BUILDING OUT the QUANTUM INTERNET
Why quantum satellites will make it harder for states to snoop
by Jacob Aron / 24 August 2016
“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. 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 low Earth 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.
“Cryptography relies on the sharing of secret keys between communicating parties, typically named Alice and Bob“
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. 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.
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 Jian-Wei Pan 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.”
“The raw key error fraction (QBER, lower panel) is dominated by source correlations at night, and starts to see a sharp increase with the rising sun shining directly in the single photon detectors.”
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.
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.
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, China’s satellite QUESS will put quantum mechanics to the test.
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.
“These two crystals, illuminated by two lasers, act as quantum memories that could synchronize future quantum communications networks. They also demonstrate that centimeter-sized objects can be entangled to share a common nonlocal feature.”
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.”
“Yttrium oxyorthosilicate crystal doped with neodymium ions illuminated by a yellow laser, demonstrating ability to store and release a photon without perturbing its quantum state.”
“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.”
“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.”
China’s 600-kilogram satellite contains a crystal that produces entangled photons.
PASSING the BELL TEST
“China has launched the world’s first satellite designed to do quantum experiments. A fleet of quantum-enabled craft is likely to follow. First up could be more Chinese satellites, which will together create a super-secure communications network, potentially linking people anywhere in the world.
But groups from Canada, Japan, Italy and Singapore also have plans for quantum space experiments. “Definitely, I think there will be a race,” says Chaoyang Lu, a physicist at the University of Science and Technology of China in Hefei, who works with the team behind the Chinese satellite.
The satellite was nicknamed “Micius,” after a fifth century B.C. Chinese scientist
The 600-kilogram craft, the latest in a string of Chinese space-science satellites, will launch from Jiuquan Satellite Launch Center in August. The Chinese Academy of Sciences and the Austrian Academy of Sciences are collaborators on the US$100 million mission. Quantum communications are secure because any tinkering with them is detectable. Two parties can communicate secretly — by sharing a encryption key encoded in the polarization of a string of photons, say — safe in the knowledge that any eavesdropping would leave its mark.
So far, scientists have managed to demonstrate quantum communication up to about 300 kilometres. Photons travelling through optical fibres and the air get scattered or absorbed, and amplifying a signal while preserving a photon’s fragile quantum state is extremely difficult. The Chinese researchers hope that transmitting photons through space, where they travel more smoothly, will allow them to communicate over greater distances.
At the heart of their satellite is a crystal that produces pairs of entangled photons, whose properties remain entwined however far apart they are separated. The craft’s first task will be to fire the partners in these pairs to ground -stations in Beijing and Vienna, and use them to generate a secret key.
UNSW qubit puts quantum operations on silicon
During the two-year mission, the team also plans to perform a statistical measurement known as a Bell test to prove that entanglement can exist between particles separated by a distance of 1,200 kilometres. Although quantum theory predicts that entanglement persists at any distance, a Bell test would prove it.
The team will also attempt to ‘teleport’ quantum states, using an entangled pair of photons alongside information transmitted by more conventional means to reconstruct the quantum state of a photon in a new location. “If the first satellite goes well, China will definitely launch more,” says Lu. About 20 satellites would be required to enable secure communications throughout the world, he adds.
The teams from outside China are taking a different tack. A collaboration between the National University of Singapore (NUS) and the University of Strathclyde, UK, is using cheap 5-kilogram satellites known as cubesats to do quantum experiments.
Last year, the team launched a cubesat that created and measured pairs of ‘correlated’ photons in orbit; next year, it hopes to launch a device that produces fully entangled pairs. Costing just $100,000 each, cubesats make space-based quantum communications accessible, says NUS physicist Alexander Ling, who is leading the project.
“Left: Artist’s rendering of a quantum communications link between a ground station and the satellite. Right: Schematic drawing of the proposed Quantum EncrYption and Science Satellite (QEYSSat) spacecraft.”
A Canadian team proposes to generate pairs of entangled photons on the ground, and then fire some of them to a microsatellite that weighs less than 30 kilograms. This would be cheaper than generating the photons in space, says Brendon Higgins, a physicist at the University of Waterloo, who is part of the Canadian Quantum Encryption and Science Satellite (QEYSSat) team. But delivering the photons to the moving satellite would be a challenge. The team plans to test the system using a photon receiver on an aeroplane first.
An even simpler approach to quantum space science, pioneered by a team at the University of Padua in Italy led by Paolo Villoresi, involves adding reflectors and other simple equipment to regular satellites.
Last year, the team showed that photons bounced back to Earth off an existing satellite maintained their quantum states and were received with low enough error rates for quantum cryptography (G. Vallone et al. Phys. Rev. Lett. 115, 040502; 2015). In principle, the researchers say, the method could be used to generate secret keys, albeit at a slower rate than in more-complex set-ups.
“…the delicate polarization states of photons, which are needed for quantum-encrypted communication, can be transmitted via a laser beam (yellow arrows) from a distant satellite (transmitter) down to a telescope on Earth (receiver)…”
Researchers have also proposed a quantum experiment aboard the International Space Station (ISS) that would simultaneously entangle the states of two separate properties of a photon — a technique known as hyperentanglement — to make teleportation more reliable and efficient.
As well as making communications much more secure, these satellite systems would mark a major step towards a ‘quantum internet’ made up of quantum computers around the world, or a quantum computing cloud, says Paul Kwiat, a physicist at the University of Illinois at Urbana–Champaign who is working with NASA on the ISS project.
In superdense teleportation of quantum information, Alice (near) selects a particular set of states to send to Bob (far), using the hyperentangled pair of photons they share.
The quantum internet is likely to involve a combination of satellite- and ground-based links, says Anton Zeilinger, a physicist at the Austrian Academy of Sciences in Vienna, who argued unsuccessfully for a European quantum satellite before joining forces with the Chinese team.
“With a bit of pre-shared entanglement, the quantum router can be realized with linear optical devices. The routing is realized with a polarization beam splitter (PBS) and a wave plate on the signal photon.”
And some challenges remain. Physicists will, for instance, need to find ways for satellites to communicate with each other directly; to perfect the art of entangling photons that come from different sources; and to boost the rate of data transmission using single photons from megabits to gigabits per second.
If the Chinese team is successful, other groups should find it easier to get funding for quantum satellites, says Zeilinger. The United States has a relatively low profile when it comes to this particular space race, but Zeilinger suggests that it could be doing more work on the topic that is classified.
Eventually, quantum teleportation in space could even allow researchers to combine photons from satellites to make a distributed telescope with an effective aperture the size of Earth — and enormous resolution. “You could not just see planets,” says Kwiat, “but in principle read licence plates on Jupiter’s moons.”
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