Quantum Measurements of Charge and Flux Qubits

Abstract

Quantum state engineering, i.e., active control over the coherent dynamics of suitable quantum systems has become a fascinating prospect of modern physics. Low-capacitance Josephson junctions offer a particularly promising way to realize quantum bits for quantum information processing since they can be embedded in electronic circuits and scaled up to large numbers of qubits. Due to the intrinsic coherence of the superconducting state long phase coherence times can be achieved. Two kinds of devices have been proposed, which use either charge or flux (phase) macroscopic quantum degree of freedom. Single- and two-qubit quantum manipulations can be controlled by gate voltages or magnetic fields, by methods established for single-charge devices or the SQUID technology, respectively. In flux qubit devices an important milestone, the observation of superpositions of different flux states in the system eigenstates, has been achieved. [6, 19] In charge qubits even coherent oscillations between the eigenstates have been demonstrated in the time domain. [16] Further work concentrated on understanding the decoherence mechanisms in these devices and on the design of Josephson circuits that diminish the dephasing effect of the environment, enable manipulations of many qubits and further release requirements on the circuit parameters.