PhD Defense: Camille Papon

Photonic circuits with multiple quantum dots: towards scalable operation of deterministic single-photon sources

Single photons represent a major asset for the development of quantum technologies, owing to their compatibility with advanced photonic integrated circuits, ultimately enabling the realization of large-scale quantum processors. Generating the necessary large photonic resource requires scaling up integrated deterministic single-photon sources (SPS), a challenging task due to emitter-to-emitter disparity in wavelength and position. Here, we experimentally implement a strategy to control multiple solid-state quantum emitters directly integrated into photonic circuits, to generate multi-photon states on-chip.

More specifically, we employ low-noise InAs quantum dots (QD) integrated into p-i-n GaAs nanophotonic waveguides, which have been developed over the past few years to generate indistinguishable single photons. The strong-light matter interaction in nanophotonic structures ensures deterministic operation, leading to a high single-photon count rate. Additionally, the planar quantum photonic platform offers the opportunity to integrate the control of SPSs through dedicated circuits, ultimately enabling the realization of a multi-QD circuit.

We first demonstrate a small-scale multi-QD photonic circuit enabling the simultaneous operation of two waveguide-integrated SPSs. To do so, we make use of dual-mode waveguides, where one mode is used for excitation and the second one for collecting single photons, enabling fully waveguide-based resonant excitation and laser filtering. We optically address these two "plug-and-play" SPSs in parallel using a polarization diversity grating to perform on-chip distribution of a single continuous-wave laser to two QDs. The pair of quantum dots is brought into mutual resonance by applying independent bias voltages across the p-i-n diode with locally-isolated electrical contacts, thereby tuning the QDs emission wavelength individually. Each of the waveguide-integrated QDs exhibit excellent single-photon generation as characterized by g(2)(0) ≪ 0.5. Two-photon quantum interference between the two mutually resonant QDs is measured, with a peak visibility of V = 79 ± 2%, limited by imperfect laser suppression.

To overcome this limit, mainly caused by fabrication disorder, we investigate a novel scheme for preparing the mode for excitation in a dual-mode waveguide based on asymmetric directional couplers. Owing to the bi-directionality of the single-photon emission, this device represents a natural source of dual-rail encoded qubits emitted by a single QD. This is confirmed by measuring the second-order correlation at the device output ports, characterized by a g(2)(0) < 0.07 in deterministic pulsed excitation. 

The results demonstrated in this thesis embody a strategy for integrating multiple quantum emitters in photonic integrated circuits, with foreseeable application in quantum simulation and quantum communication.

The thesis can be found here.