UCL Levitated Optomechanics: Quantum control and sensing on the nanoscale

Cooling and controlling the motion of levitated nanoparticles

We create nanomechanical oscillators by levitating nanoparticles using optical and electric fields. By trapping these particles in high vacuum, they are isolated from the environment allowing them to be cooled into their quantum groundstate. To cool these oscillators we use both feedback and cavity cooling to reduce their motional temperatures to the microKelvin regime. Not only do we cool their translation motion, but we have recently cooled both their rotational motion and their translational motion as well. We are part of the Atomic, Molecular, Optical and Positron Physics group at UCL.

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Quantum mechanics in the macroscopic regime

Levitated nanomechaical oscillators are a brand new, large mass, quantum system that we are exploring. Quantum mechanics is largely untested on this scale and there is a significant worldwide push to create quantum or non-classical states of motion. This includes centre-of-mass superpositions. The relatively large mass of these quantum systems makes them the ideal test bed for investigating the quantumness of gravity in the lab.

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Levitated quantum sensors

A staggering 85% of the universe's mass is dark matter, but we don’t yet know what dark matter actually is.  Huge international efforts are now underway to find the answer to this important mystery. Our team is developing a new type of quantum sensor that uses the levitation of silica nanosphere spheres in vacuum. This sensor aims to detect the tiny collisions between the dark matter around us and the spheres. We collaborate with Chamkaur Ghag in the high energy physics (HEP) group at UCL and are part of the Cosmoparticle initiative.

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Meet the Team

  • Prof. Peter Barker

    Primary Investigator, Professor of Physics

  • Prof. Tania Monteiro

    Primary Investigator, Professor of Physics

  • Dr Markus Rademacher

    Postdoctoral Fellow

  • Julian Iacoponi

    PhD student

  • Peiyao Xiong

    PhD student

  • Felipe Almeida Da Silva

    PhD student

  • Dr Eva Kilian

    Postdoctoral Fellow

  • Holly Owens

    Research Assistant

  • Margaux Piotrowski

    Research Intern

  • Saif Almazrouei

    PhD student

Previous Members

Recent publications

2026

Feedback cooling scheme for an optically levitated oscillator with controlled crosstalk

  • Levitated optical mechanical systems have demonstrated excellent force and impulse sensitivity and are currently being developed for the creation of non-classical states of motion in these new quantum systems. An important requirement in the design of these systems is the ability to independently control and cool all three translational degrees of freedom. Here, we describe the design and implementation of a stable and robust 3D velocity feedback cooling scheme with particular emphasis on creating minimal crosstalk between the independent oscillatory modes when cooling.

    https://pubs.aip.org/aip/rsi/article/97/1/013202/3377843

2025

Review Article: Roto-translational optomechanics

  • Levitated optomechanics, the interaction between light and small levitated objects, is a new macroscopic quantum system that is being used as a testing ground for fundamental physics and for the development of sensors with exquisite sensitivity. The utility of this system, when compared to other quantum optomechanical systems, is its extreme isolation from the environment and, by the relatively few degrees of freedom that a levitated object has. While work in the field has strongly focused on the three translational degrees of freedom of this system, it has become increasingly important to understand the induced rotational motion of levitated objects, particularly in optical trapping fields, but also in magnetic and electric traps. These additional three degrees of freedom, which are intrinsic to levitated systems, offer a new set of optomechanical nonlinear interactions that lead to a rich and yet largely unexplored roto-translational motion. The control and utilization of these interactions promise to extend the utility of levitated optomechanics in both fundamental studies and applications. In this review, we provide an overview of levitated optomechanics, before focusing on the roto-translational motion of optically levitated anisotropic objects. We first present a classical treatment of this induced motion, bridging the gap between classical and quantum formalisms. We describe the different types of roto-translational motion for different particle shapes via their interaction with polarized optical trapping fields. Subsequently, we provide an overview of the theoretical and experimental approaches as well as applications that have established this new field. The review concludes with an outlook of promising experiments and applications, including the creation of non-classical states of roto-translational motion, quantum-limited torque sensing and particle characterization methods. https://arxiv.org/abs/2507.20905

