HAS the era of unhackable global communication begun? Last week, the world’s first quantum communications satellite blasted into orbit from China’s Gobi desert. Known as the Quantum Science Satellite (QUESS), it is Sputnik for the ultra-paranoid.
The mission will test a way of transmitting impenetrable messages across vast distances. If successful, the next decade could see a boom in quantum satellites, resulting in a secure network that will protect its users from even the most savvy eavesdroppers.
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In an age of cyberwarfare, WikiLeaks and state-sponsored hacks, it’s easy to see the lure of a truly private way to talk to each other – something existing infrastructure just can’t ensure.
The new satellite will make its home among the thousands of communications spacecraft that already float in orbit, pumping out multiple TV channels and enabling international phone calls. But signals beamed from regular satellites to the ground via radio or microwaves can be intercepted by anyone with the necessary receiving equipment. To get around this, signals are often encrypted. Trouble is, encryption can be cracked – that’s how satellite TV pirates are able to watch channels for free.
QUESS is different. It uses a technique called quantum key distribution to encrypt signals. The laws of quantum mechanics are such that they guarantee the message is secure. So if done properly, signals can’t be hacked.
“If the first is a success, a quantum constellation of satellites could provide global coverage“
It’s a bold claim, but one backed up by hard science. Quantum key distribution works by transmitting particles of light called photons prepared in a particular quantum state. By measuring these states, the receiver on the other end can agree a stream of 0s and 1s that form a secure code or key, which can be used to encrypt data sent via conventional means – over the internet or through an ordinary communications satellite. Measuring a quantum object disturbs its state, so any attempts by an eavesdropper to intercept a photon will be detected and the key discarded, so there is no risk of being hacked (see diagram).
Quantum key distribution has already been rolled out on fibre-optic networks in the US, Europe and China, but these are limited to just a few hundred kilometres – any greater and the light signals become too faint. Photons sent through space last longer, and QUESS will extend the reach of quantum key distribution even farther by exploiting the quantum property of entanglement, which links the quantum states of two particles even when they are separated.
The satellite will first test communications between ground stations 1200 kilometres apart in China, says of the University of Science and Technology of China in Hefei. If successful, his team will look to establish a secure connection with collaborators in Austria, then in Italy and Germany, before creating “a quantum constellation for global coverage”, says Pan.
Pan’s efforts are likely to spur other launches. “You could dream of a network of satellites providing secure keys,” says Harald Weinfurter of the Ludwig Maximilian University of Munich, Germany. “Within some 10 years we could have a working network.”
Given the need for specialised receivers to pick up the faint photon signals, it’s unlikely you or I will be tapping directly into this network right away. Ordinary encryption methods based on difficult mathematical problems work fine most of the time, and underpin everyday activities like buying things online, checking your bank balance or sending WhatsApp messages.
The first users of quantum key distribution will therefore be the military, governments and banks wanting security for their most precious data. “With the budget that banks have, it’s a minor investment,” says Weinfurter.
Another attraction of quantum key distribution is that it offers protection against the march of progress. As computers get faster, there’s no guarantee that a secure message sent today won’t eventually become crackable. And if we ever develop large-scale quantum computers, many of today’s encryption techniques will be busted wide open.
Future proof
If the signal has been encrypted using a quantum satellite, these issues go away. “While quantum key distribution is considered difficult to implement, it does provide very high, long-term security for communications,” says Thomas Jennewein at the University of Waterloo, Canada.
Such a network could change the rules of financial fraud and cyberwarfare. Thanks to the Snowden revelations, we know that the US National Security Agency and its spying partners have tapped into the fibre-optic networks of firms like Yahoo and Google, allowing them to slurp up data at will. With quantum key distribution, that data would be encrypted in an unbreakable form.
And much like the internet began as a military tool, there’s no reason why this shouldn’t become the default encryption of communications for everyone in a few decades. Firms may start offering customers ultimate security as a premium product, a move Apple has already made with ordinary encryption after its battle against the FBI.
For this to happen, a quantum network will have to be made up of satellites designed differently to China’s pioneering effort. QUESS weighs 600 kilograms and is equipped to run experiments that will push quantum science to its limits (see “Testing times for quantum theory“). “If you only want to do secure communication, we can make much smaller and cheaper satellites,” says Weinfurter.
“Quantum communication would prevent things like the NSA tapping into Google’s fibre-optic cables“
A team at the Centre for Quantum Technologies in Singapore is working to put quantum key distribution equipment on CubeSats – small spacecraft that cost a fraction of their larger cousins to build and launch. The team launched its first test photon generators earlier this year, and will be watching how the Chinese mission aims photons at a ground station from a fast-moving satellite, says team member Alexander Ling.
And Jennewein and his colleagues are working on a quantum CubeSat for the Canadian Space Agency, but haven’t yet got full funding. QUESS could change that, he says.
Future satellites may go higher as well as smaller. QUESS is about 500 kilometres up, and whizzes around the globe every 90 minutes, so can be in contact with ground stations for only a short period of time. Christoph Marquardt of the Max Planck Institute for the Science of Light in Erlangen, Germany, thinks quantum satellites should be placed 36,000 kilometres up, in geostationary orbit, so they are above the same point on the ground at all times and thus always in contact. His team has shown that this is technically and economically feasible.
Whatever its form, the quantum network is coming soon. “Five years ago, I wouldn’t have thought it would be working so fast,” says Marquardt. “Now, it’s likely in the next 10 or 15 years.”
Testing times for quantum theory
The laws of quantum mechanics govern how atoms and sub-atomic particles behave. Although it is one of our most successful theories, we still don’t know whether its predictions hold in some situations – such as over very long distances or beyond Earth’s gravitational pull.
As well as showcasing the feasibility of a secure global communications network (see main story), China’s satellite QUESS will put quantum mechanics to the test.
Entanglement
The satellite is equipped with a crystal that produces entangled photons. If the theory holds, their quantum states should remain intertwined even when they are physically separated.
QUESS will fire one of these photons at a ground station in Delingha, China, and another to a station in Lijiang, more than 1200 kilometres away. If all goes to plan, measuring the state of one photon will instantly put the other in the opposite state, despite the vast separation. The record for demonstrating entanglement currently stands at 143 kilometres, the distance between the Canary Islands of La Palma and Tenerife, where such experiments are often done thanks to the still atmosphere.
Teleportation
The QUESS team will perform quantum teleportation over 1000 kilometres between the satellite and a ground station, which involves transferring or “teleporting” the quantum state of one photon to another. Although this happens instantly, the result is only apparent once the satellite and ground stations have communicated their readings via normal channels, so this can’t send messages faster than light.
Hidden variables
The team will also carry out what’s known as a Bell test – essentially, a statistical check that reality must be based on quantum mechanics, and not some other hidden theory.
This article appeared in print under the headline “A quantum of privacy”
Article amended on 16 September 2016
Correction: Â We clarified the orbits of communications and broadcasting satellites.
