[QIP-Sem] MIT Quantum Information Processing Seminar Announcement

[QIP-Sem Mailing List]

Next week's MIT QIP seminar will take place on Monday, April 11 at 
16:00 in 4-237, and features:


Large-Inductance Superconducting Flux Qubits: Coherent Oscillations 
and Controllable Coupling

by Prof. John Clarke (U. C. Berkeley Physics Dept.)

ABSTRACT

We have studied quantum coherence in a three-junction, 
superconducting flux qubit using a dc SQUID to measure its flux 
state.  The qubit and SQUID are equipped with separate, on-chip flux 
bias lines, so that we can set the flux bias of each device 
independently.  The chip is enclosed in a superconducting cavity, 
enabling us to obtain data over several days in a stable magnetic 
environment.  Detailed spectroscopic measurements reveal the expected 
avoided crossing at the degeneracy point, and resonances that are 
ascribed to tunnel barrier defects; even at millikelvin temperatures 
these resonances shift with time.  The Rabi oscillation frequency 
scales linearly with microwave amplitude.  The linewidth of 
microwave-induced resonances yields the inhomogeneously broadened 
dephasing rate 1/T2', and echoes yield the dephasing rate 1/T2; these 
times are consistent with the value of 1/T2* determined from Ramsey 
fringes.  A scheme is presented for controlling the coupling between 
two flux qubits by varying the dynamic inductance of the readout 
SQUID surrounding them.  The mutual inductance between the qubits can 
be switched from zero to a predetermined value simply by changing the 
bias current in the SQUID in the zero-voltage state.
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