Coherence in Josephson-Junction Qubits John Martinis, NIST, Boulder

The Josephson junction is an ideal solid-state system for building electrical "atoms" that can function as quantum bits for a quantum computer. I will discuss recent experimental work based on large-area “phase” qubits, where experiments have demonstrated qubit state preparation and measurement with good fidelity, as well as manipulation of the qubit state via the observation of Rabi oscillations.  However, decoherence of the qubit state is a major impediment for further progress since coherence amplitudes and times of all present experiments are lower than expected and required. Recently, we have discovered a new decoherence mechanism arising from spurious microwave resonances.  Modeling of these resonances indicates that improvements will require fabrication of junctions with lower sub-gap current and 1/f noise.  We have recently confirmed this prediction when qubits were tested with improved coherence times and amplitudes. With an understanding of the source of decoherence, we are hopeful that further materials research aimed at removing fluctuators in junctions will produce coherence good enough to build a 100-1000 qubit quantum computer.