate with a period $h/eV$. Our experiment reveals how time scales related to voltage and temperature operate on quantum transport in a coherent conductor. In complex quantum systems, the method we have developed might offer direct access to other relevant time scales related, for example, to internal dynamics, coupling to other degrees of freedom, or correlations between electrons.

cond-mat.mes-hall↗

arXiv2014

Emission of Microwave Photon Pairs by a Tunnel Junction

Generation and control of non-classical electromagnetic fields is of crucial importance for quantum information physics. While usual methods for the production of such fields rely on a non-linearity (of a crystal, a Josephson junction, etc.), a recent experiment performed on a normal conductor, a tunnel junction under microwave irradiation, has unveiled an alternative: the use of electron shot noise in a quantum conductor\cite{PAN_squeezing}. Here we show that such a device can emit \emph{pairs of microwave photons} of different frequencies with a rate as high as that of superconducting Josephson junctions\cite{Flurin}. This results in intensity fluctuations of the photon field at two different frequencies being correlated below the photon shot noise,i.e. two-mode amplitude squeezing. Our experiment constitutes a fundamental step towards the understanding of electronic noise in terms of quantum optics, and shows that even a normal conductor could be used as a resource for quantum information processing.

quant-ph↗

arXiv2013

Observation of Squeezing in the Electron Quantum Shot Noise of a Tunnel Junction

We report the measurement of the fluctuations of the two quadratures of the electromagnetic field generated by a quantum conductor, a dc- and ac-biased tunnel junction placed at very low temperature.We observe that the variance of the fluctuations on one quadrature can go below that of vacuum, i.e., that the radiated field is squeezed. This demonstrates the quantum nature of the radiated electromagnetic field.

quant-ph↗

arXiv2013

Nonsymmetrized Correlations in Quantum Noninvasive Measurements

A long-standing problem in quantum mesoscopic physics is which operator order corresponds to noise expressions like , where I(ω) is the measured current at frequency ω. Symmetrized order describes a classical measurement while nonsymmetrized order corresponds to a quantum detector, e.g., one sensitive to either emission or absorption of photons. We show that both order schemes can be embedded in quantum weak-measurement theory taking into account measurements with memory, characterized by a memory function which is independent of a particular experimental detection scheme. We discuss the resulting quasiprobabilities for different detector temperatures and how their negativity can be tested on the level of second-order correlation functions already. Experimentally, this negativity can be related to the squeezing of the many-body state of the transported electrons in an ac-driven tunnel junction.

quant-ph↗

arXiv2013

Observation of Quantum Oscillations in the Photo-assisted Shot Noise of a Tunnel Junction

We report measurements of the low frequency current fluctuations of a tunnel junction placed at very low temperature biased by a time-dependent voltage $V(t)=V(1+\cos 2πνt)$. We observe that the excess noise generated by the ac excitation exhibits quantum oscillations as a function of the dc bias, with a period given by $hν/e$ with $e$ the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.

cond-mat.mes-hall↗

arXiv2013

Noise Intensity-Intensity Correlations and the Fourth Cumulant of Current Fluctuations

We report measurements of the correlation between intensities of noise at different frequencies on a tunnel junction under ac excitation, which corresponds to two-mode intensity correlations in optics. We observe positive correlation, \emph{i.e.} photon bunching, which exist only for certain relations between the excitation frequency and the two detection frequencies, depending on the dc bias of the sample. We demonstrate that these correlations are proportional to the fourth cumulant of current fluctuations.

cond-mat.mes-hall↗

arXiv2012

Electron-photon correlations and the third moment of quantum noise

The radiation generated by a quantum conductor should be correlated with electrons crossing it. We have measured the correlation between the fluctuations of the high frequency electromagnetic power and the low frequency transport in a tunnel junction. We have explored the regimes where electromagnetic fluctuations correspond to real photons and where they correspond to vacuum at very low temperature. We deduce from our data the intrinsic third moment of quantum shot noise, which appears to be frequency independent.

cond-mat.mes-hall↗

arXiv2012

Shaping a time-dependent excitation to control the electron distribution function: noise minimization in a tunnel junction

