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In this paper we present an experiment where we measured the quantum coherence of a quasiparticle injected at a well-defined energy above the Fermi sea into the edge states of the integer quantum Hall regime. Electrons are introduced in an electronic Mach-Zehnder interferometer after passing through a quantum dot that plays the role of an energy filter. Measurements show that above a threshold injection energy, the visibility of the quantum interferences is almost independent of the energy. This is true even for high energies, up to 130~$\mu$eV, well above the thermal energy of the measured sample. This result is in strong contradiction with our theoretical predictions, which instead predict a continuous decrease of the interference visibility with increasing energy. This experiment raises serious questions concerning the understanding of excitations in the integer quantum Hall regime.

We derive fluctuation-dissipation relations for a tunnel junction driven by a high impedance microwave resonator, displaying strong quantum fluctuations. We find that the fluctuation-dissipation relations derived for classical forces hold, provided the effect of the circuit's quantum fluctuations is incorporated into a modified non-linear $I(V)$ curve. We also demonstrate that all quantities measured under a coherent time dependent bias can be reconstructed from their dc counterpart with a photo-assisted tunneling relation. We confirm these predictions by implementing the circuit and measuring the dc current through the junction, its high frequency admittance and its current noise at the frequency of the resonator., Comment: Publisehd as Physical Review Letters, 114, 126801

Nonlinear transport in the low-temperature vortex glass state of single crystal ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ has been investigated with fast current pulses driven along the $\mathrm{ab}$ plane. Field cooled preparation shows a higher threshold current (marking a jump or break in slope in the voltage response) than the zero field cooled (ZFC) one, but it is found to be metastable and convertible to the ZFC response by a small field excursion. The metastability appears on the low temperature side of the peak in the temperature dependence of the ZFC threshold current. It is suggested that the onset of metastability signals a different thermodynamic ground state.

We present an original method to measure the visibility of interferences in an electronic Mach–Zehnder interferometer in the presence of low frequency phase fluctuations. We studied the visibility of the interferences as a function of the energy and showed a gaussian variation of the decoherence and/or phase averaging at finite energy. This gaussian variation leads to a visibility modulation with a single side lobe.

We report on direct measurements of the electronic shot noise of a quantum point contact (QPC) at frequencies v in the range 4-8 GHz. Both the thermal energy and the QPC drain-source voltage V are comparable to the photon energy leading to observation of the shot noise suppression when V

International audience; We observe the suppression of the finite frequency shot-noise produced by a voltage biased tunnel junction due to its interaction with a single electromagnetic mode of high impedance. The tunnel junction is embedded in a quarter wavelength resonator containing a dense SQUID array providing it with a characteristic impedance in the kOhms range and a resonant frequency tunable in the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb blockade on the tunnel junction (roughly 30% reduction in the differential conductance) and allows an efficient measurement of the spectral density of the current fluctuations at the resonator frequency. The observed blockade of shot-noise is found in agreement with an extension of the dynamical Coulomb blockade theory.

We investigate electron dynamics at the graphene edge by studying the propagation of collective edge magnetoplasmon (EMP) excitations. By timing the travel of narrow wave-packets on picosecond time scales around exfoliated samples, we find chiral propagation with low attenuation at a velocity which is quantized on Hall plateaus. We extract the carrier drift contribution from the EMP propagation and find it to be slightly less than the Fermi velocity, as expected for an abrupt edge. We also extract the characteristic length for Coulomb interaction at the edge and find it to be smaller than for soft, depletion edge systems., Comment: 5 pages, 3 figures of main text and 6 pages, 6 figures of supplemental material

We present an experiment where the quantum coherence in the edge states of the integer quantum Hall regime is tuned with a decoupling gate. The coherence length is determined by measuring the visibility of quantum interferences in a Mach-Zehnder interferometer as a function of temperature, in the quantum Hall regime at filling factor two. The temperature dependence of the coherence length can be varied by a factor of two. The strengthening of the phase coherence at finite temperature is shown to arise from a reduction of the coupling between co-propagating edge states. This opens the way for a strong improvement of the phase coherence of Quantum Hall systems. The decoupling gate also allows us to investigate how inter-edge state coupling influence the quantum interferences' dependence on the injection bias. We find that the finite bias visibility can be decomposed into two contributions: a Gaussian envelop which is surprisingly insensitive to the coupling, and a beating component which, on the contrary, is strongly affected by the coupling., Comment: 4 pages, 5 figures

The dynamical Coulomb blockade (DCB) of tunneling is a quantum phenomenon in which tunneling of charge through a small tunnel junction is modified by its electromagnetic environment [1‐4]. This environment is described as an impedance in series with the tunnel element [see Fig. 1(a)]. The sudden charge transfer associated with tunneling can generate photons in the electromagnetic modes of the environment. In a normal metal tunnel junction, biased at voltage V, the energy eV of a tunneling electron can be dissipated both into quasiparticle excitations in the electrodes and into photons. At low temperature energy conservation forbids tunneling processes emitting photons with total energy higher than eV. This suppression reduces the conductance at low bias voltage [1,2,4]. In a Josephson junction, DCB effects are more prominent since at bias voltages smaller than the gap voltage 2� =e quasiparticle excitations cannot take away energy. Therefore, as sketched in Fig. 1(a), the entire energy 2eV of tunneling Cooper pairs has to be transformed into photons in the impedance for a dc current to flow through the junction [3,4]. Experiments have confirmed the predictions of DCB theory for the tunneling current, both in the normal [5‐7] and superconducting case [8,9] but the associated emission of photons into the environment has never been investigated. The aim of this work is to fill this gap by exploring the photonic side of DCB. We do so by embedding a Josephson junction into a well controlled electromagnetic environment provided by a microwave resonator. The resonator in turn leaks photons into an amplifier, allowing us to measure the rate and spectrum of photons emitted by the junction.

