Insider Brief
- Chinese researchers have developed a single-photon source with 71.2% efficiency, surpassing the threshold required for scalable photonic quantum computing.
- The system, based on a quantum dot embedded in a tunable microcavity, minimizes photon loss while maintaining high purity and indistinguishability.
- While the approach addresses a key scalability challenge, practical implementation still faces hurdles, including cryogenic operation and detector efficiency limits.
Chinese researchers have developed a single-photon source that meets the efficiency threshold needed for scalable photonic quantum computing, overcoming a longstanding technical barrier in the field.
Reporting their findings in Nature, the team led, by Jian-Wei Pan at the University of Science and Technology of China, demonstrated a system with an efficiency of 71.2%, surpassing the two-thirds threshold required for quantum error correction to function in photonic quantum computing. The source also maintains a low error rate and high indistinguishability, critical parameters for reliable quantum operations.
The results could significantly advance the prospects of building large-scale photonic quantum computers, the researchers report.
How It Works
Photonic quantum computers use individual particles of light — photons — to perform calculations. These systems are attractive because photons move quickly, interact weakly with their surroundings and can operate at room temperature. However, a major challenge has been photon loss, which disrupts calculations and prevents error correction from working effectively. Previous single-photon sources have failed to reach the necessary efficiency, limiting their scalability.
The research team addressed this by embedding a quantum dot—a semiconductor nanostructure that can emit single photons—inside an open microcavity that enhances photon collection. The cavity’s tunability allowed the researchers to maximize the coupling between the quantum dot and the emitted photons, increasing efficiency without sacrificing purity or indistinguishability.
“This source for the first time reaches the efficiency threshold for scalable photonic quantum computing,” the authors wrote.
The source achieved an efficiency of 71.2%, a photon indistinguishability of 98.56%, and an extremely low multi-photon error rate of 2.05%. The results suggest that this approach could be viable for building larger quantum systems based on photons.
Quantum Communication Networks
The ability to generate high-efficiency single photons is critical for advancing fault-tolerant quantum computing, which depends on quantum error correction to mitigate noise. Until now, photonic quantum computers have been limited to noisy, intermediate-scale demonstrations that quickly lose accuracy as they scale up. The new source brings photonic quantum computing closer to a future where errors can be corrected, enabling more reliable and complex calculations.
Without effective quantum error correction, the fidelity of quantum computations drops exponentially with scale, the researchers noted.
The advance might not just be of interest to photonic quantum computer developers. The new photon source could be used, for example, in quantum communication networks and cryptographic protocols. The team suggests the approach could also power computational tasks such as boson sampling, a quantum algorithm that has been used to demonstrate quantum advantage.
Fully Funable Cavity
The researchers achieved their results using a quantum dot embedded in a highly efficient open-cavity system. Unlike previous designs, this cavity is fully tunable, allowing precise spatial and spectral alignment of the quantum dot to optimize photon extraction.
The cavity consists of a top concave mirror and a bottom planar mirror, forming a Fabry-Pérot resonator. The resonator helps trap and enhance photons, making them easier to detect and use in quantum experiments. By fine-tuning the cavity length and laser excitation conditions, the researchers were able to maximize photon emission while minimizing noise.
To excite the quantum dot, they used a specially shaped laser pulse that ensured efficient photon generation. They also cooled the system to 4 kelvins to stabilize the quantum dot’s performance. The emitted photons were collected through a single-mode fiber and measured using superconducting nanowire single-photon detectors.
Limitations and Future Directions
The researchers indicate that several challenges remain. The current system operates at cryogenic temperatures, which limits practical applications. While photons themselves can function at room temperature, the quantum dot requires cooling to maintain stability. Researchers are exploring alternative materials and designs that could allow operation at higher temperatures.
Additionally, the experiment used a single quantum dot, which is not easily scalable to large numbers of qubits needed for universal quantum computing. Future work will need to integrate multiple quantum dots or alternative photon sources that can be mass-produced with high consistency.
Another limitation is the reliance on superconducting detectors with an efficiency of 79%. If detection efficiency is improved beyond 93.7%, the overall system efficiency could surpass the required threshold even further. Advancements in superconducting nanowire technology suggest this is feasible in the near future.
Research Team
The study was conducted by Xing Ding, Yong-Peng Guo, Mo-Chi Xu, Run-Ze Liu, Geng-Yan Zou, Jun-Yi Zhao, Zhen-Xuan Ge, Qi-Hang Zhang, Hua-Liang Liu, Lin-Jun Wang, Ming-Cheng Chen, Hui Wang, Yu-Ming He, Yong-Heng Huo, Chao-Yang Lu and Jian-Wei Pan. The researchers are affiliated with the Hefei National Research Center for Physical Sciences at the Microscale and the CAS Center for Excellence in Quantum Information and Quantum Physics at the University of Science and Technology of China.
