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International audience; Graphene is a very promising test bed for the field of electron quantum optics. However, a fully tunable and coherent electronic beam splitter is still missing. We report the demonstration of electronic beam splitters in graphene that couple quantum Hall edge channels having opposite valley polarizations. The electronic transmission of our beam splitters can be tuned from zero to near unity. By independently setting the beam splitters at the two corners of a graphene p−n junction to intermediate transmissions, we realize a fully tunable electronic Mach-Zehnder interferometer. This tunability allows us to unambiguously identify the quantum interferences due to the Mach-Zehnder interferometer, and to study their dependence with the beam-splitter transmission and the interferometer bias voltage. The comparison with conventional semiconductor interferometers points toward universal processes driving the quantum decoherence in those two different 2D systems, with graphene being much more robust to their effect.

Probing the dynamics of anyons A two-dimensional electron gas in the fractional quantum Hall regime has unusual excitations called anyons that carry only a fraction of the electron's charge. This fractional charge can be observed through a dynamical response to irradiation by microwaves, but such experiments require a combination of high magnetic fields with sensitive noise measurements and very low temperatures. Kapfer et al. observed this dynamical response in a GaAs/AlGaAs heterostructure hosting a high-mobility two-dimensional electron gas with fractional excitations of one-third and one-fifth of the electron's charge. The method may be of interest for use in topological quantum computing. Science , this issue p. 846

We study the Hanbury Brown and Twiss correlation of electronic quasi-particles injected in a quantum conductor using current noise correlations and we experimentally address the effect of finite temperature. By controlling the relative time of injection of two streams of electrons it is possible to probe the fermionic antibunching, performing the electron analog of the optical Hong Ou Mandel (HOM) experiment. The electrons are injected using voltage pulses with either sine-wave or Lorentzian shape. In the latter case, we propose a set of orthogonal wavefunctions, describing periodic trains of multiply charged electron pulses, which give a simple interpretation to the HOM shot noise. The effect of temperature is then discussed and experimentally investigated. We observe a perfect electron anti-bunching for a large range of temperature, showing that, as recently predicted, thermal mixing of the states does not affect anti-bunching properties, a feature qualitatively different from dephasing. For single charge Lorentzian pulses, we provide experimental evidence of the prediction that the HOM shot noise variation versus the emission time delay is remarkably independent of the temperature.

International audience; Quantum shot noise probes the dynamics of charge transfers through a quantum conductor, reflecting whether quasiparticles flow across the conductor in a steady stream, or in syncopated bursts. We have performed high-sensitivity shot noise measurements in a quantum dot obtained in a silicon metal-oxide-semiconductor field-effect transistor. The quality of our device allows us to precisely associate the different transport regimes and their statistics with the internal state of the quantum dot. In particular, we report on large current fluctuations in the inelastic cotunneling regime, corresponding to different highly correlated, non-Markovian charge transfer processes. We have also observed unusually large current fluctuations at low energy in the elastic cotunneling regime, the origin of which remains to be fully investigated.

The recent realization of single electron sources let us envision performing electron quantum optics experiments, where electrons can be viewed as flying qubits propagating in a ballistic conductor. To date, all electron sources operate in a periodic electron injection mode leading to energy spectrum singularities in various physical observables which sometimes hide the bare nature of physical effects. To go beyond this, we propose a spread-spectrum approach where electron flying qubits are injected in a non-periodic manner following a pseudorandom binary bit pattern. Extending the Floquet scattering theory approach from periodic to spread-spectrum drive, the shot noise of pseudorandom binary sequences of single electron injection can be calculated for leviton and non-leviton sources. Our new approach allows us to disentangle the physics of the manipulated excitations from that of the injection protocol. In particular, the spread spectrum approach is shown to provide a better knowledge of electronic Hong Ou Mandel correlations and to clarify the nature of the pulse train coherence and the role of the dynamical orthogonality catastrophe for non-integer charge injection., Comment: 11 pages and 5 figures including 3 new figures, results on quantum coherence and explicit formulae added

We have observed the well-kown quantum Hall effect (QHE) in epitaxial graphene grown on silicon carbide (SiC) by using, for the first time, only commercial NdFeB permanent magnets at low temperature. The relatively large and homogeneous magnetic field generated by the magnets, together with the high quality of the epitaxial graphene films, enables the formation of well-developed quantum Hall states at Landau level filling factors $\nu=\pm 2$, commonly observed with superconducting electro-magnets. Furthermore, the chirality of the QHE edge channels can be changed by a top gate. These results demonstrate that basic QHE physics are experimentally accessible in graphene for a fraction of the price of conventional setups using superconducting magnets, which greatly increases the potential of the QHE in graphene for research and applications., Comment: 9 pages, 3 figures

