摘要：Great leaps are already being made in creating a super secure quantum internet. It could overturn the role of information in our lives and give us a globe-spanning quantum supercomputer

MANY of us have uploaded our lives to the internet. Banking, work emails, social media, dating profiles, medical records – all that vital, sensitive information. So it is a little disconcerting that the internet has a fatal security flaw. Don’t panic; our private information is safe for now. But before very long the encryption algorithms that protect us online are going to crack.

That is the urgent driving force behind a new, more secure kind of internet that harnesses the power of the quantum realm. Once up and running, the system will be able to do a lot more than protect our data. It could bring us unforeseen quantum apps, and maybe become the scaffold for a world-spanning quantum computer of incredible power.

Building the quantum internet is a huge and multi-faceted engineering challenge, but the foundations are already being laid. Networks of fibres are spreading. Scientists are chatting in secret on local networks. There are even plans to use tiny satellites to enable long-distance quantum connections. Sooner or later, we could all be joining the quantum information superhighway.

Human culture and industry have long been based on information. If you could get the right kind of information, understand it and share it, you could gain power and profit. The rise of the internet as we know it cemented the role of information and we are only beginning to feel its profound effects. Now we are at the threshold of a new information age, which could change things all over again.

Conventional, classical computers deal in digital units called bits. This is the amount of information in the outcome of a coin toss, usually represented as having a value of 1 or 0. Every email, status update or photo on your phone is broken down and stored as bits.

Dealing in qubits

That is rather limited when seen from the perspective of the quantum world, where we know particles behave in ways that can seem very strange. An atom, electron or photon can be in a state where its properties aren’t determined. For example, it can have two different energies at once. These quantum states are extremely delicate, but learn to manipulate them and you can deal in particles that store a quantum unit of information, or qubit, encoding not only 0 or 1, but any blend of 0 and 1 together.

Our burgeoning ability to do just this has already produced impressive new technology, such as ultra-sensitive detectors of gravity and magnetic fields. Physicists are now able to control dozens of connected qubits at once, creating prototype quantum computers. When these grow large enough, they promise to surpass any classical computer that could ever be built – at least when it comes to certain types of calculation. Among many other things, quantum computers should be able to simulate chemistry to design new drugs and advanced materials and solve knotty problems in engineering and logistics. Their full potential is as yet unknown.

One thing we do know is that these incredible machines will mean we need a quantum internet – because it is quantum computers that threaten our security. Many encryption schemes that keep the internet secure are based on mathematical problems that are impractical for a classical computer to solve, such as factorising the product of two large prime numbers. But a big enough quantum computer could do this in a flash, using an algorithm devised by Massachusetts Institute of Technology mathematician Peter Shor in 1994. That would undermine the security of everything that relies on online communication, from email to power grids. “A lot of critical infrastructure still relies on such algorithms… including my bank,” says Siddharth Joshi at the University of Bristol, UK.

Such a dangerously powerful quantum machine is probably at least 10 years away, but the problem is urgent nonetheless. It takes a long time to change cryptosystems, and data sent today could be intercepted, stored and decrypted when a powerful enough quantum computer becomes available.

Joshi and others want to fight qubits with qubits. If you communicate using the quantum states of individual particles, then you can tell if anyone eavesdrops because the very act of looking at the signal will change those delicate states. This wouldn’t mean replacing the internet, but building an added layer of quantum communication links on top of it so users can share a key that would keep their online exchanges secret. Internet traffic would still travel through the cables it does now, it would just be encrypted and decrypted with those keys.

This kind of quantum encryption, called quantum key distribution or QKD, has been demonstrated many times in the past few decades. The first QKD bank transfer was in 2004. There are many different schemes for QKD, but some of the most secure are based on the quantum phenomenon of entanglement. You begin by putting two qubits into a shared quantum state such that when one of them has its properties measured, the outcome of measurements on its twin change in a predictable way, no matter where the two particles are. Say your two qubits are photons. Send one of the entangled pair through an optical cable, and you have a means of exchanging a secure key.

