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Strong light-matter coupling with Rydberg polaritons


School of Physics and Astronomy

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Dr H Ohadi Applications accepted all year round Competition Funded PhD Project (Students Worldwide)

About the Project

Joint PhD studentship between Scotland and Australia.
Dr Hamid Ohadi (University of St Andrews, UK) ([Email Address Removed])
A/Prof Thomas Volz (Macquarie University, Sydney, Australia)

Exploiting the laws of quantum mechanics for the benefit of humanity in the so-called "second quantum revolution" is one of the key goals of 21st-century physics. Unlike the first quantum revolution, which simply made use of the wave-particle duality that quantum mechanics dictates, the second quantum revolution harnesses entanglement, superposition and quantum measurement for creating new technologies. Ultimately, the quantum correlations at the heart of these phenomena require strong interactions between individual quantum particles. Many different quantum systems are currently being explored for applications, and photons, the quantum particles of light, are one of the most promising candidates for applications such as quantum information processing and quantum sensing. While they are easy to create, manipulate and detect, their biggest drawback, however, is the lack of direct interactions in vacuum. Yet, photons can be made to interact by binding them to matter. Photons propagating in an optically active material can couple strongly to the excitations of that material to the point where the photon and the material are entangled and form new hybrid particles that are half-matter and half-light. These chimeres are called ‘polaritons’ and have been at the forefront of modern quantum photonics research for the past two decades. While many spectacular results have been demonstrated so far, the ideal single-photon non-linear system has not been found to date.
This project aims to take a big leap forward and study a new exciting contender for inducing strong photon-photon interactions in a semiconductor system. Very recently, polaritons formed from electron hole pairs, i.e. excitons, in traditional semiconductor GaAs have been demonstrated to exhibit non-classical correlations at the single quantum level [1,2]. However, these correlations are weak due to the relatively weak interactions between the GaAs excitons. There are several avenues to induce even stronger interactions but almost all of them involve excitonic states that have a much larger interaction radius than the ground state GaAs excitons. A particularly interesting class of excitons are Rydberg excitons that in analogy to the hydrogen atom correspond to highly-excited excitons with large principal quantum numbers (and large spatial extent). Cuprous Oxides are known to have stable bulk Rydberg excitons with principal quantum numbers beyond n=20. Spectroscopy experiments have revealed effects due to exceptionally strong exciton-exciton interactions [3]. However, to date no light-matter interface with a 2D cuprous oxide film has been formed. This project will address this open problem and unlock the potential of the cuprous oxide material to form strongly interaction Rydberg polaritons.
The cotutelle project within the St-Andrew-Macquarie partnership will be conducted in two steps: In the first step, the semiconductor devices will be fabricated at the University of St Andrews. There will be two types of devices, one that will consist of a 2-dimensional microcavity formed by two highly reflective mirrors encapsulating a cuprous oxide thin film and a second type which is missing one of the mirrors and can be combined with an external mirror to form a fully-tunable system. Photons confined in the microcavity will strongly couple to the Rydberg excitons in the cuprous oxide film to form Rydberg polaritons. The second part of the project will be carried out at Macquarie University in Sydney. There the properties of the cuprous oxide polariton system will be explored with the expectation to demonstrate strong quantum correlations and photon blockade. If successful, the results will be a real breakthrough for quantum polaritonics and its perspective for real-world applications, such as networked non-linear cavities for quantum simulation with light.

Informal enquiries should be sent to Dr Hamid Ohadi ([Email Address Removed]).

Funding Notes

The funding for this project is open to aplicants worldwide.

References

[1] G. Munoz-Matutano et al, Nature Materials 18, 213–218 (2019).
[2] A. Delteil et al, Nature Materials 18, 219-222 (2019).
[3] Kazimierczuk et al, Nature 514, 343 (2014).


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