Geneva University - Superconducting flux quantum bits: fabricated quantum objects
Ecole de physique Département de physique nucléaire et corspusculaire 24, Quai Ernest-Ansermet 1211 GENEVE 4 Tél: (022) 379 62 73 Fax: (022) 379 69 92
Lundi 29 janvier 2007
COLLOQUE DE LA SECTION DE PHYSIQUE
17 heures - Auditoire Stueckelberg
Superconducting flux quantum bits: fabricated quantum objects
Prof. Hans Mooij / Kavli Institute of Nanoscience, Delft University of Technology
The quantum conjugate variables of a superconductor are the charge or number of Cooper pairs, and the phase of the order parameter. In circuits that contain small Josephson junctions, these quantum properties can be brought forward. In Delft we study so-called flux qubits, superconducting rings that contain three small Josephson junctions. When a magnetic flux of half a flux quantum is applied to the loop, there are two states with opposite circulating current. For suitable junction parameters, a quantum superposition of those macroscopic states is possible. Transitions can be driven with resonant microwaves. These quantum bits or qubits can be coupled to a harmonic oscillator. It is also possible to couple two qubits and control the transitions of the combined system; we have recently developed a so-called controlled-not gate. In principle this provides the basis for a universal quantum computer. However, decoherence is a limitation. Recently we developed a method to measure the quantum state which is purely projective. With repeated measurements on the same qubit, we demonstrated the quantum non-demolition nature of the measurement.
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