When subjected to electromagnetic radiation, the fluctuation of the electronic current across a quantum conductor increases. This additional noise, called photon-assisted shot noise, arises from the generation and subsequent partition of electron-hole pairs in the conductor. The physics of photon-assisted shot noise has been thoroughly investigated at microwave frequencies up to 20 GHz, and its robustness suggests that it could be extended to the Terahertz (THz) range. Here, we present measurements of the quantum shot noise generated in a graphene nanoribbon subjected to a THz radiation. Our results show signatures of photon-assisted shot noise, further demonstrating that hallmark time-dependant quantum transport phenomena can be transposed to the THz range., includes supplemental material

Graphene offers a unique system to investigate transport of Dirac Fermions at p–n junctions. In a magnetic field, combination of quantum Hall physics and the characteristic transport across p–n junctions leads to a fractionally quantized conductance associated with the mixing of electron-like and hole-like modes and their subsequent partitioning. The mixing and partitioning suggest that a p–n junction could be used as an electronic beam splitter. Here we report the shot noise study of the mode-mixing process and demonstrate the crucial role of the p–n junction length. For short p–n junctions, the amplitude of the noise is consistent with an electronic beam-splitter behaviour, whereas, for longer p–n junctions, it is reduced by the energy relaxation. Remarkably, the relaxation length is much larger than typical size of mesoscopic devices, encouraging using graphene for electron quantum optics and quantum information processing., Dirac fermions at a p–n junction can exhibit a wide variety of unusual properties. Here, the authors investigate the dynamics of such fermions in a graphene junction using shot noise measurements and demonstrate the crucial role of junction length.

We review the quantum noise properties of phase coherent conductors, in particular the spectacular noise suppression associated with conductance quantization. We describe general techniques used to measure the tiny quantum shot noise current fluctuations at sub-kelvin temperature for low and high frequency.

International audience; The high-frequency radiation emitted by a quantum conductor presents a rising interest in quantum physics and condensed matter. However, its detection with microwave circuits is challenging. Here, we propose to use the photon-assisted shot noise for on-chip radiation detection. It is based on the low-frequency current noise generated by the partitioning of photon-excited electrons and holes, which are scattered inside the conductor. For a given electromagnetic coupling to the radiation, the photon-assisted shot noise response is shown to be independent on the nature and geometry of the quantum conductor used for the detection, up to a Fano factor, characterizing the type of scattering mechanism. Ordered in temperature or frequency range, from few tens of mK or GHz to several hundred of K or THz respectively, a wide variety of conductors can be used like Quantum Point Contacts (this work), diffusive metallic or semi-conducting films, graphene, carbon nanotubes and even molecule, opening new experimental opportunities in quantum physics.

International audience; The search for new efficient thermoelectric devices converting waste heat into electrical energy is of major importance. The physics of mesoscopic electronic transport offers the possibility to develop a new generation of nanoengines with high efficiency. Here we describe an all-electrical heat engine harvesting and converting dissipated power into an electrical current. Two capacitively coupled mesoscopic conductors realized in a two-dimensional conductor form the hot source and the cold converter of our device. In the former, controlled Joule heating generated by a voltage-biased quantum point contact results in thermal voltage fluctuations. By capacitive coupling the latter creates electric potential fluctuations in a cold chaotic cavity connected to external leads by two quantum point contacts. For unequal quantum point contact transmissions, a net electrical current is observed proportional to the heat produced.

We report on electron transport measurements of single-wall carbon nanotubes in a four-terminal configuration with noninvasive voltage electrodes. The voltage drop is detected using multiwalled carbon nanotubes while the current is injected through nanofabricated Au electrodes. Measurements are carried at high temperature so that the four-terminal resistance directly gives the intrinsic resistance. The resistance is shown to result from weak disorder and from quantum interference effect corrections. In addition, we present Coulomb blockade measurements. The length of the quantum dot that is determined from the level spacing equals the separation between the Au electrodes.

Even when no bias voltage is applied to a quantum conductor, the electronic quantum partition noise can be investigated using giga hertz radiofrequency (RF) irradiation of a reservoir. Using a quantum point contact configuration we report an accurate determination of the partition noise Fano factor resulting from the photo-assisted shot noise. Applying both voltage bias and RF irradiation we are able to make a definitive quantitative test of the scattering theory of photo-assisted shot noise.

