Quantum Cavity Optomechanics of Levitated Nanoparticles: from Foundations to Technologies

Lead Research Organisation: University College London


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.

Planned Impact

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.


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Aranas E (2016) Split-sideband spectroscopy in slowly modulated optomechanics in New Journal of Physics

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Bhattacharya M (2017) Levitated optomechanics: introduction in Journal of the Optical Society of America B

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Bose S (2017) Spin Entanglement Witness for Quantum Gravity. in Physical review letters

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Bose S (2021) Gravitons in a box in Physical Review D

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Bullier N (2021) Quadratic optomechanical cooling of a cavity-levitated nanosphere in Physical Review Research

Description However we have further developed the hybrid trap and are working towards ground state cooling of a macroscopic oscillator. A patented accelerometer and navigation system has also resulted from this work and a spin out company by student Ying Lia Li has been launched.

We have also developed a range of methods for cooling and preparing particles in well defined states that can be used for applications from testing the limits of quantum mechanics to inertial sensing.
Exploitation Route Parts of this system are already being used by others to explore the macroscopic limit of quantum mechanics. We will also cintue to develop this work and work towards evidencing macroscopic superpositions using the technology developed in the programme.
Sectors Aerospace, Defence and Marine,Education,Other

URL https://www.smithsonianmag.com/science-nature/new-experiment-hopes-solve-quantum-mechanics-biggest-mystery-180974132/
Description Yes. An inertial sensor has been developed from the basic research conducted as part of this programme. The sensor development was supported by DSTL and the company Harris. The device is now being further developed by a PhD student on this project who then became a postdoctoral researcher in the group. This lead to the founding of a company called zero point motion that aims to develop real world devices from the expertise enabled by this grant. This work has also been an important spring board for further development of optomechanics in the UK, and particularly levitated optomechanics. On top of this, and with results also from another grant, we have developed a new spectroscopic technique for characterising a range of nanoparticles with applications in material science to biology. More recently the ability to measure small forces using this technology has led to its development as a sensor for dark matter. Unlike others sensors, it offers directional detection which promises to explore new parameter space. This work has resulted in a collaboration with the high energy physics group at UCL and to pursue funding for this via STFC.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology
Impact Types Societal

Description EU 2020 FET
Amount € 4,500,000 (EUR)
Organisation European Union 
Sector Public
Country European Union (EU)
Start 01/2018 
End 12/2021
Description Erlangen University Collaboration 
Organisation Friedrich-Alexander University Erlangen-Nuremberg
Department Department of Physics
Country Germany 
Sector Academic/University 
PI Contribution Experimental work
Collaborator Contribution Theoretical support
Impact 1 Publicatons
Start Year 2015
Description Griffith University Collaboration 
Organisation Griffith University
Country Australia 
Sector Academic/University 
PI Contribution Working out details on trapping diamond and other nanoparticles
Collaborator Contribution Transfer of expertise in ion trapping.
Impact None as yet.
Start Year 2016
Title A whispering gallery mode inertial sensor and method 
Description The patent relates to a whispering gallery mode inertial sensor and is particularly, although not exclusively, concerned with a whispering gallery mode inertial sensor configured to provide improved position, navigation and timing information. 
IP Reference GB1711156.8 
Protection Patent application published
Year Protection Granted 2017
Licensed Yes
Impact This device has been tested by the UK defence science and technology laboratory
Description Company 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Business event to explain capabilities of our research group.
Year(s) Of Engagement Activity 2017