Links that carry much larger numbers of entangled qubits could allow for even more impressive applications, such as sending messages in entirely quantum form. In the short term, quantum computers will be modest and probably housed far apart from each other, at locations like universities or research centres. But quantum communication links could connect them to create a quantum supercomputer. They could also allow users to run programs on quantum computers remotely in such a way that security would be guaranteed, with even the owners of the computer unable to snoop. This is called blind quantum computing, and it could enable anyone to use quantum computers without any risk of having sensitive data poached.

Whispering diamonds

A seed of the coming quantum internet has been sown in a laboratory in Delft, the Netherlands. There, three tiny diamonds whisper to each other, forming a miniature but fully functioning prototype network of entanglement links. Inside each diamond’s lattice of carbon atoms is a defect where a single nitrogen atom sits. A pair of electrons at this site can emit a photon that is entangled with them. Each diamond also holds a one-qubit quantum memory, which allows basic quantum information processing.

In a paper published in April, Ronald Hanson and his team at QuTech, a research institute in Delft, showed they could link three diamonds in a network and pass quantum information between them. In principle, this technology can be scaled up, allowing entanglement to be shared between any number of nodes. “This is the basic function that the quantum internet needs to perform,” says Hanson.

The hardware doesn’t have to be diamonds. Other groups are exploring different ways of handling and linking qubits. In Bristol, Joshi’s group has shown it can distribute quantum keys between eight users a few kilometres apart, all receiving entangled photons from the same laser source. It should be feasible to extend this to a few hundred people across a city, says Joshi. So far, he has demonstrated QKD and some similar protocols, but he says that with more sophisticated modules to receive the entangled photons the network would support other applications, including blind quantum computing.

Many other fledgling quantum networks are appearing, for example in Tokyo in Japan, Calgary in Canada and Los Alamos in New Mexico. These generally have only two or three nodes and are limited to QKD. But they are growing in range, with several stretching to more than 100 kilometres. The dream is to extend this to connect millions of users across the globe, carrying super-secure encryption keys across countries and continents.

Doing this will almost certainly involve piggybacking on the existing network of fibre-optic cables that carries all today’s internet traffic and other telecoms data. But here we run into a serious hitch: optical fibres aren’t completely transparent. Even if you use the optimum wavelength of light, 50 kilometres of fibre will absorb about 90 per cent of photons. That limits quantum-by-fibre to a range of a few hundred kilometres at most. Today’s fibre network uses amplifiers to boost signals. “But you can’t send quantum signals through an amplifier,” says Tim Spiller at the University of York, UK, who leads the country’s multi-institution Quantum Communications Hub. In effect, amplifiers measure the signal, which would play havoc with the delicate quantum data.

To extend the range of QKD, you can rely on trusted nodes, devices that relay a message by decrypting it and encrypting it again to send it down the next section of fibre. China already has an impressive network, with a 2000-kilometre-long backbone of 32 trusted nodes between Beijing and Shanghai, and hundreds of links in total. Problem solved? Not quite. Each node is a security risk that, if compromised, could leak your message. Worse, this is no good for fancier applications like blind computing because the original quantum information is discarded at each node.

To carry our quantum data far and wide, we need a device known as a quantum repeater. Imagine two users called Alice and Bob who want to chat. They each make a pair of entangled qubits, and each send one of their pair to a quantum repeater in the middle. The repeater performs a particular kind of simultaneous measurement of the states of the two qubits it has received, designed to entangle them. According to the rules of quantum physics, this then entangles the two qubits retained by Alice and Bob, a process called entanglement swapping. String many quantum repeaters together in a line and you can end up with entangled qubits at a much greater distance.

“The dream is to connect millions of users, carrying encryption keys across countries and continents”

If only we had a quantum repeater. They have been on the wish list for years, but have proved extraordinarily difficult to make. However, at Stony Brook University in New York, Eden Figueroa and his group are beginning to put some of the pieces together. One critical component is what’s called an in-and-out quantum memory that can catch a flying qubit and hold it until required for the simultaneous measurement. Figueroa’s quantum memory is based on a cloud of atoms that can effectively do this with a photon. The device also needs to register when it has caught a photon without disrupting the particle’s sensitive quantum state. Last year, Figueroa and his colleagues showed they could do this by sending in another photon that interacts only very weakly with the stored one.