Optical centrifuge for nanoparticles

  • We study theoretically the creation of an optical centrifuge for the controlled rotation of levitated nanorotors within an optical tweezer. The optical centrifuge's motion is simulated by rapidly rotating the linear polarization of the tightly focused optical field used to form an optical trap. We show that nanorotors, formed by anisotropic nanoparticles levitated within a trap, can be accelerated to well-defined rotational rates in excess of 100 MHz over durations of hundreds of microseconds. The initial conditions required for stable acceleration, based on optical trap properties and the anisotropic susceptibility of the nanorotor are established and confirmed by numerical simulations. We also present initial experiments that have developed tools for the rapid angular acceleration of the polarization vector of the linearly polarized beam that is required to create the centrifuge. We show that over acceleration durations in the 100𝜇⁢s range, high rotational speeds could be achieved in modest vacuum. https://journals.aps.org/prresearch/abstract/10.1103/ylw9-22jj

A Spin-Based Pathway to Testing the Quantum Nature of Gravity

  • Aaron Markowitz, Debarshi Das, Ethan Campos-Méndez, Eva Kilian, David Groswasser, Menachem Givon, Or Dobkowski, Peter Skakunenko, Maria Muretova, Yonathan Japha, Naor Levi, Omer Feldman, Damián Pitalúa-García, Jonathan MH Gosling, Ka-Di Zhu, Marco Genovese, Kia Romero-Hojjati, Ryan J Marshman, Markus Rademacher, Martine Schut, Melanie Bautista-Cruz, Qian Xiang, Stuart M Graham, James E March, William J Fairbairn, Karishma S Gokani, Joseph Aziz, Richard Howl, Run Zhou, Ryan Rizaldy, Thiago Guerreiro, Tian Zhou, Jason Twamley, Chiara Marletto, Vlatko Vedral, Jonathan Oppenheim, Mauro Paternostro, Hendrik Ulbricht, Peter F Barker, Thomas P Purdy, MV Dutt, Andrew A Geraci, David C Moore, Gavin W Morley

    A key open problem in physics is the correct way to combine gravity (described by general relativity) with everything else (described by quantum mechanics). This problem suggests that general relativity and possibly also quantum mechanics need fundamental corrections. Most physicists expect that gravity should be quantum in character, but gravity is fundamentally different to the other forces because it alone is described by spacetime geometry. Experiments are needed to test whether gravity, and hence space-time, is quantum or classical. We propose an experiment to test the quantum nature of gravity by checking whether gravity can entangle two micron-sized crystals. A pathway to this is to create macroscopic quantum superpositions of each crystal first using embedded spins and Stern-Gerlach forces. These crystals could be nanodiamonds containing nitrogen-vacancy (NV) centres. The spins can subsequently be measured to witness the gravitationally generated entanglement. This is based on extensive theoretical feasibility studies and experimental progress in quantum technology. The eventual experiment will require a medium-sized consortium with excellent suppression of decoherence including vibrations and gravitational noise. In this white paper, we review the progress and plans towards realizing this. While implementing these plans, we will further explore the most macroscopic superpositions that are possible, which will test theories that predict a limit to this. https://arxiv.org/abs/2509.01586

Searching for Ultralight Dark Matter with MOLeQuTE: a Massive Optically Levitated Quantum Tabletop Experiment

  • Many well theoretically motivated models of ultralight dark matter are expected to give rise to feeble oscillatory forces on macroscopic objects. Optically trapped sensors have high force sensitivities but have remained relatively unexplored in this context. In this work we propose a new, tunable, optically trapped sensor specifically designed to detect such forces. Our design features a high-mass (∼ mg) plate whose weight is supported by a vertical beam. We present the first systematic analysis and optimisation of quantum noises in optically trapped systems and show that our setup has the potential to operate at the standard quantum limit with current off-the-shelf technologies. We demonstrate that our sensor could offer unique access to large regions of uncharted parameter space of vector B-L dark matter, with projected sensitivities that could advance existing limits by several orders of magnitude over a broad range of frequencies. https://arxiv.org/abs/2512.00166

Levitated optomechanics with cylindrically polarized vortex beams

  • Optically levitated and cooled nanoparticles are a new quantum system whose application to the creation of non-classical states of motion and quantum limited sensing is fundamentally limited by recoil and bulk heating. We study the creation of stable 3D optical traps using optical cylindrically polarized vortex beams with radial and azimuthal polarization and show that a significant reduction in recoil heating by up to an order of magnitude can be achieved when compared with conventional single Gaussian beam tweezers. Additionally these beams allow trapping of larger particles outside the Rayleigh regime using both bright and dark tweezer trapping with reduced recoil heating. By changing the wavelength of the trapping laser, or the size of the particles, non-linear and repulsive potentials of interest for the creation of non-classical states of motion can also be created. https://arxiv.org/abs/2510.05384

