Charge fluctuations in small-capacitance junctions

Suppressing Andreev Bound State Zero Bias Peaks Using a Strongly Dissipative Lead

Hybrid semiconductor-superconductor nanowires are predicted to host Majorana zero modes that induce zero-bias peaks (ZBPs) in tunneling conductance. ZBPs alone, however, are not sufficient evidence due to the ubiquitous presence of Andreev bound states. Here, we implement a strongly resistive normal lead in InAs-Al nanowire devices and show that most of the expected Andreev bound state-induced ZBPs can be suppressed, a phenomenon known as environmental Coulomb blockade. Our result is the first experimental demonstration of this dissipative interaction effect on Andreev bound states and can serve as a possible filter to narrow down the ZBP phase diagram in future Majorana searches.

Atomic scale shot-noise using cryogenic MHz circuitry

By implementing dedicated cryogenic circuitry operating in the MHz regime, we have developed a scanning tunneling microscope (STM) capable of conventional, low frequency (<10 kHz), microscopy as well spectroscopy and shot-noise detection at 1 MHz. After calibrating our AC circuit on a gold surface, we illustrate our capability to detect shot-noise at the atomic scale and at low currents (<1 nA) by simultaneously measuring the atomically resolved differential conductance and shot-noise on the high temperature superconductor Bi₂Sr₂CaCu₂O_8+x . We further show our direct sensitivity to the temperature of the tunneling electrons at low voltages. Our MHz circuitry opens up the possibility to study charge and correlation effects at the atomic scale in all materials accessible to STM.

Photon-assisted tunnelling with nonclassical light

Among the most exciting recent advances in the field of superconducting quantum circuits is the ability to coherently couple microwave photons in low-loss cavities to quantum electronic conductors. These hybrid quantum systems hold great promise for quantum information-processing applications; even more strikingly, they enable exploration of new physical regimes. Here we study theoretically the new physics emerging when a quantum electronic conductor is exposed to nonclassical microwaves (for example, squeezed states, Fock states). We study this interplay in the experimentally relevant situation where a superconducting microwave cavity is coupled to a conductor in the tunnelling regime. We find that the conductor acts as a nontrivial probe of the microwave state: the emission and absorption of photons by the conductor is characterized by a nonpositive definite quasi-probability distribution, which is related to the Glauber–Sudarshan P-function of quantum optics. These negative quasi-probabilities have a direct influence on the conductance of the conductor.

Coherently coupling microwave photons to quantum electronic conductors could provide a useful platform for quantum information processing. Souquet et al. now theoretically demonstrate that such systems can also act as sensitive probes of the quantum properties of non-classical microwave radiation.

Bright side of the Coulomb blockade

We explore the photonic (bright) side of the dynamical Coulomb blockade (DCB) by measuring the radiation emitted by a dc voltage-biased Josephson junction embedded in a microwave resonator. In this regime Cooper pair tunneling is inelastic and associated with the transfer of an energy 2eV into the resonator modes. We have measured simultaneously the Cooper pair current and the photon emission rate at the resonance frequency of the resonator. Our results show two regimes, in which each tunneling Cooper pair emits either one or two photons into the resonator. The spectral properties of the emitted radiation are accounted for by an extension to DCB theory.

A versatile low-temperature setup for the electrical characterization of single-molecule junctions

We present a modular high-vacuum setup for the electrical characterization of single molecules down to liquid helium temperatures. The experimental design is based on microfabricated mechanically controllable break junctions, which offer control over the distance of two electrodes via the bending of a flexible substrate. The actuator part of the setup is divided into two stages. The slow stage is based on a differential screw drive with a large bending range. An amplified piezoceramic actuator forms the fast stage of the setup, which can operate at bending speeds of up to 800 μm/s. In our microfabricated break junctions this is translated into breaking speeds of several 10 nm/s, sufficient for the fast acquisition of large statistical datasets. The bandwidth of the measurement electronics has been optimized to enable fast dI/dV spectroscopy on molecular junctions with resistances up to 100 MΩ. The performance of the setup is demonstrated for a π-conjugated oligo(phenylene-ethynylene)-dithiol molecule.

Current measurement by real-time counting of single electrons

The fact that electrical current is carried by individual charges has been known for over 100 years, yet this discreteness has not been directly observed so far. Almost all current measurements involve measuring the voltage drop across a resistor, using Ohm's law, in which the discrete nature of charge does not come into play. However, by sending a direct current through a microelectronic circuit with a chain of islands connected by small tunnel junctions, the individual electrons can be observed one by one. The quantum mechanical tunnelling of single charges in this one-dimensional array is time correlated, and consequently the detected signal has the average frequency f = I/e, where I is the current and e is the electron charge. Here we report a direct observation of these time-correlated single-electron tunnelling oscillations, and show electron counting in the range 5 fA-1 pA. This represents a fundamentally new way to measure extremely small currents, without offset or drift. Moreover, our current measurement, which is based on electron counting, is self-calibrated, as the measured frequency is related to the current only by a natural constant.

Direct link between Coulomb blockade and shot noise in a quantum-coherent structure

We analyze the current-voltage characteristic of a quantum conduction channel coupled to an electromagnetic environment with arbitrary frequency-dependent impedance. In the weak blockade regime the correction to the Ohmic behavior is directly related to the channel current fluctuations, vanishing at perfect transmission in the same way as shot noise. This relation can be generalized to describe the environmental Coulomb blockade in a generic mesoscopic conductor coupled to an external impedance, as the response of the latter to the current fluctuations in the former.

Electrodynamic dip in the local density of states of a metallic wire

We have measured the differential conductance of a tunnel junction between a thin metallic wire and a thick ground plane, as a function of the applied voltage. We find that near zero voltage, the differential conductance exhibits a dip, which scales as 1/square root of [V] down to voltages V approximately 10k(B)T/e. The precise voltage and temperature dependence of the differential conductance is accounted for by the effect on the tunneling density of states of the macroscopic electrodynamics contribution to electron-electron interaction, and not by the short-ranged screened-Coulomb repulsion at microscopic scales.