High-Q resonators and quantum nondemolition measurements

J. S. Huang, L. F. Wei, C. H. Oh

We propose a high-efficiency scheme to tomographically reconstruct an unknown quantum state by using a series of quantum nondemolition (QND) measurements. The proposed QND measurements of the qubits are implemented by probing the stationary transmissions through a driven dispersively coupled resonator. It is shown that only one kind of QND measurement is sufficient to determine all the diagonal elements of the density matrix of the detected quantum state. The remaining nondiagonal elements can be similarly determined by transferring them to the diagonal locations after a series of unitary operations. Compared with the tomographic reconstructions based on the usual destructive projective measurements (wherein one such measurement can determine only one diagonal element of the density matrix), the present reconstructive approach exhibits significantly high efficiency. Specifically, our generic proposal is demonstrated by the experimental circuit quantum electrodynamics systems with a few Josephson charge qubits.

Gerard J Milburn, Daniel F. Walls

Igor Diniz, Étienne Dumur, O. Buisson +1 more

We propose a quantum nondemolition (QND) readout scheme for a superconducting artificial atom coupled to a resonator in a circuit QED architecture, for which we estimate a very high measurement fidelity without Purcell effect limitations. The device consists of two transmons coupled by a large inductance, giving rise to a diamond-shaped artificial atom with a logical qubit and an ancilla qubit interacting through a cross-Kerr-like term. The ancilla is strongly coupled to a transmission line resonator. Depending on the qubit state, the ancilla is resonantly or dispersively coupled to the resonator, leading to a large contrast in the transmitted microwave signal amplitude. This original method can be implemented with a state-of-the-art Josephson parametric amplifier, leading to QND measurements in a few tens of nanoseconds with fidelity as large as 99.9$%$.

Daniel F. Walls, Gerard J Milburn

Vladimir B. Braginsky, Farid Ya Khalili, Kip S. Thorne

Hans‐A. Bachor, M. D. Levenson, Daniel F. Walls +2 more

A quantum nondemolition (QND) measurement scheme utilizing a Kerr medium enclosed in a resonant cavity is described and analyzed theoretically. The theory demonstrates that the conditions for QND can be met in such a device. An experimental realization in the form of an optical-fiber ring resonator has been constructed and initial experimental results are presented.