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A team of researchers in China claims to have made the first conclusive demonstration of ‘quantum advantage’ – using quantum mechanics to perform calculations that would be too slow on classical computers.

The research group at the Chinese University of Science and Technology collaborated with the Shanghai Institute of Microsystem and Information Technology (SIMIT) and the National Parallel Computer Engineering Technology Research Center of the Chinese Academy of Sciences to build a quantum computer prototype in which up to 76 photons were observed exiting a network of 100 channels.

Last year, Google caused a stir by announcing that it had reached “quantum supremacy,” a term that identifies the ability of a quantum computer to perform a calculation that is practically impossible for a traditional supercomputer to solve, except by taking a disproportionate amount of time. In this specific case, Google’s quantum computer based on the Sycamore 53 qubit processor, and its algorithm was run on a 54-qubit quantum chip, each consisting of superconducting loops.

The Chinese team used laser light beams to perform a calculation that proved to be mathematically impossible on normal computers, solving it in just a few minutes. A task that would take the fourth most powerful supercomputer in the world over 2 billion years. In these cases, we are talking about laboratory tests, nothing applicable to reality for the moment.

On an experimental level, while the Google team used a superconductor cooled to near-zero temperatures, the Chinese team used photons on its Sunway TaihuLight. To define the achievement of the result, the Chinese team used a statistical test to define the path of photons traveling on an optical circuit guided by mirrors. Each photon read at the end of the process is equivalent to a qubit that reveals the result of a calculation.

The team wrote a code to simulate the behavior and quantify the resolution time compared to the most powerful computer in China. The calculation they performed is related to the boson sampling problem. This problem was devised in 2011 by two computer scientists, Scott Aaronson and Alex Arkhipov4. It involves calculating the probability distribution of many bosons whose quantum waves interfere with each other so that the position of the particles is essentially random. The probability of detecting a boson in a given position can be calculated from an equation in many unknowns.

The researchers calculated that the supercomputer would take more than 2 billion years to do what the Jiuzhang did in just over 3 minutes.

*Figure 1: An illustration of the experimental setup for generating squeezed states (Source: “Quantum computational advantage using photons”)*

**Photons like Qubit**

In quantum computers, the bits, i.e., the basic information unit of normal luminaires, are replaced by the so-called “qubits”, quantum bits, which are able to cope with enormously complex problems thanks to a greater possibility of encoding information. Problems that are largely out of reach for normal computers.

The Chinese team has demonstrated how using photons, which are the fundamental unit of light, can enhance quantum computing, going far beyond the classical counterpart. They performed the task at room temperature. Unlike Google, the optical system used is not easily reprogrammable. The advantage of a photonic quantum computer is that it does not need to be cooled down.

Starting with laser pulses, the researchers coded the information into the spatial position and polarization of particular photonic states. These states were made to interfere with each other and generate the photonic distribution representing the output. The optical circuit used is an interferometer. The team used photodetectors capable of recording individual photons to measure this distribution which, in fact, encodes calculations that are difficult to perform in a classical way.

A quantum computer bases its computing power not on microchips and circuits but on quantum computing, which is based on the principles of quantum mechanics and specifically on quantum entanglement, i.e. the ability of a subatomic particle to influence a different subatomic particle at a certain distance. The influence is practically instantaneous, and the computational speed of a quantum computer is potentially the same.

Quantum computing is still in an embryonic phase, but all these tests of supremacy promote research, making it possible to simulate large systems and, above all, to guide advances in physics. Chinese researchers are entering a competition with other companies such as Google and IBM, increasingly accelerating the race towards quantum computers.