Dispersive readout scheme for Josephson phase qubits

Superconducting qubits require appropriate isolation from the bias leads, which can be achieved by the use of superconducting transformers. A superconducting loop with a tunnel junction can be used to prepare a double-well potential, where the discrete quantum levels in one well form the qubit states. State-dependent tunneling to the other well changes the magnetic flux in the qubit loop measured by a dc-SQUID. A standard phase qubit readout by the dc-SQUID requires switching to the non-superconducting state, which generates heat and quasiparticles in the circuitry. This results in long cool-down times after each switching event and thus limits the repetition rate of the experiment. In our ongoing experiments, we replace the standard scheme by a high-frequency readout based on a weakly-dissipative measurement of the dc-SQUID Josephson inductance, which in turn depends on the flux in the dc-SQUID. This is done by detecting a phase change of the probe signal reflected from an LC-tank circuit coupled to the dc-SQUID. The expected benefits include longer coherence times, much faster measurements, and a possibility of applying non-destructive measurement schemes to Josephson phase qubits. (orig.)