Laser refrigeration on the nanoscale: From nanocryostats to quantum optomechanics

Laser refrigeration occurs when incident light absorbed by a solid material is subsequently emitted at higher energy. This blue-shifted emission of photons with respect to the excitation wavelength leads to internal cooling by converting the internal energy of the vibrations (phonons) in the solid to photons which leave the solid via fluorescence. This process lowers the entropy and temperature of the solid. One material, yttrium lithium fluoride (YLF) doped with rare earth ytterbium ions, has been shown to be the best bulk material for this process, and very recently we demonstrated the first dramatic cooling this nanoscale material to 130 K. This proof-of-principle experiment, published in Nature Photonics (doi:10.1038/s41566-017-0005-3), was accomplished by using optical levitation to both isolate the particle from the environment and to cool it.

This programme aims to capitilize on our development in two strongly interlinked strands of research. The first will develop laser refrigeration on the nanoscale, with the aim of fabricating a microscale cryogenic refrigerator using the techniques of nanophotonics to enhance the cooling and to determine the lowest minimum temperature that can be achieved using this material. We aim to lower the temperature of the solid well below 80 K with the aim to reach 10 K. By modifying the emission process using 1-D and 3-D photonic structures, we will enhance cooling while more efficiently utilising the incident light. Such a device, that can be nanofabricated, and uses laser light without direct physical contact, will rapidly find applications in cooling electronics, detectors and new quantum technologies to cryogenic temperatures .

A second strand will explore application of this technology to foundational quantum mechanics within the field of levitated optomechanics. Here, the ability to cool and manipulate the centre-of-mass motion of levitated nanoparticles is now established as a promising tool for exploring macroscopic quantum mechanics. This will allow the observation of non-classical states of motion and the creation of long-lived macroscopic quantum states, as well as a providing a testing ground for the role of gravity within quantum mechanics. However, while we and other researchers have managed to remove almost all sources of decoherence, it is the control of particle internal temperature that has yet to be mastered but for which laser refrigeration offers great promise. In this part of the programme we therefore aim to refrigerate levitated particles for use in NV nanocrystal experiments to increase both spin and motional decoherence time, and use laser refrigeration to produce narrow line widths in solids doped with other rare ions to explore both Doppler and sideband cooling of levitated nanocrystals.

Although firmly based in the UK, this programme will bring together both experimental and theoretical researchers in UK with international collaborators who will help to make this programme a success. This includes researchers with expertise in laser refrigeration, rare earth doped materials fabrication and characterisation, laser nano and micro optical fabrication, quantum optics and quantum optomechanics.

Cooling of electronic devices and detectors is routinely performed to increase speed in computation and to reduce noise in detectors and electronics. However, there is no solid state, cryogen free, device that can currently cool below approximately 140 K and therefore expensive and bulky cryostat technology must be employed. The low power, solid-state device that we will developed as part of this programme will offer a new route towards cryogen-free cooling below the thermoelectric limit, making sub-80 K cooling now feasible. This advance will have a significant impact in both industry (see letters of support) and academic research.

There is currently a significant international effort seeking to push the limits of laser refrigeration in bulk materials down below 90 K. Controlling this process on the nanoscale is new and offers a promising route to lower temperatures and therefore new devices on this scale. As we are a leading group in levitated optomechanics and its application to macroscopic quantum mechanics, our work will have significant impact on both local and international researchers in this field. We will do this through the usual channels, firstly by publishing our work in high impact journals. We will also take part in and organise conferences that are devoted to laser refrigeration and levitated optomechanics. We aim to seek out industrial and applied partners who can help us realise the refrigeration capabilities of our technology. We now have strong connections to DSTL, Harris Aerospace, Coherent Lasers Scotland and M2 lasers through our optomechanics expertise. As part of this programme, we will collaborate with project partner, P. Pauzauskie from UWA, who is a world leading expert in nanoscale fabrication of materials. We will also collaborate with Phillipe Goldner who is a world leader in the development of rare earth doped crystals for optical quantum technologies. As this programme develops, and as we demonstrate and establish the working limits of our microscale cryostat, we will seek industrial partners to commercialise this work. This will potentially have high impact for UK industry, and also for researchers in the wide range of fields that require cryogenic cooling.