Novel Superconducting Device to Isolate, Entangle and Read Out Flux Qubits

摘要

Experiments are described in which the quantum states of two flux qubits were read out by a dc SQUID (Superconducting QUantum Interference Device). Observations of the linewidths of microwave-induced peaks. Ramsey fringes and flux echoes on a single qubit yielded values of the decoherence times T(sub 2)', T(sub 2)* and T(sub 2). respectively. Within the experimental errors, the values satisfied the prediction 1/T(sub 2)* =1/T(sub 2)' + 1/T(sub 2). Spectroscopic measurements on two qubits revealed energy gaps at their degeneracy points matched to within plus or minus 1%: 8.88 plus or minus 0.01 GHz and 8.9 plus or minus 0.01 GHz, respectively. Detailed measurements were made of the entangled states of the qubits in two regimes: at the double degeneracy point (both qubits biased at a half-flux quantum phi(sub 0)/2). and at flux values away from phi(sub 0)/2 at which two qubit frequencies coincided ('intersecting anticrossing'). The energy levels were determined from microwave transitions from the ground state.