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Improving Broadband Displacement Detection with Quantum Correlations

N. S. Kampel, R. W. Peterson, R. Fischer, P.-L. Yu, K. Cicak, R. W. Simmonds, K. W. Lehnert, and C. A. Regal
Phys. Rev. X 7, 021008 – Published 18 April 2017
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Abstract

Interferometers enable ultrasensitive measurement in a wide array of applications from gravitational wave searches to force microscopes. The role of quantum mechanics in the metrological limits of interferometers has a rich history, and a large number of techniques to surpass conventional limits have been proposed. In a typical measurement configuration, the trade-off between the probe’s shot noise (imprecision) and its quantum backaction results in what is known as the standard quantum limit (SQL). In this work, we investigate how quantum correlations accessed by modifying the readout of the interferometer can access physics beyond the SQL and improve displacement sensitivity. Specifically, we use an optical cavity to probe the motion of a silicon nitride membrane off mechanical resonance, as one would do in a broadband displacement or force measurement, and observe sensitivity better than the SQL dictates for our quantum efficiency. Our measurement illustrates the core idea behind a technique known as variational readout, in which the optical readout quadrature is changed as a function of frequency to improve broadband displacement detection. And, more generally, our result is a salient example of how correlations can aid sensing in the presence of backaction.

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  • Received 3 October 2016

DOI:https://doi.org/10.1103/PhysRevX.7.021008

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic, Molecular & Optical

Authors & Affiliations

N. S. Kampel1,*, R. W. Peterson1, R. Fischer1, P.-L. Yu1,†, K. Cicak2, R. W. Simmonds2, K. W. Lehnert1, and C. A. Regal1,‡

  • 1JILA, University of Colorado and National Institute of Standards and Technology, and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
  • 2National Institute of Standards and Technology, Boulder, Colorado 80305, USA

  • *nir.kampel@jila.colorado.edu
  • Current address: School of Electrical and Computer Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907, USA.
  • regal@jila.colorado.edu

Popular Summary

A wide range of devices, from atomic clocks to the interferometers used to detect gravitational waves, require measurements so precise that quantum effects must be taken into account. In the case of continuous displacement measurements, one key problem is to consider the consequences of quantum backaction, where random kicks from photons jiggle the measurement device (e.g., the interferometer mirrors). This imposes an apparent limit, known as the standard quantum limit (SQL), on the precision of the measurement. Many techniques show promise for advancing beyond the SQL, but progress remains elusive. Our experimental investigation focuses on probing displacements or forces that oscillate at a range of frequencies, a requirement for many types of measurements.

In our model system, we probe the motion of a silicon nitride membrane using an optical cavity with a modified readout configuration. Light from the cavity carries with it information about the motion, and this light beam is compared to another overlapping beam to extract the displacement information. Through the right readout configuration, we are able to show sensitivity better than the SQL would dictate for our quantum efficiency. Our technique illustrates the core idea behind a method known as variational readout, in which the optical readout quadrature (how the beams are compared) is changed as a function of frequency.

Optomechanical systems continue to elucidate long-standing questions in quantum metrology. Improved quantum efficiency of detection will enable even greater measurement precision with this technique.

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Vol. 7, Iss. 2 — April - June 2017

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