Quantum mechanics dictates that a measurement always disturb the measured system. In weak continuous measurements, the trade-off between measurement precision and back-action onto the system yields an optimal measurement sensitivity, which is known as the Standard Quantum Limit (SQL) in opto-mechanical measurements, such as gravitational-wave detection. It corresponds to finding the optimal optical power in a compromise between quantum shot noise and quantum radiation-pressure noise. Coherent quantum-noise cancellation (CQNC) aims at overcoming the SQL and reducing back-action noise via the introduction of an effective negative-mass oscillator. In an alloptical set-up, this oscillator is realised by a detuned optical resonator coupled to incoming light with a beam-splitter and a down-conversion interaction and needs to be matched to the measured system in resonance frequency, damping and coupling strengths. This thesis explores the nature of CQNC and a potential all-optical realisation in theory and experiment, with a particular emphasis on the beam-splitter and the down-conversion interaction. Two possible set-ups are compared theoretically and critical parameters determined. Available opto-mechanical devices were characterised and confirmed to be suitable for CQNC. The down-conversion coupling strength gDC is linked to experimentally obtainable parameters. More than 2.3 dB reduction in uncertainty of two-mode squeezed light were observed. The squeezing measurements yielded gDC = 2\pi\times200 kHz at 100mW pump power, which is well within the initially required range and is in agreement with results from two other measurement methods. Optical resonators coupled via a beam-splitter interaction are studied theoretically and experimentally. In this work, the beam-splitter interaction of strength gBS was realised by a wave plate. A simplified experiment design enabled stabilisation of the coupled resonators. Our experimental observations accurately confirmed our theoretical predictions. The observed mode splitting yielded gBS = 2\pi\times235 kHz, within the updated requirements. Losses and a limited measurement strength will be the limiting factors for CQNC. The updated set of parameters, backed by the conducted experiments, paves the way towards a reduction of radiation-pressure noise of up to 4.8 dB.
Quantum mechanics dictates that a measurement always disturb the measured system. In weak continuous measurements, the trade-off between measurement precision and back-action onto the system yields an optimal measurement sensitivity, which is known as the Standard Quantum Limit (SQL) in opto-mechanical measurements, such as gravitational-wave detection. It corresponds to finding the optimal optical power in a compromise between quantum shot noise and quantum radiation-pressure noise. Coherent quantum-noise cancellation (CQNC) aims at overcoming the SQL and reducing back-action noise via the introduction of an effective negative-mass oscillator. In an alloptical set-up, this oscillator is realised by a detuned optical resonator coupled to incoming light with a beam-splitter and a down-conversion interaction and needs to be matched to the measured system in resonance frequency, damping and coupling strengths. This thesis explores the nature of CQNC and a potential all-optical realisation in theory and experiment, with a particular emphasis on the beam-splitter and the down-conversion interaction. Two possible set-ups are compared theoretically and critical parameters determined. Available opto-mechanical devices were characterised and confirmed to be suitable for CQNC. The down-conversion coupling strength gDC is linked to experimentally obtainable parameters. More than 2.3 dB reduction in uncertainty of two-mode squeezed light were observed. The squeezing measurements yielded gDC = 2\pi\times200 kHz at 100mW pump power, which is well within the initially required range and is in agreement with results from two other measurement methods. Optical resonators coupled via a beam-splitter interaction are studied theoretically and experimentally. In this work, the beam-splitter interaction of strength gBS was realised by a wave plate. A simplified experiment design enabled stabilisation of the coupled resonators. Our experimental observations accurately confirmed our theoretical predictions. The observed mode splitting yielded gBS = 2\pi\times235 kHz, within the updated requirements. Losses and a limited measurement strength will be the limiting factors for CQNC. The updated set of parameters, backed by the conducted experiments, paves the way towards a reduction of radiation-pressure noise of up to 4.8 dB.
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 the realization of an all-optical, tabletop effective-negative-mass oscillator (ENMO) scheme capable of canceling quantum noise when cascaded with an opto-mechanical sensor susceptible to (quantum) radiation pressure noise. Our coherent quantum noise cancellation (CQNC) scheme offers a broadband cancellation capability with a tunable, wavelength-flexible, and compact system. This is achieved through the implementation of an optical equivalent of an opto-mechanical interaction, facilitated by a down-conversion and a beam-splitting process. The intricate nature of the system and its multiple interacting components made characterizing the interdependent parameters with conventional methods ineffective, leading to the development of an in-situ characterization scheme. The obtained parameters meet the targets for CQNC set in previous studies. With our current realization, we project a broadband quantum noise reduction of 3.6 dB, corresponding to a 77% reduction in quantum back-action noise at the optimal frequency of maximum reduction, indicating the readiness of the ENMO for application. We discuss the prospects for new applications in quantum information and communication using the same platform.
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 semitransparent mirror, an ensemble of quantum dots (QDs) 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.
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.
We theoretically study how quantum measurement noise can be engineered in a hybrid cavitymagnomechanical platform for precision force sensing. The proposed configuration consists of a driven optomechanical cavity, with a movable mirror on one side plus a fixed semi-transparent mirror on the other side, coupled to a magnon mode, with an OPA placed inside the cavity. We show that the magnon mediated dynamics reshapes the added-noise spectrum leading to improved sensitivity compared to a conventional optomechanical sensor. In particular, by satisfying the coherent quantum noise cancellation (CQNC) criterion, radiation-pressure back-action can be fully suppressed. In addition, a larger OPA pump gain permits operation beyond the standard quantum limit at substantially reduced laser power, thereby mitigating power-related constraints without sacrificing performance. These combined advantages provide a practical pathway to below-SQL weak force detection and can outperform existing approaches based on squeezing in magnomechanics.
Coherent quantum-noise cancellation (CQNC) can be used in optomechanical sensors to surpass the standard quantum limit (SQL). In this paper, we investigate an optomechanical force sensor that uses the CQNC strategy by cascading the optomechanical system with an all-optical effective negative-mass oscillator. Specifically, we analyze matching conditions and losses and compare the two possible arrangements in which either the optomechanical or negative-mass system couples first to light. While both of these orderings yield a sub-SQL performance, we find that placing the effective negative-mass oscillator before the optomechanical sensor will always be advantageous for realistic parameters. The modular design of the cascaded scheme allows for better control of the subsystems by avoiding undesirable coupling between system components while maintaining a performance similar to the integrated configuration proposed earlier. We conclude our work with a case study of a micro-optomechanical implementation.