Chinese scientists demonstrate quantum supremacy

Boson sampling device can accomplish a specific task in 200 seconds that would take the most powerful supercomputer 600 million years

A team of scientists, led by Jian-Wei Pan and Chao-Yang Lu of the University of Science and Technology of China (USTC) in Shanghai, claims to have demonstrated the feat of quantum supremacy by performing certain computations nearly 100 trillion times faster than the world's most advanced supercomputer.

The concept of quantum supremacy was developed by John Preskill, the Richard P. Feynman professor of theoretical physics at the California Institute of Technology, as a test that would show that a quantum computer can complete a complex calculation that a conventional computer cannot do within a reasonable length of time.

In the latest research, USTC scientists said that they have designed a boson sampling device (an optical circuit involving mirrors, lasers, prisms and photon detectors), dubbed Jiuzhang, which is capable of achieving quantum supremacy.

Boson sampling is a technique that involves computing the output of a linear optical circuit that has multiple inputs and multiple outputs. A single beam of light is passed through a beam splitter, which splits the beam into two separate beams which then propagate in different directions.

If two identical photons hit the beam splitter at the same time, they do not split from one another; instead they stick together and start moving in the same direction.

A boson sampling device can be considered a type of quantum computer, although one with a very limited purpose.

In the current study, published in Science, the scientists used Jiuzhang to send laser pulses into a maze of 300 beam splitters and 75 mirrors arranged in a random layout, and were able to predict the interference patterns with a fidelity of 0.99 over many trials, the maximum possible measure being 1 (meaning that all theoretical predictions are realised).

Such predictions are extremely difficult using classical computers, and the team calculated that the world's most powerful super computer - Japan's Fugaku - would take about 600 million years to do what Jiuzhang can accomplish in just 200 seconds. Sunway TaihuLight, the world's fourth most powerful supercomputer, would take around 2.5 billion years to do the same computation.

The experiment has little practical use as Jiuzhang was set up specifically to achieve this one task, but it does demonstrate the potential of quantum computing to solve problems that are practically insoluble with current computing technologies.

This is the second time that researchers have claimed to achieve quantum supremacy. Last year, Google said that its Sycamore quantum processor was able to achieve "quantum supremacy" for the first time, surpassing the performance of conventional devices. Google researchers claimed that Sycamore performed a specific task in 200 seconds that would take the world's best supercomputer nearly 10,000 years to complete.

IBM, however, disputed the 'quantum supremacy' claims made by Google. IBM quantum computing specialists Edwin Pednault, John Gunnels and Jay Gambetta stated in a blog post that a conventional computer would take just days, not 10,000 years, to do the same task completed by Google's Sycamore quantum machine in 200 seconds.

Nevertheless, most scientists believe the Google experiment was a true demonstration of quantum supremacy.