When a team of Chinese scientists beamed entangled photons from the nation’s Micius satellite to conduct the world’s first quantum-secured video call in 2017, experts declared that China had taken the lead in quantum communications. New research suggests that lead has extended to quantum computing as well.
In three preprint papers posted on arXiv.org last month, physicists at the University of Science and Technology of China (USTC) reported critical advances in both quantum communication and quantum computing. In one of the studies, researchers used nanometer-scale semiconductors called quantum dots to reliably transmit single photons—an essential resource for any quantum network—over 300 kilometers of fiber, well over 100 times farther than previous attempts. In another, scientists improved their photonic quantum computer from 76 detected photons to 113, a dramatic upgrade to its “quantum advantage,” or how much faster it is than classical computers at one specific task. The third paper introduced Zuchongzhi, made of 66 superconducting qubits, and performed a problem with 56 of them—a figure similar to the 53 qubits used in Google’s quantum computer Sycamore, which set a performance record in 2019.
“It’s an exciting development. I did not know that they were coming out with not one but two of these [quantum computing results] in the same week,” says Scott Aaronson, a theoretical computer scientist at the University of Texas at Austin. “That's pretty insane.”
All three achievements are world-leading, but Zuchongzhi in particular has scientists talking because it is the first corroboration of Google’s landmark 2019 result. “I’m very pleased that someone has reproduced the experiment and shown that it works properly,” says John Martinis, a former Google researcher who led the effort to build Sycamore. “That’s really good for the field, that superconducting qubits are a stable platform where you can really build these machines.”
Quantum computers and quantum communication are nascent technologies. None of this research is likely to be of practical use for many years to come. But the geopolitical stakes of quantum technology are high: full-fledged quantum networks could provide unhackable channels of communication, and a powerful quantum computer could theoretically break much of the encryption currently used to secure e-mails and Internet transactions.
Tensions between the U.S and China are currently at their highest point in decades, with the countries clashing over trade, human rights issues, concerns about espionage, COVID and Taiwan. After China’s demonstration of the Micius satellite in 2017, American politicians responded by pushing hundreds of millions of dollars into quantum information science via the National Quantum Initiative. It was an eerie bit of déjà vu. About 60 years earlier, the U.S. was similarly spurred to fund another pie-in-the-sky initiative—space exploration—because of fearmongering over a little Soviet satellite named Sputnik.
But this struggle for quantum advantage need not be a perfect mirror of the space race. Zuoyue Wang, a science historian at California State Polytechnic University, Pomona, notes that China and the U.S. are intimately intertwined in many areas—science among them—that could prevent a hostile new competition in the quantum realm. Today hundreds of thousands of students travel from China to study in the U.S., and scientists in both countries collaborate closely on research ranging from agriculture to zoology. In spite of rising geopolitical tensions between the two countries, “they’re each other’s biggest international collaboration partners,” Wang says.
Qubit by Qubit
Forty years ago physicist Richard Feynman made a straightforward proposition: Classical computers trying to simulate a fundamentally quantum reality might be outdone by a computer that, like reality, is itself quantum. In 2019 a team at Google led by Martinis realized this so-called quantum advantage by demonstrating that the company’s Sycamore system really could perform a specific, limited task exponentially faster than even powerful classical supercomputers (though a competing team at IBM disputed that Google’s achievement represented a true quantum advantage). A year later USTC researchers performed a similar experiment with a quantum computer made from photons.
Why can rudimentary quantum computers beat classical supercomputers at specific tasks? The common refrain goes something like this: Instead of classical bits that are 0 or 1, a quantum computer uses qubits, whose state is somewhere in between 0 and 1 prior to measurement—a so-called quantum superposition. To work together within a computer, qubits must also be entangled, or quantum correlated with one another.
It might be more intuitive to consider the task Zuchongzhi and Sycamore have performed. “It’s almost embarrassingly simple,” Aaronson says. “All you do is a random sequence of quantum operations.” This chaotic set of instructions entangles all the qubits into one big, messy state. Describing that state is easier for qubits than bits. Describing two entangled qubits requires four classical bits. (There are four possible outcomes: 00, 01, 10, or 11.) The state complexity scales exponentially, so what takes 50 qubits requires 250, or about one quadrillion, bits to describe. Photonic quantum computers create a similarly entangled messy state but with photons instead acting as qubits.
