A quantum memory for light is expected to play a crucial role in quantum communication protocols and distributed quantum computing. In addition to storage and buffering, a quantum memory can be used for manipulations of stored states to allow more complex quantum network operations. In this work, we demonstrate a quantum-compatible in-memory Fourier transform using a combination of two well-established quantum memory protocols: Gradient Echo Memory and Electromagnetically Induced Transparency. Our experiment is realized using an ensemble of rubidium atoms that are laser cooled in an elongated magneto-optic trap to maximize optical depth. The results of our time-frequency Fourier transform can be understood as a temporal double slit. We show that the interference between time-separated pulses depends on the relative phase and time between the pulses of light. The use of quantum memory protocols enables us to illuminate exactly where and how interference occurs between time-separated pulses. Time-frequency Fourier manipulation is a well-established technique in classical optical systems. Our combination of Fourier manipulation and quantum-compatible memory could be used to bring similar capability to quantum optical systems. (c) 2026 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Aquantummemoryforlightisexpectedtoplayacrucialroleinquantumcommunicationprotocols and distributed quantum computing. In addition to storage and buffering, a quantum memory can be used for manipulations of stored states to allow more complex quantum network operations. In this work, we demonstrate an in-memory Fourier transform using a combination of two well-established quantum memory protocols: Gradient Echo Memory and Electromagnetically Induced Transparency. Our experiment is realised using an ensemble of rubidium atoms that are laser cooled in an elongated magneto-optic trap to maximise optical depth. The results of our time-frequency Fourier transform can be understood as a temporal double slit. We show that the interference between time-separated pulses depends on the relative phase and time between the pulses of light. The use of a quantum memory enables us to illuminate exactly where and how interference occurs between time separated pulses. Time-frequency Fourier manipulation is a well established technique in classical optical systems. Our combination of Fourier manipulation and quantum-compatible memory could be used to bring similar capability to quantum optical systems.

As free-space optical systems grow in scale and complexity, troubleshooting becomes increasingly time-consuming and, in the case of remote installations, perhaps impractical. An example of a task that is often laborious is the alignment of a high-finesse optical resonator, which is highly sensitive to the mode of the input beam. In this work, we demonstrate how machine learning can be used to achieve autonomous mode-matching of a free-space optical resonator with minimal supervision. Our approach leverages sample-efficient algorithms to reduce data requirements while maintaining a simple architecture for easy deployment. The reinforcement learning scheme that we have developed shows that automation is feasible even in systems prone to drift in experimental parameters, as may well be the case in real-world applications.

Alkali-noble-gas comagnetometers have become an essential tool for tests of fundamental physics and offer a compact platform for precision gyroscopy. They are, however, limited by technical noise at low frequencies, commonly due to their limited suppression of magnetic noise. Here we investigate a new method for co-magnetometry between a single noble gas and alkali species. While similar to well-known devices using self-compensation, our scheme introduces magnetic pulses that controllably perturb the noble gas and pulsed optical pumping to polarise the alkali atoms. These applied pulses allow our scheme to measure, rather than just suppress, the effect of magnetic noise thereby offering reduced cross-talk. We show numerically that our scheme retrieves four signals (rotations and magnetic fields on two transverse axes) with similar sensitivity to a single axis device. We also present a proof-of-principle experiment based on a 87Rb-129Xe cell. Our data shows a low magnetic-rotation cross-talk of 0.2 ± 0.1μHz/pT, which is already on par with the most sensitive devices relying on self-compensation.

Optical quantum memories are essential for quantum communications and photonic quantum technologies. Ensemble optical memories based on 3-level interactions are a popular basis for implementing these memories. All such memories, however, suffer from loss due to scattering. In off-resonant 3-level interactions, such as the Raman gradient echo memory (GEM), scattering loss can be reduced by a large detuning from the intermediate state. In this work, we show how electromagnetically induced transparency adjacent to the Raman absorption line plays a crucial role in reducing scattering loss, so that maximum efficiency is in fact achieved at a moderate detuning. Furthermore, the effectiveness of the transparency, and therefore the efficiency of GEM, depends on the order in which gradients are applied to store and recall the light. We provide a theoretical analysis and show experimentally how the efficiency depends on gradient order and detuning.

