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Feb 2011

May 2014

Towards observing the standard quantum limit in a macroscopic acoustic oscillator

The aim of the project is to construct an experiment eventually capable of measuring the motion of an acoustic harmonic oscillator using a parametric microwave transducer such that the precision of the measurement is limited by the intrinsic quantum noise of the macroscopic body. This limit is referred to as the Standard Quantum Limit (SQL) and is deﬁned by Heisenberg’s famous Uncertainty Principle.

The experiment is based around establishing resonant Whispering Gallery modes (WGMs) inside a suspended HEMEX Laboratories grade sapphire transducer and then establishing an acoustic resonance in the crystal as well. The crystal oscillating in its acoustic mode of vibration will change the frequency of the internal electromagnetic resonance (the WGMs) as its radius and permittivity ﬂuctuate. These frequency ﬂuctuations can be viewed as sidebands in the frequency domain on either side of the electric resonant frequency oﬀset by the mechanical resonant frequency. A shift in the WG resonance frequency due to oscillation of the transducer in its acoustic mode will manifest as a voltage peak read from our microwave readout system, which can then be converted to a displacement x(t) if the ratio dV/dx is known. This displacement will have an associated uncertainty which it is hoped will be limited by the intrinsic quantum uncertainty predicted by the Heisenberg uncertainty principle; the SQL, if all other noise sources are sufficiently suppressed.

With the need for higher precision measurements within the scientiﬁc community, obtaining displacement measurements on the order of the standard quantum limit has become a major goal. A measurement this precise has never been obtained using our proposed methodology and to do so on such a macroscopic level would be an exciting prospect not only for the aﬃrmation of fundamental theories of quantum physics but also the possible applications in gravity wave detection, quantum information technologies and hence also quantum computing.

- Muriel and Colin Ramm Medal and Scholarship in Experimental Physics
- ARC Centre of Excellence for Engineered Quantum Systems