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Conference on Lasers and Electro-Optics

Optomechanical Quantum Thermometry

Thomas Purdy, Robinjeet Singh, Nikolai N. Klimov, Zeeshan Ahmed, Karen E. Grutter, Kartik A. Srinivasan, Jacob M. Taylor

Published Jan 1, 2018• 1 citation
Publisher

Abstract

We demonstrate quantum-backaction-noise-calibrated Brownian motion thermometry as a metrological application of quantum optomechanics. We present recent work as well as on-going efforts to enhance the bandwidth, sensitivity, and range of these devices.
DOI
10.1364/cleo_at.2018.jf2b.5

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Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâ s uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity ∼ ⁣104\sim\!10^4∼104 times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called â variational measurementâ , which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10\% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.
By shining laser light through a nanomechanical beam, we measure the beam's thermally driven vibrations and perturb its motion with optical forces at a level dictated by the Heisenberg measurement-disturbance uncertainty relation. Such quantum backaction is typically difficult to observe at room temperature where the motion driven by optical quantum intensity fluctuations is many orders of magnitude smaller than the thermal motion. We demonstrate a cross-correlation technique to distinguish optically driven motion from thermally driven motion, observing this quantum backaction signature up to room temperature. While it is often difficult to absolutely calibrate optical detection, we use the scale of the quantum correlations, which is determined by fundamental constants, to gauge the size of thermal motion, demonstrating a path towards absolute thermometry with quantum mechanically calibrated ticks.
Abstract Cavity optomechanics has achieved the major breakthrough of the preparation and observation of macroscopic mechanical oscillators in non-classical states. The development of reliable indicators of the oscillator properties in these conditions is important also for applications to quantum technologies. We compare two procedures to infer the oscillator occupation number, minimizing the necessity of system calibrations. The former starts from homodyne spectra, the latter is based on the measurement of the motional sideband asymmetry in heterodyne spectra. Moreover, we describe and discuss a method to control the cavity detuning, that is a crucial parameter for the accuracy of the latter, intrinsically superior procedure.
Detailed understanding of physical measurements is essential for devising efficient metrological strategies and measurement-feedback schemes, as well as finding fundamental limitations on measurement sensitivity. In the quantum regime, measurements modify the state of the system of interest through measurement backaction as a direct consequence of the Heisenberg principle. In cavity optomechanics and electromechanics, a plethora of strategies exist for measuring the mechanical motion using electromagnetic fields, each leading to different competition between measurement imprecision and backaction. While this range of techniques allows broad applications of optomechanical and electromechanical devices, it makes direct comparison of different measurement methods difficult. We develop a formalism for quantifying the performance of optomechanical measurements using a few relevant figures of merit. Our approach is inspired by similar characterizations in quantum optics and quantifies the main properties of quantum measurements -- the ability to distinguish different quantum states and preservation of signal in the presence of measurement noise. We demonstrate our concept on the most common optomechanical measurements -- displacement detection, coherent quantum noise cancellation, and quantum nondemolition measurements -- and perform detailed analysis of errors in optomechanical nondemolition measurements. This newly acquired knowledge allows us to propose a strategy for quantum nondemolition measurements in levitodynamics using coherent scattering. Our results complement existing knowledge of linear optomechanical interactions and open the way to new understanding of optomechanical measurements, thus allowing also novel applications of optomechanical devices in fundamental physics and quantum technologies.
The motion of sub-atomic particles is characteristically quantum mechanical in nature. In contrast, quantum aspects of the motion of massive, many-atom objects are typically obscured by high phonon occupation and thermal noise. This contrast is now diminishing, owing to research efforts to bring the control and measurement of macroscopic motion fully into the quantum regime, using objects with masses from attograms to kilograms. This achievement will be critical for constructing force detectors sensitive to gravitational waves, verifying the correspondence principle at macroscopic scales, and realizing protocols that mechanically store and exchange quantum information. Here, we directly observe the quantization of the collective motion of an ultracold atomic ensemble by measuring the inability of a 0.6 attogram centre-of-mass mode to emit energy from its ground state. Moreover, the optical output of our strongly coupled cavity-optomechanical system contains a spectroscopic record of the energy exchanged between light and motion, directly quantifying the heat deposited by a quantum position measurement's backaction. These results, in addition to providing a method for calibrating the motion of low-occupation mechanical systems, offer new possibilities for investigating collective modes of degenerate gases and for diagnosing optomechanical measurement backaction.