Nano-scale imaging with Hong-Ou-Mandel Interferometry

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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.