Olivier Buisson

Observation of Transition from Escape Dynamics to Underdamped Phase Diffusion in a Josephson Junction

We have investigated the dynamics of underdamped Josephson junctions. In addition to the usual crossover between macroscopic quantum tunnelling and thermally activated (TA) behaviour we observe in our samples with relatively small Josephson coupling E_J, for the first time, the transition from TA behaviour to underdamped phase diffusion. Above the crossover temperature the threshold for switching into the finite voltage state becomes extremely sharp. We propose a (T,E_J) phase-diagram with various regimes and show that for a proper description of it dissipation and level quantization in a metastable well are crucial.

Coherent oscillations in a superconducting multilevel quantum system

We have observed coherent oscillations in a multilevel quantum system, formed by a current-biased dc SQUID. These oscillations have been induced by applying resonant microwave pulses of flux. Quantum measurement is performed by a nanosecond flux pulse that projects the final state onto one of two different voltage states of the dc SQUID, which can be read out. The number of quantum states involved in the coherent oscillations increases with increasing microwave power. The dependence of the oscillation frequency on microwave power deviates strongly from the linear regime expected for a two-level system and can be very well explained by a theoretical model taking into account the anharmonicity of the multilevel system.

Measurement of the effect of quantum phase slips in a Josephson junction chain

Coulomb interactions can cause a rapid change in the phase of the wavefunction along a very narrow superconducting system. Such a phase slip is now measured in a chain of Josephson junctions.

Junction fabrication by shadow evaporation without a suspended bridge

We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by E-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in-situ fabrication of junctions and capacitors without the use of the well-known suspended bridge[1]. The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 0.01 to more than 10000 micron square. We have fabricated Al/AlOx/Al Josephson junctions, phase qubit and capacitors using a 100kV E- beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20kV E-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillations decay and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this controlled undercut technique.We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by e-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in situ fabrication of junctions and capacitors without the use of the well-known suspended bridge (Dolan 1977 Appl. Phys. Lett. 31 337-9). The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 10(-2) µm(2) to more than 10(4) µm(2). We have fabricated Al/AlO(x)/Al Josephson junctions, phase qubit and capacitors using a 100 kV e-beam writer. Although this high voltage enables a precise control of the undercut, implementation using a conventional 20 kV e-beam is also discussed. The phase qubit coherence times, extracted from spectroscopy resonance width, Rabi and Ramsey oscillation decays and energy relaxation measurements, are longer than the ones obtained in our previous samples realized by standard techniques. These results demonstrate the high quality of the junction obtained by this bridge-free technique.

Coherent low-energy charge transport in a diffusive S-N-S junction.

We have studied the current-voltage characteristics of diffusive mesoscopic Nb-Cu-Nb Josephson junctions with highly transparent Nb-Cu interfaces. We consider the low-voltage and high-temperature regime eV < epsilon(c) < k(B)T, where epsilon(c) is the Thouless energy. The observed excess current as well as the observed subharmonic Shapiro steps under microwave irradiation suggest the occurrence of low-energy coherent multiple Andreev reflection.

Entangled States in a Josephson Charge Qubit Coupled to a Superconducting Resonator

We study the dynamics of a quantum superconducting circuit which is the analogue of an atom in a high-Q cavity. The circuit consists of a Josephson charge qubit coupled to a superconducting resonator. The Josephson charge qubit can be treated as a two level quantum system whose energy separation is split by the Josephson energy E j . The superconducting resonator in our proposal is the analogue of a photon box and is described by a quantum harmonic oscillator with characteristic frequency ω r . The coupling between the charge qubit and the resonator is realized by a coupling capacitance C c . We have calculated the eigenstates and the dynamics of the quantum circuit. Interesting phenomena occur when the Josephson energy equals the oscillator frequency, E j = ħ ω r . Then the quantum circuit is described by entangled states. We have deduced the time evolution of these states in the limit of weak coupling between the charge qubit and the resonator. We found Rabi oscillations of the excited charge qubit eigenstate. This effect is explained by the spontaneous emission and re-absorption of a single photon in the superconducting resonator.

Strong tunable coupling between a superconducting charge and phase qubit

We have realized a tunable coupling over a large frequency range between an asymmetric Cooper pair transistor (charge qubit) and a dc SQUID (phase qubit). Our circuit enables the independent manipulation of the quantum states of each qubit as well as their entanglement. The measurement of the charge qubits quantum states is performed by an adiabatic quantum transfer from the charge to the phase qubit. The measured coupling strength is in agreement with an analytic theory including a capacitive and a tunable Josephson coupling between the two qubits.

One-shot quantum measurement using a hysteretic dc SQUID

We propose a single shot quantum measurement to determine the state of a Josephson charge quantum bit (qubit). The qubit is a Cooper pair box and the measuring device is a two junction superconducting quantum interference device (dc SQUID). This coupled system exhibits a close analogy with a Rydberg atom in a high Q cavity, except that in the present device we benefit from the additional feature of escape from the supercurrent state by macroscopic quantum tunneling, which provides the final readout. We test the feasibility of our idea against realistic experimental circuit parameters and by analyzing the phase fluctuations of the qubit.

Evidence of two-dimensional macroscopic quantum tunneling of a current-biased dc SQUID

The escape probability out of the superconducting state of a hysteretic dc SQUID has been measured at different values of the applied magnetic flux. At low temperatures, the escape current and the width of the probability distribution are temperature independent but they depend on flux. Experimental results do not fit the usual one-dimensional macroscopic quantum tunneling (MQT) law but are perfectly accounted for by the two-dimensional MQT behavior as we propose here. Near zero flux, our data confirms the recent MQT observation in a dc SQUID [Phys. Rev. Lett. 89, 98301 (2002)]].

Etching suspended superconducting tunnel junctions from a multilayer

A method to fabricate large-area superconducting hybrid tunnel junctions with a suspended central normal metal part is presented. The samples are fabricated by combining photo-lithography and chemical etch of a superconductor—insulator—normal metal multilayer. The process involves few fabrication steps, is reliable and produces extremely high-quality tunnel junctions. Under an appropriate voltage bias, a significant electronic cooling is demonstrated. We analyze semi-quantitatively the thermal behavior of a typical device.