D. Esteve

Rabi oscillations, Ramsey fringes and spin echoes in an electrical circuit

We present a superconducting tunnel junction circuit which behaves as a controllable atom, and whose ground and first excited state form an effective spin 1/2. By applying microwave pulses, we have performed on this circuit experiments demonstrating the controlled manipulation of the spin : Rabi precession, Ramsey interferences, and spin echoes.

On the observability of Coulomb blockade and single-electron tunneling

Abstract Coulomb blockade and single-electron tunneling are manifestations, at the macroscopic level, of the granularity of charge. These effects can occur in small capacitance tunnel junction systems, under conditions which are discussed in this article. We examine in particular how the zero-point electromagnetic fluctuations can eventually wash out the single-electron effects.

Emission of non-classical radiation by inelastic Cooper pair tunneling

We show that a properly dc-biased Josephson junction in series with two microwave resonators of different frequencies emits photon pairs in the resonators. By measuring auto- and intercorrelations of the power leaking out of the resonators, we demonstrate two-mode amplitude squeezing below the classical limit. This nonclassical microwave light emission is found to be in quantitative agreement with our theoretical predictions, up to an emission rate of 2 billion photon pairs per second.

Observation of the effect of an electromagnetic environment with sharp resonances on the current-voltage characteristics of a small capacitance tunnel junction

Abstract We have measured the current-voltage characteristic of a small capacitance tunnel junction coupled to a transmission line resonator. We calibrate the resonator using the sharp resonances displayed by the junction in the superconducting state, which correspond to the pumping of the modes of the resonator by the AC Josephson current. With this calibration, we explain quantitatively the non-linearity of the junction characteristic in the normal state as being due to the process by which a single electron tunnels by emitting a photon, the basic process of the theory of the effect of the electromagnetic environment on tunneling.

Frequency-determined current with a turnstile device for single electrons

Abstract We have fabricated a device in which the current is to a high accuracy determined by an external frequency f as I=ef. This device consists of an array of ultrasmall tunnel junctions. An rf voltage is applied to a gate and causes the transfer of a single electron per cycle through the array. The locking of the electron transfer is obtained by using Coulomb blockade of electron tunneling.

The Proximity Effect in Mesoscopic Diffusive Conductors

The “proximity effect” is a loose term describing the presence of a superconducting order in a piece of normal metal (N) placed in electrical contact with a superconducting one (S). Because of an action-reaction like principle, the order induced on the N side of the NS interface due to the presence of S is correlated with a weakening of the superconducting order on the S side due to the presence of N. Thus, the proximity effect is actually the dilution of superconducting order at an NS interface.

Observability of the coulomb blockade in single tunnel junctions

The finite temperature I-V-characteristic of a normal tunnel junction coupled to its electromagnetic environment is calculated. It is shown that due to quantum fluctuations of the charge the Coulomb gap is only observable provided the impedance Z(ω) of the environment exceeds the resistance quantum RQ = h/2e2. The observability of the Coulomb blockade in single junctions is also severely limited by finite temperature effects. Apart from general results for an environment with arbitrary impedance, detailed predictions are presented for a case of practical interest where the leads attached to the junction may be represented through a series inductance and a shunt capacitance.

The superconducting proximity effect probed on a mesoscopic length scale

We have measured by tunneling spectroscopy the electronic density of states in a non-superconducting wire in good contact with a superconductor, at distances of 200, 300 and 800 nm from the interface. Closest to the interface, the density of states near the Fermi energy is reduced to 55% of its normal value. At the farthest measurement point, this dip has nearly completely disappeared. We compare our data to predictions based on the Usadel equations.

Measurement of the incremental charge of a superconducting island

Abstract We have measured the total charge of a superconducting electrode or ‘island’ which is free to exchange electrons with a metallic ground through a tunnel junction and which is capacitively coupled to a voltage source at potential U . We find that the island charge varies stepwise with U , the incremental charge being equal to 2 e inside a low-temperature, low-magnetic-field domain, and to e elsewhere. In this latter case, steps corresponding to different parities of the number of electrons inside the island have unequal lengths. The odd-even free energy difference deduced from the asymmetry of the steps is well explained by the theory of dirty superconductors.

Manipulating Electrons One by One

We describe three circuits based on nanoscale tunnel junctions with which we manipulate single electrons. Their operating principle is based on “macroscopic charge quantization”: at low temperature, the charges in the metallic islands between the junctions correspond to integer numbers of electrons and their fluctuations are quenched due to the electrostatic energy gap arising from the small value of the island capacitances. The simplest circuit we have operated is the “single electron box”: one junction and one capacitor define a single island which we have charged with one electron at a time. To monitor the entrance of one extra electron on the island we have used a single electron transistor, which consists of two nanojunctions in series, as a very sensitive electrometer. We have designed and operated two other devices, the pump (3 junctions) and the turnstile (4 junctions), through which electrons are transferred one by one by means of gate voltages capacitively coupled to the islands. When this controlled transfer of electrons is clocked at a frequency /, both devices produce a d.c. current I = ef.