In the Centre's Quantum optics group, we investigate how to generate, manipulate, and transmit quantum states of light with high fidelity. Using specialist optical fibres fabricated within the Centre for Photonics, we develop high-performance sources of single photons, entangled photon pairs, and squeezed light. These quantum resources underpin applications in quantum computing, precision measurement, and secure communications.

Quantum optics in photonic crystal fibre

Photonic crystal fibre (PCF) is a type of specialised optical fibre with a lattice of voids surrounding a light-guiding core of solid glass. The intense contrast in optical properties between glass and empty space, combined with design flexibility, equips these fibres with extremely high optical nonlinearity and controllable chromatic dispersion. These properties make PCF an ideal platform for four-wave mixing- a nonlinear optical process which, at the quantum level, allows individual photons to be created or annihilated at desired wavelengths.

At Bath, we are harnessing PCF for a range of applications in quantum technology. As part of the National Quantum Technologies Programme, we are using PCF to modify the spectrum of single photons from quantum emitters while preserving their quantum coherence, making them compatible with long-range fibre communications or quantum memories. We are also experimenting with reflective structures written directly into the cores of the fibres, creating cavities to enhance the nonlinear interaction or improve performance by reflecting away unwanted noise photons. By engineering the dispersion of our fibres, we can also eliminate spectral correlations in the photon pairs they emit. This enables scalable multiplexing of heralded single-photon sources, or "twin beam" sources with photon-number entanglement for use in quantum repeaters.

Light-matter interaction

Optical switching experiment in a photonics laboratory. A transparent gas-filled cell mounted on an optical table is illuminated with purple light as part of a laser-based switching setup — Optical setup featuring a rubidium vapour cell for studies of light–matter interactions.

We investigate non-linear interactions between light and matter, and how those interactions can be harnessed to yield phase shifts suitable for single-photon switching and photon-photon gates. In particular, we develop high-speed low-loss optical switches, critical for scaling up quantum photonic architectures that enable high computational clock rates. By utilising the strong two-photon absorption in warm rubidium vapour, we implement phase shifts and all-optical control of weak signal fields. This technology will find applications within active multiplexing, loop-based quantum memory and feed-forward for quantum error correction protocols.

We are investigating a range of platforms that offer scalability for quantum networks, such as cavities and fibre-integration. By confining rubidium atoms within the core of anti-resonant hollow-core fibre, we find an increased light-matter interaction strength with low optical loss. Utilising fibre also allows for more seamless integration with other fibre technology where an alkali gas such as rubidium could be the basis for a single photon source, a frequency converter, a quantum memory, all in fibre.

Complementing these efforts, we develop optical methods to rapidly control the density of alkali-metal vapours used in quantum technologies. Using plasmonic gold nanoparticles as nanoscale atomic dispensers, we have demonstrated a sixteen-fold increase in rubidium vapour pressure in less than 20 milliseconds. Our recent research investigates the microscopic origin of this effect through a combination of time-resolved vapour measurements, ultrafast spectroscopy, and first-principles calculations. The results indicate a key role for energetic “hot” electrons at early times, alongside slower thermal processes. This work opens new routes towards faster, lower-power, and more compact atomic devices for quantum photonics, sensing, and nonlinear optics, while providing advanced tools for controlling the atomic media that underpin our light-matter interaction and quantum network research.

Microresonator frequency combs

Integrated lithium niobate ring resonator on a SiO₂/Si chip, pumped by a CW laser to generate optical frequency combs spanning the visible and infrared spectra. — Ring-shaped optical cavity on a chip, pumped by a continuous-wave laser to generate broadband optical frequency combs spanning visible and infrared wavelengths.

Optical frequency combs (OFCs), consisting of hundreds or thousands of equally spaced, narrow spectral lines, have witnessed remarkable progress through the development of photonic integrated circuit (PIC)-based microresonator technology and the discovery and exploitation of microresonator frequency combs, or microcombs. Microcombs, together with the dissipative optical solitons that underpin their operation, are emerging as a disruptive technology for precision metrology, spectroscopy, waveform synthesis, and optical information processing. Their implementation on compact, chip-scale platforms enables a wide range of applications and promises to underpin the technological and economic transformation driven by the rapid development of artificial intelligence.

From a fundamental perspective, further scientific and technological progress requires a deeper understanding of the physical principles governing OFC generation in emerging photonic circuit designs based on novel material platforms, as well as a better understanding of the interplay between soliton dynamics, nonlinear instabilities, and mode-locking mechanisms.

Our Bath group conducts theoretical and experimental research on microcombs and dissipative optical solitons, with a particular emphasis on frequency conversion and spectral tunability, which are critical for applications in precision frequency metrology, optical clocks, and coherent optical communications.