We present shot noise measurements in the fractional quantum Hall regime at ν=13 when the fractional edges are weakly coupled by an artificial impurity, namely a quantum point contact. At low voltage, when the weak coupling is renormalized by the interaction and leads to strong backscattering, shot noise measurements show super Poissonian shot noise with a charge close to 2e or even above.

International audience; We investigate resonant edge magnetoplasmons (EMPs) and their decay in graphene by high-frequency electronic measurements. From EMP resonances in disk shaped graphene, we show that the dispersion relation of EMPs is nonlinear due to interactions, giving rise to the intrinsic decay of EMP wave packets. We also identify extrinsic dissipation mechanisms due to interaction with localized states in bulk graphene from the decay time of EMP wave packets. We indicate that, owing to the linear band structure and the sharp edge potential, EMP dissipation in graphene can be lower than that in GaAs systems.

International audience; The complete knowledge of a quantum state allows the prediction of the probability of all possible measurement outcomes, a crucial step in quantum mechanics. It can be provided by tomographic methods which have been applied to atomic, molecular, spin and photonic states. For optical or microwave photons, standard tomogra-phy is obtained by mixing the unknown state with a large-amplitude coherent photon field. However, for fermions such as electrons in condensed matter, this approach is not applicable because fermionic fields are limited to small amplitudes (at most one particle per state), and so far no determination of an electron wavefunction has been made. Recent proposals involving quantum conductors suggest that the wavefunction can be obtained by measuring the time-dependent current of electronic wave interferometers or the current noise of electronic Hanbury-Brown/Twiss interferometers. Here we show that such measurements are possible despite the extreme noise sensitivity required, and present the reconstructed wavefunction quasi-probability, or Wigner distribution function, of single electrons injected into a ballistic conductor. Many identical electrons are prepared in well-controlled quantum states called levitons by repeatedly applying Lorentzian voltage pulses to a contact on the conductor. After passing through an electron beam splitter, the levitons are mixed with a weak-amplitude fermionic field formed by a coherent superposition of electron–hole pairs generated by a small alternating current with a frequency that is a multiple of the voltage pulse frequency 16. Antibunching of the electrons and holes with the levi-tons at the beam splitter changes the leviton partition statistics, and the noise variations provide the energy density matrix elements of the levitons. This demonstration of quantum tomography makes the developing field of electron quantum optics with ballistic conductors a new test-bed for quantum information with fermions. These results may find direct application in probing the entanglement of electron flying quantum bits, electron decoherence and electron interactions. They could also be applied to cold fermionic (or spin-1/2) atoms.

In a quantum conductor, electrons propagate coherently, i.e. the electronic waves keep a well defined phase. Thus one can observe a wealth of new interesting effects, directly related to the quantum nature of the system. Transport measurements provide information on the wave nature of the electrons, via the transmission of the associated waves, in the frame of the so-called scattering theory. Alternatively, Shot-Noise gives information on the corpuscular behaviour of the electrons, by reflecting the random transfer of charge quanta. Moreover, in a quantum conductor, the noise is sensitive to the particle statistics : the Pauli exclusion principle correlates the flow of electrons, thus reducing the Shot-Noise below the classical value 2eI observed in usual classical electronic devices. More generally, it is expected that quantum conductors are less noisy than their classical analog. In this paper, we present an experiment on the simplest quantum conductor, a Quantum Point Contact ; the fine control of the transmission of electronic waves, allows observation of wave-like as well as particle-like effects.

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

The existence of fractional charges carrying the current is experimentally demonstrated. Using a 2-D electron system in high perpendicular magnetic field we measure the shot noise associated with tunneling in the fractional quantum Hall regime at Landau level filling factor 1 3 . The noise gives a direct determination of the quasiparticle charge, which is found to be e ∗ =e/3 as predicted by Laughlin. The existence of e/3 Laughlin quasiparticles is unambiguously confirmed by the shot noise to Johnson–Nyquist cross-over found for temperature Θ=e ∗ V ds /2k B .

The experimental requirements for low frequency electrical noise measurements of single electron tunneling (SET) semiconductor samples are discussed and a two-channel cross-correlation spectrum analysis method combined with ultralow noise detection is proposed which gives 10 mK noise temperature resolution. We emphasize the effect of the high frequency photon radiation originating from the external circuit which may strongly affect sub-kelvin low frequency noise measurements if not filtered. We quantitatively show that hot photons are important as a mechanism of activation of electron traps for semiconductor SET devices, increasing low frequency 1/f or telegraphic noise, and as a mechanism to increase the electronic temperature. We describe a new type of cryogenic coaxes whose attenuation drastically thermalizes the photons. As a check, we show for the first time resonant tunneling peak linewidths in the Coulomb blockade regime decreasing with temperature down to ≃60 mK.

