Chip-Integrated Nanowire Quantum Dot Emitters for Scalable Quantum Photonics

Abstract

Single-photons are a fundamental component in many quantum optical technologies. An example of where single-photons are needed is in quantum cryptographic technologies such as the BB84 quantum key distribution protocol. This involves encoding data in the polarization of single-photons to share a key between the two parties. Such applications require a single-photon source that ideally can emit exactly one photon on-demand with a well-defined polarization and spatiotemporal mode. Among the various candidates, semiconductor quantum dots are two-level systems that can be tailored to emit single-photons at desired wavelengths. However, since the quantum dots emit randomly in all directions, they are typically embedded into different photonic structures for more efficient coupling to optical fibre systems.

In particular, bottom-up III-V nanowires embedded with quantum dots are promising single-photon sources for integration with large-scale quantum photonic circuits. To test the feasibility of a nanowire-Si₃N₄ waveguide integration, simulations of the power transfer from the fundamental HE₁₁ mode of a tapered nanowire to an underlying ridge waveguide predicted >95% coupling efficiency. The mode within the nanowire is expanded as the light propagates up the nanowire taper, making it compatible for both collection with a free-space objective or to evanescently couple to an in-plane straight waveguide. In this thesis, different hybrid quantum photonic integrated circuit devices are demonstrated to efficiently harvest single-photons generated on-chip.

Through a pick-and-place method, individual nanowires can be transferred from the growth substrate to a chip containing Si₃N₄ photonic waveguides. This hybrid integrated device combines the efficient emission of quantum dots with the low-loss characteristics of waveguides. Coupling light on and off the chip is performed using lensed fibres. An evanescent coupling efficiency of η_c ~ 84.8% was measured with one device. Indistinguishable photons were successfully generated on-chip using above-band excitation and quasi-resonant (p-shell) excitation to obtain two-photon visibilities of 9.2% and a much improved 19.2%, respectively, over the temporal extent of the emitted photons.

A major limitation of these devices is that single-photons are more efficiently collected from the tapered end of the nanowire as opposed to the base. To collect the photons emitted in both directions, a curved waveguide with a long untapered nanowire can be used. This requires the precise placement of the quantum dot adjacent to the curved waveguide. To increase the tolerance for the quantum dot position along the curve, a curved segmented waveguide was investigated. A coupling efficiency of 63% was obtained from the combined counts of both the X and X⁻ transitions in a single dot.

For large-scale quantum networks, stable single-photon sources with a small footprint are desirable. The need for active alignment is eliminated with a plug-and-play source which allows for easy access to multiple fibre-coupled sources simultaneously. A fibre-packaged chip consisting of straight waveguides and tapered nanowire-quantum dots is demonstrated to have a total plug-and-play efficiency of 3.6 ± 0.7% simultaneously collecting from both ends of one device. The multiphoton emission probability of the device is g⁽²⁾(0) = 7.2%. The fibre-packaged chip was robust against multiple cooldown cycles and showed stable alignment over 27 hours of continuous operation.

The devices demonstrated in this thesis serve as proof-of-concept platforms for in-plane single-photon generation. Based on this work, single photons can then be manipulated with standard photonic components including ring-resonators, Mach-Zehnder interferometers, phase shifters for more complex applications on-chip.