Cavity optomechanics: towards sensing at the quantum limit

The grand challenge of attempting to cool a small mechanical device towards its quantum ground state is driving intense activity in many leading experimental groups worldwide. What seemed an unfeasible target only a decade ago, now appears tantalisingly close: by means of optomechanical techniques, micromechanical resonators such as small mirrors and cantilevers have been cooled by several orders of magnitude, down to occupation numbers of order n~30. The ultimate goal of approaching the ground state (n~1) now seems a realistic prospect, although serious obstacles remain; among these, thermal coupling to the environment is the most serious.However, within the last year, three groups (including the PI's) have independently proposed a novel scheme which has a fundamental new design: a dielectric nanosphere, optically levitated in a cavity and cooled by dipole forces arising from the optical field. The lack of mechanical connection to the cavity structure in a sense insulates the device from important sources of thermal noise and gives this scheme a unique edge in relation to conventional devices. The project brings together experimental and theory groups from London and Southampton with the ultimate goal of successfully implementing this scheme experimentally, for the first time. In addition, we aim to thoroughly understand the underlying physics theoretically by undertaking complete and realistic simulations of the optically cooled nanosphere system.Once the quantum limit is achieved, the main target is to operate the device in this regime. The rewards are potentially great. This is an attainable quantum technology which offers the prospect of unparalleled sensitivity in measurement, limited only by the Heisenberg uncertainty principle. For example, it is for this reason that these devices are used for gravitational-wave detectors, which require extraordinarily precise detections of displacement. They offer also the possibility of fundamental insights into the quantum-classical border: it may be possible to investigate superpositions which differ only by the displacement of a macroscopic object. Some experimental groups are investigating dipole-force coupling cavity optomechanics using a BEC (Bose Einstein Condensate) as the mechanical oscillator. In this case, the target is already in the ground state so it is already possible to explore the quantum regime. We will also investigate this regime theoretically, in order to establish whether quantum effects like squeezing (which improve sensing in the quantum regime) may be viably generated in such a scheme, as two of the co-applicants have already identified a potentially promising regime.Finally, taking the long view, we note that in parallel to this work, small sensors such as micron-sized cantilevers are actively being developed for biosensing applications (for ultra-sensitive detection of biomolecules or as force sensors). UCL, in particular the LCN (London Centre for Nanotechnology) is a leader in this field. On the otherhand, groups (such as the Caltech group of Vahala) working to cool optomechanical devices to the quantum limit are already testing their potential as biosensors.A desirable ambition, in the long-term would be to achieve a merger of these two directions: quantum limited detection and biosensing. We will explore the viability of employing schemes based on our dielectric nanospheres.

An important long-term aim of this project is to develop a new type of quantum technology, offering unrivalled sensitivity of measurement. We envisage that the ability to detect tiny forces and displacement (at the single molecular level) will be of significant interest in chemical and bio-sensing. We have specifically chosen our team and project partners, who range from physicists to engineers, so that we can both study the fundamental aspects of this work while also recognising and exploiting important sensing applications when they arise. This programme will initiate UK participation in the development of this new technology where it currently has little or no presence. Our research will thus provide expertise in this area and we will be in a good position to provide expert advice to UK government and industry on potential applications and current worldwide trends in this technology. This research capability will add substantially to the UK's competitiveness in this emerging new quantum technology. As part of this programme, we aim to develop stronger links with the London Nanotechnology Centre (LCN) and medical faculties at UCL who are developing cantilever type bio-senors specifically for health care. The contact will allow us access to potential users of our technology outside the physical sciences, while they will also benefit from our expertise in quantum sensing. The technology and expertise in both experiment and theory will be useful in quantum metrology and therefore eventually also to standards laboratories. Advances here will be communicated to the UK's National Physical Laboratory. Any commercially useful intellectual property will be patented. This service, as well as assistance in technology transfer, is offered through the UCL Business office and at Southampton via Research and Innovation services. As well as publishing our results through the usual established channels such as journal publications, conferences and talks, we aim to bring together interested groups within the UK, and also key international groups, for a quantum sensing sandpit/workshop. This event will also involve a public lecture on quantum sensing and will be given by an esteemed visiting scientist in the field. Other beneficiaries of this work are the students and PDRAs who will be trained during the project. It will provide them with valuable experimental experience in the application of lasers, optics and electronics as well as vacuum technique, all of which are of importance and relevant to many experimental labs in physics and chemistry. We point out that these skills are also important to research and development in government labs, but also to many high tech industries and eventually to wealth creation in the UK. The significant theory component of this research will also provide important training in modern, advanced theoretical quantum physics. These acquired skills will be transferable to a wide range of subject areas which are reliant on skilled graduates in the physical sciences. The students and PDRA's will develop a range of programming and modelling skills, which will have wider application, particularly in the financial sector. The applicants have a particularly good record in this area with past students in jobs ranging from the health and eye care industry to finance, as well as in academia. This emerging field has great potential for public outreach activities, as the long term aim of demonstrating a mesocale particle cooled to the quantum limit inspires wide public interest. The issue of whether it is possible to observe quantum behavior in a small mechanical device is inspiring interest beyond the specialist science community. We aim therefore to disseminate our results and progress by our press releases, websites, a research blog devoted to this project, as well as lectures through the UCL Science Centre for the general public, teachers and school children.