Interferometers enable ultrasensitive measurement in a wide array of applications from gravitational wave searches to force microscopes. The role of quantum mechanics in the metrological limits of interferometers has a rich history, and a large number of techniques to surpass conventional limits have been proposed. In a typical measurement configuration, the trade-off between the probe’s shot noise (imprecision) and its quantum backaction results in what is known as the standard quantum limit (SQL). In this work, we investigate how quantum correlations accessed by modifying the readout of the interferometer can access physics beyond the SQL and improve displacement sensitivity. Specifically, we use an optical cavity to probe the motion of a silicon nitride membrane off mechanical resonance, as one would do in a broadband displacement or force measurement, and observe sensitivity better than the SQL dictates for our quantum efficiency. Our measurement illustrates the core idea behind a technique known as variational readout, in which the optical readout quadrature is changed as a function of frequency to improve broadband displacement detection. And, more generally, our result is a salient example of how correlations can aid sensing in the presence of backaction.
Cavity optomechanical systems have emerged as a promising platform for quantum sensing. Quantum mechanics imposes a standard quantum limit (SQL) on the force-sensitivity for the standard homodyne phase quadrature measurement of the cavity's output field. In this paper, we investigate ways to enhance weak force sensitivity beyond SQL by employing a variational homodyne quadrature readout and quantum squeezing. Our study reveals a remarkable improvement in the force sensitivity of a cavity optomechanical sensor at a suitable homodyne angle, compared with standard phase quadrature detection of the cavity output field within a specific frequency band. We further demonstrate improved force sensitivity via intra-cavity squeezing (ICS) or injected external squeezing (IES) of the cavity mode. Both variational homodyne readout and quantum squeezing induce quantum correlations between the amplitude and phase quadratures of the cavity's output field, thereby improving force sensitivity. Our results suggest that IES is preferable to ICS for sub-SQL force sensing with system stability and lower probe power requirements. The squeezing-enhanced variational homodyne detection scheme can enable high-precision quantum sensing across various hybrid quantum platforms.
We propose an ultrasensitive displacement measurement scheme to overcome the standard quantum limit (SQL) in the unresolved sideband cavity optomechanical system with nonlinear optomechanical coupling and squeezed light injection. By introducing the optimized quantum correlation, which is enabled by suitable choices of the squeezing angle, squeezing level, power of the probe light, and measurement angle of homodyne detection, the off-resonant displacement sensitivity reaches 6 dB below the SQL in linear optomechanical coupling. In contrast, displacement sensitivities with a coherent probe plus variational readout and squeezed probe plus fixed measurement angle (phase quadrature) are 2.6 dB and 4.6 dB below the SQL, respectively. By combining linear and quadratic optomechanical coupling, we show that the displacement sensitivity can be further improved to 9.6 dB below the SQL. Our results have potential applications in gravitational-wave detectors, quantum metrology, and the search for dark matter.
Quantum noise limits high-precision sensing, such as in gravitational wave detectors. Caves first proposed beating this shot-noise limit by injecting squeezed vacuum at the dark port of an interferometer. Frequency-dependent squeezing through a filter cavity now delivers broadband quantum noise reductions of up to 5.2 dB at Hanford and 6.1 dB at Livingston in the LIGO detectors' fourth observing run. Building on these advances, this thesis develops techniques for interferometric sensing under realistic loss and phase noise, spanning Gaussian state preparation, optomechanical readout and feedback control, and non-Gaussian state preparation. In Parts I and II, we work in the two-photon formalism, where states are Gaussian and dynamics are linear. In Part I, we present a waveguide squeezer as an alternative to the traditional bulk cavity-based source; it is single-pass, broadband, and compact, and its modularity opens a path to non-Gaussian state generation. We report the detection of squeezed light from waveguide sources at 1064 nm in the sub-MHz regime, and we phase-stabilize the squeezed vacuum with quantum noise locking. We also analyze the loss and phase-noise tolerance of Gaussian states. In Part II, we design a tabletop phase-sensitive optomechanical amplifier and analyze its performance in terms of cavity stability, control system, and noise budget. We then implement the locking scheme, including Pound-Drever-Hall locking of the cavity, and characterize the noise budget at the initial stage. We also propose using optomechanics for coherent feedback control, which could evade back-action and broaden the detection bandwidth of future gravitational wave detectors. In Part III, we turn to non-Gaussian states. Phase noise does not preserve Gaussianity, so the best input state under combined loss and phase noise need not be Gaussian. We numerically optimize the input state for displacement sensing over the
(η,σϕ) landscape, integrating the Lindblad master equation in a truncated Fock basis and computing the quantum Fisher information. Within the Gaussian sub-family, the optimizer returns a displaced squeezed state once phase noise is appreciable, rather than pure squeezed vacuum. Without the Gaussian restriction, the optimized states sort roughly into four classes (Fock-like, cubic-phase-like, discrete-rotational, and squeezed), each dominant in a different region of the parameter space. In the experimentally relevant region of 1% loss and 100 mrad phase noise, cat states outperform squeezed vacuum even with balanced homodyne readout, an improvement implementable in the near term. We also show that the non-Gaussian advantage persists at higher photon budgets, well above
Nˉ=5. Together, these results show how state preparation, readout, and feedback choices trade off against realistic loss and phase noise, and identify the regimes where non-Gaussian states perform better.