Numerous observations suggest that there exist undiscovered beyond-the-Standard-Model particles and fields. Because of their unknown nature, these exotic particles and fields could interact with Standard Model particles in many different ways and assume a variety of possible configurations. Here we present an overview of the Global Network of Optical Magnetometers for Exotic physics searches (GNOME), our ongoing experimental program designed to test a wide range of exotic physics scenarios. The GNOME experiment utilizes a worldwide network of shielded atomic magnetometers (and, more recently, comagnetometers) to search for spatially and temporally correlated signals due to torques on atomic spins from exotic fields of astrophysical origin. We survey the temporal characteristics of a variety of possible signals currently under investigation such as those from topological defect dark matter (axion-like particle domain walls), axion-like particle stars, solitons of complex-valued scalar fields (Q-balls), stochastic fluctuations of bosonic dark matter fields, a solar axion-like particle halo, and bursts of ultralight bosonic fields produced by cataclysmic astrophysical events such as binary black hole mergers.

Efficient quantum memories will be an essential building block of large-scale networked quantum systems and provide a link between flying photonic qubits and atomic or quasi-atomic local quantum processors. Memory efficiencies above 50% are required to be operating above the quantum no-cloning limit. High efficiency operation necessitates tailored photon source and memory pairs with matched bandwidths. In this paper, we explore the combination of an ultralow spectral bandwidth source of single photons from cavity-enhanced spontaneous parametric down-conversion with a hot gas-ensemble atomic memory. Our rubidium vapor gradient echo memory achieves 84% ± 3% recall efficiency of single photons: a record for single photon storage in a warm vapor platform that avoids bulky and complex systems, such as high vacuum and low temperature cryogenics.

Optical systems are often subject to parametric instability caused by the delayed response of the optical field to the system dynamics. In some cases, parasitic photothermal effects aggravate the instability by adding new interaction dynamics. This may lead to the possible insurgence or amplification of parametric gain that can further destabilize the system. In this paper, we show that the photothermal properties of an optomechanical cavity can be modified to mitigate or even completely cancel optomechanical instability. By inverting the sign of the photothermal interaction to let it cooperate with radiation pressure, we achieve control of the system dynamics to be fully balanced around a stable equilibrium point. Our study provides a feedback solution for optical control and precise metrological applications, specifically in high-sensitivity resonating systems that are particularly susceptible to parasitic photothermal effects, such as our test case of a macroscopic optical levitation setup. This passive stabilization technique is beneficial for improving system performance limited by photothermal dynamics in broad areas of optics, optomechanics, photonics, and laser technologies.

Efficient quantum memories will be an essential building block of large scale networked quantum systems and provide a link between flying photonic qubits and atomic or quasi-atomic local quantum processors. To provide a path to scalability avoidance of bulky, difficult to maintain systems such as high vacuum and low temperature cryogenics is imperative. Memory efficiencies above 50% are required to be operating above the quantum no-cloning limit. Such high efficiencies have only been achieved in systems with photon sources tailored to the memory bandwidth. In this paper we explore the combination of an ultralow spectral bandwidth source of single photons from cavity-enhanced spontaneous parametric down-conversion with a gas-ensemble atomic memory. Our rubidium vapour gradient echo memory achieves 84 ± 3 % recall efficiency of single photons: a record for an always-ready, hot, and vacuum system free optical memory.

Strong nonlinear interactions between single photons have important applications in optical quantum information processing. Demonstrations of these interactions in cold atomic ensembles have largely been limited to exploiting slow light generated using electromagnetically induced transparency (EIT). However, these EIT implementations have limited achievable phase shifts due to spontaneous emission. Here, we demonstrate and characterize a scheme free from these limitations using gradient echo memory with inferred single photon phase shifts of 0.07±0.02 μrad. Excellent agreement with theoretical modelling was observed. Degradation of memory efficiency was observed for large phase shifts but strategies to overcome that are presented.

Backward Brillouin scattering in whispering-gallery-mode micro-resonators offers an exciting avenue to pursue both classical and quantum optomechanics applications. Our team—the Quantum Measurement Lab—together with our collaborators, are currently utilizing this regime and the favourable properties it affords for non- Gaussian motional state preparation of the acoustic field. In particular, the high mechanical frequencies, and low optical absorption and heating provide a promising route to overcome current hindrances within optomechanics. Three of our recent experimental results in this area include: (i) Brillouin optomechanical strong coupling, (ii) single-phonon addition or subtraction to a thermal state of the acoustic field, and (iii) performing phase-space tomography of non-Gaussian states generated by single- and multi-phonon subtraction. This SPIE presentation will cover these three results, what they enable, and the broader direction of our lab including the prospects of this platform for quantum-memory applications.

