Quantum nondemolition measurements of a flux qubit coupled to a noisy detector

D. V. Averin

Wei Jiang, Yang Yu, Lian-Fu Wei

We theoretically study the quantum nondemolition measurements of a flux qubit coupled to a noisy superconducting quantum interference device (SQUID). The obtained analytical results indicate that the measurement probability is frequency-dependent in a short time scale and has a close relationship with the measurement-induced dephasing. Furthermore, when the detuning between the driven and bare resonator equals the coupling strength, we can obtain the maximum measurement rate that is determined by the character of the noise in the SQUID. Finally, we analysed the mixed effect caused by coupling between the non-diagonal term and the external variable. It is found that the initial information of the qubit is destroyed due to quantum tunneling between the qubit states.

D. V. Averin

Kohji Takashima, Munehiro Nishida, Shigemasa Matsuo +1 more

Quantum nondemolition (QND) measurements allow us to measure an observable of a quantum system without introducing a back‐action on this observable due to the measurement itself. Here we propose a method for the QND measurement of a superconducting flux qubit by extending the spin QND measurement. Under an adequate condition, qubit and interaction Hamiltonian satisfy QND conditions and then QND measurement of a superconducting flux qubit is possible.

D. V. Averin

The concept of quantum nondemolition (QND) measurement is extended to coherent oscillations in an individual two-state system. Such a measurement enables direct observation of an intrinsic spectrum of these oscillations avoiding the detector-induced dephasing that affects the standard (non-QND) measurements. The suggested scheme can be realized in Josephson-junction qubits which combine flux and charge dynamics.

Gerard J Milburn, Daniel F. Walls