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Conference on Lasers and Electro-Optics

Optomechanical sensor at cryogenic temperatures

G. Breutebbach, T. Kalkbrenner, Ilkka Juhani Tittonen, J. Mlynek, Stephan Schiller

Published Nov 27, 2002• 1 citation
Publisher

Abstract

Summary form only given.We present an optomechanical system consisting of a Fabry-Perot cavity with a movable mirror, which has the essential features to allow the observation of quantum backaction in the measurement of a macroscopic system. The Fabry-Perot cavity consists of a rigid in-coupling mirror and a torsional oscillator fabricated from a highly polished silicon wafer. At cryogenic temperatures, the mechanical Q factor of the oscillator's torsional mode at 25 kHz exceeds 2/spl middot/10/sup 6/. The finesse of the cavity is /spl Ffr/=13,000, and a diode-pumped monolithic Nd:YAG laser is employed as laser source, with typical laser powers incident onto the cavity /spl sim/5 mW. The laser frequency is locked to the cavity, using a standard FM technique. The change in the cavity's length caused by the motion of the torsional oscillator is detected by reading out the error signal of the feedback loop at the oscillator's resonance frequency. The Brownian random motion of the oscillator at room temperature is shown. At cryogenic temperatures (4.5 K), we measured a rms displacement of /spl Delta/x=1.4/spl middot/10/sup -14/ m, in agreement with the theory of Brownian noise. We demonstrated the operation of the cryogenic high-finesse interferometric force sensor. With planned improvements of the finesse and a reduction of the laser's amplitude and phase noise, our sensor opens up the possibility of observing, for the first time, the standard quantum limit of position measurement in interferometry.

