Single-shot Andreev read-out of entangled superconducting qubits

The basic entity of quantum information is a qubit with states [0>and [1>, where the great power of quantum information processing (QIP) derives from algorithms that use linear combinations of these states to perform calculations that are orders of magnitude faster than in ordinary computers. The ground state and first excited state of an atom could easily serve as qubit states, but it is difficult to control the coupling between qubits based on such microscopic degrees of freedom. It is easier to couple to macroscopic quantum superconducting integrated circuits made using modern nanofabrication techniques. Such circuits behave like artificial atoms , and it is possible to measure the atom-like properties (Rabi oscillations and Ramsey fringes) associated with the lowest two quantised energy levels. Our qubits are superconducting aluminium loops interrupted by Josephson junctions with deliberately engineered critical currents and coupling energies. When a magnetic field is applied, a persistent current will flow in the loop either in the clockwise (the [0> state) or anticlockwise (the [1> state) direction. Such qubits are named persistent current or flux qubits; they are regarded as the most promising solid-state candidates for quantum computation and the preparation, manipulation and measurement of the qubit can be achieved by incorporating the artificial atom into a high frequency (up to 10 GHz) circuit. In superconducting qubits the ability to manipulate of the quantum states for QIP is limited by the extrinsic decoherence introduced by the read-out measurement. We have developed a new type of read-out, which consists of a nanometre scale normal metal cross joined to superconducting leads attached to the qubit loop. Electrons in the metal undergo Andreev reflections at the metal-superconductor interfaces, and the normal metal cross acts as an interferometer that is sensitive to the superconducting macroscopic phase from which the quantum state of the qubit can be measured. Due to the weak coupling between the normal electrons in the interferometer and the Cooper pairs in the superconductor leads, the Andreev probe is expected to have little or no back-action, and the estimated timescales make high fidelity single-shot measurements feasible. We also aim to reduce intrinsic sources of decoherence; we will use laser ablation to improve fabrication techniques, and we will investigate new materials for fabricating superconductor qubits. The culmination of our proposed research will be simultaneous dynamical measurements of coupled superconducting qubits and determination of their quantum entanglement, basic to implementation of a universal quantum gate, and a necessary step towards QIP.