A laser cooled nanocryostat: Refrigeration, alignment and rotation of levitated Yb⁺³:YLF nanocrystals

The ability to cool and manipulate levitated nano-particles in vacuum is a promising new tool for exploring macroscopic quantum mechanics\cite{WanPRL2016,Scala2013,Zhang2013}, precision measurements of forces, \cite{GambhirPRA2016} and non-equilibrium thermodynamics \cite{GieselerNatNano2014,MillenNat2014}. The extreme isolation afforded by optical levitation offers a low noise, undamped environment that has to date been used to measure zeptonewton forces \cite{GambhirPRA2016}, radiation pressure shot noise,\cite{Jain2016} and to demonstrate the cooling of the centre-of-mass motion \cite{LiNatPhys2011,Gieseler2012}. Ground state cooling, and the creation and measurement of macroscopic quantum superpositions, are now within reach, but control of both the center-of-mass and internal temperature is required. While cooling the centre-of-mass motion to microKelvin temperatures has now been achieved, the internal temperature has remained at or well above room temperature. Here we demonstrate refrigeration of levitated Yb$^{3+}$:YLF from room temperature to 130 K using anti-Stokes fluorescence cooling, while simultaneously using the optical trapping field to align the crystal to maximise cooling.