Quantum theory of laser noise in coherent-scattering levitated optomechanics
Davide Candoli, Andreu Riera-Campeny, Lorenzo Dania, Oscar Schmitt Kremer, Johannes Piotrowski, Jayadev Vijayan, Lukas Novotny, Martin Frimmer, Oriol Romero-Isart, Carlos Gonzalez-Ballestero
quant-phSubmitted Oct 23, 2026• Announced Oct 26, 2026• 0 citations
State purity is a key resource for experimental tests of quantum mechanics with massive objects. Cavity cooling of a levitated nanoparticle via coherent scattering stands out as a strategy to prepare highly pure states of its roto-translational motion. In several cases, though, the performance of cooling by coherent scattering is limited by the classical fluctuations of the laser field. Here we present a quantum theory of coherent-scattering experiments which includes the effect of the laser amplitude and phase noise. We derive a master equation for the noise-averaged particle's dynamics, valid for both center-of-mass and rotational degrees of freedom, and quantify the cooling performance for different levels of noise in state-of-the-art experiments. We compute approximate analytical expressions for the laser-noise-induced heating rates and excess phonon number in both regimes of weak and strong optomechanical coupling. Moreover, our model can be applied to laser noise processes with generic power spectral densities and finite correlation times. This work offers a toolbox to quantify and optimize the state purity in coherent-scattering experiments, and opens new avenues towards the preparation of quantum states with levitated systems.
State purity is a key resource for experimental tests of quantum mechanics with massive objects. Cavity cooling of a levitated nanoparticle via coherent scattering stands out as a strategy to prepare highly pure states of its roto-translational motion. In several cases, though, the performance of cooling by coherent scattering is limited by the classical fluctuations of the laser field. Here we present a quantum theory of coherent-scattering experiments which includes the effect of the laser amplitude and phase noise. We derive a master equation for the noise-averaged particle's dynamics, valid for both center-of-mass and rotational degrees of freedom, and quantify the cooling performance for different levels of noise in state-of-the-art experiments. We compute approximate analytical expressions for the laser-noise-induced heating rates and excess phonon number in both regimes of weak and strong optomechanical coupling. Moreover, our model can be applied to laser noise processes with generic power spectral densities and finite correlation times. This work offers a toolbox to quantify and optimize the state purity in coherent-scattering experiments, and opens new avenues towards the preparation of quantum states with levitated systems.
ABSTRACT Levitodynamics, i.e., the levitation of objects of mesoscopic size has made huge progress in the last decade allowing to address some questions of fundamental physics for the first time. It has become possible to cool a levitated particle of mesoscopic size down to its motional ground state and to observe its motion driven by quantum fluctuations. This defines a quantum system that is ideally suited for an analysis within Nelson's stochastic mechanics approach to quantum mechanics, which allows for a dynamic description of individual particle paths. In good approximation, a levitated nanoparticle is a quantum harmonic oscillator. The coherent states of such an oscillator are superpositions of classical paths and the stochastic ground state process. They are exact solutions not only of the Schrödinger equation but also of the quantum Hamilton equations of Nelson's stochastic mechanics, and they are amenable to path simulations. All model parameters are typically available from experiment. Analyzing the quantum motion of the levitated nanoparticle in terms of the coherent states of a quantum harmonic oscillator allows for a quantitative prediction of the experimentally measured power spectral density of its position fluctuations and predicts new experimentally accessible measures for the mean number of phonons present in the center of mass motion. It also explains the observed phase space trajectory of the nano‐particle during cooling.
Recoil heating from photon scattering is a fundamental source of decoherence in optical trapping, severely limiting the preparation of nonclassical motional states. In cavity setups in the coherent scattering configuration, a predictive theory of recoil heating rate is missing, as usual perturbative approaches fail in the presence of sharp optical resonances. Hence, current works assume that the recoil heating rate is approximately equal to its free-space value. Here we show that this is not the case, as the electromagnetic environment can strongly modify recoil heating rate by the Purcell effect. Specifically, we predict that this rate can be significantly suppressed in state-of-the-art microcavities, for both center-of-mass and librational motion. To establish these results, we develop a general theoretical framework based on macroscopic quantum electrodynamics and on the few-mode quantization approach developed in nanophotonics. Our method applies to particles trapped in the presence of arbitrary electromagnetic structures, thus providing a route to engineering motional decoherence in levitated optomechanics by photonic structure design.
