Microwave Measurement beyond the Quantum Limit with a Nonreciprocal Amplifier

F. Lecocq, L. Ranzani, G. A. Peterson, K. Cicak, A. Metelmann, S. Kotler, R. W. Simmonds, J. D. Teufel, J. Aumentado

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14 Scopus citations


The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by loss or excess noise on the way to the detector. Even a quantum-limited detector that simultaneously measures a second noncommuting observable would double the output noise, therefore limiting the efficiency to 50%. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction-based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of 62-9+17% that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.

Original languageAmerican English
Article number044005
JournalPhysical Review Applied
Issue number4
StatePublished - Apr 2020
Externally publishedYes

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© 2020 American Physical Society. © 2020 American Physical Society. US.


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