A theoretical study of the single electron coherence properties of Lorentzian and rectangular pulses is presented. By combining bosonization and the Floquet scattering approach, the effect of interactions on a periodic source of voltage pulses is computed exactly. When such excitations are injected into one of the channels of a system of two copropagating quantum Hall edge channels, they fractionalize into pulses whose charge and shape reflects the properties of interactions. We show that the dependence of fractionalization induced electron/hole pair production in the pulses amplitude contains clear signatures of the fractionalization of the individual excitations. We propose an experimental setup combining a source of Lorentzian pulses and an Hanbury Brown and Twiss interferometer to measure interaction induced electron/hole pair production and more generally to reconstruct single electron coherence of these excitations before and after their fractionalization., 18 pages, 10 figures, 1 table

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

International audience; The on-demand generation of pure quantum excitations is important for the operation of quantum systems, but it is particularly difficult for a system of fermions. This is because any perturbation affects all states below the Fermi energy, resulting in a complex superposition of particle and hole excitations. However, it was predicted nearly 20 years ago1, 2, 3 that a Lorentzian time-dependent potential with quantized flux generates a minimal excitation with only one particle and no hole. Here we report that such quasiparticles (hereafter termed levitons) can be generated on demand in a conductor by applying voltage pulses to a contact. Partitioning the excitations with an electronic beam splitter generates a current noise that we use to measure their number. Minimal-excitation states are observed for Lorentzian pulses, whereas for other pulse shapes there are significant contributions from holes. Further identification of levitons is provided in the energy domain with shot-noise spectroscopy, and in the time domain with electronic Hong–Ou–Mandel noise correlations4, 5, 6, 7, 8. The latter, obtained by colliding synchronized levitons on a beam splitter, exemplifies the potential use of levitons for quantum information: using linear electron quantum optics9 in ballistic conductors, it is possible to imagine flying-qubit10, 11 operation in which the Fermi statistics are exploited12, 13, 14 to entangle synchronized electrons emitted by distinct sources15, 16, 17, 18. Compared with electron sources based on quantum dots19, 20, 21, the generation of levitons does not require delicate nanolithography, considerably simplifying the circuitry for scalability. Levitons are not limited to carrying a single charge, and so in a broader context n-particle levitons could find application in the study of full electron counting statistics22, 23, 24, 25. But they can also carry a fraction of charge if they are implemented in Luttinger liquids3 or in fractional quantum Hall edge channels26; this allows the study of Abelian and non-Abelian quasiparticles in the time domain. Finally, the generation technique could be applied to cold atomic gases27, 28, leading to the possibility of atomic levitons.

The quantum suppression of shot noise predicted for mesoscopic conductors is observed using absolute measurements of the equilibrium and nonequilibrium electrical fluctuations of a quantum point contact. The small energy ( $30\char21{}600\mathrm{mK}$) and low frequency ( $1\char21{}10\mathrm{kHz}$) used for measurements allow for a reliable quantitative experimental confirmation of the quantum noise theory. The noise reduction factor is found to be in excellent agreement with theoretical expectations, evolving from nearly unity at low electronic wave transmission to nearly zero on a conductance plateau.

We have realized a quantum optics like Hanbury Brown and Twiss (HBT) experiment by partitioning, on an electronic beam-splitter, single elementary electronic excitations produced one by one by an on-demand emitter. We show that the measurement of the output currents correlations in the HBT geometry provides a direct counting, at the single charge level, of the elementary excitations (electron/hole pairs) generated by the emitter at each cycle. We observe the antibunching of low energy excitations emitted by the source with thermal excitations of the Fermi sea already present in the input leads of the splitter, which suppresses their contribution to the partition noise. This effect is used to probe the energy distribution of the emitted wave-packets., 5 pages, 4 figures

We report on investigations about limitations of the Hall resistance quantization accuracy (4×10−7) measured in Hall bars based on monolayer and bilayer exfoliated graphene deposited on Si/SiO 2 substrates (best results in such devices). Transport measurements at low magnetic field (including quantum corrections) reveal that charge carrier scattering is mainly caused by charged impurities. In the QHE regime, dissipation occurs through quasi-elastic inter-Landau level (LL) scattering assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-LL transitions and cause the low breakdown currents (≈1µA) observed in the narrow (

