Quantum Cavity Optomechanics of Levitated Nanoparticles: from Foundations to Technologies

Processes in the microscopic world are extremely well described by quantum theory, but yet little is known about the transition to the classical world at macroscopic scales. For example, can a macroscopic object such as a virus be put into a quantum superposition, and if not, what are the processes at these length and mass scales that prevent this? These types of questions are not only important for our fundamental understanding of the world but they will also impact on the development of future engineered macroscopic quantum systems. Until very recently these questions remained a primarily theoretical pursuit because the experimental methods required to prepare and maintain the delicate quantum states in the presence of environmental noise did not exist. This is because even weak interactions between a quantum system and its environment can rapidly destroy them. As such, these systems must be prepared in well controlled isolation, and typically, this often requires cooling to very low temperatures. New experimental techniques now offer the prospect for laboratory tests of macroscopic quantum mechanics. This field, collectively known as quantum cavity optomechanics, uses the controlled interaction of light with the mechanical motion of nanoscale and microscale oscillators, to coherently control their motion. To date quantum ground state cooling has been demonstrated in only a handful of these solid-state devices but a macroscopic superposition, and even non-classical motion, has yet to be observed.

A new optomechanical oscillator system that is levitated in vacuum has recently been developed by the UCL group. It uses a novel configuration of electric and optical
fields to achieve extremely good isolation from the environment. Cooling from room temperatures down to milliKelvin temperatures has been achieved for the first time, by employing a technique called cavity cooling, with quantum ground state cooling now within reach. Our aim in this research programme is to build on this initial success by using the hybrid technologies to create a well controlled, low dissipation macroscopic oscillator, that can be prepared in its absolute ground state. This system will allow us to explore macroscopic quantum mechanics by preparing and measuring its nonclassical motion. For the first time, we will undertake laboratory tests of theoretical models for macroscopic wavefunction collapse. This will be possible even when the system is not in the ground state. The very low noise and high mechanical Q of this oscillator system also offers significant promise for sensing applications. Therefore as part of this research programme we will begin to explore these more classical applications which includes the development of a new type of in-trap spectrometer capable of measuring mass, charge and shape of nanoparticles, while another strand will seek to use the tunable interactions between the levitated particle for controlling, switching and storing light fields.

Our fundamental and applied research programme is anticipated to have a wide range of both short and long term impact as outlined below.


Scientific and technological impact
The research programme that we have outlined will have significant impact not only within the optomechanical and photonics research community, but also well beyond. The development of new techniques to cool and stably trap levitated nanoparticles allows not only basic science, but also in the future, for new quantum technologies. In addition, there are other applications envisaged in this programme, such as techniques for characterisation of nanoparticles which, albeit not in the quantum domain, offer impact in the shorter term. Demonstrating ground state cooling and measuring the non-classical nature of a macroscopic system for the first time will have a very considerable impact in the foundational quantum mechanics community; the prospect of levitated quantum particles is already stimulating a large number of theoretical proposals to test basic physics, including by our project partners. Cooling to the quantum regime, combined with the sensitive readout allowed by an optical cavity allows for a new generation of quantum limited sensors for weak forces and even biophysics applications. Finally, we expect that by extending the modality of in-trap mass spectrometry, as outlined in this programme of research, will have long term impact in the analytical sciences such as materials science, chemistry and bioscience.

Impacts on the UK's future workforce, industry and the public sector
The high level of technical training in this programme, that ranges from the complex modelling of optomechanical systems in the quantum regime to the design and construction of intricate instruments to measure these quantum states provides a wealth of training and skills for PDRAs, PhD and undergraduate students. The graduates will be highly valued over a wide range of sectors as evidenced by the varied industries and government agencies where our past graduates find employment. This includes the energy, defence, and IT industries but also in the educational and academic sectors at universities and schools.

Societal Impact
There is currently considerable interest in physics and quantum mechanics in particular outside academia. This is evidenced by the large number of television and radio programmes on these topics as well as the very positive reaction to the quantum workshop (http://thequantumworkshop.com/) an outreach programme run by members of our research group on quantum mechanics and optical trapping. Our research programme is of particular interest to the general public because at its heart it seeks to understand, in a very accessible way, the transition from the quantum world to the classical world that we all understand. Our programme will have added societal impact because we can demonstrate how the pursuit of fundamental curiosity driven enquiry also leads to the development of real world applications such as the enhanced mass spectrometry that we develop as part of this programme.