Cavity Quantum Electrodynamics
The interaction between a quantum emitter and a single optical cavity mode, termed cavity quantum electrodynamics (QED), has allowed for a number of key experimental advances in quantum optics, including the observation of an enhancement of spontaneous emission, the demonstration of the photon blockade effect and vacuum-induced transparency. Using homemade fibre cavity platforms, we are investigating cavity-enhanced light-matter interactions in a range of solid-state materials.
Description: Photons in free space are exceptional carrier of information and are ideal candidates for quantum communication. However, they barely interact at low energies, and this limits our ability to exploit them for quantum applications. When photons are strongly coupled to matter excitations (excitons), half-light half-matter quasi-particles are formed, named polaritons. Two independent photons can now “see” each other through the interaction of their corresponding excitonic parts. In these conditions, light behaves as a gas of interacting photons, and exciting phenomena, like Bose-Einstein condensation and superfluidity of polaritons can be observed. Yet, polariton interactions are weak, and photon nonlinearities are observed only when a large number of polaritons is populated. Therefore, experiments remains described within the semiclassical limit.
At the low-temperature cavity QED lab, we aim to enter the quantum regime by increasing nonlinearites up to a level where the presence of a polariton blocks the excitation of a second one, a phenomenon known as polariton blockade . Our group has recently made a significant step forward towards the blockade regime , by using of a home-built semi-integrated fibre cavity . Such a system enables strong photonic confinement and in-situ tuning of the exciton interaction, and it is ideal for quantum polaritonics. Our research focuses mainly on two aspects: on one side we work on photonic engineering to achieve stronger photonic confinement; on the other side, we use novel materials, such as two-dimensional materials, to achieve stronger exciton-exciton interactions.
 Verger et al., Phys. Rev. B, 73, 193306 (2006)
 Matutano et al., Nat. Mater., 18, 213-218 (2019)
 Besga et al., Phys. Rev. Applied, 3, 014008 (2015)
Contact: Lorenzo Scarpelli, Thomas Volz
Light-matter interactions with diamond colour centres and fibre cavities
Description: We study cavity-enhanced light-matter interactions with defects in diamond and mechanically tunable open fibre cavities, in the context of quantum sensing and quantum information processing.
Room temperature quantum magnetic sensing with lasing from nitrogen-vacancy (NV) centres in diamond attracted a lot of research interest in recent years [1, 2]. The main challenge until now is the experimental realization of such a laser. As a strong step towards this, we explored amplification by stimulated emission of diamond NV centres in our room-temperature fibre cavity platforms . Our study addresses the challenge of light induced charge state switching of NV centres in realizing the lasing.
We are extending our experimental studies towards realizing strong coupling between single diamond colour centres and single photons inside the fibre cavities for quantum information processing.
 Jeske, Jan, Jared H. Cole, and Andrew D. Greentree. "Laser threshold magnetometry." New Journal of Physics 18.1 (2016): 013015.
 Jeske, Jan, Desmond WM Lau, Xavier Vidal, Liam P. McGuinness, Philipp Reineck, Brett C. Johnson, Marcus W. Doherty et al. "Stimulated emission from nitrogen-vacancy centres in diamond." Nature communications 8, no. 1 (2017): 1-8.
 Sarath R, Rogers, Lachlan J., Xavier Vidal, Reece P. Roberts, Hiroshi Abe, Takeshi Ohshima, Takashi Yatsui, Andrew D. Greentree, Jan Jeske, and Thomas Volz. "Amplification by stimulated emission of nitrogen vacancy centres in a diamond-loaded fiber cavity." arXiv preprint arXiv:1912.05801 (2019).
Contact: Sarath Raman Nair, Lachlan Rogers, Thomas Volz