# Highlights

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

Y. L. Li, J. Millen, and P. F. Barker

Optics Express 24, 1392-1401 (2016)

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.

## Cavity Cooling a Single Charged Levitated Nanosphere

J. Millen, P. Z. G. Fonseca, T. Mavrogordatos, T. S. Monteiro, and P. F. Barker

Phys. Rev. Lett. 114, 123602 (2015)
Accompanying Physics Focus article: Physics 8, 28 (2015)

Optomechanical cavity cooling of levitated objects offers the possibility for laboratory investigation of the macroscopic quantum behavior of systems that are largely decoupled from their environment. However, experimental progress has been hindered by particle loss mechanisms, which have prevented levitation and cavity cooling in a vacuum. We overcome this problem with a new type of hybrid electro-optical trap formed from a Paul trap within a single-mode optical cavity. We demonstrate a factor of 100 cavity cooling of 400 nm diameter silica spheres trapped in vacuum. This paves the way for ground-state cooling in a smaller, higher finesse cavity, as we show that a novel feature of the hybrid trap is that the optomechanical cooling becomes actively driven by the Paul trap, even for singly charged nanospheres.

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

J. Millen, T. Deesuwan, P. Barker, J. Anders

Nature Nanotechnology 9, 425–429 (2014)
News & Views article: Nature Nanotechnology 9, 415–417 (2014)

Einstein realised that the fluctuations of a Brownian particle can be used to ascertain 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 optically levitated nanospheres that are heated to investigate the non-equilibrium properties of the gas surrounding them. Analysing the sphere’s 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 new opportunities for accurate temperature measurements with spatial resolution on the nanoscale, and a new means for testing non-equilibrium thermodynamics

## Matter Wave Interferometry of a Levitated Thermal Nano-Oscillator Induced and Probed by a Spin

M. Scala, M. S. Kim, G. W. Morley, P. F. Barker, S. Bose

Phys. Rev. Lett. 111, 180403 (2013)

We show how the interference between spatially separated states of the center of mass (COM) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey Interferometry). We propose to use an optically levitated diamond bead containing an NV center spin. The nano-scale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the COM state is required and thereby one dispenses with cavities, spatially resolved detection and low mass-dispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nano-crystal being in spatially distinct locations.

## Dynamics of levitated nanospheres: towards the strong coupling regime

T. S. Monteiro, J. Millen, Florian Marquardt, G. A. T. Pender, D. Chang, P. F. Barker

New J. Phys. 15 015001(2013)

Selected as a Highlight of 2013!

Video Abstract:

The use of levitated nanospheres represents a new paradigm for the optomechanical cooling of a small mechanical oscillator, with the prospect of realising quantum oscillators with unprecedentedly high quality factors. We investigate the dynamics of this system, especially in the so-called self-trapping regimes, 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 $\omega_M$ and single-photon optomechanical coupling strength parameters $g$ are ahttp://iopscience.iop.org/1367-2630/15/1/015001 function of the optical field intensities, in contrast to usual set-ups where $\omega_M$ 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 $r=20-500$\,nm, illustrating a protocol for loading single nanospheres into a standing wave optical trap that would be formed by an optical cavity. We use this data to confirm the dependence of the effective optomechanical coupling strength on sphere radius for levitated nanospheres in an optical cavity and discuss the prospects for reaching regimes of strong light-matter coupling. Theoretical semiclassical and quantum displacement noise spectra show that for larger nanospheres with $r \gtrsim 100$\,nm a range of interesting and novel dynamical regimes can be accessed. These include simultaneous hybridization of the two optical modes with the mechanical modes and parameter regimes where the system is bistable. We show that here, in contrast to typical single-optical mode optomechanical systems, bistabilities are independent of intracavity intensity and can occur for very weak laser driving amplitudes.

## Doppler Cooling a Microsphere

P. F. Barker

Phys. Rev. Lett. 105, 073002 (2010)

Doppler cooling the center-of-mass motion of an optically levitated microsphere via the velocity-dependent scattering force from narrow whispering gallery mode resonances is described. Light that is red detuned from the whispering gallery mode resonance can be used to damp the center-of-mass motion in a process analogous to the Doppler cooling of atoms. The scattering force is not limited by saturation but can be controlled by the incident power. Cooling times on the order of seconds are calculated for a 20 μm diameter silica microsphere trapped within optical tweezers.

# 2016

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

Y. L. Li, J. Millen, and P. F. Barker
Opt. Express 24, 1392-1401 (2016)

# 2015

### Cavity Cooling a Single Charged Levitated Nanosphere

J. Millen, P. Z. G. Fonseca, T. Mavrogordatos, T. S. Monteiro, and P. F. Barker
Phys. Rev. Lett. 114, 123602 (2015)
Accompanying Physics Focus article: Physics 8, 28 (2015)

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

Marco G. Genoni, Jinglei Zhang, James Millen, Peter F. Barker, Alessio Serafini
ArXiv:1503.05603

# 2014

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

Y. Lia Li, J. Millen, P. F. Barker
Proc. SPIE 9164, Optical Trapping and Optical Micromanipulation XI, 916404 (September 16, 2014)

### Cavity cooling a trapped nanosphere in vacuum.

P. F. Barker, James Millen, Piergiacomo Z. G. Fonseca, T. Mavrogordatos, T. S. Monteiro
Proc. SPIE 9164, Optical Trapping and Optical Micromanipulation XI, 916403 (September 16, 2014)

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

J. Millen, T. Deesuwan, P. Barker, J. Anders
Nature Nanotechnology 9, 425–429 (2014)
News & Views article: Nature Nanotechnology 9, 415–417 (2014)

# 2013

### Matter Wave Interferometry of a Levitated Thermal Nano-Oscillator Induced and Probed by a Spin

M. Scala, M. S. Kim, G. W. Morley, P. F. Barker, S. Bose
Phys. Rev. Lett. 111, 180403 (2013)

### Dynamics of levitated nanospheres: towards the strong coupling regime

T. S. Monteiro, J. Millen, Florian Marquardt, G. A. T. Pender, D. Chang, P. F. Barker
New J. Phys. 15 015001 (2013)

# 2012

### Cooling optically trapped particles

P. F. Barker, J. Millen, Y. Lia Li, M. Trivedi, T. S. Monteiro
Proc. SPIE 8458, Optical Trapping and Optical Micromanipulation IX, 845808 (October 9, 2012)

### Optomechanical cooling of levitated spheres with doubly-resonant fields

G. A. T. Pender, P. F. Barker, Florian Marquardt, J. Millen and T. S. Monteiro
Phys. Rev. A 85, 021802 (2012)

# 2011 and earlier

### Doppler Cooling a Microsphere

P. F. Barker
Phys. Rev. Lett. 105, 073002 (2010)

### Cavity cooling of an optically trapped nanoparticle

P. F. Barker and M. N. Shneider
Phys. Rev. A 81, 023826 (2010)

### Landauer’s principle in the quantum regime

Stefanie Hilt, Saroosh Shabbir, Janet Anders, and Eric Lutz
Phys. Rev. E 83, 030102(R) (2011)