James Millen

Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere

Einstein realized that the fluctuations of a Brownian particle can be used to ascertain the properties of its environment. A large number of experiments have since exploited the Brownian motion of colloidal particles for studies of dissipative processes, providing insight into soft matter physics and leading to applications from energy harvesting to medical imaging. Here, we use heated optically levitated nanospheres to investigate the non-equilibrium properties of the gas surrounding them. Analysing the spheres Brownian motion allows us to determine the temperature of the centre-of-mass motion of the sphere, its surface temperature and the heated gas temperature in two spatial dimensions. We observe asymmetric heating of the sphere and gas, with temperatures reaching the melting point of the material. This method offers opportunities for accurate temperature measurements with spatial resolution on the nanoscale, and provides a means for testing non-equilibrium thermodynamics.

Perspective on quantum thermodynamics

We are grateful for discussions with Janet Anders, Lidia del Rio, and Mathis Friesdorf. JM would like to acknowledge support from the Marie Sklodowska-Curie Action H2020-MSCA-IF-2014. This work was partly supported by the European COST network MP1209.

Dynamics of levitated nanospheres: towards the strong coupling regime

The use of levitated nanospheres represents a new paradigm for the optomechanical cooling of a small mechanical oscillator, with the prospect of realizing quantum oscillators with unprecedentedly high quality factors. We investigate the dynamics of this system, especially in the so-called self- trapping regime, where one or more optical fields simultaneously trap and cool the mechanical oscillator. The determining characteristic of this regime is that both the mechanical frequency !M and single-photon optomechanical coupling strength parameters g are a function of the optical field intensities, in contrast to usual set-ups where !M and g are constant for the given system. We also measure the characteristic transverse and axial trapping frequencies of different sized silica nanospheres in a simple optical standing wave potential, for spheres of radii r = 20-500nm, illustrating a protocol for loading single nanospheres into a standing wave optical trap that would be formed by an optical cavity.

Nonlinear Dynamics and Strong Cavity Cooling of Levitated Nanoparticles

Optomechanical systems explore and exploit the coupling between light and the mechanical motion of macroscopic matter. A nonlinear coupling offers rich new physics, in both quantum and classical regimes. We investigate a dynamic, as opposed to the usually studied static, nonlinear optomechanical system, comprising a nanosphere levitated in a hybrid electro-optical trap. The cavity offers readout of both linear-in-position and quadratic-in-position (nonlinear) light-matter coupling, while simultaneously cooling the nanosphere, for indefinite periods of time and in high vacuum. We observe the cooling dynamics via both linear and nonlinear coupling. As the background gas pressure was lowered, we observed a greater than 1000-fold reduction in temperature before temperatures fell below readout sensitivity in the present setup. This Letter opens the way to strongly coupled quantum dynamics between a cavity and a nanoparticle largely decoupled from its environment.

Full rotational control of levitated silicon nanorods

We study a nanofabricated silicon rod levitated in an optical trap. By manipulating the polarization of the light we gain full control over the ro-translational dynamics of the rod. We are able to trap both its centre-of-mass and align it along the linear polarization of the laser field. The rod can be set into rotation at a tuned frequency by exploiting the radiation pressure exerted by elliptically polarized light. The rotational motion of the rod dynamically modifies the optical potential, which allows tuning of the rotational frequency over hundreds of Kilohertz. This ability to trap and control the motion and alignment of nanoparticles opens up the field of rotational optomechanics, rotational ground state cooling and the study of rotational thermodynamics in the underdamped regime.

Optomechanical cooling of levitated spheres with doubly resonant fields

Optomechanical cooling of levitated dielectric particles represents a promising new approach in the quest to cool small mechanical resonators toward their quantum ground state. We investigate two-mode cooling of levitated nanospheres in a self-trapping regime. We identify a structure of overlapping, multiple cooling resonances and strong cooling even when one mode is blue-detuned. We show that the best regimes occur when both optical fields cooperatively cool and trap the nanosphere, where cooling rates are over an order of magnitude faster compared to corresponding single-resonance cooling rates.

Simultaneous cooling of coupled mechanical oscillators using whispering gallery mode resonances.

We demonstrate simultaneous center-of-mass cooling of two coupled oscillators, consisting of a microsphere-cantilever and a tapered optical fiber. Excitation of a whispering gallery mode (WGM) of the microsphere, via the evanescent field of the taper, provides a transduction signal that continuously monitors the relative motion between these two microgram objects with a sensitivity of 3 pm. The cavity enhanced optical dipole force is used to provide feedback damping on the motion of the micron-diameter taper, whereas a piezo stack is used to damp the motion of the much larger (up to 180 μm in diameter), heavier (up to 1.5 × 10(-7) kg) and stiffer microsphere-cantilever. In each feedback scheme multiple mechanical modes of each oscillator can be cooled, and mode temperatures below 10 K are reached for the dominant mode, consistent with limits determined by the measurement noise of our system. This represents stabilization on the picometer level and is the first demonstration of using WGM resonances to cool the mechanical modes of both the WGM resonator and its coupling waveguide.

Quantum cooling and squeezing of a levitating nanosphere via time-continuous measurements

With the purpose of controlling the steady state of a dielectric nanosphere levitated within an optical cavity, we study its conditional dynamics under simultaneous sideband cooling and additional time-continuous measurement of either the output cavity mode or the nanosphere’s position. We find that the average phonon number, purity and quantum squeezing of the steady-states can all be made more non-classical through the addition of time-continuous measurement. We predict that the continuous monitoring of the system, together with Markovian feedback, allows one to stabilize the dynamics for any value of the laser frequency driving the cavity. By considering state of the art values of the experimental parameters, we prove that one can in principle obtain a non-classical (squeezed) steady-state with an average phonon number .

Cooling the centre-of-mass motion of a silica microsphere

We describe cooling of the center-of-mass (c.o.m.) motion of silica microspheres using the morphology dependent whispering gallery mode (WGM) resonances excited by light coupled from a tapered optical fibre. This scheme uses passive cooling via the velocity dependent scattering force from the excitation of WGM resonances in one direction1 and active feedback cooling via cavity enhanced optical dipole forces (CEODF)2 along a perpendicular axis. Initial experiments have shown successful laser frequency locking to a WGM using relatively high coupled powers despite thermal bistability and thermally induced frequency shifts in the WGM. We also demonstrate the optomechanical transduction required for feedback by monitoring the transmission through the tapered fibre, demonstrating the ability to resolve displacements of less than a nanometer and velocities less than 40X10-6 ms-1.

Cooling and manipulation of nanoparticles in high vacuum

Optomechanical systems, where the mechanical motion of objects is measured and controlled using light, have a huge range of applications, from the metre-scale mirrors of LIGO which detect gravitational waves, to micron scale superconducting systems that can transduce quantum signals. A fascinating addition to this field are free or levitated optomechanical systems, where the oscillator is not physically tethered. We study a variety of nanoparticles which are launched through vacuum (10−8 mbar) and interact with an optical cavity. The centre of mass motion of a nanoparticle can be cooled by the optical cavity field. It is predicted that the quantum ground state of motion can be reached, leaving the particle free to evolve after release from the light field, thus preparing nanoscale matter for quantum interference experiments.