A section of the light-based quantum computer developed by researchers at the University of Science and Technology of China. [PhotoXinhua]
HEFEI – At a lab in the east Chinese city of Hefei, near a canopy-like freezer suspended from the ceiling, a team of scientists are busy installing a quantum computer.
Li Shaowei has played a big role in this task. The research engineer takes a palm-sized cube from a box. The 3D-encapsulated device, a 176-qubit quantum processor, is designed to achieve computational acceleration not currently possible for conventional computers, a capability known as “quantum supremacy.” Instead of labyrinthine transistors, this processor contains meticulously printed Josephson junctions, a structure less than a micron in size that features a thin layer of aluminum oxide sandwiched between two layers of aluminum.
As the temperature approaches absolute zero, two layers of aluminum exhibit zero resistance, or superconductivity, to form quantum states that can support exponentially growing computing capacity. But now the quantum-based circuit wafer is sealed in a metal case, with nearly 200 holes allowing it to connect to the wires dangling from the freezer above. Standing on a table, it takes Li about two hours to complete the connection work.
Over the past six years, more than 10 superconductivity-driven prototypes have been created. Among them is Zuchongzhi 2.1, which is currently running and making a repetitive noise in another lab in the building. Zuchongzhi 2.1 is a 66-qubit programmable quantum computer system manufactured in 2021 that can run large-scale random quantum circuits sampling about 10 million times faster than the fastest supercomputer at the time.
“Like Zuchongzhi 2.1, the new system has 66 digital qubits plus 110 coupling qubits,” says Li, who works at the University of Science and Technology of China. “I have to connect each qubit to the appropriate wire, otherwise they could lose control.”
The team then manages to package the quantum system in matryoshka doll-like vacuum vessels of different sizes and use liquid helium to realize an absolute zero working environment.
The cylinder-shaped computer prototype reminds Xu Yu, a post-doctoral researcher, of Zordon from Eltar, the protagonist in the US superhero TV series Power Rangers, whose head is soaked in a glass.
According to Xu, the assembled system, a replica of Zuchongzhi 2.1, will be connected to a quantum computing cloud platform to make it available to other research institutions and even to the public.
In 2019, Google reported the existence of a 53-qubit superconducting processor called the Sycamore that performed the quantum random sampling task in 200 seconds, and the designers said at the time that the world’s fastest supercomputer would take 10,000 years to complete to produce a similar output.
The 66-qubit Zuchongzhi 2.1 is 1 million times faster than the speed Sycamore was able to achieve in 2019.
“Superconducting computing is a technical avenue believed to be the best strategy to advance quantum computing to a stage where it can be used for tasks akin to general-purpose computers,” says Xu.
Another team of USTC scientists has developed a prototype quantum computer called Jiuzhang using a photon-driven strategy. In 2021, Jiuzhang 2.0 was shown to have implemented Gaussian boson sampling, a classic simulation algorithm, with 113 photons detected, which was a trillion times faster than the world’s fastest existing supercomputer.
These two technical avenues have placed China at the forefront of quantum computing development and commercialization, making the country one of the world leaders in the sector.
China is also home to a number of startups working on quantum technology, such as Origin Quantum, a Hefei-based unicorn company that has launched OriginQ Cloud, a full-stack cloud service platform for quantum computing.
Currently, quantum computers excel at certain less practical tasks such as quantum random sampling and Gaussian boson sampling. In the near future, more practical quantum computers are expected to take artificial intelligence to new heights, developing entirely new materials, chemicals and medicines.
Having achieved “quantum supremacy,” scientists in the global quantum computing race are now putting more emphasis on improving machine performance by focusing on error correction and qubit lifetime.
“With well-known algorithms, high-quality automatic error correction for a quantum computing prototype can require thousands of qubits,” says Xu.