China’s Quantum Leap

Researchers in China claim to have achieved quantum supremacy, the point where a quantum computer completes a task that would be virtually impossible for a classical computer to perform. The device, named Jiuzhang, reportedly conducted a calculation in 200 seconds that would take a regular supercomputer a staggering 2.5 billion years to complete. Traditional computers process data as binary bits — either a zero or a one. Quantum computers, on the other hand, have a distinct advantage in that their bits can also be both a one and a zero at the same time. That raises the potential processing power exponentially, as two quantum bits (qubits) can be in four possible states, three qubits can be in eight states, and so on. That means quantum computers can explore many possibilities simultaneously, while a classical computer would have to run through each option one after the other. Progress so far has seen quantum computers perform calculations much faster than traditional ones, but their ultimate test would be when they can do things that classical computers simply can’t. And that milestone has been dubbed “quantum supremacy.”

For the uninitiated, quantum supremacy is a term used in the world of quantum computing to describe the moment when a programmable quantum device can solve a problem that no classical computer can feasibly solve. At the start of this decade, Google was the first to claim it had cracked quantum supremacy. The company said that its 53-qubit Sycamore processor had performed a certain computation within 200 seconds — a task that Google estimated would have taken the world’s most powerful supercomputer 10,000 years. Later, China claims to have joined the quantum supremacy club. The Jiuzhang computer reportedly found the solution to a particularly challenging problem within three minutes and 20 seconds. A traditional supercomputer, on the other hand, would take no less than 2.5 billion years to do the same — for reference, that’s more than half the current age of the Earth. The calculation was what’s known as boson sampling, which computes the output of a complex optical circuit. Basically, photons are sent into the system via many inputs, and once inside they’re split by beam splitters and bounced around by mirrors. Boson sampling takes all those variables into account and calculates the possible output of this maze – an incredibly difficult task for a regular computer, but a good test for quantum computers.

In this case, Jiuzhang was working with 50 photons, 100 inputs, 100 outputs, 300 beam splitters and 75 mirrors. The computer managed to calculate a distribution sample in about as long as it takes to make a coffee, whereas Sunway TaihuLight – currently the fourth most powerful supercomputer in the world – would have needed a fifth of the entire age of the universe to do the same job. That’s clearly a huge achievement, but it doesn’t mean that traditional computers are going anywhere any time soon. These kinds of calculations aren’t particularly useful work in themselves — they’re mostly tests to show off the potential power of quantum computers. Plus, they still have some stability issues that need addressing and won’t necessarily be better at everything than their predecessors. In a more recent development, researchers in China have developed a quantum processing unit (QPU) that is 1 quadrillion (10¹⁵) times faster than the best supercomputers on the planet. The new prototype 105-qubit chip, dubbed “Zuchongzhi 3.0”, which uses superconducting qubits, represents a significant step forward for quantum computing, scientists at the University of Science and Technology of China (USTC) in Hefei have been quoted as saying. It rivals the benchmarking results set by Google’s latest Willow QPU in December 2024 that allowed scientists to stake a claim for quantum supremacy — where quantum computers are more capable than the fastest supercomputers — in lab-based benchmarking. The scientists used the processor to complete a task on the widely used quantum computing random circuit sampling (RSC) benchmark in just a few hundred seconds, they said in a new study published March 3 in the journal Physical Review Letters.

This test, an 83-qubit, 32-layer random circuit sampling task, was also completed 1 million times faster than the result set by Google’s previous generation Sycamore chip, published in October 2024. Frontier, the second-fastest supercomputer in the world, would only be able to complete the same task in 5.9 billion years, by contrast. The latest iteration of Zuchongzhi includes 105 transmon qubits — devices made from metals like tantalum, niobium, and aluminium that have reduced sensitivity to noise — in a 15-by-7 rectangular lattice. This builds on the previous chip, which included 66 qubits. One of the most important areas critical to the viability of quantum computing in real-world settings is coherence time, a measure of how long a qubit can maintain its superposition and tap into the laws of quantum mechanics to perform calculations in parallel. Longer coherence times mean more complicated operations and calculations are possible.

Another major improvement was in gate fidelity and quantum error correction, which has been an obstacle to building useful quantum computers. Gate fidelity measures how accurately a quantum gate performs its intended operation, where a quantum gate is analogous to a classical logic gate, performing a specific operation on one or more qubits, manipulating their quantum state. Higher fidelity qubits mean fewer errors and more accurate computations. Zuchongzhi 3.0 performed with an impressive parallel single-qubit gate fidelity of 99.90%, and a parallel two-qubit gate fidelity of 99.62%. Google’s Willow QPU edged it slightly, with results of 99.97% and 99.86% respectively. These improvements were largely possible due to engineering improvements, including enhancements in fabrication methods and better-optimised qubits design, the scientists said in the study. For instance, the latest iteration lithographically defines qubit components using tantalum and aluminium, bonded through an indium bump flip-chip process. This improves accuracy and minimises contamination.

Currently, China and the United States are the two global frontrunners in quantum computing research, with each country alternately achieving groundbreaking advancements. The global scientific community has outlined a three-step roadmap for experimental quantum computing development. The first step is achieving quantum supremacy; the second step involves developing quantum simulators with hundreds of controllable qubits to tackle real-world problems beyond the capabilities of supercomputers; and the third step focuses on substantially improving qubit control precision, integration scale and error correction to develop programmable, general-purpose quantum computers. Quantum advantage represents a critical foundation for near-term applications and scalable quantum error correction, both of which are essential for the future of practical quantum computing.

Quantum computing really began to garner interest in 1994, when mathematician Peter Shor developed a quantum algorithm that could find the prime factors of large numbers efficiently, meaning these algorithms could solve the problems in a time of practical relevance, something that is beyond the capability of “state-of-the-art classical algorithms.” It was made possible when both quantum computers and classical computers tried to solve problems; however, the means by which they manipulate data to get answers are fundamentally very different. As you probably guessed, quantum computing looks to the quantum world to function, driven by two principles of quantum mechanics—superposition and entanglement. Superposition can be a little counterintuitive but bear with us. In short, superposition refers to the quantum phenomenon where a quantum system can exist in multiple states or places at the same time. In other words, something can be “here” and “there,” or “up” and “down” at the same time. Think of it as having a computer system that can be both 1 and 0 at the same time. This principle lays the framework for a quantum computer’s basic component of information, a qubit.

Qubits play a similar role to bits in a computer. As described in the Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2018 Symposium, “In classical computing, bits are transistors that can be off or on, corresponding to the states 0 and 1. In qubits such as electrons, 0 and 1 simply correspond to states like the lower and upper energy levels discussed above.” “Qubits are distinguished from classical bits, which must always be in the 0 or 1 state, by their ability to be in superpositions with varying probabilities that can be manipulated by quantum operations during computations.” In short, quantum computers perform calculations based on the probability object’s state before it is measured allowing them to have the potential to process exponentially more data compared to classical computers and this is exactly what makes quantum computing special.

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