Deterministic photon source interfaced with a programmable silicon-nitride integrated circuit

Single photons are crucial for advancements in quantum technologies like secure communication and quantum computing. A major hurdle in creating scalable photonic quantum hardware is effectively interfacing high-quality single-photon sources (SPSs) with advanced PICs. Solid-state quantum emitters, such as quantum dots (QDs) in materials like GaAs or defect centers in diamond, offer deterministic generation of near-ideal single photons. However, large-scale PIC fabrication on these materials is challenging. A hybrid approach, where a source chip feeds a PIC made from a mature platform like SiN, is more promising. SiN offers several advantages: it's CMOS-compatible, enabling scalability; it exhibits low losses; and it's compatible with complex programmable circuits and cryogenic operation. While programmable SiN circuits have been demonstrated, they previously used probabilistic photon sources. This work presents a modular photonic platform that integrates an InGaAs QD in a GaAs photonic-crystal waveguide with a programmable, low-loss SiN PIC. The platform successfully performs high-fidelity multiphoton on-chip operations showcased through bosonic suppression laws and probabilistic entangled Bell state generation.

The paper reviews existing literature on single-photon sources and their integration with photonic integrated circuits. It highlights the challenges of using III-V semiconductors like GaAs for large-scale PICs and the advantages of SiN as a mature and CMOS-compatible alternative. The authors cite previous work on SiN PICs, focusing on their programmability and low loss characteristics, but also acknowledging the lack of integration with high-quality, deterministic single-photon sources like quantum dots. Existing work on heterogeneous integration of different material platforms for quantum photonic circuits is also reviewed, emphasizing the need for improved efficiency and reduced losses in coupling the photon sources to the PICs.

The researchers used InAs QDs embedded in a p-i-n GaAs diode heterostructure as the solid-state single-photon emitters. A distributed Bragg reflector (DBR) was implemented to enhance vertical emission. High-quality metal gates were used to control charge noise and tune the emission wavelength via the Stark effect. A photonic crystal waveguide (PCW) planar structure interfaced with the QDs, boosting coupling efficiency. A photonic crystal mirror ensured single-sided emission. The QD source operated at 1.6 K and was excited by a pulsed laser. The emitted photons were collected using a shallow-etched grating (SEG) and coupled to a SiN PIC via optical fibers. The SiN PIC used rectangular waveguides with SiO2 cladding and incorporated thermal phase shifters for reconfigurability. The coupling to and from the chip was done using a fiber array and on-chip waveguide tapers. Directional couplers were designed for 50:50 splitting. The waveguide propagation loss was measured using the cut-back method. A free-space demultiplexer spatially separated and synchronized consecutive photons from the QD to create simultaneous photon pairs for input into the PIC. The single-photon source was characterized using free-space optics (measuring lifetime, single-photon purity, and HOM interference), and then compared to on-chip characterization using the SiN PIC (Hanbury-Brown and Twiss experiment, on-chip HOM interference). Experiments were conducted to demonstrate bosonic suppression laws using a four-mode DFT interferometer, and to generate and characterize photonic entanglement (using state tomography to reconstruct the density matrix of the generated Bell state).

The integrated quantum photonic platform demonstrated high performance. The single-photon source exhibited a neutral exciton lifetime of 917 ps, near-unity single-photon purity (99.2% in free-space, 99.5% on-chip), and a high HOM interference visibility (94.5 ± 1.7% in free-space, 94.3 ± 1.2% on-chip). The SiN PIC showed a propagation loss of ~0.3 dB/cm at the QD emission wavelength and coupling losses between 4.25 and 7 dB (primarily attributed to a mismatch between the device's optimal wavelength and the target wavelength). The experiment successfully demonstrated bosonic suppression laws in a four-mode DFT interferometer, showing good agreement with theoretical predictions. High-fidelity generation of entangled Bell states was also achieved, with a fidelity of 92% with the ideal |ψ⁺⟩ state, as determined by quantum state tomography. The platform exhibited minimal degradation of photon properties when using the SiN PIC.

The results demonstrate a successful integration of a high-quality QD single-photon source with a programmable SiN PIC, enabling on-chip multiphoton experiments relevant to quantum information processing. The successful demonstration of bosonic suppression laws and entangled Bell state generation using a solid-state photon source for the first time highlights the platform's potential. The observed infidelities are mainly attributed to partially distinguishable photons and the non-ideal coupling efficiency, both of which can be improved in future work. This work represents significant progress toward building large-scale photonic quantum technologies.

This research successfully demonstrated a modular quantum photonic platform combining a high-quality quantum dot single-photon source with a low-loss, scalable SiN PIC. The platform's capabilities were shown through on-chip multiphoton experiments, including bosonic suppression laws and photonic entanglement generation. Future improvements, such as optimizing component design and reducing coupling losses through heterogeneous integration, will pave the way for experiments with a larger number of photons and the development of large-scale photonic quantum technologies. The use of more suitable phase shifters for cryogenic operation is also suggested for future iterations.

The primary limitations are the coupling losses between the QD source and the SiN PIC, and the limited indistinguishability of the emitted photons. While the coupling losses are partially due to a mismatch in optimal wavelengths and can be addressed with design improvements, further optimization is needed to significantly reduce them. The limited photon indistinguishability contributes to the infidelity in the Bell state generation and the incomplete suppression in the bosonic suppression law experiments. Additionally, the thermal phase shifters used are not optimal for cryogenic operation, which would be necessary for direct heterogeneous integration of the source with the SiN PIC.