Hefei, China · 3 December 2020 — A research team from the University of Science and Technology of China (USTC), led by Pan Jianwei and Lu Chao-Yang, published a paper in the journal Science demonstrating that their photonic quantum computer, named Jiuzhang, achieved quantum computational advantage using a technique called Gaussian boson sampling. The system detected up to 76 photons in approximately 200 seconds, performing a calculation that the team estimated would take China's Sunway TaihuLight supercomputer approximately 2.5 billion years to complete.
The result marked the second independent claim of quantum computational advantage, following Google's Sycamore processor in October 2019. However, while Google's system used superconducting loops of metal to form qubits, Jiuzhang used photons themselves as the quantum carriers — a fundamentally different physical approach to quantum computation.
The Experiment: Gaussian Boson Sampling
Boson sampling is a specialised quantum computational problem that is not useful for general-purpose computing but serves as a benchmark to demonstrate quantum advantage. The task involves sending photons through a complex optical network — a maze of beam splitters and phase shifters — and measuring the probability distribution of where they emerge. For a sufficiently large number of photons, the calculation of this distribution becomes intractable for classical computers.
Gaussian boson sampling, the specific variant used by Jiuzhang, employs squeezed light — a quantum state of light where the uncertainty in one optical property is reduced below the classical limit at the expense of increased uncertainty in the conjugate property. The experimental setup involved:
- A Verdi-pumped Mira 900 titanium-sapphire laser split into 13 paths of equal intensity
- 25 periodically poled potassium titanyl phosphate (PPKTP) crystals to produce 25 two-mode squeezed states
- Hybrid encoding equivalent to 50 single-mode squeezed states
- 12 nm filtering to increase purity from 98% to 99%
- A 100-mode interferometer through which the squeezed states were routed
- 100 single-photon detectors with 81% efficiency sampling the output
The system was named after Jiuzhang Suanshu (The Nine Chapters on the Mathematical Art), an ancient Chinese mathematical text dating to approximately the 1st century CE that laid foundations for algebra and geometry in Chinese mathematics.
Photonic vs. Superconducting Quantum Computing
The Jiuzhang result was notable for demonstrating quantum advantage through a completely different hardware platform than Google's Sycamore. As Scientific American explained: "Sycamore uses superconducting loops of metal to form qubits; in Jiuzhang, the photons themselves are the qubits."
| Feature | Google Sycamore (2019) | USTC Jiuzhang (2020) |
|---|---|---|
| Qubit type | Superconducting transmon | Photons |
| Task | Random circuit sampling | Gaussian boson sampling |
| Claimed runtime | 200 seconds | 200 seconds |
| Estimated classical time | 10,000 years (Summit) | 2.5 billion years (Sunway TaihuLight) |
| Operating environment | Dilution refrigerator (~15 mK) | Room temperature optical bench |
| Programmability | Gate-based, programmable | Fixed optical network |
The Quantum Advantage Debate
The concept of "quantum supremacy" or "quantum computational advantage" — the point at which a quantum device solves a problem that is infeasible for classical computers — has been a subject of both intense pursuit and scientific debate. Critics noted that both Google's and USTC's claims relied on specific, narrowly defined benchmark problems rather than practically useful computations.
IBM challenged Google's 2019 claim, arguing that an optimised classical algorithm could perform the same random circuit sampling in 2.5 days rather than 10,000 years. Similarly, subsequent advances in classical algorithms for boson sampling have narrowed the gap between quantum and classical performance estimates.
Nevertheless, the Jiuzhang result was significant as an independent verification that quantum advantage could be achieved on a different hardware platform, using a different computational problem, by a different research group. This suggested that quantum advantage was not an artefact of a single experimental setup but a reproducible physical phenomenon.
Research Team and Context
Pan Jianwei, the project's lead researcher, is one of China's most prominent quantum physicists and a pioneer in quantum communication and quantum computing. He led the development of the Micius satellite, which demonstrated quantum key distribution over intercontinental distances in 2017. Lu Chao-Yang, co-lead, designed the optical network architecture.
The USTC team has continued to upgrade Jiuzhang since 2020. Subsequent versions achieved higher photon counts and improved performance. The original 76-photon result was later improved to 113 detected photons with Jiuzhang 2.0 in 2021, and further upgrades continued through 2023 and beyond.
Implications for Quantum Technology
The Jiuzhang demonstration reinforced China's position as a leading nation in quantum technology investment and research. The Chinese government has invested billions of yuan in quantum science through the National Laboratory for Quantum Information Sciences, under construction in Hefei.
The result also intensified the global quantum computing race. The United States, European Union, and China have each launched multi-billion-dollar quantum technology initiatives, with photonic quantum computing emerging as a complementary approach to the superconducting and trapped-ion platforms being developed by companies like IBM, Google, IonQ, and Rigetti.
For the broader scientific community, Jiuzhang demonstrated that the path to quantum advantage was not monolithic — multiple physical platforms and computational paradigms could reach this threshold, each with distinct trade-offs in programmability, scalability, and error correction.
Sources
- Han, H.-S. et al., "Quantum computational advantage using photons," Science, vol. 370, pp. 1460-1463, 3 December 2020. DOI: 10.1126/science.abe8770
- Wikipedia, "Jiuzhang (quantum computer),"
- Science News, "Light-based quantum computer Jiuzhang achieves quantum supremacy," December 2020
- Scientific American, "Light-Based Quantum Computer Exceeds Fastest Classical Supercomputers," December 2020
- China Daily, "Quantum computer created," 5 December 2020
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Institutional Research Desk · Foresight Institute of Research and Translation
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