U. Gennser

Tomonaga–Luttinger physics in electronic quantum circuits

In one-dimensional conductors, interactions result in correlated electronic systems. At low energy, a hallmark signature of the so-called Tomonaga–Luttinger liquids is the universal conductance curve predicted in presence of an impurity. A seemingly different topic is the quantum laws of electricity, when distinct quantum conductors are assembled in a circuit. In particular, the conductances are suppressed at low energy, a phenomenon called dynamical Coulomb blockade. Here we investigate the conductance of mesoscopic circuits constituted by a short single-channel quantum conductor in series with a resistance, and demonstrate a proposed link to Tomonaga–Luttinger physics. We reformulate and establish experimentally a recently derived phenomenological expression for the conductance using a wide range of circuits, including carbon nanotube data obtained elsewhere. By confronting both conductance data and phenomenological expression with the universal Tomonaga–Luttinger conductance curve, we demonstrate experimentally the predicted mapping between dynamical Coulomb blockade and the transport across a Tomonaga–Luttinger liquid with an impurity.

Electroluminescence from strain-compensated Si0.2Ge0.8/Si quantum-cascade structures based on a bound-to-continuum transition

Intersubband electroluminescence from strain-compensated Si/Si0.2Ge0.8 quantum cascade (QC) structures, consisting of up to 30 periods grown by molecular beam epitaxy on Si0.5Ge0.5 pseudosubstrates is reported. The design of the active region is based on a so-called “bound-to-continuum transition.” The intersubband radiation is emitted at a wavelength of 7 μm and is polarized, as expected for intersubband transitions between heavy hole states. A good agreement with photocurrent measurements is also found.

Quantum Limit of Heat Flow Across a Single Electronic Channel

Quantum Heating Mesoscopic wires exhibit peculiar properties at low temperatures. Their electric conductance can show plateaus at evenly spaced values, which reflects the sequential opening of “quantum transport channels,” each of which can only carry a finite amount of charge or heat. Whereas the step size for the electric conductance depends on the type of the particle carrying the charge, for heat conduction this “quantum” is universal. Jezouin et al. (p. 601, published online 3 October; see the Perspective by Sothmann and Flindt) measured the quantum of heat conduction through a single electronic channel by comparing the amount of heat needed to heat a small metal plate to a constant temperature, while varying the number of electronic channels through which the heat was dissipated from the plate. Encouragingly, the measurement was in agreement with the theoretical prediction. The unit of heat carried by electrons is measured using noise thermometry and found to be consistent with predictions. [Also see Perspective by Sothmann and Flindt] Quantum physics predicts that there is a fundamental maximum heat conductance across a single transport channel and that this thermal conductance quantum, GQ, is universal, independent of the type of particles carrying the heat. Such universality, combined with the relationship between heat and information, signals a general limit on information transfer. We report on the quantitative measurement of the quantum-limited heat flow for Fermi particles across a single electronic channel, using noise thermometry. The demonstrated agreement with the predicted GQ establishes experimentally this basic building block of quantum thermal transport. The achieved accuracy of below 10% opens access to many experiments involving the quantum manipulation of heat.

Direct measurement of the coherence length of edge states in the integer quantum Hall regime.

We have determined the finite temperature coherence length of edge states in the integer quantum Hall effect 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 one 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.

Non-equilibrium edge-channel spectroscopy in the integer quantum Hall regime

Gapless edge-state excitations known as one-dimensional chiral fermions explain many experimental observations of the behaviour of integer quantum Hall systems. But prevailing theory suggests the emergence of extra edge states as well. A new spectroscopic technique for probing the flow of energy in the edge channels of a quantum Hall device finds no loss of energy to such extra states.

Energy Relaxation in the Integer Quantum Hall Regime

We investigate the energy exchanges along an electronic quantum channel realized in the integer quantum Hall regime at a filling factor of νL=2. One of the two edge channels is driven out of equilibrium and the resulting electronic energy distribution is measured in the outer channel, after several propagation lengths 0.8  μm≤L≤30  μm. Whereas there are no discernible energy transfers toward thermalized states, we find efficient energy redistribution between the two channels without particle exchanges. At long distances L≥10  μm, the measured energy distribution is a hot Fermi function whose temperature is lower than expected for two interacting channels, which suggests the contribution of extra degrees of freedom. The observed short energy relaxation length challenges the usual description of quantum Hall excitations as quasiparticles localized in one edge channel.

Electrically injected cavity polaritons

We have realised an electroluminescent device in which electron are injected into intersubband polariton branches. We reproduce electroluminescence spectra by using a phenomenological model, in which a voltage dependent injection is taken into account.

Tuning Energy Relaxation along Quantum Hall Channels

The chiral edge channels in the quantum Hall regime are considered ideal ballistic quantum channels, and have quantum information processing potentialities. Here, we demonstrate experimentally, at a filling factor of ν(L)=2, the efficient tuning of the energy relaxation that limits quantum coherence and permits the return toward equilibrium. Energy relaxation along an edge channel is controllably enhanced by increasing its transmission toward a floating Ohmic contact, in quantitative agreement with predictions. Moreover, by forming a closed inner edge channel loop, we freeze energy exchanges in the outer channel. This result also elucidates the inelastic mechanisms at work at ν(L)=2, informing us, in particular, that those within the outer edge channel are negligible.

Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states.

Many-body correlations and macroscopic quantum behaviours are fascinating condensed matter problems. A powerful test-bed for the many-body concepts and methods is the Kondo effect, which entails the coupling of a quantum impurity to a continuum of states. It is central in highly correlated systems and can be explored with tunable nanostructures. Although Kondo physics is usually associated with the hybridization of itinerant electrons with microscopic magnetic moments, theory predicts that it can arise whenever degenerate quantum states are coupled to a continuum. Here we demonstrate the previously elusive ‘charge’ Kondo effect in a hybrid metal–semiconductor implementation of a single-electron transistor, with a quantum pseudospin of 1/2 constituted by two degenerate macroscopic charge states of a metallic island. In contrast to other Kondo nanostructures, each conduction channel connecting the island to an electrode constitutes a distinct and fully tunable Kondo channel, thereby providing unprecedented access to the two-channel Kondo effect and a clear path to multi-channel Kondo physics. Using a weakly coupled probe, we find the renormalization flow, as temperature is reduced, of two Kondo channels competing to screen the charge pseudospin. This provides a direct view of how the predicted quantum phase transition develops across the symmetric quantum critical point. Detuning the pseudospin away from degeneracy, we demonstrate, on a fully characterized device, quantitative agreement with the predictions for the finite-temperature crossover from quantum criticality.

Finite bias visibility of the electronic Mach-Zehnder interferometer

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.