Nano-scale imaging with Hong-Ou-Mandel Interferometry

This project is targeted at establishing the fundamental limits of quantum interferometry, with particular emphasis on the specific and widespread Hong-Ou-Mandel (HOM) interferometer. We will show that quantum HOM interferometry enables extremely precise depth and thickness measurements in an optical microscope. We then propose to use this approach to build a non-invasive optical imaging system that will provide sub-nanometer precision, improving upon the state-of-the-art by three orders of magnitude. To achieve our goal, we will combine customised quantum optical interference with new advanced statistical analysis tools. We will also integrate the latest ultra-sensitive single-photon detector array sensors into the imaging system to provide unprecedented sensitivity and temporal resolution. This interdisciplinary research brings together experimental and theoretical physicists to develop the optical systems, sources and underlying models, and biologists as end users of the technology.
Our research relies on quantum interference of indistinguishable single photons, known as Hong-Ou-Mandel interference, which can give very precise information about the thickness of an unknown sample. The principle works by using two identical photons, which are produced at exactly the same time. If one of the photons is delayed with respect to the other due to transmission through a sample of unknown thickness, the properties of the sample can be established by detailed analysis of the interference pattern when the two photons are brought back together. Furthermore, the precise form of the interference pattern, and consequently the precision of the measurement, can be controlled by customising the spectral properties of the single photons. Generally, this method provides high temporal precision with a large dynamic range, yet does not suffer from phase instability between the two photons. While this phenomenon has been known for many years, the tools to reach its fundamental limits have not yet been developed.
To reach the boundaries of this optical method, we will develop custom photon sources to provide tailored quantum interference patterns and develop new analysis procedures based on the Fisher information associated with the data. The Fisher information is a statistical approach for assessing how much information about an unknown parameter is available in measured data. In any physical system, one builds a model that includes a number of parameters, and in our imaging system, the thickness of the sample will be the key quantity that we wish to establish. Small changes to the thickness of the sample will result in small changes to the observed data and by analysing the Fisher information, we will be able to reach the ultimate precision provided by information theory. We predict this ultimate limit to be sub-nanometer in precision.
In the final stages of the project, we will also measure a series of biological samples. Accurately establishing cell, protein, and DNA morphology is vital for determining the performance of biological systems. It is well known that the structural form of DNA plays a crucial role in its functionality. DNA can be prepared in various forms and can take the shape of strands or more convoluted structures, such as for example DNA origami. The DNA strands therefore occupy different volumes and thicknesses at the nanometer level. After metrology of defined 'ground truth' DNA origami structures, we will extend our study to that of chromatin structures in vitro.

Our goal is to develop a broad range of tools that have wide-ranging impact across multiple fields. The advanced statistical analysis routines will have applications in metrology and precision measurement; the new quantum sources will have impact in the fields of quantum simulations, networks and computation; and the optical HOM-microscope will have impact in the field of microscopy. The team is well placed to take advantage of the impact of the research, with a track record of success in this area. The existing research network of the team, which covers quantum technology and super-resolution imaging, will enable us to deliver the impact in a timely manner.
Significant impact will also come through working closely with two companies, Renishaw and Photon Force, and one international research center, ICFO, as detailed in the letters of support. Renishaw is a world-leading metrology company. Despite their involvement with precision measurement, they are yet to establish a quantum research division. Consequently, they have indicated their interest and support for this proposal and will follow this research closely. Photon Force is a start-up company, who produce and sell single-photon sensitive cameras. They are looking to establish their products in a wide range of disciplines, and precision quantum sensing is one such area. ICFO, The Institute of Photonic Sciences in Barcelona, is a world-leading research institute. Their interests in our proposal relate to combining our quantum methods with their classical interferometry approach to microscopy and we will benefit from their recently developed wide-field, lensless imaging technique.
Impact on the quantum technology side will be led by Daniele Faccio, Jonathan Matthews, Jonathan Leach, and Erik Gauger. Daniele Faccio leads one of the workpackages of the Quantum Imaging hub, Jonathan Matthews holding and EPSRC quantum technology fellowship, Jonathan Leach is a researcher in the Quantum Imaging hub, and Erik Gauger holds a Royal Society of Edinburgh fellowship in bio-inspired quantum technologies. Access to the Quantum Hub network provides contacts with over twenty national and international companies.
One example of impact in quantum technology is in the development of bright sources of indistinguishable photons, a key objective of our proposal. Such a source is relevant to our HOM imaging system but also extremely relevant to many aspects of the UK's quantum technology program. For example, quantum interference is the fundamental principle behind quantum computing, quantum simulations, and quantum networks. The development of sources from our program will feed into the research in other areas.
Impact in bio-imaging will be led by Rory Duncan, the director of the Edinburgh super-resolution imaging consortium (www.esric.org), which is funded through multiple channels, e.g. the MRC, BBSRC, EPSRC, and Wellcome. This is a world-leading research facility targeted at developing and implementing new super-resolution imaging technics for microscopy. Rory is also Director of Edinburgh Biosciences Ltd, a company established to commercialise his biophotonic technologies for use in diagnostics and healthcare applications.
Our team also has an outstanding track record in public engagement. In 2017, Daniele Faccio was awarded the Royal Society of Edinburgh Senior Public Engagement medal. He has given multiple public talks on science, including talks at TEDx and the Edinburgh International Science Festival, and produced a number of public engagement videos, with more the 1M total views. He has also appeared in a Discovery Channel documentary (Strip the Cosmos) that has been aired internationally and is currently consulting with the BBC regarding future Horizon documentaries. Jonathan Leach and Erik Gauger have been key members of teams at the Royal Society Summer Science Exhibition, and Rory Duncan has developed low-cost microscopes for schools based on camera-phone technology.