This is why even a small 50-qubit quantum computer can beat a massive classical supercomputer. “If you look at the West—the U.S., Europe—there haven’t been a lot of people talking about repeating [Google’s 2019] experiment,” Martinis says. “I admire, in China, that they want to do this seriously.”
With 56 qubits and 113 detected photons, the USTC systems detailed in two of the new preprints are now technically the most powerful quantum computers in the world—with two big caveats. First, neither quantum computer can do anything useful. (Photonic quantum computing is not a universal computer platform, so even scaled up, it would not be a conventional programmable computer.) Second, it is not clear exactly how much of a quantum advantage they actually have over classical computers. Over the past few months, several studies have claimed the ability to approximate that messy entangled state, especially for photonic quantum computers.
Despite the difficulties of working with photonic quantum computers, USTC researchers have good incentive to master the platform because photons are the medium of China’s emerging quantum network. Already, thousands of kilometers of fiber-optic cables have created an initial quantum link between Beijing and Shanghai. The link is not a fully realized quantum connection: it is divided up by nodes because photons can only go so far without succumbing to noise in the fiber. A bona fide quantum network could have a variety of applications, but the two main ones are precision synchronization and unhackable communications.
To fulfill that promise, quantum networks will require—among other things—entangled single photons that can be used for quantum key distribution or other operations that require entanglement. Quantum dots are thought to be ideal sources for single photons. Until now, quantum dots had never sent single photons through more than about a kilometer of fiber. (Typically, the longer the fiber, the greater the noise.) But the USTC team managed to increase the transmission distance while also decreasing the noisiness of the single photon. Its success came from taking strenuous measures, such as stabilizing the temperature of the 300-kilometer fiber to within a tenth of a degree Celsius.
Racing in the Quantum Realm
Is China ahead of the U.S. in quantum information technology? The answer depends on how you measure it. While estimates vary, both countries appear to fund the research to the tune of more than $100 million per year. China has more total patents across the full spectrum of quantum technology, but U.S. companies have a dramatic lead in quantum computing patents. And of course, China has a more sophisticated quantum network and now claims the top two quantum computers.
“It's such a new problem for the U.S. to be facing,” says Mitch Ambrose, a science policy analyst at the American Institute of Physics. “It was ahead for so long, and in so many areas, that it hasn’t really had to do much thinking about what it means to be behind.”
Broadly speaking, quantum research in China is almost entirely state driven—concentrated into a few universities and companies. Research in the U.S., in comparison, is much more disparate—spread across dozens of funding agencies, universities and private companies.
“The Chinese government is thinking about science technology very seriously, probably more than the U.S. administration” Wang says. “No one else will pick up the tab.”
Currently, the U.S. government is determining how to fund the future of quantum information science in proposed bills such as the Innovation and Competition Act of 2021, which would provide $1.5 billion for communications research, including quantum technology. In response to security concerns about China, the bill also prioritizes semiconductor manufacturing and includes a provision that would restrict cooperation with China on nuclear energy and weaponry. This is not the first restriction on scientific collaboration between the two countries. Since 2011 NASA has been under the thumb of the Wolf Amendment, which bans any cooperation with China’s space agency without a waiver. Conversely, China and the U.S. have also spent more than four decades cooperating officially on scientific matters, because of the U.S.-China Agreement on Cooperation in Science and Technology of 1979.
As tensions between the two nations continue to rise, quantum research occupies an awkward spot: although it remains basic research with limited current applications, its future strategic potential is clear and immense. “What are the rules of the road for scientific exchanges going forward in any field, let alone quantum?” Ambrose asks. Hawkish funding of quantum technology could further inflame relations, but it could also stimulate more cooperation and transparency between competing countries seeking to prove their quantum prowess.
During the cold war, the U.S. and the Soviet Union sought to demonstrate parity with, if not supremacy over, each other in nuclear weaponry, spaceflight and other strategically important technical pursuits. Olga Krasnyak, an expert in science diplomacy at the National Research University Higher School of Economics in Moscow, argues the resulting U.S.-Soviet scientific exchanges helped end the cold war. “Science diplomacy has this advantage—it uses science, which is universal,” Krasnyak says. And just as importantly, it uses scientists—who historically have leveraged their common humanity and shared quest for knowledge to overcome the strain of any ideological differences. Quantum computing and communications may indeed have the power to reshape the world. But, Krasnyak says, “I believe in the power of human communication, too.”