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arXiv:1903.11732 · Science

Quantum Back-Action of an Individual Variable-Strength Measurement

Michael Hatridge, Shyam Shankar, Mazyar Mirrahimi, Flavius Schackert, K. Geerlings, T. Brecht, Katrina Sliwa, Baleegh Abdo, Luigi Frunzio, S. M. Girvin, R. J. Schoelkopf, Michel Devoret

Published Jan 10, 2013• 278 citations
arXivHTMLPDF

Abstract

Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depend on the quantity that is measured. In a partial measurement performed by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrated this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures was observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.

DOI
10.1126/science.1226897

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Measuring a quantum system can randomly perturb its state. The strength and nature of this back-action depend on the quantity that is measured. In a partial measurement performed by an ideal apparatus, quantum physics predicts that the system remains in a pure state whose evolution can be tracked perfectly from the measurement record. We demonstrated this property using a superconducting qubit dispersively coupled to a cavity traversed by a microwave signal. The back-action on the qubit state of a single measurement of both signal quadratures was observed and shown to produce a stochastic operation whose action is determined by the measurement result. This accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.
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We study the back action on a coupled multiple-qubit system induced by a quantum cavity probe in a non-demolition quantum measurement scheme. The exact quantum state stochastic Schrödinger equation is derived to systematically investigate the dynamics of quantum entanglement of the multiple-qubit system inside the cavity probe. Although quantum entanglement cannot be directly measured through experiments, the back action on quantum entanglement in the multi-qubit system is witnessed by applying the exact quantum state stochastic equations. Our results demonstrate that the analysis on the sensitivity of the quantum measurement should include not only the standard quantum limit of the canonical operators, but also the back action on the quantum entanglement. Our new method proposed a theoretical approach to investigate the effects of the measurement back action on the multi-qubit systems induced by the cavity probe.