Superconducting flux quantum bits: fabricated quantum objects

The conjugate variables of a superconductor are the charge and the phase of the order parameter. In circuits containing Josephson junctions, these quantum properties can be displayed.

We study flux qubits, superconducting rings that contain three small Josephson junctions. When a magnetic flux of half a flux quantum is applied, there are two states with opposite currents. For suitable junction parameters, a superposition of these states is possible. Transitions can be driven with resonant microwaves. The 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 controlled-not gate. In principle this provides the basis for a universal quantum computer. However, decoherence is a limitation. The quantum measurements can be carefully designed in these systems, and we have 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.