George publishes in Science Advances
Working in collaboration with experimentalists at Heriot-Watt and Glasgow Universities, the paper entitled Attosecond-resolution Hong-Ou-Mandel interferometry (DOI:10.1126/sciadv.aap9416) has been published in Science Advances.
The team investigated an optical sensor that uses a type of interferometry based on the Hong-Ou-Mandel effect, whereby two identical photons deterministically bunch together at a balanced beam splitter. They were able to measure the optical thickness of an object by looking at the change in coincidence rate of a pair of photodetectors placed at the output of the beam splitter.
By manually varying the length of one arm of the interferometer a characteristic coincidence dip can be resolved. Usually, the whole dip is scanned through with and without the unknown sample in order to estimate its thickness. George has helped design a new protocol, which relies on a single calibration of the dip followed by fine-tuning of the interferometer to a particular optical delay.
The theoretical model of the interferometer takes account of imperfect loss and residual distinguishability of photons, and was successfully shown to be consistent with experimental trials. The fundamental precision limit (known as the Cramér-Rao bound) can be calculated from this statistical model, and then used to calculate the minimum uncertainty in the estimate produced by the experiment.
Comparing the experimental and theoretical uncertainties their Hong-Ou-Mandel interferometer approached the best precision available with the given photon sources and loss rates. Pushing for the best possible results, the new method was implemented experimentally and enabled the sensor to reach attosecond resolution. This sort of performance means in the future similar sensors could be used to image cell membranes, which are of the order of 1nm thick. A press release is available here https://warwick.ac.uk/newsandevents/pressreleases/nanoscale_measurements_100x/