The controlled and accurate emission of coherent electronic wave packets is of prime importance for future applications of nano-scale electronics. Here we present a theoretical and experimental analysis of the finite-frequency noise spectrum of a periodically driven single electron emitter. The electron source consists of a mesoscopic capacitor that emits single electrons and holes into a chiral edge state of a quantum Hall sample. We compare experimental results with two complementary theoretical descriptions: On one hand, the Floquet scattering theory which leads to accurate numerical results for the noise spectrum under all relevant operating conditions. On the other hand, a semi-classical model which enables us to develop an analytic description of the main sources of noise when the emitter is operated under optimal conditions. We find excellent agreement between experiment and theory. Importantly, the noise spectrum provides us with an accurate description and characterization of the mesoscopic capacitor when operated as a periodic single electron emitter., Comment: 17 pages, 15 figures

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature ($T, Comment: 14 pages, 9 figures

We present here the experimental study of the short time correlations of the current fluctuations generated by a periodic single electron emitter. The electron emitter is a mesoscopic capacitor, a top gated quantum dot connected to a conductor via a tunable tunnel barrier. We observe a new fundamental noise for electrons which is associated with the quantum fluctuations of the electron emission time from one emission cycle to the other. This random jitter between the emission trigger and the single particle emission is related to the random nature of single particle tunneling and is intrinsic to any single particle emitter. When the emitter emits a single particle at each cycle with unit probability, the noise reduces to this fundamental jitter limit which demonstrates single particle emission.

The minima of the oscillations of the tunnel conductance through a small electronic system present a rich variety of quantum conduction mechanisms when the charging energy E c and the electrochemical potential difference eV between the resevoirs compete with the single particle energy level separation Δ. We emphasize the strong relation between off-resonance tunneling and elastic co-tunneling in the linear conductance regime while, in the non-linear regime, inelastic co-tunneling has no equivalent for the case of resonant tunneling. We also review the recent experimental results which clearly identify this latter regime and we discuss the intermediate regime, valid for few electron systems, where E c ⋍ Δ.

We report on quantization tests of the quantum Hall effect in monolayer and bilayer graphene based devices which are fabricated from natural graphite by micromechanical exfoliation. Measurements of the Hall resistance R H with relative uncertainties in the range of 10−7 performed using a cryogenic current comparator based resistance bridge have been combined with high-precision longitudinal resistance R xx measurements to demonstrate that the quantization of R H in such graphene based devices agrees with the theoretical prediction with a relative accuracy of about 2 parts in 107. For a BL sample, the characterizations which have been carried out reveal the existence of a ν=−4 Hall resistance plateau flat within 2 parts in 106 over a finite range of electron density. At the center of the plateau, the Hall resistance agrees with R K /4 within 1.5 part in 107. Besides, a sample fabricated from a ML was also characterized: an agreement between the Hall resistance and R K /2 on the ν=2 plateau within 3 parts in 107 has been demonstrated.

We report on the fabrication of graphene based devices using the micromechanical exfoliation of natural graphite and their electrical characterization including metallic contact resistance measurements, mobility and density charge carriers measurements. The first and major application aimed is the quantrum Hall effect (QHE) metrology. The samples fabricated, from both monolayer and bilayer graphene flakes, with rather low contact resistance (typically a few tens or hundreds of Ω) enable the performance of QHE quantization tests exact within uncertainties of some part in 107. The limitations of this record accuracy are suggested by the results of the extensive characterizations performed (small size samples (

We report on spin dependent transport measurements in carbon nanotubes based multi-terminal circuits. We observe a gate-controlled spin signal in non-local voltages and an anomalous conductance spin signal, which reveal that both the spin and the orbital phase can be conserved along carbon nanotubes with multiple ferromagnetic contacts. This paves the way for spintronics devices exploiting both these quantum mechanical degrees of freedom on the same footing., 8 pages - minor differences with published version

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.

In analogy with quantum optics, short-time correlations of the current fluctuations are measured and used to assess the quality of the single-particle emission of a recently introduced on-demand electron source. We observe, in the context of electronics, the fundamental noise limit associated with the quantum fluctuations of the emission time of single particles, or quantum jittering. In optimum operating conditions of the source, the noise reduces to the quantum jitter limit, which demonstrates single-particle emission. Combined with the coherent manipulations of single electrons in a quantum conductor, this electron quantum optics experiment opens the way to explore new problems including quantum statistics and interactions at the single-electron level.

