Department of Physics, University of California, Berkeley
Materials Sciences Division, Lawrence Berkeley National Laboratory
I describe experiments on single and coupled superconducting flux qubits (quantum bits) in which the quantum state is measured with a Superconducting QUantum Interference Device (SQUID). The flux qubit exhibits the properties of a spin-1/2 system, including superposition of quantum states. Two qubits, coupled by their mutual inductance and by screening currents in the readout SQUID, produce a ground state |0> and three excited states |1>, |2> and |3>. Microwave spectra reveal an anticrossing between the |1> and |2> energy levels. The level repulsion can be reduced to zero by means of a current pulse in the SQUID that changes its dynamic inductance and hence the coupling between the qubits. The results are in good agreement with predictions. The ability to switch the coupling between qubits on and off permits efficient realization of universal quantum logic. A major source of decoherence in the flux qubit is 1/f flux noise (f is frequency). The source of this noise, first observed in SQUIDs, has until now remained a puzzle for a quarter of a century. I describe a model for the origin of flux noise that involves the locking of single electron spins in traps. The model explains the essential experimental facts.
This work is in collaboration with T. Hime, R.H. Koch, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, A. Ustinov, D. DiVincenzo, K.B. Whaley, F.K. Wilhelm and C.-E. Wu, and supported by AFOSR, ARO, DOE and NSF.