“China's quantum-secured internet will combine satellites and fiber optics”
In only fifteen years, China has come to the forefront in quantum information technologies. A success marked this year with the first functional quantum internet backbone, and an experimental quantum satellite —coupled with a similar instrument aboard the Chinese space station.
Photo Credit China News Service
La Recherche How could quantum physics change people's life?
Jian-Wei Pan First of all, quantum communications will be a key feature of our information exchanges. In the past, Homo sapiens won the battle of evolution over Neanderthals because they invented the language. They were able to share their knowledge, coordinate and cooperate. Today, we need to guarantee our communication privacy. It is necessary to be free to think, to talk and to exchange. It is critical as cell phone is increasingly used for bank accounts, car access, home control and so on. Privacy is no longer a privilege for the elites, it is needful for anyone! But using the conventional (mathematical) approach, whenever you design something to secure communications, there will always be someone smart enough to beat your method. That's why quantum techniques are so important as nobody can break the fundamental laws of physics!
Where do your passion for quantum physics comes from?
I am curious and quantum mechanics is fascinating. It has amazing features, such as superposition —that permits a quantum particle to be in different states at the same time—, and entanglement, a tricky property that allows quantum correlations. Imagine we play the dice. If your dice and mine are entangled, it means that every time I get a six , you'll also get a six, whatever the distance between us. My interest in quantum mechanics is double: to explore its fundamental concepts, and to build devices that could change people's everyday life.
Are there other practical applications?
Our very basic tool is the manipulation of quantum systems. We can handle single particles, and entangle them as we wish. Once we are able to do that, we can provide secure quantum communications. Controlling many quantum states also allows to perform quantum computation, which will greatly extend our computational power, for example to design new pharmaceutical molecules or materials for industry. Quantum physics could also greatly impact life as it permits much more precise measurements than what we achieve today. Quantum systems are very sensitive to their environment. Thus, we could find nice ways to carry out very high precision measurements of gravity, time, distance… It could improve navigation accuracy. In biology, it might scale the resolution of nuclear magnetic resonance imaging down to a single cell.
Quantum physics research has always been made on Earth. Why did you choose space, using a satellite?
The first reason is to overcome the fiber optics limits. As an unknown quantum state cannot be measured and copied precisely, a quantum-based signal cannot be amplified. The signal drops exponentially with the distance, so that in a fiber, after 1000 km, the absorption of the light prevent us from sending more than a single quantum bit every few hundred years! In 2003, I thought we had first to build a quantum repeater to extend the communication distances in fibers. It is a relay which would read the state of a light particle before it is absorbed, and transfer it to another photon. It requires teleportation of the quantum state from one particle to another. We also need a quantum memory to store the state of the incoming photon before transferring it to another. But these technologies are still not available. Back in 2003, I chose to follow two paths: building a quantum repeater and developing the free-space channel, where absorption is much lower. Our first calculations showed that 80% of a satellite's quantum signal can reach a ground station. In a 100 kilometers fiber, it is only 1%. That's why, back in 2003, we envisioned to go to space. But it took thirteen years to do it.
For which reason?
In 2007, Anton Zeilinger, my former PhD supervisor in Austria, and I were both considering building a quantum satellite. I was targeting the Chinese Space Agency, and Anton's had the European Space Agency in mind. We set up an agreement: we would collaborate on the first project to be funded. Both Anton's and my group worked very hard independently to demonstrate the feasibility of such space-based quantum experiments. We discovered that much less of the estimated 80% of the signal could effectively reach a ground station, due to the diffraction of laser beams. In 2013, my group demonstrated that a 99.99% loss —40 dB— does not forbid quantum key distribution (1). We had to develop the technology to be sure that the signal can be seen, despite the satellite high velocity in the sky. We called it ATP, for acquisition, positioning and tracking. Imagine that I am the satellite. Acquisition means that you see me. Positioning means that I see you. And tracking means that both can receive the signals as we move. In particular, we had to create tools to overcome the flickering linked to atmospheric fluctuations, a phenomenon that you can see when you observe stars in the night.
After the Micius satellite launch, you've been able to secure a videoconference between China and Austria in September 2017…
Years ago, Anton thought that it was too risky to mix three different experiments onboard, and that I should focus on one. But I considered that these were necessary to justify the cost of a satellite. We worked very hard and we were lucky enough to succeed in distributing a quantum key (QKD) using single photons from satellite to ground, that allowed our secured videoconference (2); in sending entangled photons from the satellite to two ground stations separated by 1200 km (3). And in teleporting a quantum state from ground to the satellite, over a 1400 km distance (4). We still perform experiments as long as the weather allows it. For example, to send the secure key to encrypt the videoconference, we used the single photon scheme, proposed in 1984 by Bennet and Brassard, the inventors of quantum cryptography. We are now performing experiments with the 1991 Eckart scheme, based on entanglement. It is much more difficult to achieve. I'm confident that we'll succeed within the next weeks or months.
The first intercontinental teleportation of particles through the Micius quantum communications satellite that allowed to hold a conference call impossible to hack.
Is this scheme more secure?
Since the safety of quantum key distribution relies on the impossibility to cut a photon into two, both single photon-based and entanglement-based QKD have the same security. However, entanglement-based QKD is still safe even if the entangled photon sources are provide by your enemy! As long as you can verify that the tool respects the non-locality principle, no-one can fudge the result.
But it supposes that there are no loopholes in the non-locality verification process…
The non-locality principle states that two quantum particles can be correlated regardless of the distance between them. That means that entanglement can be preserved at any distance. Einstein strongly disagreed with this idea but French physicist Alain Aspect demonstrated it, for the first time, in 1982, with 13 meters-separated entangled photons. Together with other pioneers in the fields of quantum foundations, he gave birth to quantum information technology, The second quantum revolution as we entitled a seminar, here in Berlin, to celebrate Alain's 70th birthday. But several loopholes can alter our trust in quantum-secured communications. Nature can be tricky: some hidden variables may influence our experiments, sending signals without our knowledge that rigs our measurements.
