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From the Archives: The Dawn of the Quantum Internet
Satellite experiments help bring an era of quantum communication closer
Long ago, they say, tulips came from the Tian Shen mountains. Over millennia they made their way west; carried first to the gardens of Persia and then onwards, in the hands of princes, to the palaces of the Ottoman sultans and the courts of Europe. Eventually they became precious; so valuable that a single bulb was once exchanged for twelve acres of land, and the sudden collapse in their price left the Dutch economy in ruins.
This is not a story about tulips, however. It is a story about the Internet, about quantum technology, satellite and lasers. All of these, today, can be found in the Tian Shen mountains; a place that at first glance seems little unchanged from the days when gardeners first gathered tulip bulbs.
Buried deep within those mountains is an astronomical observatory, and it was from there, some years ago, that a laser beam was fired into a star-strewn night sky. Encoded within the beam was a message; a secret key transmitted in the individual photons of the laser beam.
Over ten minutes the beam traced its way across the sky, marking the passage of QUESS, a satellite flying far above. One day, the researchers hope, the message they are sending will pave the road towards a new quantum Internet; a network of guaranteed privacy and security.
Sometime later, as QUESS flies over the city of Graz, Austria, the satellite activates its own laser. Once again the message is encoded into photons and sent in a stream of light to a waiting receiver. This time, however, it is read; and the key it holds opens an ultra-secure communications channel: a link stretching from the centre of Europe to the wilderness of Tian Shen.
For as long as people have communicated, they have sought to conceal what is said. Evidence of cryptography stretches back to the very earliest known writing. Today it provides the foundation of the economy, from banking to bitcoin, powers revolutions and coups across the planet, and underpins the Internet, the greatest mass communication device ever created.
All encryption, however, has a weakness. Coded messages require a key, a piece of secret information known only to those trusted to read the message. Should the key be stolen or intercepted, then the code is suddenly worthless; its secrets betrayed to the thief.
Imagine, for example, that Alice wants to send a message to Bob. She’s worried that a third person, Eve, is trying to intercept and read the message. To avoid this, she writes the message in code. That works to keep the message hidden, but now Bob has a problem. He cannot read Alice’s message without also knowing the key, which he doesn’t yet have.
Somehow Alice needs to provide the key to Bob. She can’t send the key with the message, since Eve, if she were to intercept the message, would also capture the key. She could send them separately, though there is still a risk that Eve manages to intercept them both. Perhaps she can send the key via a more secure method - but if she can do that, why bother with the code at all?
The answer, we discovered in the 1970s, is something called public key cryptography. Instead of one key, Alice and Bob both have two keys of their own; one which is kept secret and one which is revealed to everyone and anyone. When Alice wants to send a message, she uses Bob’s public key to encode the message. Thanks to a piece of mathematical magic, only Bob’s secret, or private, key can decode it.
That mathematical magic relies on the use of large prime numbers. As long as the numbers are large enough, no modern computer can crack the code in a reasonable time. Yet quantum computers are different. Whereas a supercomputer might take a billion years to crack the key, a reasonably powerful quantum computer could do it in minutes.
That is a problem. Quantum computers are, perhaps fortunately, several years, even decades, from mainstream use. But they will eventually be here, and with them the entire infrastructure underlying the modern Internet, economy and democracy will be under threat.
Throughout history, code makers and code breakers have fought a long running battle. Every step forward in cracking codes has, sooner or later, been met by a new advance in secrecy. Quantum computing will be no exception. Indeed, researchers have already started working on new forms of quantum cryptography, secure against any form of attack.
Almost all forms of encryption depend on mathematics for their security. Modern codes, for the most part, rely on calculations that are easy to do - like multiplying two large prime numbers - but hard to reverse. Quantum cryptography, however, relies on physics; trusting security to the quantum properties of photons.
To understand the benefit, let’s return to the key exchange problem. Eve, somehow, has obtained a quantum computer, rendering Alice’s efforts at keeping the key secret worthless. But Alice, knowing this, decides to turn to quantum cryptography and encode the key in a stream of photons.
The details of how she does this are complex, but you can imagine that photons spin in two possible directions. By making the photons spin in one direction or the other you create a series of (quantum) 1s and 0s, allowing data transmission. These photon spins are quantum states, and they obey quantum laws.
Alice sends her series of photons to Bob, but Eve, aware they are coming, manages to intercept them. Now, though, she has a problem. By measuring the spin of each photon, she triggers something known as wavefunction collapse; a quantum effect that irreversibly alters the state of the photons. Bob, seeing that, realises the key has been intercepted and asks Alice to send a fresh one.
This is the magic of quantum key distribution, or QKD. Eve can intercept the key as much as she wants, but she cannot do so secretly. Alice and Bob will always know, thanks to the quantum laws of nature, that she has stolen the key. Once they do get a key through without interception, they are free to start communicating without risk of being overheard.
The idea behind QKD has already been demonstrated in practice. In 2006 teams of researchers exchanged keys between two Spanish islands, establishing secure communication over a few dozen miles. But the technology does have some rather severe limitations. Quantum states are hard to maintain, and transmitting the keys over long distances is tricky.
Instead researchers have turned to satellites. As long as the quantum state can be maintained for a few hundred miles, then the key can be sent via a laser from Earth to an orbiting satellite. The satellite stores the key until it happens to fly over the intended destination, at which point the key is sent back to Earth.
The first such long-distance quantum key distribution took place between Vienna and China in 2016, covering a distance of almost five thousand miles. Researchers first created the secret key in China, encoded it into photons and then fired them in a laser beam towards the orbiting QUESS satellite. When QUESS flew over Vienna some time later, the satellite shot a second laser beam back down to Earth, carrying the key to its intended destination.
That was enough to prove the concept worked, but it is not yet enough to establish a full scale quantum internet. For that we will need a constellation of satellites; sufficient to ensure a key can be exchanged whenever it is needed. Indeed one day, not too long from now, such a constellation may become a reality - one as essential to everyday life as GPS.
For the moment, however, QKD is still under development. Much of the work is being done in China - a nation eager to secure its messages from prying Western eyes. But Europe and America are pursing quantum satellites of their own. Eagle-1, a European QKD satellite, is planned for launch in 2024; it, like QUESS, will test the technology.
Put together, these efforts mark the slow beginnings of the quantum era. Some day researchers may come to think of Tian Shen as not just the birthplace of the tulip; but also as a marker of the dawn of a new Internet.
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