The anti-ferromagnetic coupling of a magnetic impurity carrying a spin with the conduction electrons spins of a host metal is the basic mechanism responsible for the increase of the resistance of an alloy such as Cu${}_{0.998}$Fe${}_{0.002}$ at low temperature, as originally suggested by Kondo . This coupling has emerged as a very generic property of localized electronic states coupled to a continuum . The possibility to design artificial controllable magnetic impurities in nanoscopic conductors has opened a path to study this many body phenomenon in unusual situations as compared to the initial one and, in particular, in out of equilibrium situations. So far, measurements have focused on the average current. Here, we report on \textit{current fluctuations} (noise) measurements in artificial Kondo impurities made in carbon nanotube devices. We find a striking enhancement of the current noise within the Kondo resonance, in contradiction with simple non-interacting theories. Our findings provide a test bench for one of the most important many-body theories of condensed matter in out of equilibrium situations and shed light on the noise properties of highly conductive molecular devices., Comment: minor differences with published version

The low temperature, high magnetic field phase of the two-dimensional electron system at the GaAs/GaAlAs heterojunction is distinguished by the appearance of both non-linear electrical conduction and a low frequency magneto-phonon branch with a small gap. The relationship between the gap and the threshold electric field for conduction is investigated and shown to be well accounted for by a simple model of a magnetically induced Wigner solid in a random field representing host disorder.

The 2D electron gas of a GaAs/GaAlAs heterojunction is confined to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number of the island. Conductance oscillations, periodic with the electron number, are observed in the Coulomb blockade regime with negligible energy level spacing. At low temperature, the blockade of the conductance by the island is shown to be limited by the quantum process of co-tunneling.

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 $h\nu/e$., Comment: Submitted to Phys.Rev.Lett

We report on direct measurements of the electronic shot noise of a Quantum Point Contact (QPC) at frequencies ν in the range 4–8 GHz. The very small energy scale used ensures energy independent transmissions of the few transmitted electronic modes and their accurate knowledge. Both the thermal energy and the QPC drain‐source voltage Vds are comparable to the photon energy leading to the observation of the shot noise suppression when Vds

A double barrier Single Electron Transistor is realized in two dimensions by confining the 2-D electron gas of a GaAs/GaAlAs heterojunction to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number in the island. The island has small single-particle energy level spacing and forms a metallic island. Periodic conductance oscillations characteristic of Coulomb blockade are observed when the central gate voltage is varied. The ability to vary the tunnel conductance allows us to study the basic physics of the Coulomb blockade: our results show that the quantum charge fluctuation mechanism which limits the tunneling blockade at low temperature is of second order in tunnel barrier transparencies in agreement with the charge Macroscopic Quantum Tunneling (q-MQT) or co-tunneling model.

Electrons confined at the interface of a GaAs/GaAlAs heterojunction form a 2D quantum electron liquid. Under a strong magnetic field a phase transition to an electron (Wigner) solid takes place in the low filling factor regime of the Fractional Quantum Hall Effect (FQHE). We describe experimental evidence for such electron solid formation obtained both by radiofrequency (RF) study of the low-frequency collective excitations and by conductivity measurements. A finite-threshold electric field for DC conduction reflecting the electron crystalline pinning in the sample random potential is found associated to a small gap in the solid phase low-lying collective excitation branch. The v = 1 5 FQHE liquid reenters the solid domain at low temperature.

Graphene Hall bars prepared from exfoliated natural graphite of large size and high quality have been measured in the quantum Hall effect regime. The monolayer Graphene sheet shows Hall quantization robust upon applying very large current. Low longitudinal resistance is found up to 10 muA with finite width h/2e2 Hall plateau at 4.2 K and 16 T. The TiAu/Graphene contact resistance is found low, typically 20 Omega to 50 Omega. The results strongly indicate that Graphene is a promising new material for quantum metrology.

We report on the realization of a single electron source similar to single photon sources in optics providing an important brick for the implementation of flying quantum bits in ballistic conductors. On demand single electron injection is obtained using a quantum dot connected to the conductor via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy.

Experimental studies of the magnetically induced Wigner solidification for electrons at high quality GaAs/Ga(Al)As heterojunctions in the extreme quantum limit and of the classical electron solidification for electrons on the helium surface are described. The use of a radiofrequency broadband spectroscopy technique to observe the low lying electronic collective excitations has led to the observation of the phase diagram of the quantum phase, the shear modulus and the specific heat of the classical phase.