We develop the theory describing the quantum coupled dynamics of the center-of-mass motion of a nanoparticle and an ensemble of ions co-trapped in a dual-frequency linear Paul trap. We first derive analytical expressions for the motional frequencies and classical trajectories of both nanoparticle and ions. We then derive a quantum master equation for the ion-nanoparticle system and quantify the sympathetic cooling of the nanoparticle motion enabled by its Coulomb coupling to a continuously Doppler-cooled ion. We predict that motional cooling down to sub-kelvin temperatures is achievable in state-of-the-art experiments even in the absence of motional feedback and in the presence of micromotion. We then extend our analysis to an ensemble of N ions, predicting a linear increase of the cooling rate as a function of N and motional cooling of the nanoparticle down to tenths of millikelvin in current experimental platforms. Our work establishes the theoretical toolbox needed to explore the ion-assisted preparation of non-Gaussian motional states of levitated nanoparticles.
We theoretically show that laser recoil heating in free-space levitated optomechanics can be arbitrarily suppressed by shining squeezed light onto an optically trapped nanoparticle. The presence of squeezing modifies the quantum electrodynamical light-matter interaction in a way that enables us to control the amount of information that the scattered light carries about a given mechanical degree of freedom. Moreover, we analyze the trade-off between measurement imprecision and back-action noise and show that optical detection beyond the standard quantum limit can be achieved. We predict that, with state-of-the-art squeezed light sources, laser recoil heating can be reduced by at least 60% by squeezing a single Gaussian mode with an appropriate incidence direction, and by 98% by squeezing a properly mode-matched mode. Our results, which are valid both for motional and librational degrees of freedom, will lead to improved feedback cooling schemes as well as boost the coherence time of optically levitated nanoparticles in the quantum regime.
Abstract Coherent scattering of photons is a novel mechanism of optomechanical coupling for optically levitated nanoparticles promising strong, versatile interactions with light and between nanoparticles. We show that it allows efficient deterministic generation of Gaussian entanglement between two particles in separate tweezers. A combination of red- and blue-detuned tweezers brings a mechanical Bogoliubov mode to its ground state. An additional, dispersively coupled cavity mode can reduce noise in the orthogonal mode, resulting in strong entanglement as quantified by the logarithmic negativity and verifiable with the Duan criterion for realistic experimental parameters. Such an important resource for quantum sensing and quantum simulations is pivotal for current experiments and presents an important step towards optomechanics with multiple particles in the quantum regime.
Quantum mechanics, since its very beginning, has totally changed the way we understand nature. The past hundred years have seen great successes in the application of quantum physics, including atomic spectra, laser technology, condensed matter physics and the remarkable possibility for quantum computing, etc. This thesis is dedicated to a small regime of quantum physics. In the first part of the thesis, I present the studies of atomic quasi-stable states, which refer to those Rydberg states of an atom that are relatively stable in the presence of strong fields. Through spectrally probing the quasi-stable states, series of survival peaks are found. If the quasi-stable electrons were created by ultraviolet (UV) lasers with two different frequencies, the survival peaks could be modulated by continuously changing the phase difference between the UV and the IR laser. The quantum simulation, through directly solving the Schrödinger equation, matches the experimental results performed with microwave fields, and our studies should provide a guidance for future experiments. Despite the huge achievements in the application of quantum theory, there are still some fundamental problems that remain unresolved. One of them is the so-called quantum-to-classical transition, which refers to the expectation that the system behaves in a more classical manner when the system size increases. This basic question was not well answered until decoherence theory was proposed, which states that the coherence of a quantum system tends to be destroyed by environmental interruptions. Thus, if a system is well isolated from its environment, it is in principle possible to observe macroscopic quantum coherence. Quite recently, testing quantum principles in the macroscale has become a hot topic due to rapic technological developments. A very promising platform for testing macroscale quantum physics is a laser levitated nanoparticle, and cooling its mechanical motion to the ground state is the first step. In the second part of this thesis, we develop the theory of decoherence for a mesoscopic system's rotational degrees of freedom. Combining decoherence in the translational degrees of freedom, the system's shot noise heating is discussed. We then focus on cooling the nanoparticle in the laser-shot-noise-dominant regime using two different feedback cooling schemes: the force feedback cooling and the parametric feedback cooling. Both quantum and classical calculations are performed, and an exact match is observed. We also explore the parameters that could possibly affect the cooling trend, where we find that the cooling limit for both cooling schemes strongly depends on the position measurement efficiency, and it poses good questions for researchers interested in achieving ground state cooling: what is the best measurement efficiency for a given measurement setup and what can be done to get a better measurement efficiency?