Condensed Matter Physics Seminar

2 p.m., Thursday, September 23, 2004
Room 1201, Physics Building

 1,2,3 Superconducting Qubits

Fred Wellstood

(Center for Superconductivity Research, Department of Physics, University of Maryland, College Park)

Abstract:  I describe our groupís use of microwave spectroscopy and time domain techniques to observe the interaction between two coupled superconducting quantum bits (qubits). Each qubit is a single current-biased Nb/AlOx/Nb Josephson tunnel junction that is about 10 mm on a side and cooled to 20 mK. We couple the two qubits together using thin-film capacitors and achieve a coupling coefficient of greater than 10%. By applying microwaves in the 4-6 GHz frequency range, we can determine the lowest allowed transition frequencies between energy levels in the system. We find clear spectral evidence for the existence of an avoided level crossing when the junctions are in resonance and can compare our results in detail to what is expected from quantum mechanics. In theory, near the region of maximum splitting, the upper and lower branches correspond to transitions from the ground state of the system to symmetric and anti-symmetric entangled states of the two junctions. The observation of these transitions provides experimental support for the existence of entangled states of the 2 qubits. Extending the frequency to 4- 14 GHz reveals additional levels due to the two junctions coupling to a resonant LC (inductor-capacitor) mode formed by the wires and capacitors used to couple the junctions together. This three body system can displays an avoided crossing between three levels at a triple degeneracy point, where the quantum theory predicts entangled states involving all three systems. Perhaps the most remarkable aspect of these results is that the two junctions in the system are separated by a distance of about 0.7 mm. Despite this large distance and the macroscopic number of Cooper pairs participating in the system, quantum effects are observed. Work funded by the NSF, DOD and CSR.

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