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Optical and Quantum Sensing and Precision Metrology II

Quantum limitations and back action evading measurements in classical force and rotation detection

Andrey B. Matsko, Sergey P. Vyatchanin, Anatoliy A. Savchenkov, Skip Williams

Published Mar 1, 2019• 1 citation
Publisher

Abstract

We theoretically study both the technical and the fundamental quantum limitations of the sensitivity of a resonant optical gyroscope based on a high finesse optical cavity. We show that the quantum back action associated with the resonantly enhanced optical cross and self-phase modulation results in the nonlinear optics-mediated standard quantum limit (SQL) of the angle random walk of the gyroscope. We also found that the measurement sensitivity of a generic optical gyroscope is fundamentally limited due to the opto-mechanical properties of the device. Ponderomotive action of the light interrogating the gyroscope cavity leads to the opto-mechanical SQL of the rotation angle detection. The uncorrelated quantum fluctuations of power of clockwise and counterclockwise light waves result in optical power-dependent uncertainty of the angular gyroscope position.

DOI
10.1117/12.2515797

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We theoretically investigate quantum measurement noise in a hybrid optomechanical system, focusing on radiation pressure back action and its impact on force sensing. The setup consists of an optomechanical cavity with a movable mirror, a fixed semi transparent mirror, an ensemble of quantum dots (QD) coupled to the cavity mode, and an intracavity optical parametric amplifier (OPA). We show how the QD induced response, together with the system nonlinearity, modifies the noise spectral density and thereby improves the force measurement sensitivity. In this setup, coherent quantum noise cancellation (CQNC) can completely remove the back action noise. In addition, increasing the OPA pump gain enables sensitivity beyond the standard quantum limit (SQL) at reduced laser power. These combined effects allow weak force sensing beyond the SQL.

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We theoretically study both the technical and the fundamental quantum limitations of the sensitivity of a resonant optical gyroscope based on a high finesse optical cavity. We show that the quantum back action associated with the resonantly enhanced optical cross and self-phase modulation results in the nonlinear optics-mediated standard quantum limit (SQL) of the angle random walk of the gyroscope. We also found that the measurement sensitivity of a generic optical gyroscope is fundamentally limited due to the opto-mechanical properties of the device. Ponderomotive action of the light interrogating the gyroscope cavity leads to the opto-mechanical SQL of the rotation angle detection. The uncorrelated quantum fluctuations of power of clockwise and counterclockwise light waves result in optical power-dependent uncertainty of the angular gyroscope position.
We found that the measurement sensitivity of an optical integrating gyroscope is fundamentally limited due to ponderomotive action of the light leading to the standard quantum limit of the rotation angle detection. The uncorrelated quantum fluctuations of power of clockwise and counterclockwise electromagnetic waves result in optical power-dependent uncertainty of the angular gyroscope position. We also show that, on the other hand, a quantum back action evading measurement of angular momentum of a gyroscope becomes feasible if proper measurement strategy is selected. The angle is perturbed in this case. This observation hints on fundamental inequivalency of integrating and rate gyroscopes.
We find that the measurement sensitivity of an optical integrating gyroscope is fundamentally limited due to ponderomotive action of the light leading to the standard quantum limit of the rotation angle detection. The uncorrelated quantum fluctuations of power of clockwise and counterclockwise electromagnetic waves result in optical power-dependent uncertainty of the angular gyroscope position. We also show that, on the other hand, a quantum backaction evading measurement of the angular momentum of a gyroscope becomes feasible if the proper measurement strategy is selected. The angle is perturbed in this case. This observation hints at the fundamental inequivalence of integrating and rate gyroscopes.
The Standard Quantum Limit (SQL) in classical force detection using mechanical systems arises from quantum back-action, which disturbs the oscillator’s evolution during measurement. We present a theoretical analysis of a scheme that enables broadband back-action evading (BAE) force sensing by employing a multidimensional optical transducer. The proposed system consists of a mechanical oscillator coupled to an optical structure supporting three distinct optical modes, with mode spacing approximately matched to the mechanical resonance. The central mode is coherently pumped, and the signal is extracted by monitoring the light emitted from the adjacent sideband modes. By selecting appropriate optical quadratures for detection and applying tailored post-processing to the measurement data, our approach suppresses back-action noise across a wide frequency range, enabling sensitivity beyond the SQL. The architecture can be interpreted as a composite system hosting quantum-mechanics-free subsystems (QMFS), which effectively isolate the measured observable from quantum fluctuations by exploiting correlations akin to those found in systems with engineered “negative mass” response. We also show that the adjacent sideband modes are entangled and discuss possible methods for detecting this entanglement. This configuration opens a promising route toward high-precision quantum metrology without being constrained by quantum measurement noise.
The high-sensitivity quantum detection of a resonant classical force acting on a quantum harmonic oscillator can be significantly improved by employing a resonant optical parametric transducer. This technique leverages the unique properties of parametric interactions to enhance the response of the measurement system while maintaining quantum-limited performance. We demonstrate that such a configuration not only enables quantum back-action evading (BAE) measurements, surpassing the Standard Quantum Limit (SQL) of sensitivity, it also provides a mechanism for noiseless amplification of the classical signal. The key to this enhancement lies in the ability of the parametric transducer to generate and separately measure the upper and lower modulation sidebands induced by the external force. By decoupling and analyzing these sidebands independently, the system becomes capable of amplifying the signal without introducing additional noise, effectively circumventing quantum measurement constraints.
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.
We theoretically investigate quantum measurement noise in a hybrid optomechanical system, focusing on radiation pressure back action and its impact on force sensing. The setup consists of an optomechanical cavity with a movable mirror, a fixed semi transparent mirror, an ensemble of quantum dots (QD) coupled to the cavity mode, and an intracavity optical parametric amplifier (OPA). We show how the QD induced response, together with the system nonlinearity, modifies the noise spectral density and thereby improves the force measurement sensitivity. In this setup, coherent quantum noise cancellation (CQNC) can completely remove the back action noise. In addition, increasing the OPA pump gain enables sensitivity beyond the standard quantum limit (SQL) at reduced laser power. These combined effects allow weak force sensing beyond the SQL.