(Inside Science) -- On Sept. 29, 2017, in Beijing, as physicist Jian-Wei Pan laid his hand on a ceremonial glass globe, the first long-distance quantum communication landline came online, connecting the capital city of China with the coastal city of Shanghai over a distance of more than 1,200 miles. For quantum physicists and tech geeks around the world, it was a day for celebration. But China wasn’t done yet.
On the same day in Vienna, Austria, Pan’s former doctoral advisor Anton Zeilinger received a video call from his colleagues in China. This was no ordinary Skype chat -- it was the first quantum-encrypted video call, made possible by the Chinese quantum communication satellite known as Micius.
The quantum-secure intercontinental video call held between China and Austria on Sept. 29, 2017.
Chinese Academy of Sciences
The two feats were hailed as major achievements by scientists and the news media around the world. To understand the importance, you first need to know: What exactly is quantum communication?
The short answer is that it is a new and supersecure way to communicate or exchange data. It has been called a potential game-changer during a time when cybersecurity and privacy have become a constant concern for individuals, companies and governments.
For the long answer, you’ll need to understand a little bit about a bizarre quantum phenomenon called entanglement first.
Quantum communications for dummies
Although so-called hack-proof quantum communication sounds like the stuff of science fiction, the operating principle has actually been known for quite some time. In 1982, the scientists William Wootters and Wojciech Zurek, and Dennis Dieks first articulated the "no-cloning theorem," which states that quantum information carried by particles such as photons cannot be replicated exactly. That means that if we can somehow transmit data as quantum information, there is no way for a hacker to secretly listen in.
The idea works this way: In quantum communication, two users directly share pairs of particles in a so-called entangled state, meaning their quantum properties are linked. Think of the pairs, which are most often photons, as pairs of candies with matching flavors. The pairs are generated in a random order and the ones that are successfully shared between the sender and the receiver will then spell out a secret phrase -- lemon, cherry, grape, cherry -- used to encrypt a subsequent data transmission. Since the only way to "detect" the flavor of the candy is to eat it, any candies that have been "detected" by a hacker are automatically omitted from the key phase, making it impossible for a hacker to eavesdrop.
People have long known that quantum communication was possible, but the Chinese were the first to actually build large scale infrastructure for the task, said Tim Spiller, the director for the U.K. Quantum Technology Hub for Quantum Communications, an ongoing multi-institutional program in the U.K.
Indeed, smaller scale quantum communication systems have been around since at least 2004. Nowadays, you can literally buy a quantum encryption box online with a credit card. Some banks have used the technology to send encrypted messages over short distances, but the range of applications is small because of technical limitations. Now China is overcoming one of those limitations: It has taken the technology and made it bigger -- much bigger.
A hack-proof game of telephone
The newly launched Beijing-Shanghai quantum link connects Beijing to Jinan to Hefei to Shanghai, a distance of more than 1,200 miles. Several major Chinese banks are already using the link to transfer their most sensitive data.
However, even though the newly opened quantum link is technically quantum, it is still not 100 percent secure. Photons, or light, can only go through about 100 kilometers of optic fiber before getting too dim to reliably carry data. As a result, the signal needs to be relayed by a node, which decrypts and re-encrypts the data before passing it on. This process makes the nodes susceptible to hacking. There are 32 of these nodes for the Beijing-Shanghai quantum link.
This is still a significant security improvement over traditional fiber optics, said Yu-Ao Chen, a quantum physicist at the University of Science and Technology of China in Shanghai.
"Before the Beijing-Shanghai line, hackers could in principle wiretap anywhere along the entire length of the optic fiber. Now, the number of vulnerable points has been narrowed down to just 32," he said. Chen works in the research group in charge of China’s large-scale quantum communication projects, which is led by Jian-Wei Pan, known in China by his audacious nickname, “Father of Quantum.”
For a quantum communication system to be 100 percent secure, the nodes themselves would have to be hack-proof as well. Scientists are already working on a solution: the quantum repeater.
A quantum repeater essentially serves the same purpose as an ordinary relay node, except it works in a slightly different way. A network using quantum repeaters is shaped more like a family tree than a linear chain. In this family tree-shaped game of telephone, the quantum repeater is the parent who distributes identical pairs of quantum keys between two children, therefore doubling the possible distance between users. Moreover, these "parents" can also have their own “parents,” which can then double the key-sharing distance between the children at the bottom for every extra level created atop the family tree. This in effect increases the distance a quantum message can be sent without ever having to decrypt it.
Although the principle is straightforward, each additional layer of parents creates a new set of technological challenges, such as additional noise in the data. A recent paper by Chen and his colleagues in Physical Review Letters reports how they built an experimental system up to the grandparent layer, quadrupling the theoretical distance limit for a hack-proof quantum communication link. But for international distances over thousands of miles, one’ll need to go higher up the family tree, perhaps up to the great-great-great-great grandparent level.
Or you can go to space.
