C. Urbina

Manipulating the Quantum State of an Electrical Circuit

We have designed and operated a superconducting tunnel junction circuit that behaves as a two-level atom: the “quantronium.” An arbitrary evolution of its quantum state can be programmed with a series of microwave pulses, and a projective measurement of the state can be performed by a pulsed readout subcircuit. The measured quality factor of quantum coherenceQ ϕ ≅ 25,000 is sufficiently high that a solid-state quantum processor based on this type of circuit can be envisioned.

Electron transport through a metal-molecule-metal junction

Molecules of bisthiolterthiophene have been adsorbed on the two facing gold electrodes of a mechanically controllable break junction in order to form metal-molecule(s)-metal junctions. Current-voltage

The signature of chemical valence in the electrical conduction through a single-atom contact

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Phase controlled superconducting proximity effect probed by tunneling spectroscopy.

characteristics have been recorded at room temperature. Zero bias conductances were measured in the 10--100 nS range and different kinds of nonlinear

Exciting Andreev pairs in a superconducting atomic contact

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Observation of the Temporal Decoupling Effect on the Macroscopic Quantum Tunneling of a Josephson Junction

curves with steplike features were reproducibly obtained. Switching between different kinds of

Measurement of the current-phase relation of superconducting atomic contacts.

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Single electron tunneling rates in multijunction circuits

curves could be induced by varying the distance between the two metallic electrodes. The experimental results are discussed within the framework of tunneling transport models explicitly taking into account the discrete nature of the electronic spectrum of the molecule.

Single Cooper pair pump

Fabrication of structures at the atomic scale is now possible using state-of-the-art techniques for manipulating individual atoms, and it may become possible to design electrical circuits atom by atom. A prerequisite for successful design is a knowledge of the relationship between the macroscopic electrical characteristics of such circuits and the quantum properties of the individual atoms used as building blocks. As a first step, we show here that the chemical valence determines the conduction properties of the simplest imaginable circuit—a one-atom contact between two metallic banks. The extended quantum states that carry the current from one bank to the other necessarily proceed through the valence orbitals of the constriction atom. It thus seems reasonable to conjecture that the number of current-carrying modes (or ‘channels’) of a one-atom contact is determined by the number of available valence orbitals, and so should strongly differ for metallic elements in different series of the periodic table. We have tested this conjecture using scanning tunnelling microscopy and mechanically controllable break-junction techniques, to obtain atomic-size constrictions for four different metallic elements (Pb, Al, Nb and Au), covering a broad range of valences and orbital structures. Our results demonstrate unambiguously a direct link between valence orbitals and the number of conduction channels in one-atom contacts.

Evidence for long-lived quasiparticles trapped in superconducting point contacts

Using a dual-mode STM-AFM microscope operating below 50 mK we measured the local density of states along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.