Towards maximally entangled photon pairs and single cycle quantum nonlinear interferometers

Abstract

This work contributes to the development of maximally entangled photon pairs and single-cycle quantum nonlinear interferometers. It is both theoretical and experimental in nature.

Under pulsed pumping conditions, the entanglement of photon pairs degrades with increased pump bandwidth. This effect can be overcome by producing a difference beam state where the photon pairs are correlated instead of anti-correlated in frequency. This work identifies the criteria required to produce the maximally entangled photon pair state known as a difference beam state using four-wave mixing. It is shown that the key requirement to produce such a state experimentally is to ensure that Ω_c ≪ Ω_p ≪ Ω_EPM, where Ω_c is the width of the phasematching function, Ω_p is the pump bandwidth and Ω_EPM quantities the contribution of the second-order dispersion. I build an experimental set-up to produce photon pairs and measure the signal photons. I identify the next steps to build a set-up capable of measuring the entanglement of signal and idler photons which is necessary to demonstrate a difference beam state has been produced. This set-up would use a direct detection system that measures both the frequency and arrival time of photon pairs. This would allow for the two photon amplitude to be calculated and the Schmidt number to be determined.

A second aim of this project was to produce a quantum nonlinear interferometer using two nonlinear crystals. I provide a theoretical description of the output of a single-cycle quantum nonlinear interferometer composed of two aperiodically poled lithium niobate crystals, for the first time to our knowledge. I build this interferometer, observing nonlinear interference which is modified using a 4mm thick BK7 sample. A broadband phase measurement was determined from this measurement with a bandwidth of 20nm. This provides an important step towards carrying out a broadband homodyne measurement and thus quantifying the squeezing of the light produced by these crystals. This would allow the crystals to be used in an interferometer that beats the standard quantum noise limit.

Date of Award	30 Jun 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Hamid Ohadi (Supervisor)