DOI
10.1109/iqec.1998.680404

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Summary form only given.We present an optomechanical system consisting of a Fabry-Perot cavity with a movable mirror, which has the essential features to allow the observation of quantum backaction in the measurement of a macroscopic system. The Fabry-Perot cavity consists of a rigid in-coupling mirror and a torsional oscillator fabricated from a highly polished silicon wafer. At cryogenic temperatures, the mechanical Q factor of the oscillator's torsional mode at 25 kHz exceeds 2/spl middot/10/sup 6/. The finesse of the cavity is /spl Ffr/=13,000, and a diode-pumped monolithic Nd:YAG laser is employed as laser source, with typical laser powers incident onto the cavity /spl sim/5 mW. The laser frequency is locked to the cavity, using a standard FM technique. The change in the cavity's length caused by the motion of the torsional oscillator is detected by reading out the error signal of the feedback loop at the oscillator's resonance frequency. The Brownian random motion of the oscillator at room temperature is shown. At cryogenic temperatures (4.5 K), we measured a rms displacement of /spl Delta/x=1.4/spl middot/10/sup -14/ m, in agreement with the theory of Brownian noise. We demonstrated the operation of the cryogenic high-finesse interferometric force sensor. With planned improvements of the finesse and a reduction of the laser's amplitude and phase noise, our sensor opens up the possibility of observing, for the first time, the standard quantum limit of position measurement in interferometry.
An optomechanical sensor suitable for the study of quantum effects has been developed and characterized. The sensor reads out the vibrations of a microfabricated miniature silicon mechanical oscillator which forms one end mirror of a high finesse Fabry-P\'erot cavity. The mechanical quality factor is up to Q=300000Q=300000Q=300000 at 300 K and rises up to $Q=4\ifmmode\times\else\texttimes\fi{}{10}^{6}$ at 4 K. The thermal noise of the oscillator has been measured in the time and frequency domains at room temperature and at 4.5 K. The prospects for observing the standard quantum limit are discussed.
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.
Cavity Optomechanics, that is the study of the interaction between an optical cavity mode and a mechanical degree of freedom, has known impressive evolution over the past decade, to become a new field at the union of condensed matter physics and optics. One of the major goals of this discipline is to test and study quantum mechanics using macroscopic systems. Among the most fundamental problems the community aims to address is the question of the quantum limits in position measurement. Quantum mechanics predicts that any measurement comes along with a backaction, which perturbs the state of the measured system. Moreover, it is expected to be conjugated with the quantum noise of the measurement apparatus (called measurement noise) used to probe the system. The optimal sensitivity is reached whenever both the measurement and the backaction noise are identical, a situation which can be assimilated to the acceptance of Heisenberg's inequality for the measurement apparatus. In cavity optomechanics, the mode of an optical cavity is used as a measurement apparatus of the position of a mechanical resonator which is expected to be responsible for the backaction imprecision. However, this so-called radiation pressure quantum backaction has never been observed to date, while it remains a decisive step towards understanding quantum measurement processes. We describe in this manuscript the study of radiation pressure effects in cavity optomechanics. We introduce the optomechanical system we have developed, which consists in a cm-scale ultra high Q (~ 1 000 000) plano-convex mechanical resonator incorporated into a ultra-high finesse (~ 300 000) Fabry-Prot cavity. We present two important results we obtained with this system. First, we were able to report the first direct observation of radiation pressure in real-time, based on establishing pump-probe correlations. We were also able to demonstrate for the first time nonlinear backaction effects related to substantial improvement of position measurement sensitivity. We explain why demonstrating quantum backaction requires ultra-high stability of the optical mode. We present important changes made to the previous experimental setup, notably on the laser source, on the detection and the stabilization of the experiment. We then describe a new optomechanical detection technique providing an independent measurement of the cavity detuning. Finally, we present a proof-of-principle experiment allowing to extract quantum optomechanical correlations at room temperature.
) mechanical modes in a phononic-bandgap shielded membrane resonator. An optical mode of a compact Fabry-Perot resonator detects these modes' motion with a measurement rate (96 kHz) that exceeds the mechanical decoherence rates already at moderate cryogenic temperatures (10 K). Reaching this quantum regime entails, inter alia, quantum measurement backaction exceeding thermal forces and thus strong optomechanical quantum correlations. In particular, we observe ponderomotive squeezing of the output light mediated by a multitude of mechanical resonator modes, with quantum noise suppression up to -2.4 dB (-3.6 dB if corrected for detection losses) and bandwidths ≲90 kHz. The multimode nature of the membrane and Fabry-Perot resonators will allow multimode entanglement involving electromagnetic, mechanical, and spin degrees of freedom.
We have developed an experiment of ultrasensitive interferometric measurement of small displacements based on a high-finesse Fabry-Perot cavity. We have observed the internal thermal noise of mirrors and fully characterized their acoustics modes with a sensitivity of 3 × 10 −20 m/Hz 1/2 . This unique sensitivity is a step towards the first observation of the radiation pressure effects and the resulting standard quantum limit in interferometric measurements. Our experiment may become a powerful facility to test quantum noise reduction schemes such as the use of squeezed light or quantum locking of mirrors. As a first result, we present the observation of a cancellation of radiation pressure effects in our cavity. This back-action noise cancellation was first proposed within the framework of gravitational-wave detection with dual resonators, and may drastically improve the sensitivity of such measurements.
In order to observe quantum radiation pressure noise and reduce it by measuring the ponderomotively squeezed light on a table-top experiment, we are developing a laser interferometer with Fabry-Perot cavities with very small suspended mirrors. As a preliminary setup, we have constructed a Fabry-Perot cavity of finesse 1300 with a suspended mirror of 20 mg. The cavity was locked stably at low laser power for which the classical radiation pressure caused little effect on the dynamics of the small mirror. For the stable operation of this cavity with higher laser power, a technique to control the motion of the small mirror, especially its yaw motion, is necessary. We describe that the motion can be stabilized through the radiation pressure of light inside the cavity, by controlling the motion of the front mirror of a Fabry-Perot cavity properly.