IQC PhD thesis defence - Sayan Gangopadhyay

Candidate: Sayan Gangopadhyay
Supervisor: Michael Reimer
Location: QNC 2101

Quantum photonic technologies require bright, deterministic sources of entangled photons. Applications such as quantum networks further demand high single-photon indistinguishability, a key requirement for quantum interference in protocols such as entanglement swapping. The highest values of indistinguishability are achieved using resonant excitation. Semiconductor nanowire quantum dots are among the brightest on-demand sources of high-fidelity entangled photon pairs generated through the biexciton-exciton cascade. However, implementing resonant excitation in these systems has remained a long-standing challenge due to the stringent laser suppression required in nanowire geometries. 

In this thesis, we establish a robust technique for resonant excitation of a quantum dot embedded in a tapered single-mode nanowire waveguide. By engineering mode matching between the incident laser and the nanowire-guided mode, efficient coupling to and from the quantum dot is achieved while simultaneously suppressing back-scattered laser light. This approach enables the realization of a one-dimensional atom, in which coherent single-photon reflection is observed. Furthermore, by combining mode matching with polarization-based rejection, we achieve laser suppression on the order of 10⁶, enabling the generation of single photons under pulsed resonant excitation. 

Under these conditions, we observe clear Rabi oscillations and strong antibunching in second-order correlation measurements. Two-photon interference measurements yield an indistinguishability of 0.41 at a temporal separation of 12.5 ns, indicating that residual decoherence mechanisms, such as charge noise, limit performance in the current devices. Motivated by this limitation, we propose a broadband nanowire cavity based on a quasi-bound state in the continuum (quasi-BIC) design. This cavity leverages interference between two resonances to simultaneously achieve a Purcell enhancement of 17, high extraction efficiency of 74%, and a directional emission with an 88% Gaussian overlap over a spectral bandwidth of 4 nm, sufficient to enhance both photons in the biexciton–exciton cascade. 

These results establish resonant excitation in nanowire quantum dots as a viable route toward generating indistinguishable single photons, while highlighting the role of residual decoherence mechanisms that currently limit performance. The proposed quasi-BIC cavity design further provides a pathway toward enhancing emission rates and photon indistinguishability in this platform. Furthermore, the realization of a one-dimensional atom in a nanowire platform opens new opportunities for exploring waveguide quantum electrodynamics, including emitter-mediated photon-photon interactions.