Site-controlled quantum dot arrays edge-coupled to integrated silicon nitride waveguides and devices

TL;DR

This work demonstrates the first active, cryogenic alignment and edge-coupling of arrays of site-controlled GaAs quantum dots to silicon nitride waveguides, using self-aligned nanopillars to achieve deterministic, scalable SPS integration with on-chip photonic devices. The ten-QD to ten-waveguide coupling yields a consistent waveguide-coupled signal of $0.17 \pm 0.02$ relative to direct collection, corresponding to an absolute coupling efficiency of about $0.05 \pm 0.02$ when accounting for all steps; the emitters show low inhomogeneous broadening with spectral overlap across adjacent dots. High-purity single-photon emission is verified via $g^{(2)}(0)$ measurements (e.g., $0.04 \pm 0.01$ off-chip, $0.11 \pm 0.02$ on-chip with an MMI beamsplitter), and an on-chip beamsplitter confirms functional distribution of photons. The approach offers scalable, reversible hybrid integration of deterministic SPSs with PICs, enabling potential scaling to hundreds or thousands of emitters and providing a platform for practical quantum information protocols, with future improvements in pillar design, tapering, and 3D photonic integration.

Abstract

The scalability of quantum photonic integrated circuits opens the path towards large-scale quantum computing and communication. To date, this scalability has been limited by the stochastic nature of the quantum light sources. Moreover, hybrid integration of different platforms will likely be necessary to combine state-of-the-art devices into a functioning architecture. Here, we demonstrate the active alignment and edge-coupling of arrays of ten site-controlled gallium arsenide quantum dots to an array of ten silicon nitride single-mode waveguides, at cryogenic temperatures. The coupling is facilitated by the fabrication of nanopillars, deterministically self-aligned around each quantum dot, leading to a high-yield and regular array of single-photon sources. An on-chip beamsplitter verifies the triggered emission of single photons into the silicon nitride chip. The low inhomogeneous broadening of the ensemble enables us to observe the spectral overlap of adjacent site-controlled emitters. Across the array of waveguides, the signal collected from each coupled quantum dot is consistently and reproducibly 0.17 relative to the free-space collection from the very same single-photon source. Comparing measurement with waveguide simulations, we infer that absolute coupling efficiencies of $\approx 5 \%$ are currently obtained between our quantum dots and the waveguides.

Figures (6)

• Figure 1: Experimental arrangement and sample details. (a) The QD-nanopillar array positioned orthogonally underneath a SiN single-mode waveguide array, with SEM and optical images of the chips, respectively. Both the SiN waveguides and the QD-pillars are separated with 10 µm pitch. The top inset shows an optical image of the waveguide facets. (b) Photo of the experimental arrangement used to achieve the edge-coupling, with a red laser coupled through a SiN waveguide down to the QD chip. (c) A close-up of one of the fabricated nanopillars. (d) A schematic of the nanopillar, showing the lateral etching. For clarity, the carrier confinement layers immediately above and below the QD have been omitted.
• Figure 2: Active alignment of a QD-pillar to a single-mode waveguide. (a) Final stages of alignment. The spectra are offset but shown on the same scale. The excitation power was set to give the maximum signal and the QD-pillar was aligned in $xyz$, and these steps were iteratively optimized. After the signal was close to maximum, further improvement was noticeable by a several-fold reduction in the excitation powers, which are indicated in the figure. Also shown (dotted lines) are spectra taken during over-contact, where the spectrum was significantly broadened -- this was then reversed by increasing the $z$ distance. (b) In the final stages of chip-chip alignment, the QD chip could come into contact with the PIC, causing a small redshift due to strain. Small redshifts were generally reversible by increasing the separation.
• Figure 3: Waveguide-coupled single-photon measurement. (a) Normalized spectra of QD emission obtained through the single-mode waveguide under above-band excitation (black). The quasi-resonant laser (blue) was tuned to one of the hot trion states $X^{+*}$, and subsequently filtered with notch filter(s). (b) Under this excitation the $X^+$ line was selectively excited. TCSPC: time-correlated single-photon counter (c) Raw autocorrelation measurement of the $X^+$ line of a QD-pillar when excited quasi-resonantly through the hot trion $X^{+*}$ state, obtained through a waveguide and an off-chip beamsplitter.
• Figure 4: Single-photon verification using the on-chip beamsplitter. (a) Normalized (and effectively background-subtracted) spectra of two adjacent QDs taken directly (red and blue), and through the two output ports of the MMI (green and orange) while exciting both QDs non-resonantly through one port with an above-band laser. (b) Spectra of the QD emission obtained through the two output ports of the MMI when exciting through the hot-trion $X^{+*}$ state. (c) Raw $G^{(2)}(\tau)$ measurement. Correlation of the $X^+$ line from the two output ports of the MMI.
• Figure 5: Coupling to the waveguide array. (a) Comparison of the spectrum of a single QD obtained through a SiN waveguide (red) and directly via an objective lens (blue), at 13 K. (b) Under the same chip-chip alignment, we obtained 10 signals from 10 sequential QDs through the single-mode waveguide array. (c) Spectra of 10 sequential QDs obtained through a SiN single-mode waveguide array (red) and directly via an objective lens (blue). (d) The ratio of the integrated signals observed in (c), with uncertainties due to the peak fitting procedure.