International audience; The quantum Hall effect is universal and expected to occur in all two-dimensional electron systems in perpendicular high-magnetic field. We revisit quantum localization thanks to the high-energy scale of the quantum Hall effect in graphene, where the electron dynamics obey the Dirac equation. We solve a long debated question on the nature of electron transport in the transition region between Hall resistance plateaus. Is it of metallic or of variable-range hopping type as proposed, respectively, by Pruisken and Polyakov-Shklovskii? To the best of our knowledge, no experiment was able to discriminate between these models. Here, measurements of the conductance peak width scaling exponents with both temperature and current and determination of the localization length validates the variable-range hopping scenario. This shows that the usual assumption of a metallic behavior of the two-dimensional electron gas (2DEG) between Hall resistance plateaus is unnecessary for macroscopic samples.

We report on conductance measurements in carbon nanotube based double quantum dots connected to two normal electrodes and a central superconducting finger. By operating our devices as beam splitters, we provide evidence for Crossed Andreev Reflections \textit{tunable in situ}. This opens an avenue to more sophisticated quantum optics-like experiments with spin entangled electrons., submitted

The decoupled layer quasi-two-dimensional glass phase of c-axis vortices in optimally doped Bi2Sr2CaCu2O8+delta (BSCCO) exhibits low-temperature metastability, the onset temperature of which is coincident with the peak in depinning current for zero-field-cooled sample preparation. We present experiments in strongly underdoped BSCCO showing similar metastability and develop a model for pinning dynamics where metastability results from progressive loss of thermal contact. The model accounts well for the field-temperature locus of the metastability and for the irreversibility and memory effects. We conclude that the metastability arises from a nonsingular dynamic crossover and does not mark a phase transition.

An electronic Mach Zehnder interferometer is used in the integer quantum hall regime at filling factor 2, to study the dephasing of the interferences. This is found to be induced by the electrical noise existing in the edge states capacitively coupled to each others. Electrical shot noise created in one channel leads to phase randomization in the other, which destroys the interference pattern. These findings are extended to the dephasing induced by thermal noise instead of shot noise: it explains the underlying mechanism responsible for the finite temperature coherence time $\tau_\phi(T)$ of the edge states at filling factor 2, measured in a recent experiment. Finally, we present here a theory of the dephasing based on Gaussian noise, which is found in excellent agreement with our experimental results., Comment: ~4 pages, 4 figures

We have determined the finite temperature coherence length of edge states in the Integer Quantum Hall Effect (IQHE) regime. This was realized by measuring the visibility of electronic Mach-Zehnder interferometers of different sizes, at filling factor 2. The visibility shows an exponential decay with the temperature. The characteristic temperature scale is found inversely proportional to the length of the interferometer arm, allowing to define a coherence length $\l_\phi$. The variations of $\l_\phi$ with magnetic field are the same for all samples, with a maximum located at the upper end of the quantum hall plateau. Our results provide the first accurate determination of $\l_\phi$ in the quantum Hall regime., Comment: 4 pages, 4 figures

We present an original statistical method to measure the visibility of interferences in an electronic Mach-Zehnder interferometer in the presence of low frequency fluctuations. The visibility presents a single side lobe structure shown to result from a gaussian phase averaging whose variance is quadratic with the bias. To reinforce our approach and validate our statistical method, the same experiment is also realized with a stable sample. It exhibits the same visibility behavior as the fluctuating one, indicating the intrinsic character of finite bias phase averaging. In both samples, the dilution of the impinging current reduces the variance of the gaussian distribution., Comment: 4 pages, 5 figures

Experiments on potential differences in the low-temperature vortex solid phase of monocrystalline platelets of superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ subjected to currents driven either through an $\mathrm{ab}$ surface or from one such surface to another show evidence of a resistive/nonresistive front moving progressively out from the current contacts as the current increases. The depth of the resistive region has been measured by an in-depth voltage probe contact. The position of the front associated with an injection point appears to depend only on the current magnitude and not on its withdrawal point. It is argued that enhanced nonresistive superconducting anisotropy limits current penetration to depths less than the London length and results in a flat rectangular resistive region with simultaneous $\mathrm{ab}$ and c current breakdown which moves progressively out from the injection point with increasing current. Measurements in $\mathrm{ab}$ or c configurations are seen to give the same information, involving both $\mathrm{ab}$-plane and c-axis conduction properties.

Nonlinear charge transport in the low temperature vortex glass state of single crystal Bi 2 Sr 2 CaCu 2 O 8 superconductor has been investigated with fast current pulses driven along the ab plane. Field cooled (FC) preparation shows a higher threshold current than zero field cooled (ZFC) one, but is found to be metastable and convertible to the ZFC response by a small field excursion. The metastability appears on the low temperature side of the peak in the temperature dependence of the ZFC threshold current. It is suggested that the onset of metastability signals a new thermodynamic ground state.