These quantum memories have three big pluses for practicality. They are portable, coming in handy 40-centimetre modules. They work at room temperature rather than the frigid temperatures needed for many atomic-cloud devices. They can also work at normal telecoms wavelengths, as the team showed last year when they connected two of these devices 158 kilometres apart. “We are getting close to entanglement swapping, where everything has to work together,” says Figueroa. Useful repeaters will not only have to do all of this, but do it very efficiently.

Even boosted by repeaters, the fibre quantum internet will be patchy. Links across the ocean will be a particular problem because existing undersea cables have built-in amplifiers, spelling doom for qubits. If you laid a dedicated quantum undersea cable, it would have to include quantum repeaters that could be relied on to work for a long time.

So researchers are also looking at quantum links using satellites. The front runner here is China, which in 2016 launched the Micius satellite, carrying a quantum communications toolkit. “When Micius launched, that got everyone else to sit up,” says Daniel Oi at the University of Strathclyde, UK.

Micius encrypted a videoconference between Beijing and Vienna, Austria, in 2017, based on a form of QKD that has a high data rate, but in which the satellite acts as a trusted node. This will be fine for some users, such as governments and corporations that can afford their own satellites, but it won’t guarantee security for all the users in a highly connected future quantum internet. Then in 2019, Micius was used to form a link between two ground stations in China, at Nanshan and Delingha, 1200 kilometres apart, by splitting each entangled pair of photons and sending one to each station. This form of QKD is particularly secure. Even if the satellite were compromised, the key would be immune to hacking.

The disadvantage is that it works slowly. The two parties can only use an entangled pair when both photons in the pair make it to them, and in any satellite link, the majority of the light is lost because most photons either miss the receiver or get absorbed by the atmosphere. The Chinese ground stations are at high altitude and have large telescopes to act as receivers; and the satellite generates about 6 million entangled pairs per second. But even then, the secret key was generated at a rate of only a fraction of a bit per second. Jian-Wei Pan at the University of Science and Technology of China in Hefei, who leads the work on Micius, says he is now working to boost this rate with several improvements including brighter sources of entangled light.

Quantum constellation

Pan and Oi both foresee a network with many quantum terminals, including moving ones on ships and planes. “If you have many ground stations, a few big satellites won’t be able to service them,” says Oi. Instead, we will need a sprawling constellation of small satellites. Several projects are blazing the trail, including a UK-Singapore mission called SPEQTRE, and ROKS, a satellite built by a private consortium. Both are due to launch in 2022.

To weave a world-wide quantum web on top of all this hardware, we will need the kind of software that lets us blithely use apps on the classical internet. Several layers of software, known as the internet stack, route data around the existing network, so the average user doesn’t need to worry about the plumbing. Stephanie Wehner at QuTech is one of those working to build a quantum internet stack. Then there’s the fun stuff, the actual apps. We still don’t know what could be possible. New types of gaming? Novel forms of communication?

When these extraordinary technologies have girdled the world, we might not notice at first. The effect should mainly be an absence of problems: you don’t lose access to your bank account, elections aren’t hacked, the lights don’t go out. In time, there will be more tangible benefits too, especially for science. Quantum data links could allow telescopes to exchange information instantaneously to give astronomers a sharper view of the universe. They could synchronise atomic clocks more accurately, and so make gravitational wave detectors more sensitive. Not to mention the promise of shackling quantum computers together to boost their power.

On the other hand, the quantum web will surely turn the dark web darker still, and some people are bound to take advantage. One worrying suggestion is that terrorists could use blind quantum computing to design new weapons – and nobody would know. Governments might consider putting back doors into the hardware, “but that would defeat the purpose of all this”, says Wehner.

Perhaps in the end, this new form of internet will make the world simultaneously safer and more dangerous. How very quantum.