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Sensing with optical and acoustic waves

November 1, 2021 - 11:00am
Thomas Purdy
University of Pittsburgh

In this seminar, I will discuss several recent and ongoing experimental efforts in the Purdy Lab (which specializes in quantum sensors and transducers with optical, mechanical, and microwave systems), with a focus on some often overlooked or least under-appreciated aspects of relatively simple measurements.  We have recently completed the first detailed study of acoustic blackbody radiation interacting with a nanomechanical system.  While the acoustic equivalent of the well-known electromagnetic blackbody radiation should be equally ubiquitous, there have been almost no experimental investigations.  You might look at something and say: “That is glowing red, it must be hot”, but you don’t typically turn your head, cup your ear, and getting close (but not so close as to burn yourself) mention “Wow, that is so loud, it must be really hot”.  We have constructed and studied an optomechanical resonator, whose dominant source of mechanical dissipation is the exchange of energy with a remote “acoustic blackbody” (i. e. a mechanically lossy bit of material deposited on the substrate, that efficiently absorbs and emits thermal acoustic radiation) via acoustic waves travelling through an acoustically transparent substrate.  Ultimately, the concept of optomechanically detected acoustic blackbody radiation will be incorporated with our previous experimental efforts in quantum-noise-calibrated optomechanical, Brownian-motion thermometry to create a chip-scale primary thermometer that can sense the temperature of a macroscopic volume via a nano-opto-mechanical transducer.

Another ongoing effort is to understand and eventually surpass the quantum limits of optical lever detection of a nanobeam.  The optical lever, where the angular deviation reflected light is used to measure the tilt of a surface, is arguably one of the oldest precision optical measurement techniques.  It is still in common use today in atomic force microscopy, industrial sensing applications, and precision metrology because of its simplicity, robustness, and excellent sensitivity.  However, there has been very little work to understand the fundamental quantum limits of this measurement technique.  Due to recent advances in ultralow dissipation nanomechanical systems, it has now become experimentally relevant to consider and overcome such limits.  A Heisenberg measurement—disturbance uncertainty relation is enforced by the optical torque noise from photons recoiling off a surface into different spatial modes.  By a simple rearrangement of the lenses in our optical lever detection system, we find that the effects of optical backaction can be evaded by exploiting optomechanically induced optical correlations.  We have demonstrated this effect to cancel classical laser noise and are working to upgrade our system to perform optical lever detection below the standard quantum limit.  Time permitting, I will also briefly discuss other ongoing efforts in the lab to transduce quantum signals between the mechanical, optical, and microwave domains for applications in quantum information.

Joint Quantum Institute Seminars take place live each Monday during Fall and Spring Semesters, 11:00 a.m. - 12:00 p.m. Eastern Time, in Room 2400 of the Atlantic Building. University of Maryland affiliates may participate using Zoom. The Seminars are also simulcast world-wide on the Joint Quantum Institute YouTube channel,, which supports audience participation in the chat interface.


ATL 2400