Robert J. A. Francis-Jones

Photon-pair generation in photonic crystal fibre with a 1.5 GHz modelocked VECSEL

Four-wave mixing (FWM) in optical fibre is a leading technique for generating high-quality photon pairs. We report the generation of photon pairs by spontaneous FWM in photonic crystal fibre pumped by a 1.5 GHz repetition-rate vertical-external-cavity surface-emitting laser (VECSEL). The photon pairs exhibit high count rates and a coincidence-to-accidental ratio of over 80. The VECSELs high repetition-rate, high average power, tunability, and small footprint make this an attractive source for quantum key distribution and photonic quantum-state engineering.

All-fiber multiplexed source of high-purity single photons

Single-photon sources based on spontaneous photon-pair generation have enabled pioneering experiments in quantum physics. However, next-generation photonic quantum technologies require higher generation probabilities of photons in well-controlled pure states capable of high-visibility interference. We have harnessed bespoke fiber technology to develop virtually alignment-free sources that deliver high-purity heralded single photons in telecoms single-mode fiber. The resulting access to low-loss optical delay enabled us to actively route the heralded output from two almost identical sources to enhance the delivery probability of single photons relative to one individual source. Our results indicate how the scale of photonic quantum technologies might be increased via guided-wave multiplexing of high-purity photons.

Characterizing the variation of propagation constants in multicore fiber

We demonstrate a numerical technique that can evaluate the core-to-core variations in propagation constant in multicore fiber. Using a Markov Chain Monte Carlo process, we replicate the interference patterns of light that has coupled between the cores during propagation. We describe the algorithm and verify its operation by successfully reconstructing target propagation constants in a fictional fiber. Then we carry out a reconstruction of the propagation constants in a real fiber containing 37 single-mode cores. We find that the range of fractional propagation constant variation across the cores is approximately ± 2 × 10(-5).

Characterisation of longitudinal variation in photonic crystal fibre

We present a method by which the degree of longitudinal variation in photonic crystal fibre (PCF) may be characterised through seeded four-wave mixing (FWM). Using an iterative numerical reconstruction, we created a theoretical model of the PCF that displays FWM phasematching properties that are similar to experiment across all measured length scales. Our results demonstrate that the structure of our PCF varies by less than ±1 % and that the characteristic length of the variations is approximately 15 cm.

Photon Pair Generation via Four-Wave Mixing in Photonic Crystal Fibres

The four-wave mixing process is well known in classical non-linear optics but has also been studied extensively within the framework of quantum theory, due to its usefulness as a source of heralded single photons.

Design, Fabrication, and Characterisation of PCFs for Photon Pair Generation

In this chapter the design, fabrication and characterisation of a PCF for photon pair generation is described. The PCF structure was carefully designed to generate the long wavelength photon (idler) near 1550 nm, with the corresponding signal photon near 800 nm. The 1550 nm wavelength region is of particular importance in the telecommunications industry as it lies in the low loss window of silica optical fibres.

Numerical Modelling of Multiplexed Photon Pair Sources

In this Chapter, the theory and performance of two different multiplexing schemes is discussed in detail.

Characterisation of a Multiplexed Photon Pair Source

In this Chapter we will build on the work presented in Chap. 5 and characterise the individual sources and the complete multiplexed device.

Construction of an Integrated Fibre Source of Heralded Single Photons

In this Chapter the construction of an integrated fibre photon pair source in PCF is described.

Fibre-integrated noise gating of high-purity heralded single photons

We present an all-fibre source of high-purity heralded single photons with an integrated conditional optical gate that reduces uncorrelated noise by almost an order of magnitude. Generating photon pairs by four-wave mixing in photonic crystal fibre, we observe with the noise gate active a factor of 7 reduction in the rate of single counts in the heralded channel with no measurable drop in coincidence count rate. In contrast to electronic post-selection of coincidence events, the real reduction in the flux of unwanted photons is beneficial for example to avoid bleaching light-sensitive samples or in generating entangled states.