We have measured flux-noise correlations in a tunable superconducting flux qubit. The device consists of two loops that independently control the qubit's energy splitting and tunnel coupling. Low-frequency flux noise in the loops causes fluctuations of the qubit frequency and leads to dephasing. Since the noises in the two loops couple to different terms of the qubit Hamiltonian, a measurement of the dephasing rate at different bias points provides a way to extract both the amplitude and the sign of the noise correlations. We find that the flux fluctuations in the two loops are anticorrelated, consistent with a model where the flux noise is generated by randomly oriented unpaired spins on the metal surface.
Flux qubits (also known as persistent current qubits), one kind of superconducting tunnel junction circuits, are recently demonstrated as a promising candidate to realize quantum computations.In order to do large-scale quantum information processing, it is necessary to switch on and off the couplings between individual flux qubits in the time-domain while those qubits retain coherence.This report contains two parts: one is to describe my understanding of models, fabrications and measurements for one single flux qubit; the other is to compare two proposed schemes and the corresponding experimental realizations of coherent tunable couplings between flux qubits.
We experimentally confirm the functionality of a coupling element for flux-based superconducting qubits, with a coupling strength
J whose sign and magnitude can be tuned in situ. To measure the effective
J, the ground state of a coupled two-qubit system has been mapped as a function of the local magnetic fields applied to each qubit. The state of the system is determined by directly reading out the individual qubits while tunneling is suppressed. These measurements demonstrate that
J can be tuned from antiferromagnetic through zero to ferromagnetic.