The ability to cool and manipulate levitated nanoparticles in vacuum is a promising tool for exploring macroscopic quantum mechanics 1,2 , precision measurements of forces 3 and non-equilibrium thermodynamics 4,5. The extreme isolation afforded by optical levitation offers a low noise, undamped environment that has to date been used to measure zeptonewton forces 3 , radiation pressure shot noise 6 , and to demonstrate centre-of-mass motion cooling 7,8. Ground state cooling, and the creation of macroscopic quantum superpositions, are now within reach, but control of both the centre-of-mass and internal temperature is required. While cooling the centre-of-mass motion to micro Kelvin temperatures has now been achieved, the internal temperature has remained at or above room temperature. Here we realise a nanocryostat by refrigerating levitated Yb 3+ :YLF nanocrystals to 130 K using anti-Stokes fluorescence cooling, while simultaneously use the optical trapping field to align the crystal to maximise cooling.

A nitrogen-vacancy (NV−) centre in a nanodiamond, levitated in high vacuum, has recently been proposed as a probe for demonstrating mesoscopic centre-of-mass superpositions and for testing quantum gravity. Here, we study the behaviour of optically levitated nanodiamonds containing NV− centres at sub-atmospheric pressures and show that while they burn in air, this can be prevented by replacing the air with nitrogen. However, in nitrogen the nanodiamonds graphitize below ≈10 mB. Exploiting the Brownian motion of a levitated nanodiamond, we extract its internal temperature (Ti) and find that it would be detrimental to the NV− centre’s spin coherence time. These values of Ti make it clear that the diamond is not melting, contradicting a recent suggestion. Additionally, using the measured damping rate of a levitated nanoparticle at a given pressure, we propose a new way of determining its size.

Navigation, bio-tracking devices and gravity gradiometry are amongst the diverse range of applications requiring ultrasensitive measurements of acceleration. We describe an accelerometer that exploits the dispersive and dissipative coupling of the motion of an optical whispering gallery mode (WGM) resonator to a waveguide. A silica microsphere-cantilever is used as both the optical cavity and inertial test-mass. Deflections of the cantilever in response to acceleration alter the evanescent coupling between the microsphere and the waveguide, in turn causing a measurable frequency shift and broadening of the WGM resonance. The theory of this optomechanical response is outlined. By extracting the dispersive and dissipative optomechanical rates from data we find good agreement between our model and sensor response. A noise density of 4.5 µg·Hz − 1 2 with a bias instability of 31.8 µg (g = 9.81 m·s −2 ) is measured, limited by classical noise larger than the test-mass thermal motion. Closedloop feedback is demonstrated to reduce the bias instability and long term drift. Currently this sensor outperforms both commercial accelerometers used for navigation and those in ballistocardiology for monitoring blood flowing into the heart. Further optimization would enable short-range gravitational force detection with operation beyond the lab for terrestrial or space gradiometry.

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