Which kind of loophole could bias measurements?
There are several. First there is a locality loophole. When a measurement takes too long, a signal has enough time to travel and correlate your particles to convince you that non-locality is respected. A second loophole rest on our ability to detect quantum particles. As long as you don't probe 100% of the particles you created, you cannot be sure that a hidden variable cheats on us. It could let us only see the particles that do not observe non-locality. Those two loopholes have been closed in 1998 and 2001 (5)(6). But others are still opened. That's why I intend to pursue space experiments. It will require sending a quantum device to the Moon! (read box).
Micius satellite works only at night, as the sun light dazzle sensors. Can we imagine 24/7 quantum communications from space?
This is a serious drawback of our first device. But last June, we published a day-and-night free space quantum communication result on Earth, over a 53 km distance (7), much more than the atmosphere effective thickness (*), which is about 10 km. We'll have to design higher orbits as a satellite on low-earth orbit has smaller area and time coverage. Micius has only about 300 seconds to communicate when it flies over a ground station. If you put a small fleet of satellites at several tens of thousands of kilometers, like the GPS instruments which fly about 20200 km from the ground, you can perform 24/7 quantum communications. We are very confident we can build that, as our technology progresses and shrinks in size. Micius' payload is about 600 kilograms. But we sent an second quantum device in September 2016, one month after Micius, onboard the Tiangong 2 Chinese space station. With only 58 kilograms, it could sent quantum keys over a 719 km distance, at a 100 bits per second rate (8).
Xinglong's ground observatory follows the experimental Micius quantum science satellite. (Source: Institute of Innovation Science of Miniature Satellites of the Chinese Academy of Sciences)
You are also involved, as the chief scientist, in China first quantum-secured optical fiber backbone. Can you describe it?
This is the first step of China's future quantum internet that would combine ground lines and satellites. We built a 2000 km branch between Shanghai and Beijing and connected the nearby main cities to it. These links are used by the government, universities, banks and big companies such as Alibaba. About every 70 km, we installed a relay to amplify the signals. As we do not have quantum repeaters today, these stations converts the quantum keys in classical ones, then recreate the quantum keys to be sent to the next relay. This is not a 100% quantum network, but trusting the thirty government-protected relays is much easier and more secure than relying on a multi-thousand-kilometer-long communication line.
How do you explain the spectacular progress made by China in quantum technologies?
This is very easy to answer. In the past, China has no choice but to import its cybersecurity technologies from the USA. And we knew that our communications were monitored, even our prime minister's cell phone. As our high-speed train system and power plants, most of our infrastructures and businesses need trustful technology. So China had no choice, but to develop its own know-how, and the government people came to us. We decided to take advantage of the devices that we would build for answering their request, to make fundamental science. It was a true opportunity for us!
Did Europe helped much?
Indeed. In 1996, as I couldn't fund my research in China, I was lucky enough that Anton Zeilinger accepted me in his group, to prepare my PhD. One year after my arrival, we published the first quantum teleportation experiment in Nature, and I was the second author! (9) Then I worked on entanglement swapping and purification —some techniques used to design a quantum repeater— before moving to Heidelberg (Germany) to develop a quantum memory. There, I got the Marie Curie Excellence Grant from the European Union to create my own group. When I went back to China in 208, the government decided to fund our efforts, so that we advanced from the knowledge that I and my colleagues had learned abroad.
(1) Nature Photonics, 7, 387, 2013
(2) Nature, 549, 43, 2017
(3) Science, 356, 6343, 2017
(4) Nature, 549, 70, 2017
(5) Physical Review Letters, 81, 5039, 1998
(6) Nature, 409, 791, 2001
(7) Nature Photonics, 11, 509, 2017
(8) China Physics Letters, vol 34, issue 9, 2017.
(9) Nature, 390, 575, 1997
(*) This thickness corresponds to an atmosphere with same mass as Earth's but whose barometric pressure does not decrease when the altitude rises.
QUANTUM PHYSICS ON THE MOON
Verifying non-locality in an experiment supposes that the measurement are chosen using perfect random number generators. Otherwise there is a “freedom-of-choice loophole”. There is also a so-called “Schrodinger's cat loophole”, based on the thought-experience proposed in 1935 by Erwin Schrodinger: as the superposition principle states, a cat shunt in a closed box can be alive, dead, or both dead alive and dead. The result is known when opening the box, and after the scene has been interpreted by the scientist' brain, say one-tenth of a second. If photons used in an experiment are too close, the brain could have been —theoretically— influenced in the meantime! «We plan to add a quantum device aboard Chinese's future Moon lander, within ten or fifteen years, explains Jian-Wei Pan. We will also put a quantum communication probe in one of the Earth-Moon Lagrangian points, where the gravity of the two bodies cancels. There, we will be able to create entangled photons and send them to Earth and the Moon. As the distance between the two detectors will be greater than a light-second, and thanks to our know-how, we hope being able to close both loopholes.»
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CONTEXT
As Europe and the USA struggle to take the issues at stake, China massively invests in quantum physics and its astonishing properties, called to revolutionize areas as diverse as computer science, medical imaging and —Beijing's prime goal— cybersecurity, since quantum techniques can secure telecommunications networks from eavesdropping once and for all.
> INTERVIEWEE
Jian-Wei Pan
Physicist
1970 ▪ Born in Dongyang (China)
1999 ▪ PhD in Anton Zeilinger's group in Vienna (Austria)
2003 ▪ Move to Heidelberg
2008 ▪Goes back to China at USTC
