Nano-scale imaging with Hong-Ou-Mandel Interferometry

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science


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.


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Description In 2019, we discovered the limitations to monitoring the polarisation state of light in the context of Hong-Ou-Mandel interference. This type of interference is the building block of optical quantum networks and quantum computing, so it is vital to understand how it behaves. The work on the fundamental limits was published in January 2020 in Optics Express.

In 2020, we published work on the theoretical limits of Hong-Ou-Mandel measurements when considering the effects of number-resolving and increased temporal resolution of the detectors. This work was published in PRA in September 2020.
Exploitation Route The work that we have published will be used by our collaborators when designing new experiments that use Hong-Ou-Mandel interference.
Sectors Other

Description Invited talk at Photonics for Quantum Workshop at RIT, USA 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Dr Leach gave an invited talk on applications of single-photon detector arrays and measurement techniques. The workshop was on the topic of photonics for quantum science, and it was held at RIT, USA.
Year(s) Of Engagement Activity 2019
Description Public engagement at IOP Scotland Festival of Physics 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Dr Gauger ran a public engagement event on Quantum Technology at the IOP Scotland Festival of Physics. This was held at Dynamic Earth in October.
Year(s) Of Engagement Activity 2018