Research

Cavity cooling free nanoparticles

An excellent way of studying the limits of quantum physics is to see if increasingly more massive objects can be made to behave in a quantum way. A highly sensitive way to do this is to perform matter-wave interferometry; to create a cold beam of single-particles and pass them through an interferometer.

As a Marie Curie Fellow in the group of Markus Arndt at the University of Vienna I will be exploring methods to produce and control various nanoscale objects in high-vacuum, including nano-rods, spheres and even viruses.

Key publication:
Cavity cooling of free silicon nanoparticles in high vacuum
P Asenbaum, S Kuhn, S Nimmrichter, U Sezer & M Arndt
Nature Communications 4, 2743 (2013)

Funding:
“Cavity Cooling of Dielectric Nanoparticles” FWF Austria No. P 27297 (M Arndt & S Kuhn)
Marie Curie European Fellowship (J Millen)

 

Non-equilibrium thermodynamics with levitated nanospheres

One of the biggest recent theoretical breakthroughs in statistical mechanics is the
extension of the second law of thermodynamics, valid for processes between
equilibrium states, to non-equilibrium processes. Fluctuation theorems link
averages of stochastically fluctuating quantities in a non-equilibrium process, such
as entropy, work or heat, for a process with equilibrium properties, or those of the
time-reversed process. A large number of colloidal experiments have been used
to test their predictions and employ them for the measurement of forces on the
molecular scale.

We have found that levitated nanospheres are an excellent system for studying non-equilibrium processes, and that by studying the fluctuations of the levitated particle one can learn about the environment.

millen_HiRes

Key publication:
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
News & Views article: Levitating nanoparticles: Non-equilibrium nano-thermometry
arXiv

Funding:
“Non-Equilibrium Processes in the Underdamped Regime” Royal Society grant RG2014 R2 (J Anders & J Millen)

 

Optomechanical cooling of trapped nanospheres

During my PostDoc in the group of Professor Peter Barker at University College London we theoretically and experimentally studied the optomechanical cooling of levitated nanospheres. The goal of this work is to cool the centre-of-mass motion of these nano-scale objects to the quantum level. It is by no means clear that such a massive object could exhibit quantum behaviour, and if it does it will shed light on the transition between quantum and classical physics, and the role of gravity in quantum science.

We developed a unique hybrid experiment, where an electric Paul trap is used to levitate the nanospheres, avoiding instabilities that occur when levitating with optical fields. We could then use the field of a high-finesse optical cavity to cool the nanospheres. This technique is extremely promising for reaching the quantum level.

More information can be found on the UCL Optomechanics group webpage.

paul trap

Key publication:
Cavity Cooling a Single Charged Levitated Nanosphere
J Millen, PZG Fonseca, T Mavrogordatos, TS Monteiro, PF Barker
Editor’s suggestion Phys. Rev. Lett. 114, 123602
Physics focus story: How to Stop a Nanosphere
Nature Photonics research highlight:
Cool Levitation
arXiv

Funding:
“Cavity optomechanics: towards sensing at the quantum limit” EPSRC grant EP/H050434/1 (PF Barker & T Monteiro)
“Quantum feedback control of levitating opto-mechanics” EPSRC grant EP/K026267/1 (PF Barker & A Serafini)

 

Rydberg states in Strontium

During my Ph. D. at Durham University, under the supervision of Dr. Matt Jones, we developed an experiment to study highly excited (Rydberg) states in cold atomic Strontium. Using Strontium is novel, as it is a group 2 element, and this means that there are two valence electrons. As well as developing spectroscopic tools and techniques to work with this species we were able to unravel complicated interatomic interactions within a Rydberg gas by utilizing the second valence election, through two-electron excitation leading to autoionization.

Key publication:
Two-electron excitation of an interacting cold Rydberg gas
J Millen, G Lochead, MPA Jones
Phys. Rev. Lett. 105, 213004
arXiv

Funding:
“Charge delocalisation and hopping in an ultra-cold atomic lattice” EPSRC grant EP/D070287/1 (MPA Jones)