Experimental demonstration of an oscillator stabilized Josephson flux qubit

We experimentally demonstrate the use of a superconducting transmission line, shorted at both ends, to stabilize the operation of a tunable flux qubit. Our qubit consists of three Josephson junctions and three loops coupled to a fixed-length superconducting transmission line. The bare qubit has two control parameters, the flux and the control flux. This allows the qubit to have a tunable difference frequency between the ground and first excited states and at the same time to be biased at a degenerate point with respect to the flux parameter. This condition can be met for a wide range of junction critical currents. This flexibility of our structure is a very desirable property for a scalable qubit. To stabilize the operation of our qubit and increase its coherence time, we couple the bare qubit to the lowest mode of a superconducting transmission line, which we model as a harmonic oscillator. Using harmonic oscillator stabilization and pulsed dc operation, we have observed Larmor oscillations with a single shot visibility of 90 percent and a coherence time of 100 ns. In another qubit the visibility was 60 percent and there was no measurable visibility reduction after 35 ns.