Quantum communication in space
The first quantum communication satellite -- called Micius in English -- was launched in 2016. The satellite was named after a famous ancient Chinese scientist and philosopher, who around 400 B.C. was the first person to document the operating principle of a pinhole camera, including a description of the straight-line propagation of light.
More than two millennia later his namesake satellite allowed scientists in Beijing and Vienna to host the first quantum-encrypted video conference across a distance of more than 4,600 miles.
As Micius flew over the night sky at 18,000 miles per hour, it split up pairs of entangled photons and sent half of them down to Xinglong Observatory, located less than 10 miles from the Great Wall. Less than an hour later, Micius would pass over Austria, repeating the process of splitting entangled photon pairs and casting them down to the Lustbühel Observatory, perched on the Alpine slopes outside Graz. This completed the quantum key distribution.
A time-lapse photo of Xinglong Observatory communicating with the Micius quantum satellite. The ground observatory tracks the Micius satellite using a 810 nanometer red laser. The satellite uses a 532 nanometer green laser, and in this photo the green dashes show the satellite's path across the sky.
Courtesy of Ying-Wei Chen
"The success of these projects and experiments has made China the leader in quantum communication. It showcases exactly the kind of technological advances that can be driven by scientific discoveries, as well as scientific discoveries through technological advances," wrote Juan Yin, one of the authors of the latest Physical Review Letters paper that detailed the science behind the landmark video call, in an email to Inside Science. Yin is also a scientist in Pan’s group at the University of Science and Technology of China.
Despite the latest milestones, there is still much room for improvement in the technology, according to Yin. For instance, since a quantum satellite needs line-of-sight to transfer data, communication coverage by the satellite is very limited -- it has to fly directly over the user. A satellite with a higher orbit can increase its coverage, but will demand higher location tracking accuracy from both the ground receivers and the satellite itself.
One solution would be to send up more satellites to form a global satellite network, according to Hoi-Kwong Lo, a quantum information scientist from the University of Toronto. If different institutions or governments sent up the satellites in the network, participants would have to worry about software protocols and trustworthiness between them, he said.
There is also a limitation on bandwidth. Right now, only about one in 6 million photons sent by the satellite reach the ground receiver. And you can forget about using the system during the day -- the brightness of the sun would overwhelm the already dim signal of the satellite. It would be like trying to spot a flashing firefly on a sunny afternoon from across a football field.
The bandwidth also depends on the number of entangled photons the satellite can generate and store. Currently, the top speed for Micius is just a few kilobytes per second -- enough for transmitting a few quantum keys between two science teams, but hardly enough for millions of internet users to simultaneously encrypt sensitive transactions. The Beijing-Shanghai quantum link is a little bit faster than Micius, but essentially still in the same range.
"It's like the internet. Once you've gone beyond a point-to-point set up and try to build a network, there will be a lot of practical issues we will need to think about,” said Lo.
Bamboos after the rain
A common Chinese expression says, “after rain, bamboo springs.” Pan referenced the expression at a conference in Shanghai earlier this year to describe the state of quantum communication, which he predicted would, like well-watered bamboo shoots, soon enter the age of rapid growth.
However, phones or laptops with the technology won't be available for a while.
“At the minute, these devices are still quite expensive and quite big. If you're a telecommunications service provider you might be prepared to buy one, but if you're an individual you're probably not going to,” said the U.K. Quantum Technology Hub's Spiller.
"One day we may be able to make one that can fit on a chip, then we could stick one in every computer. There's still got to be significant technological advances before we can do that, but it's not crazy to think about," he said.
As the practicality of the technology becomes more and more apparent, competition is heating up among nations to take the next step. Pan’s group is considered by many to be the leader in quantum communication, partly thanks to the support it receives from the Chinese government. The government is now doubling down on the technology by investing another $10 billion dollars for a brand-new research center dedicated to quantum information sciences. The National Laboratory for Quantum Information Sciences will be located in Hefei, a city in the province of Anhui, with construction expected to finish by 2020.
Groups from Europe, the U.S., Canada, Japan and Singapore now have plans to conduct their own quantum communication experiments in space, each trying to win a leg of the ongoing quantum leapfrog race.
The director of the U.S. National Institute of Standards and Technology, Walter Copan, told a roundtable of industry and scientific society representatives in December that expanding his agency’s work in quantum science and engineering is among his priorities.
“We can’t fool ourselves -- it’s a long journey to actually get to quantum-based devices and systems,” Copan said, according to a summary of the remarks by FYI, a news and resource center for federal science policy that is funded by the American Institute of Physics (Editor's note: Inside Science is also supported by the American Institute of Physics). But, he added, “it’s very clear that we are in a race as a nation, and we have China and the European Union dramatically investing in this area.”
Editor’s Note: The interview with Yu-Ao Chen was conducted in Chinese and translated by Yuen Yiu.
Update: This story was amended on April 28, 2020 to clarify that the photons sent down to Xinlong are not entangled with the ones sent to Lustbuhel.