2024

Optimal superpositions for particle detection via quantum phase

  • Exploiting quantum mechanics for sensing offers unprecedented possibilities. State-of-the-art proposals for novel quantum sensors often rely on the creation of large superpositions and generally detect a field. However, what is the optimal superposition size for detecting an incident particle from a specific direction? This question is nontrivial as, in general, this incident particle will scatter off with varied momenta, imparting varied recoils to the sensor, resulting in decoherence rather than a well-defined measurable phase. By considering scattering interactions of directional particulate environments with a system in a quantum superposition, we find that there is an optimal superposition size for measuring particles via a relative phase. As a consequence of the anisotropy of the environment, we observe a feature in the limiting behavior of the real and imaginary parts of the system’s density matrix, linking the optimality of the superposition size to the wavelength of the scatterer. https://doi.org/10.1103/PhysRevResearch.6.023037

A levitated atom-nanosphere hybrid quantum system

  • Near-field, radially symmetric optical potentials supported by a levitated nanosphere can be used for sympathetic cooling and for creating a bound nanosphere-atom system analogous to a large molecule. We demonstrate that the long range, Coulomb-like potential produced by a single blue detuned field increases the collisional cross-section by eight orders of magnitude, allowing fast sympathetic cooling of a trapped nanosphere to microKelvin temperatures using cold atoms. By using two optical fields to create a combination of repulsive and attractive potentials, we demonstrate that a cold atom can be bound to a nanosphere creating a new levitated hybrid quantum system suitable for exploring quantum mechanics with massive particles. https://doi.org/10.1088/1367-2630/ad19f6

Dark Matter Searches with Levitated Sensors

  • Motivated by the current interest in employing quantum sensors on Earth and in space to conduct searches for new physics, we provide a perspective on the suitability of large-mass levitated optomechanical systems for observing dark matter signatures. We discuss conservative approaches of recoil detection through spectral analysis of coherently scattered light, enhancements of directional effects due to crosscorrelation spectral densities, and the possibility of using quantum superpositions of mesoscopic test particles to measure rare events. https://doi.org/10.1116/5.0200916

Sensing directional noise baths in levitated optomechanics

  • Optomechanical devices are being harnessed as sensors of ultraweak forces for applications ranging from inertial sensing to the search for the elusive dark matter. For the latter, there is a focus on detection of either higher energy single recoils or ultralight, narrow-band sources; a directional signal is expected. However, the possibility of searching for a stochastic stream of weak impulses, or more generally a directional broadband signal, need not be excluded; with this and other applications in mind, we apply Gaussian white noise impulses with a well defined direction to a levitated nanosphere trapped and 3D cooled in an optical tweezer. We find that cross-correlation power spectra offer a calibration-free distinctive signature of the presence of a directional broadband force and its orientation quadrant, unlike normal power spectral densities (PSDs). We obtain excellent agreement between theoretical and experimental results. With calibration we are able to measure the angle , akin to a force compass in a plane. We discuss prospects for extending this technique into the quantum regime and compare the expected behavior of quantum baths and classical baths. https://doi.org/10.1103/PhysRevResearch.6.013129

Levitodynamic spectroscopy for single nanoparticle characterization

  • Fast detection and characterization of single nanoparticles such as viruses, airborne aerosols and colloidal particles are considered to be particularly important for medical applications, material science and atmospheric physics. In particular, non-intrusive optical characterization, which can be carried out in isolation from other particles, and without the deleterious effects of a substrate or solvent, is seen to be particularly important. Optical characterization via the scattering of light does not require complicated sample preparation and can in principle be carried out in-situ. We describe the characterization of single nanoparticle shape based on the measurement of their rotational and oscillatory motion when optically levitated within vacuum. Using colloidally grown yttrium lithium fluoride nanocrystals of different sizes, trapped in a single-beam optical tweezer, we demonstrate the utility of this method which is in good agreement with simulations of the dynamics. Size differences as small as a few nanometers could be resolved using this technique offering a new optical spectroscopic tool for non-contact characterization of single nanoparticles in the absence of a substrate.
    https://doi.org/10.48550/arXiv.2401.11551

