Single Photon and Entangled Photon Light Sources towards Quantum Sensing with Optical Resonators

Abstract

Quantum optical sensors exploiting properties such as entanglement offer the potential to make enhanced measurements of physical parameters with a higher precision per photon used than achievable with coherent states of light. This thesis is on building sources of single photons and entangled photon pairs, towards developing quantum optical sensors using whispering gallery mode (WGM) resonators. These optical interferometers are used as highly sensitive optical sensors, including as biosensors which can detect single molecules when combined with localised surface plasmon resonances. Therefore, developing WGM sensors which use quantum optical states of light could enable measurements on biological samples and even single molecules with higher precision using fewer photons.

This thesis has three parts. First, we discuss some motivations for quantum sensing schemes using WGM resonators, and compare with previous quantum biosensing experiments. Theoretical modelling results are shown for WGM optical resonators coupled to one arm of a Mach-Zehnder interferometer (MZI). This setup is predicted to produce an interesting double resonance dip when an indistinguishable photon pair is incident on the two input ports; a variation on the behaviour of N00N states in an MZI but now also with an optical resonance. Using a computational model, an example of a WGM measurement scheme is shown that can achieve a factor of two enhancement in signal-to-noise ratio (SNR) by using entangled photon pairs.

In the second part we discuss single photon emitters in hexagonal boron nitride (hBN). Atomic vacancy defects in hBN have been demonstrated as room temperature single photon sources. These single photon emitters are characterised with a view to sensing applications and the fluorescence blinking behaviour is studied. The intensity stability of the single photon emission is measured using the time-dependent Mandel Q parameter. Although these sources can produce nonclassical states of light, it was found that a source of entangled photons would be more versatile for sensing experiments.

The final part is on building a source of entangled photon pairs using the spontaneous parametric down-conversion (SPDC) nonlinear optical process in a periodically-poled KTP crystal (PPKTP). Polarisation entangled states are generated using a PPKTP Sagnac loop setup, and characterised using quantum state tomography and Hong-Ou-Mandel (HOM) interference. We show an example of this source being applied to a refractive index sensor by using HOM interference of photon pairs coupled to a tapered optical fibre sensor. Tapered fibres can provide a means to couple entangled photon pairs to WGM resonators, and to close the thesis we discuss the potential for realising the experiment investigated in the theory chapter: using entangled photon pairs for quantum optical sensing with WGM sensors. The main challenge is achieving narrow bandwidth and wavelength-tuneable entangled photon pairs, however using cavity-enhanced down conversion sources this is possible in future work.