Mehul Malik: High-Dimensional Quantum Technologies: Harnessing Structured Light for the Quantum Networks of Tomorrow

Quantum states of light entangled in high dimensions offer the potential for noise-robust quantum communication networks that harness the full information carrying capacity of a photon [1]. A central challenge in the realisation of such networks is the ability to precisely control and reliably transport high-dimensional entangled states of light. I will begin this talk by introducing the paradigmatic source of high-dimensional photonic entanglement—a nonlinear crystal generating a pair of photons entangled in the position-momentum degrees-of-freedom [2]. I will go on to discuss how knowledge of the continuous bi-photon wavefunction allows us to tailor discrete high-dimensional entangled states of light with record quality in a variety of spatial mode bases. As an example, I will present the generation and measurement of high-dimensional “pixel” entanglement with fidelities exceeding 94% and entanglement dimensionalities up to 55 [3]. In the second part of this talk, I will discuss an experiment where we transport high-dimensional entanglement through a complex scattering medium: a 2-metre section of commercial multi-mode fibre [4]. Interestingly, by using a property unique to high-dimensional entanglement, the medium is rendered transparent to entanglement by carefully “scrambling” the photon that did not enter it, rather than unscrambling the photon that did. I will go on to discuss how the same complex medium can be used for the precise manipulation of high-dimensional entangled states of light. I will present our “top-down” approach that allows us to program a general photonic quantum circuit within a commercial multi-mode fibre using methods of inverse-design. I will demonstrate how this programmability allows us to turn the multi-mode fibre into a generalised multi-outcome device, enabling us to both transport, manipulate, and certify entanglement within the transmission channel itself [5]. [1] V. Srivastav et al, “Quick Quantum Steering: Overcoming Loss and Noise with Qudits,” Physical Review X 12, 041023 (2022). [2] V. Srivastav et al, “Characterising and Tailoring Spatial Correlations in Multi-Mode Parametric Downconversion,” Physical Review Applied 18, 054006 (2022). [3] N.H. Valencia et al. “High-Dimensional Pixel Entanglement: Efficient Generation and Certification,” Quantum 4, 376 (2020). [4] N.H. Valencia et al. “Unscrambling Entanglement through a Complex Medium,” Nature Physics 16, 1112-1116 (2020). [5] S. Goel et al, “Inverse-design of high-dimensional quantum optical circuits in a complex medium,” arXiv:2204.00578 (2022).