Nonclassical states of light are important tools for quantum technologies and the main instrument for quantum communications. In this project we aim to generate three-photon states (triplets) using the Kerr effect. Generation of triplet states by spontaneous decay has not yet been demonstrated and will open up striking possibilities, for example, heralding the arrival of pair-photons. Generation of triplets can be viewed as the reverse of third harmonic generation, requiring a similar phase-matching condition. However, it is linearly dependent on the input power so that increasing the length of the nonlinear medium is important. This makes optical fibre very attractive, although chromatic dispersion prevents phase-matching between pump and signal within the LP01 mode. As a result it is necessary for the third harmonic pump to be in a higher-order mode, which is impractical and yields a poor overlap. We have designed a fibre in which the short wavelength is guided by an all-solid bandgap structure, while the long wavelength is guided by total internal reflection. It consisted of a periodic arrangement of high-refractive index Schott lead-silicate SF6 glass rods (index 1.76 in the IR) embedded in a host with lower refractive index (Schott LLF1, index 1.54), the central core being made from LLF1 glass. The structure supported phase-matched third harmonic generation between the fundamental mode at 1512 nm and a single-lobed mode of the bandgap structure at 504 nm [Cavanna (2016)] with a conversion efficiency of ~0.02% for 12.5 µW average pump power (pulse duration 20 ps and repetition rate 1 kHz). 

Highly excited Rydberg atoms display exceptionally large polarisability and are not only very sensitive to electric fields, but can also undergo long-range interactions with other Rydberg atoms. This makes them of great interest as sensitive electric field sensors or for optical nonlinearities down to the single photon level. We have introduced a thermal vapour of caesium into a hollow core kagomé-PCF and used light-induced atomic desorption (LIAD) to modulate the atom density over several orders of magnitude. After coupling an intense light beam into the fibre, atoms previously adsorbed onto the core walls are released and added to the vapour density. No cryogenic set-up (typically a magneto-optical trap) is required, raising the possibility of a fully fibre-based system. A broad-band guiding kagomé-style hollow-core PCF was used. After demonstrating the feasibility of the system we focused on understanding certain unexpected energy shifts in Rydberg states, which we attribute to surface charges on the core walls [Epple (2014)]. In parallel we have actively modulated the Rydberg states using an external electric field, permitting active switching of atoms in and out resonance by Stark shifting their energy levels, and sideband modulation [Veit (2016)].