Photothermal effects can alter the response of an optical cavity, for example, by inducing self-locking behavior or unstable anomalies. The consequences of these effects are often regarded as parasitic and generally cause limited operational performance of the cavity. Despite their importance, however, photothermal parameters are usually hard to characterize precisely. In this work we use an optical cavity strongly coupled to photothermal effects to experimentally observe an optical back-action on the photothermal relaxation rate. This effect, reminiscent of the radiation-pressure-induced optical spring effect in cavity optomechanical systems, uses optical detuning as a fine control to change the photothermal relaxation process. The photothermal relaxation rate of the system can be accordingly modified by more than an order of magnitude. This approach offers an opportunity to obtain precise in-situ estimations of the parameters of the cavity, in a way that is compatible with a wide range of optical resonator platforms. Through this back-action effect we are able to determine the natural photothermal relaxation rate and the effective thermal conductivity of the cavity mirrors with unprecedented resolution.

Results are reported from the first full-scale search for transient signals from exotic fields of astrophysical origin using data from a newly constructed Earth-scale detector: the Global Network of Optical Magnetometers for Exotic physics searches (GNOME). Data collected by the GNOME consist of correlated measurements from optical atomic magnetometers located in laboratories all over the world. GNOME data are searched for patterns of signals propagating through the network consistent with exotic fields composed of ultralight bosons such as axion-like particles (ALPs). Analysis of data from a continuous month-long operation of the GNOME finds no statistically significant signals consistent with those expected due to encounters with topological defects (axion domain walls), placing new experimental constraints on such dark matter scenarios.

Quantum optical measurement techniques offer a rich avenue for quantum control of mechanical oscillators via cavity optomechanics. In particular, a powerful yet little explored combination utilizes optical measurements to perform heralded non-Gaussian mechanical state preparation followed by tomography to determine the mechanical phase-space distribution. Here, we experimentally perform heralded single-phonon and multiphonon subtraction via photon counting to a laser-cooled mechanical thermal state with a Brillouin optomechanical system at room temperature and use optical heterodyne detection to measure the s-parametrized Wigner distribution of the non-Gaussian mechanical states generated. The techniques developed here advance the state of the art for optics-based tomography of mechanical states and will be useful for a broad range of applied and fundamental studies that utilize mechanical quantum-state engineering and tomography.

We experimentally explore backward Brillouin scattering with high-frequency acoustic fields for optomechanics applications. We (i) demonstrate strong coupling between the optical and acoustic fields and (ii) perform single-phonon addition and subtraction operations.

Adding or subtracting a single quantum of excitation to a thermal state of a bosonic system has the counter-intuitive effect of approximately doubling its mean occupation. We perform the first experimental demonstration of this effect outside optics by implementing single-phonon addition and subtraction to a thermal state of a mechanical oscillator via Brillouin optomechanics in an optical whispering-gallery microresonator. Using a detection scheme that combines single-photon counting and optical heterodyne detection, we observe this doubling of the mechanical thermal fluctuations to a high precision. The capabilities of this joint click-dyne detection scheme adds a significant new dimension for optomechanical quantum science and applications.

Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios.

Optical levitation of mechanical oscillators has been suggested as a promising way to decouple the environmental noise and increase the mechanical quality factor. Here, we investigate the dynamics of a free-standing mirror acting as the top reflector of a vertical optical cavity, designed as a testbed for a tripod cavity optical levitation setup. To reach the regime of levitation for a milligram-scale mirror, the optical intensity of the intracavity optical field approaches 3 MW cm −2 . We identify three distinct optomechanical effects: excitation of acoustic vibrations, expansion due to photothermal absorption, and partial lift-off of the mirror due to radiation pressure force. These effects are intercoupled via the intracavity optical field and induce complex system dynamics inclusive of high-order sideband generation, optical bistability, parametric amplification, and the optical spring effect. We modify the response of the mirror with active feedback control to improve the overall stability of the system.

Induced transparency is a common but remarkable effect in optics. It occurs when a strong driving field is used to render an otherwise opaque material transparent. The effect is known as electromagnetically induced transparency in atomic media and optomechanically induced transparency in systems that consist of coupled optical and mechanical resonators. In this work, we introduce the concept of photothermally induced transparency (PTIT). It happens when an optical resonator exhibits nonlinear behavior due to optical heating of the resonator or its mirrors. Similar to the established mechanisms for induced transparency, PTIT can suppress the coupling between an optical resonator and a traveling optical field. We further show that the dispersion of the resonator can be modified to exhibit slow or fast light. Because of the relatively slow thermal response, we observe the bandwidth of the PTIT to be 2π × 15.9 Hz, which theoretically suggests a group velocity of as low as 5 m/s.

We characterise the dynamics of a vertical Fabry-Pérot cavity, where the upper mirror levitates due to radiation pressure force. The system is found naturally unstable, and we study two-laser trapping/damping schemes and photothermal effects. © 2020 The Authors