It is well established that quantum back action (QBA) can be reduced (or even completely canceled) in composite systems by engineering so-called quantum-mechanics-free subspaces (QMFSs) of commuting variables. An important ingredient in this approach is the negative-mass oscillator, a phenomenon that arises in, e.g., polarized spin ensembles and two-tone-driven optomechanical systems. Entanglement established between a system of interest and an auxiliary negative-mass reference system allows measurements of motion, fields and forces with, in principle, unlimited precision. To date, these principles have been developed theoretically and demonstrated experimentally for a number of composite systems. However, the utility of the concept has been limited by the dominating requirement of close proximity of the resonance frequencies of the system of interest and the negative-mass reference system, and by the need to embed the subsystems in a narrowband cavity, which could be problematic while at the same time achieving good overcoupling. Here we propose a general approach which overcomes these limitations by employing periodic modulation of the driving fields (e.g., two-tone driving) in combination with coherent or measurement-based anti-noise paths. This approach makes it possible to engineer a QMFS of two systems with vastly different spectra and with arbitrary signs of their masses, while dispensing with the need to embed the subsystems in a sideband-resolving cavity. We discuss the advantages of this novel approach for applications such as QBA evasion in gravitational wave detection, force sensing, and entanglement generation between disparate systems.
It is well established that quantum back action (QBA) can be reduced (or even completely canceled) in composite systems by engineering so-called quantum-mechanics-free subspaces (QMFSs) of commuting variables. An important ingredient in this approach is the negative-mass oscillator, a phenomenon that arises in, e.g., polarized spin ensembles and two-tone-driven optomechanical systems. Entanglement established between a system of interest and an auxiliary negative-mass reference system allows measurements of motion, fields and forces with, in principle, unlimited precision. To date, these principles have been developed theoretically and demonstrated experimentally for a number of composite systems. However, the utility of the concept has been limited by the dominating requirement of close proximity of the resonance frequencies of the system of interest and the negative-mass reference system, and by the need to embed the subsystems in a narrowband cavity, which could be problematic while at the same time achieving good overcoupling. Here we propose a general approach which overcomes these limitations by employing periodic modulation of the driving fields (e.g., two-tone driving) in combination with coherent or measurement-based anti-noise paths. This approach makes it possible to engineer a QMFS of two systems with vastly different spectra and with arbitrary signs of their masses, while dispensing with the need to embed the subsystems in a sideband-resolving cavity. We discuss the advantages of this novel approach for applications such as QBA evasion in gravitational wave detection, force sensing, and entanglement generation between disparate systems.
Quantum mechanics is potentially advantageous for certain information-processing tasks, but its probabilistic nature and requirement of measurement back action often limit the precision of conventional classical information-processing devices, such as sensors and atomic clocks. Here we show that by engineering the dynamics of coupled quantum systems, it is possible to construct a subsystem that evades the measurement back action of quantum mechanics, at all times of interest, and obeys any classical dynamics, linear or nonlinear, that we choose. We call such a system a quantum-mechanics-free subsystem (QMFS). All of the observables of a QMFS are quantum-nondemolition (QND) observables; moreover, they are dynamical QND observables, thus demolishing the widely held belief that QND observables are constants of motion. QMFSs point to a new strategy for designing classical information-processing devices in regimes where quantum noise is detrimental, unifying previous approaches that employ QND observables, back-action evasion, and quantum noise cancellation. Potential applications include gravitational-wave detection, optomechanical force sensing, atomic magnetometry, and classical computing. Demonstrations of dynamical QMFSs include the generation of broad-band squeezed light for use in interferometric gravitational-wave detection, experiments using entangled atomic spin ensembles, and implementations of the quantum Toffoli gate.
Quantum mechanics dictates that a measurement without perturbation is not possible. A textbook example is the observation of the position of an object, which imposes a random back action perturbation on the momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. Here we demonstrate that the back action on an oscillator measured in a reference frame of another oscillator with an effective negative mass can be evaded in both position and momentum variables simultaneously. The mechanical oscillator is a millimeter-sized membrane and the reference negative mass oscillator is the collective spin of an atomic ensemble precessing in a magnetic field. Laser light transmitted through the hybrid system of these two disparate oscillators serves as the meter. We first observe the quantum measurement back action on each oscillator. We then demonstrate that back action at the single noise photon level is efficiently suppressed or enhanced depending on the sign of the effective mass of the reference spin oscillator. The two oscillators are separated by one meter but can be placed at a much larger distance as they are interfaced by laser light. The reference spin oscillator is insensitive to gravity and acceleration which can be efficiently detected by the mechanical oscillator in the absence of the measurement back action. The novel hybrid quantum system presented here paves the road to generation of entanglement and distant quantum communication between mechanical and spin systems and to back action free sensing of acceleration and force.
Quantum mechanics is potentially advantageous for certain information-processing tasks, but its probabilistic nature and requirement of measurement backaction often limit the precision of conventional classical information-processing devices, such as sensors and atomic clocks.Here we show that, by engineering the dynamics of coupled quantum systems, it is possible to construct a subsystem that evades the measurement backaction of quantum mechanics, at all times of interest, and obeys any classical dynamics, linear or nonlinear, that we choose.We call such a system a quantum-mechanics-free subsystem (QMFS).All of the observables of a QMFS are quantum-nondemolition (QND) observables; moreover, they are dynamical QND observables, thus demolishing the widely held belief that QND observables are constants of motion.QMFSs point to a new strategy for designing classical information-processing devices in regimes where quantum noise is detrimental, unifying previous approaches that employ QND observables, backaction evasion, and quantum noise cancellation.Potential applications include gravitational-wave detection, optomechanical-force sensing, atomic magnetometry, and classical computing.Demonstrations of dynamical QMFSs include the generation of broadband squeezed light for use in interferometric gravitational-wave detection, experiments using entangled atomic-spin ensembles, and implementations of the quantum Toffoli gate.
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.
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.
We propose that the dispersion management of coherent atomic matter waves can be exploited to overcome quantum back-action in condensate-based optomechanical sensors. The effective mass of an atomic Bose-Einstein condensate modulated by an optical lattice can become negative, resulting in a negative-frequency optomechanical oscillator, a negative environment temperature, and optomechanical properties opposite to those of a positive-mass system. This enables a quantum-mechanics-free subsystem insulated from quantum back-action.