2023

Controlling mode orientations and frequencies in levitated cavity optomechanics

  • Cavity optomechanics offers quantum ground state cooling, control and measurement of small mechanical oscillators. However optomechanical backactions disturb the oscillator motions: they shift mechanical frequencies and, for a levitated oscillator, rotate the spatial orientation of the mechanical modes. This introduces added imprecisions when sensing the orientation of an external force. For a nanoparticle trapped in a tweezer in a cavity populated only by coherently scattered (CS) photons, we investigate experimentally mode orientation, via the Sxy (ω) mechanical cross-correlation spectra, as a function of the nanoparticle position in the cavity standing wave. We show that the CS field rotates the mechanical modes in the opposite direction to the cavity backaction, canceling the effect of the latter. It also opposes optical spring effects on the frequencies. We demonstrate a cancellation point, where it becomes possible to lock the modes near their unperturbed orientations and frequencies, independent of key experimental parameters, while retaining strong light-matter couplings that permit ground state cooling. This opens the way to sensing of directionality of very weak external forces, near quantum regimes.

Sympathetic cooling and squeezing of two colevitated nanoparticles

  • Levitated particles are an ideal tool for measuring weak forces and investigating quantum mechanics in macroscopic objects. Arrays of two or more of these particles have been suggested for improving force sensitivity and entangling macroscopic objects. In this article, two charged, silica nanoparticles, that are coupled through their mutual Coulomb repulsion, are trapped in a Paul trap, and the individual masses and charges of both particles are characterized. We demonstrate sympathetic cooling of one nanoparticle coupled via the Coulomb interaction to the second nanoparticle to which feedback cooling is directly applied. We also implement sympathetic squeezing through a similar process showing nonthermal motional states can be transferred by the Coulomb interaction. This work establishes protocols to cool and manipulate arrays of nanoparticles for sensing and minimizing the effect of optical heating in future experiments. https://doi.org/10.1103/PhysRevResearch.5.013070

Simultaneous cavity cooling of all six degrees of freedom of a levitated nanoparticle

  • Controlling the motional degrees of isolated, single nanoparticles trapped within optical fields in a high vacuum are seen as ideal candidates for exploring the limits of quantum mechanics in a new mass regime. These systems are also massive enough to be considered for future laboratory tests of the quantum nature of gravity. Recently, the translational motion of trapped particles has been cooled to microkelvin temperatures, but controlling all the observable degrees of freedom, including their orientational motion, remains an important goal. Here we report the control and cooling of all the translational and rotational degrees of freedom of a nanoparticle trapped in an optical tweezer, accomplished by cavity cooling via coherent elliptic scattering. We reached temperatures in the range of hundreds of microkelvins for the translational modes and temperatures as low as 5 mK for the librational degrees of freedom. This work brings within reach applications in quantum science and the study of single isolated nanoparticles via imaging and diffractive methods, free of interference from a substrate. https://doi.org/10.1038/s41567-023-02006-6

Measurement of the motional heating of a levitated nanoparticle by thermal light

  • We report on measurements of the photon-induced heating of silica nanospheres levitated in a vacuum by a thermal light source formed by a superluminescent diode. Heating of the nanospheres motion along the three trap axes was measured as a function of gas pressure for two particle sizes and recoil heating was shown to dominate other heating mechanisms due to relative intensity noise and beam pointing fluctuations. Heating rates were also compared with the much lower reheating of the same sphere when levitated by a laser. https://doi.org/10.1103/PhysRevA.107.013521

Sympathetic cooling and squeezing of two co-levitated nanoparticles

  • Levitated particles are an ideal tool for measuring weak forces and investigating quantum mechanics in macroscopic objects. Arrays of two or more of these particles have been suggested for improving force sensitivity and entangling macroscopic objects. In this article, two charged, silica nanoparticles, that are coupled through their mutual Coulomb repulsion, are trapped in a Paul trap, and the individual masses and charges of both particles are characterized. We demonstrate sympathetic cooling of one nanoparticle coupled via the Coulomb interaction to the second nanoparticle to which feedback cooling is directly applied. We also implement sympathetic squeezing through a similar process showing nonthermal motional states can be transferred by the Coulomb interaction.