Indistinguishable photons from an artificial atom in silicon photonics

Integrated silicon photonics offers a promising path towards scalable quantum technologies due to its compatibility with advanced semiconductor manufacturing. Current approaches to fault-tolerant photonic quantum computation rely on probabilistic methods, lacking deterministic quantum light sources, efficient photon-photon gates, and quantum memories. This limitation significantly hinders scalability and requires substantial resource overhead. The integration of carefully controlled quantum emitters within reconfigurable photonic circuits promises hardware-efficient universal quantum computation and time-multiplexed quantum networking. Silicon photonics provides a mature platform for low-loss, reconfigurable integrated photonics. However, a crucial missing element has been a deterministic atomic source of indistinguishable photons in silicon. This research addresses this challenge by demonstrating the generation of telecom-band indistinguishable photons from an artificial atom—a G center—embedded within a silicon photonic waveguide.

The paper extensively reviews the current landscape of silicon-based quantum technologies, highlighting the limitations of existing approaches that rely on probabilistic methods for photon pair generation and two-qubit gate implementation. It cites various research efforts focusing on developing fault-tolerant photonic quantum computation, deterministic quantum light sources, photon-photon gates, and quantum memories, emphasizing the need for scalable solutions. The authors underscore the potential of integrating quantum emitters into reconfigurable photonic circuits for hardware-efficient quantum computation and networking, referencing relevant studies in this area. The lack of a deterministic atomic source of indistinguishable photons in silicon is identified as a major hurdle, motivating the current research.

The researchers fabricated a device consisting of a G center—an artificial atom—created within a silicon photonic waveguide using ion implantation. The G center's optical properties were characterized using various techniques. Spatial scanning with a free-space excitation beam (635 nm) and waveguide-coupled photon detection identified an isolated G center. The photon emission rate, saturation response, and lifetime were measured using time-resolved single-photon detection with both continuous-wave and pulsed (705 nm) excitation. The G center's emission linewidth was determined by analyzing the spectrum, deconvolving the cavity response to obtain the intrinsic linewidth. The single-photon nature of the emitted light was confirmed by measuring the second-order autocorrelation function, g(2)(0). Long-term stability of the single-photon emission was assessed by analyzing intensity correlations over extended time scales. Finally, the indistinguishability of successive photons from the G center was evaluated through a Hong-Ou-Mandel (HOM) interference experiment using a time-delayed Mach-Zehnder interferometer. This experiment involved manipulating the relative polarizations of the photons to optimize interference. The device fabrication involved standard microfabrication techniques, including electron beam lithography, etching, and deposition. The detailed fabrication steps are outlined in the supplementary material.

The study successfully demonstrated the generation of indistinguishable single photons from a G center in a silicon waveguide. Key findings include: (1) High-purity telecom-band single photons were generated. The measured photon rate was 18 kHz using a bandpass filter centered at the G center's zero-phonon line (ZPL). The saturation count rate was 35 kcps with a saturation power of 2.44 μW. (2) The G center exhibited a measured PL lifetime of 4.61 ns and an estimated radiative lifetime of 1.1 ns. (3) The emission linewidth of the G center was determined to be 2.0 ± 0.5 GHz, confirming single-photon emission. (4) Second-order autocorrelation measurements revealed antibunching behavior, confirming single-photon emission with g(2)(0) < 0.4. Long-term stability was also demonstrated. (5) The Hong-Ou-Mandel (HOM) interference experiment showed clear quantum interference, confirming the indistinguishability of successive photons emitted from the G center. The high degree of indistinguishability supports the use of this system for quantum applications. These results provide experimental confirmation of a key element for scalable silicon-based quantum photonic technologies.

The successful generation of indistinguishable single photons from a G center in a silicon waveguide directly addresses a critical limitation in scalable silicon photonics-based quantum technologies. The achievement of high-purity, telecom-band single photons with demonstrable indistinguishability opens up new avenues for building integrated photonic quantum networks and processors. The long-term stability of the single-photon emission further enhances the practical applicability of this system. The results highlight the potential of using artificial atoms like the G center for deterministic quantum light sources in silicon. This platform can be extended by introducing emerging silicon artificial atoms with electron and nuclear spins, leading to the implementation of quantum processors and repeater building blocks. The integration of these components with advanced manufacturing capabilities promises to enable the scalability of spin- and photon-based quantum processors and repeaters.

This research presents a significant advancement in silicon photonics by demonstrating the generation of indistinguishable single photons from an artificial atom (G center) embedded in a silicon waveguide. This achievement overcomes a major hurdle in scaling silicon-based quantum technologies. Future research could focus on improving the photon emission rate, exploring different types of artificial atoms in silicon, and integrating these sources into more complex quantum photonic circuits for building quantum networks and processors.

While the study demonstrates the generation of indistinguishable photons, the photon generation rate could be further improved. The g(2)(0) value, while below 0.4, is not ideally close to 0, which may be partially due to imperfect extinction in pulsed laser downsampling and dark counts. Further optimization of the device design and experimental setup might lead to higher indistinguishability and efficiency. The research focused on a single G center; further studies are needed to investigate the scalability and reproducibility of the results across multiple centers.