Advances in precision optomechanical measurement underpin a wide range of technologies, from inertial sensors to the detection of gravitational waves and the exploration of fundamental physics. For such applications, the ultimate sensitivity is limited by quantum noise: shot noise, which stems from the stochastic arrival of photons, dominates at high frequencies, while Quantum Back-Action (QBA) noise—arising from the random radiation-pressure force exerted by the light field—prevails at low frequencies. The interplay between these two contributions defines the Standard Quantum Limit (SQL) of interferometry, a power-dependent trade-off that constrains further performance improvements. Although several sub-SQL techniques have been demonstrated, they typically enhance narrow-band sensitivity or require long filter cavities. Coherent Quantum Noise Cancellation (CQNC), first proposed by Tsang and Caves, offers a broadband solution by introducing an Effective Negative-Mass Oscillator (ENMO) that destructively interferes with the QBA noise across the entire spectrum except at the mechanical resonance of the opto-mechanically coupled system. A mathematically equivalent scheme for broadband QBA evasion has been demonstrated—for example, using atomic ensembles. The operation of this scheme is restricted to specific atomic transition wavelengths and is therefore unsuitable for many optomechanical platforms. This thesis reports the first all-optical, table-top realisation of an ENMO capable of Coherent Quantum Noise Cancellation in quantum optomechanical sensors. The scheme employs an optical analogue of the optomechanical interaction, generated through a combination of parametric down-conversion and beam-splitting processes, yielding a tunable, wavelength-flexible, and compact system. The achieved parameters satisfy the CQNC targets established in earlier theoretical work, and the current system predicts a broadband quantum-noise reduction of 3.6 dB (77% suppression of QBA noise(in dB)) at the frequency of maximum suppression. With further reduction of intracavity losses, the projected cancellation improves to 11.9 dB (81% suppression (in dB)). The experimental realisation features two polarisation-coupled optical resonators that host a two-mode squeezing interaction; the optical resonators possess distinct detunings and linewidths. The interdependent nature of the ENMO system motivated the development of an in-situ characterisation method, which is essential for integrating the ENMO with an optomechanical system in practical applications. Adequately addressing the system’s complexity required the development of five specialised experimental techniques for parameter tuning and comprehensive characterisation. Beyond quantum-noise cancellation, this system lends itself to application in quantum-information and quantum-communication tasks, including an on-demand, high-purity single-photon source and a quantum memory. Owing to the optical platform’s immunity to thermal decoherence, it can be operated at room temperature, and the parametric down-conversion process affords flexibility in wavelength selection. The results presented here advance the state of the art in broadband quantum noise reduction in a broad class of optomechanical devices, with immediate relevance for emerging quantum-technology applications.
We report an experimental demonstration of quantum-enhanced displacement measurement of a membrane in an optical cavity by using bright quantum correlated light to surpass the shot-noise limit (SNL). The optical cavity enhances the displacement signal and the twin beams suppress the noise background 3 dB below the SNL at an equivalent optical power. Thus, the signal-to-noise ratio is significantly improved by simultaneously using optical cavities and twin beams, and a sensitivity of 200 am/Hz is achieved. The frequency fluctuations of twin beams are analyzed and reduced by using optical twin cavities. These results provide a useful method for precision measurements as well as to control cavity optomechanical systems with nonclassical light.