Self-Aligned Heterogeneous Quantum Photonic Integration

TL;DR

Self-aligned heterogeneous integration addresses the mismatch between solid-state quantum emitters and scalable photonics by enabling near-unity coupling at interfaces and leveraging broad material compatibility. The authors demonstrate a diamond-$TiO_2$ platform with a self-guided nanobeam insertion and conformal $TiO_2$ deposition to realize a heterogeneous photonic crystal cavity, achieve Purcell enhancement of SiV centers, and implement chip-scale optical spin control. They further show inverse-design-based broadband single-photon extraction into a heterogeneous waveguide, achieving high collection efficiency across a narrow spectral window and favorable orientation dependence. Collectively, these results establish a practical route to scalable quantum photonic integrated circuits that combine high-quality solid-state emitters with mature thin-film photonics and programmable device functionality for quantum networking and processing.

Abstract

Integrated quantum photonics holds significant promise for scalable photonic quantum information processing, quantum repeaters, and quantum networks, but its development is hindered by the mismatch between materials hosting high-quality quantum emitters and those compatible with mature photonic technologies. Heterogeneous integration offers a potential solution to this challenge, yet practical implementations have been limited by inevitable insertion losses at material interfaces. Here, we present a self-aligned heterogeneous quantum photonic integration approach that can deterministically achieve near-unity coupling efficiency at the interface. To showcase our approach, we demonstrate Purcell enhancement of a silicon vacancy (SiV) center in diamond induced by a heterogeneous photonic crystal cavity defined by titanium dioxide (TiO2), as well as optical spin control and readout via a TiO2 photonic circuit. We further show that, when combined with inverse photonic design, our approach enables efficient and broadband collection of single photons from a color center into a heterogeneous waveguide. Our approach is not restricted to SiV centers or TiO2; it can be broadly applied to integrate diverse solid-state quantum emitters with thin-film photonic devices where conformal deposition is possible. Together, these results establish a practical route to scalable quantum photonic integrated circuits that combine high-quality quantum emitters with technologically mature photonic platforms.

Figures (5)

• Figure 1: Self-aligned heterogeneous photonic integration. (a) Workflow of the self-aligned heterogeneous photonic integration. Step 1: patterning of the inverse of the full photonic circuit into a photoresist layer. Step 2: pick-and-place of a diamond nanobeam near the entrance of the slot defined by the photoresist. Step 3: insertion of the diamond nanobeam into the slot via a self-guided process. Step 4: conformal deposition of TiO$_2$ over the entire substrate via atomic layer deposition. Step 5: removal of excess TiO$_2$ via back etching. Step 6: removal of the photoresist. (b) Optical microscope images showing different stages during the nanobeam insertion process. (c) SEM image of the device right after the insertion of the diamond nanobeam (Step 3).
• Figure 2: Analysis of external coupling loss for our self-aligned heterogeneous photonic devices. (a) Schematic illustration of the interface at the diamond-TiO$_2$ heterogeneous waveguide and a monolithic TiO$_2$ waveguide. (b), (c) Calculated electric-field intensity profile in the cross-section plane of the diamond-TiO$_2$ heterogeneous and TiO$_2$ monolithic waveguides. (d) Calculated insertion loss at the interface between the two waveguides.
• Figure 3: Purcell enhancement induced by a heterogeneous photonic crystal cavity. (a) SEM image of the diamond-TiO$_2$ heterogeneous photonic crystal cavity. (b) Calculated longitudinal electric field intensity profile of the heterogeneous photonic cavity. (c) Transmission spectrum of a heterogeneous photonic crystal cavity, yielding a $Q$ factor of 4600(180). (d) Photoluminescence spectra of an embedded SiV center as the cavity resonance is tuned in time via gas condensation. The white dashed line indicates the resonant frequency of the cavity mode. (e) Photoluminescence spectra of a SiV center when it is resonant (blue) and far detuned (red) from the cavity mode. For (c) - (e), the spectra was obtained by exciting the cavity or the SiV center via free space and collecting the signal from the inverse-designed grating coupler on the right end.
• Figure 4: Optical spin initialization and readout via a TiO$_2$ photonic circuit. (a) SEM image of a TiO$_2$ 80/20 insertion coupler integrated with a diamond nanobeam. The diamond nanobeam is embedded inside the upper arm of the TiO$_2$ waveguide. The inset shows the scanning confocal photoluminescence image of the region with the inserted diamond nanobeam, showing a large density of SiV centers embedded inside. (b) Photoluminescence spectra of the SiV centers collected from the top of the integrated diamond nanobeam via free space. (c) Photoluminescence spectra of the same SiV centers collected from the inverse-designed grating coupler at port C1. (d) Photoluminescence excitation spectrum of the optical C transition of a specific SiV center under a magnetic field around 0.3 T. The two peaks correspond to the two spin-conserving transitions. (e) Optical initialization and readout of the SiV spin via the photonic circuit. (f) Measurement of spin relaxation time via a pump-probe sequence.
• Figure 5: An inverse-designed one-way quantum light extractor. (a) Design geometry of the quantum light extractor overlaid with the calculated electric field distribution when excited by a dipole (blue arrow) polarized along the TE-mode of the waveguide. (b),(c) Calculated collection efficiency (b) and Purcell factor (c) for four possible orientations of a SiV center.