We report measurements of shot noise in a tunnel junction under bi-harmonic illumination, Vac(t) = Vac1 cos(2πft) + Vac2 cos(4πft+ ϕ). The experiment is performed in the quantum regime, hf >> k_BT at low temperature T = 70 mK and high frequency f = 10 GHz. From the measurement of noise at low frequency, we show that we can infer and control the non-equilibrium electronic distribution function by adjusting the amplitudes and phase of the excitation, thus modeling its shape. In particular, we observe that the noise depends not only on the amplitude of the two sine waves but also on their relative phase, due to coherent emission/absorption of photons at different frequencies. By shaping the excitation we can minimize the noise of the junction, which no longer reaches its minimum at zero dc bias. We show that adding an excitation at frequency 2f with the proper amplitude and phase can reduce the noise of the junction excited at frequency f only.

cond-mat.mes-hall↗

arXiv2008

The Noise Susceptibility of a Photo-excited Coherent Conductor

We report the theory of the \emph{dynamical response of current fluctuations} of a photo-excited conductor. We have performed the calculation for a coherent conductor described by arbitrary energy-dependent transmissions and for arbitrary frequencies. We consider two experimental setups that correspond to different ways of symmetrizing the current operators, leading to different predictions. Our results are in very good, quantitative agreement with a recent measurement. We demonstrate that the dynamical response of noise that we have calculated is the key concept that relates Dynamical Coulomb Blockade, i.e. the electron-electron correction to the conductance due to the presence of an external impedance, to quantum noise.

cond-mat.mes-hall↗

arXiv2005

Higher moments of noise

In this article we present simple approaches to the calculation of P(i) (the probability distribution of current fluctuations) in a tunnel junction, and to the effect of the environment on noise measurements in terms of the modification of P. We do not provide rigorous calculations, but simple considerations that bear the essential ingredients of the phenomena. We also discuss the effect of a finite measurement bandwidth. We report experimental results of the first measurement of the third moment of voltage fluctuations in tunnel junctions, from room temperature down to 50mK. Then we discuss extensions of that measurements to finite frequencies and to the study of other systems. We show the first data of the third moment in the regime where the frequency is larger than the temperature. Finally we discuss a new quantity, the "noise thermal impedance", which links the second and third moment.

cond-mat.mes-hall↗

arXiv1997

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.

cond-mat.mes-hall↗

arXiv1996

Magneto-polarisability of mesoscopic rings

We calculate the average polarisability of two dimensional mesoscopic rings in the presence of an Aharonov-Bohm flux. The screening is taken into account self-consistently within a mean-field approximation. We investigate the effects of statistical ensemble, finite frequency and disorder. We emphasize geometrical effects which make the observation of field dependent polarisability much more favourable on rings than on disks or spheres of comparable radius. The ratio of the flux dependent to the flux independent part is estimated for typical GaAs rings.

cond-mat.mes-hall↗

arXiv1995

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 $GaAs/GaAlAs$ 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 $Φ_0/2$ 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 $G_0$. The real part of the periodic magnetoconductance is of the order of $0.2 G_0$ 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.

cond-mat↗

arXiv1994

Electron-electron interactions in one- and three-dimensional mesoscopic disordered rings: a perturbative approach

We have computed persistent currents in a disordered mesoscopic ring in the presence of small electron-electron interactions, treated in first order perturbation theory. We have investigated both a contact (Hubbard) and a nearest neighbour interaction in 1D and 3D. Our results show that a repulsive Hubbard interaction produces a paramagnetic contribution to the average current (whatever the dimension) and increases the value of the typical current. On the other hand, a nearest neighbour repulsive interaction results in a diamagnetic contribution in 1D and paramagnetic one in 3D, and tends to decrease the value of the typical current in any dimension. Our study is based on numerical simulations on the Anderson model and is justified analytically in the presence of very weak disorder. We have also investigated the influence of the amount of disorder and of the statistical (canonical or grand-canonical) ensemble.

cond-mat↗

arXiv1993

Conductance of Aharonov--Bohm Rings: From the Discrete to the Continuous Spectrum Limit

The dissipative conductance of an array of mesoscopic rings, subject to an a.c. magnetic flux is investigated. The magneto--conductance may change sign between canonical and grand-canonical statistical ensembles, as function of the inelastic level broadening and as function of the temperature. Differences between canonical and grand-canonical ensembles persist up to temperature of the order of the Thouless energy.

cond-mat↗