In the September 2017 issue of Nature Photonics, Dr A. Rahman and Professor P. Barker from Physics and Astronomy at UCL report on the first demonstration of optical refrigeration of a submicrometre particle suspended by laser light. The tiny levitated cryostat, consists of a single nanocrystal of Yb3+:YLF crystal cooled to temperatures as cold as −143 °C by illuminating, trapping and aligning it with a single laser beam. Their work has shown that the internal temperature of the nanocrystal can be controlled over a wide range using different trapping laser wavelengths. The laser polarization controls the orientation of the trapped crystal and maximizes its cooling. This development is a further important step towards bringing these macroscopic systems into the quantum regime.


Citation: Laser refrigeration, alignment and rotation of levitated Yb3+:YLF nanocrystals, A.T.M. Anishur Rahman and P. F. Barker, Nature Phonics, Nature Photonics 11, 634–638 (2017)


In a 3 minute video with the online journal, Erika Aranas and other group members explain the work


Our paper “Nonlinear dynamics and strong cavity-cooling of levitated nanoparticles” has been recently published in Physical Review Letter! It has been also selected to be a PRL Editor’s suggestion!

You can find the paper here.


14054221_10100615704706855_8825210623784134533_nCongratulations to Lia (Ying Lia Li) for passing her Ph.D. viva on the 22nd September! Her Ph.D. thesis is titled ‘Cooling and sensing using whispering gallery mode resonators’.

Dr. Li has taken on a post-doc position with Prof. Barker to further her research on whispering gallery modes.


Members of the group attended Quantum Engineering of Levitated systems meeting (Benasque)benasque

 (left) Giacomo Fonseca, Peter Barker, Nathanel, Antonio, Tania Monteiro. (right) Giacomo, Nathanael, Antonio.

spherecoolingOur paper on using optical whispering gallery modes (WGM) to detect and cool mechanical modes of a microsphere-cantilever and the motion of the tapered fibre used to couple light into the WGM has been published in Optics Express!
Ying Lia Li, James Millen, and P. F. Barker, “Simultaneous cooling of coupled mechanical oscillators using whispering gallery mode resonances,” Opt. Express 24, 1392-1401 (2016) 

We have just uploaded a new experimental paper on the ArXiv, click here to view it.
he paper is titled ‘Macroscopic Center-of-Mass Cooling using Whispering Gallery Mode Resonances’ and covers the main achievements from Lia’s Ph.D. project.

As technology continues to miniaturize, we need new ways to control micro-devices. Light is an excellent tool for both measuring the motion of small objects as well as manipulating them. We show that by trapping light from an ultra-thin optical fiber within a glass sphere (diameter of human hair), we can detect the displacement between them that is smaller than the size of an atom! Using this transduction we reduce the natural thermal vibrations from the environment by cooling the motion of both objects by almost 300 C. This is the first time this has been achieved on two objects at once, or with objects this large and heavy.

Our UCL theory team comprised of Marco and Alessio, collaborating with Jinglei Zhang at Scuola Normale Superiore and James (now with Markus Arndt) have published a paper titled ‘Quantum cooling and squeezing of a levitated nanosphere via time-continuous measurements’, which is in the New Journal of Physics.

This work is particularly interesting for the many nanosphere cavity cooling experiments being conducted around the world, including our very own! In the paper they find that the addition of continuous measurement of the system can help create more non-classical states, squeeze the nanosphere’s position, and enhance cooling, helping us to reach a mechanical ground state.

Direct link to the pdf: here

We recently released a new paper titled ‘Testing Wavefunction Collapse Models using Parametric Heating of a Trapped Nanosphere’ written by Dan Goldwater (one of our Ph.D students) in collaboration with Mauro Paternostro at Queen’s University.

In this paper we propose a protocol which we could use to test novel theories which alter quantum mechanics, so-called collapse theories. The strengths of our proposal are that it could be implemented using our already existing hybrid trap (see previous posts), although we would require a lower pressure and temperature than we have achieved so far; and that it appears capable of testing the leading collapse theory (CSL) over a large parameter range, giving it an edge over other experiments designed to the same end. This work has been interesting – we’re seeking to address foundational questions in the lab using new experimental techniques, leading us to study both the philosophical aspects of quantum mechanics and the technical details of proposed experimental set-ups.

It is currently on the arXiv:


11334000_924866107534569_548118734246015486_oOur Ph.D. student Lia (Ying Lia Li) went to ONNA2015 last week, which is a conference held by OIST University on tapered optical fibres. OIST University is based on the sub-tropical island of Okinawa, Japan and is thriving with great scientific research, as well as surrounded by beaches and jungle. Lia presented her new results on active feedback cooling of a microsphere-cantilever using a piezo element, as well as feedback cooling the mechanical modes of the coupling taper using the cavity enhanced optical dipole force. Tapered optical fibres are crucial for her work, as they allow her to excite whispering gallery modes within the microsphere and these resonances both enhance optical forces as well as transduce the centre-of-mass motion of both the microsphere-cantilever and the tapered optical fibre.

The ONNA2015 programme was crammed with invited speakers from around the world, on a broad range of subjects from nano-fabrication, non-linear optics, plasmonics and cold atoms.