Applying the theory of quantum nonlinear multimode optics to optimising integrated photonic devices
Creators: name: Blay, Daniel
Publisher: name: Macquarie University
Year: 2025
Subjects: classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: Quantum integrated photonics seeks to generate and manipulate non-classical states of light, a critical resource for several emerging technologies, including quantum computing and cryptography. Integrated photonic devices can leverage the stability of a solid-state platform, as well as the mature design and manufacturing capabilities of the optics industry, to create quantum devices at scales not possible for non-integrated systems. We focus on the generation of heralded single photons via parametric processes, which ideally can produce photons in pure quantum states at a high rate and on-demand, with the desired spectral properties, and suffering from minimal noise. We address three challenges faced by these parametric sources of photons.In order to herald photons with high purity using non-frequency resolving detectors, it is common practice to use spectral filters on the heralding channel to reduce the classical uncertainty in the spectral mode of the heralded photon. This improvement in the purity comes at the cost of throwing away light, reducing the heralding probability. This relationship is well known, but not thoroughly quantified in existing texts. To fill this gap, we derive expressions for the purity and heralding probability that are easy to apply to any filter or biphoton state, as well as closed-form approximations that can be applied to a wide range of states. This includes closed-form expressions for states described by simple shapes, which approximate a wide range of existing sources.The second challenge we address is broadband spontaneous Raman scattering, a strong source of noise in the form of uncorrelated single photons for amorphous material platforms. These Raman photons can be generated up to some tens of terahertz away from the pump laser, making them difficult to avoid. We propose a novel third-order nonlinear optical process that allows for a large spectral separation between the pump laser, the strongest source of Raman noise, and the biphoton generation band. We call this process seeded three photon downconversion, as a photon in the pump laser at the third harmonic frequency is annihilated to generate three photons around the fundamental frequency, with an additional weak seed laser at the fundamental frequency stimulating the generation of photon pairs. We develop a theoretical description of seeded three photon down conversion, including the full multimode and dispersive behaviour. Using this model we demonstrate a viable proof-of-principle source in microfibre, and that the signal-to-noise ratio of this example source is higher than that of a comparable “typical” source.Finally, we analyse the use of stop-band engineering to suppress noise from parasitic nonlinear processes. In the case of dual-pump four wave mixing, where one photon is annihilated from either pump to generate a pair of photons at the same frequency, it is also possible for each pump to individually generate pairs of photons. These single-pump pairs are a strong source of noise, and they cannot be filtered out after they are generated. Instead, a scheme has been proposed that suppresses the parasitic processes before the noise photons are generated. This involves introducing periodic index modulations to the waveguide, opening stop-bands to suppress the generation of either the signal or idler photon outside of the degenerate frequency band, which also suppresses the generation of its correlated twin inside the degenerate band. This removes a strong source of noise but without affecting the desired dual-pump photon pairs. We develop a new framework for analysing this process, based on the Heisenberg equations. Under continuous wave conditions, we reduce these equations to a set of simple coupled differential equations, with known solutions. We solve these equations for an example system, and demonstrate that the stop-bands opened by the grating leads to an order of magnitude reduction in the noise from parasitic photons.