Written by Gloria Wang

From hundreds of millions of private investment to grand claims about a computing revolution, quantum computing has changed people’s approach to the future of technology, and rightfully so. Just this month, a team of researchers in China have developed and tested a photonic quantum computer claimed to be one trillion times faster than the most powerful supercomputer. Called Jiuzhang, the quantum computer is the first-ever definitive demonstration of quantum supremacy, the goal of being able to perform tasks that no classical computer could in a reasonable amount of time. In a 2020 paper published in the journal Science, Zhong et al. present their new invention.

The team used laser beams to perform Gaussian boson sampling (GBS), a computation that is estimated to take classical computers at least 0.6 billion years to complete. Jiuzhang took only 200 seconds. Gaussian boson sampling is the calculation of the probability distribution of bosons — a fundamental class of particles that includes photons — where the position of the particles is essentially arbitrary due to wave interference. The boson states have a strong connection to the Gaussian distribution, hence the name.

Setup of the photonic quantum computer Jiuzhang used by Zhong et. al. (Science).

As a ‘#P-hard problem,’ the calculation is extraordinarily difficult due to the sheer number of solutions. There are no shortcuts in classical computing for this impossibly long calculation. However, a quantum computer can simulate the quantum process instead of doing brute-force calculations. In order to allow bosons to interfere directly in the quantum process, the team decided to use photons as qubits⁠— the basic unit of quantum information. Single photons enter the circuit in parallel and encounter optical components such as beam splitters. Due to their bosonic nature, if two photons arrive at a beam splitter at the same time, they will both follow the same path. By doing so, the time it took to complete the calculation was reduced drastically. 

“We have shown that we can use photons, the fundamental unit of light, to demonstrate quantum computational power well beyond the classical counterpart,” explains Jian-Wei Pan at the University of Science and Technology of China in Hefei (Ball 2020).

Although GBS calculations are very practical in terms of real-life application in physics, the computer is not particularly useful on its own due to its use of non-programmable optical circuits. If the team is able to create a programmable chip, more computational problems could be solved, such as predicting how proteins dock to one another (Ball 2020). One thing is for sure, this achievement serves as a milestone in quantum computing, pushing the field in the right direction.

References

Zhong, H., et al. (2020). Quantum computational advantage using photons. Science, Vol. 370, Issue 6523, pp. 1460-1463, DOI: 10.1126/science.abe8770. Retrieved 23 December 2020, from https://science.sciencemag.org/content/sci/370/6523/1460.full.pdf 

Ball, P. (2020). Physicists in China challenge Google’s ‘quantum advantage’. (2020). Nature. Retrieved 23 December 2020, from https://www.nature.com/articles/d41586-020-03434-7  

Chen, S. (2020). China claims quantum computing lead with Jiuzhang photon test, creating a machine ‘one trillion times faster’ than the next best supercomputer. South China Morning Post. Retrieved 23 December 2020, from https://www.scmp.com/news/china/science/article/3112649/china-claims-quantum-computing-lead-jiuzhang-photon-test