Quantum cryptography is considered a fully secure encryption method, but researchers from Linköping University and Stockholm University have discovered that this is not always the case. They found that energy-time entanglement - the method that today forms the basis for many systems of quantum cryptography - is vulnerable to attack. The results of their research have been published in Science Advances.
"With this security hole, it's possible to eavesdrop on traffic without being detected. We discovered this in our theoretical calculations, and our colleagues in Stockholm were subsequently able to demonstrate it experimentally," says Jan-Åke Larsson, professor at Linköping University's Division of Information Coding.
Quantum cryptography is considered a completely safe method for information transfer, and theoretically it should be impossible to crack. Many research groups around the world are working to make quantum cryptography resistant to various types of disturbance, and so far it has been possible to handle the disturbance that has been detected. Quantum cryptography technology is commercially available, but there is much doubt as to whether it is actually used.
Entanglement over global distances via quantum repeaters with satellite links
Authors:
Boone et al
Abstract:
We study entanglement creation over global distances based on a quantum repeater architecture that uses low-earth orbit satellites equipped with entangled photon sources, as well as ground stations equipped with quantum non-demolition detectors and quantum memories. We show that this approach allows entanglement creation at viable rates over distances that are inaccessible via direct transmission through optical fibers or even from very distant satellites.
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One possible future for communication is to create a quantum version of the internet that will have the ability, among other things, to send information with perfect security. This network will use entangled photons to transmit information from one locations to another without it passing through the space in between, hence the security.
Photons can only travel hundred kilometres or so through optical fibres before being absorbed. Conventional optical networks get around this with repeaters that boost the optical signal as it passes by.
This is more difficult with a quantum network but physicists have already tested many of the building blocks necessary to make quantum repeaters work. Nevertheless, quantum repeaters will be delicate pieces of kit, operating close to absolute zero with all the necessary cooling and power that is also required.
That should be relatively straightforward for quantum networks that stretch across land. But it is entirely unsuitable for undersea cables where conditions are far more hostile and the absence of infrastructure is a potential showstopper.
In fact, nobody is quite sure how it will be possible to operate the number of quantum repeaters necessary to carry one half of an entangled photon pair across the Atlantic or the Pacific. And without any technology even on the horizon that can do this job, there is a very real possibility that the quantum Internet might only ever consist of isolated quantum islands on different continents.
Today, Simon Devitt from Ochanomizu University in Japan and a few pals have come up with a way to solve this problem. Their idea is to transport the quantum bits or qubits across the ocean on a containership, a kind of quantum Clipper, that will shuttle back and forth across the seas with a ghostly quantum load.
Quantum hacking is the latest fear in the world of information security. Not so long ago, physicists were claiming that they could send information with perfect security using a technique known as quantum key distribution.
This uses the laws of quantum mechanics to guarantee perfectly secure communication. And perfectly secure communication is what you get, at least in theory.
The trouble is that in practice the equipment used to carry out quantum key distribution has a number of weaknesses that an eavesdropper can exploit to gain information about the messages being sent. Various groups have demonstrated how quantum hacking presents a real threat to “perfectly secure” communication.
So in the cat and mouse game of information security, physicists have been fighting back by designing equipment that is more secure. Today, Nitin Jain at the Max Planck Institute for the Science of Light in Erlangen, Germany, and a few pals show how the changes still leave the equipment open to attack but at the same time reveal how the next generation of quantum cryptography could be made better.
Europe and China are gaining the upper hand in the race to bounce perfectly secure messages off satellites in low Earth orbit.
One of the great benefits of quantum communication is the ability to send messages from one point in space to another with perfect security. Not so great is the fact that so-called quantum cryptography is limited to distances of around 100 kilometers.
That’s because over longer distances, photons tend to be absorbed by the glass in fiber-optic cables and by the atmosphere when beamed from one location to another. That causes errors that are too great for perfect privacy.
But there is a potential way around this–to send photons to an orbiting spacecraft, which then retransmits the message securely when it is over another part of the planet. That’s possible because the photons traveling straight up only have to negotiate a few tens of kilometers of the atmosphere before reaching space.
So it’s not surprising that governments all over the world are keen on exploiting space-based quantum cryptography. Indeed, last year we reported on a Chinese team that had successfully reflected individual photons off an orbiting satellite, to simulate a satellite sending photons to the ground.
The Chinese team said the demonstration was a crucial step toward space-based quantum cryptography. However, the ability to send single photons from orbit and receive them on the ground is not enough.
A key factor is the error rate in this process. If the error rate is above 11 percent, quantum cryptography does not work.
So an important unanswered question is whether the error rate is small enough.
Today, we get an answer thanks to the work of Giuseppe Vallone at the University of Padova in Italy and a few pals. These guys have bounced polarized photons off a number of different satellites and measured the error rate in the photons that return to Earth.
And they have good news. They clearly show that the error rate can be made smaller than the critical threshold.
A breakthrough in quantum cryptography demonstrates that information can be encrypted and then decrypted with complete security using the combined power of quantum theory and relativity - allowing the sender to dictate the unveiling of coded information without any possibility of intrusion or manipulation.
Scientists sent encrypted data between pairs of sites in Geneva and Singapore, kept "perfectly secure" for fifteen milliseconds - putting into practice what cryptographers call a 'bit commitment' protocol, based on theoretical work by study co-author Dr Adrian Kent, from Cambridge's Department of Applied Mathematics and Theoretical Physics.
Researchers describe it as the first step towards impregnable information networks controlled by "the combined power of Einstein's relativity and quantum theory" which might one day, for example, revolutionise financial trading and other markets across the world.
'Bit commitment' is a mathematical version of a securely sealed envelope. Data are delivered from party A to party B in a locked state that cannot be changed once sent and can only be revealed when party A provides the key – with security guaranteed, even if either of the parties tries to cheat.
The technique could one day be used for everything from global financial trading to secure voting and even long-distance gambling, although researchers point out that this is the "very first step into new territory".
This is a significant breakthrough in the world of 'quantum cryptography' – one that was once believed to be impossible. The results are published in the journal Physical Review Letters.
"This is the first time perfectly secure bit commitment – relying on the laws of physics and nothing else – has been demonstrated," said Adrian Kent.
"It is immensely satisfying to see these theoretical ideas at last made practical thanks to the ingenuity of all the theorists and experimenters in this collaboration."
Any signal between Geneva and Singapore takes at least fifteen milliseconds – with a millisecond equal to a thousandth of a second. This blink-of-an-eye is long enough with current technology to allow data to be handed over encrypted at both sites, and later decrypted – with security "unconditionally guaranteed" by the laws of physics, say the team.
The researchers have exploited two different areas of physics: Einstein's special relativity – which interprets uniform motion between two objects moving at relative speeds – combined with the power of quantum theory, the new physics of the subatomic world that Einstein famously dismissed as "spooky".
Completely secure 'bit commitment' using quantum theory alone is known to be impossible, say researchers, and the "extra control" provided by relativity is crucial.
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One of the dreams for security experts is the creation of a quantum internet that allows perfectly secure communication based on the powerful laws of quantum mechanics.
The basic idea here is that the act of measuring a quantum object, such as a photon, always changes it. So any attempt to eavesdrop on a quantum message cannot fail to leave telltale signs of snooping that the receiver can detect. That allows anybody to send a “one-time pad” over a quantum network which can then be used for secure communication using conventional classical communication.
That sets things up nicely for perfectly secure messaging known as quantum cryptography and this is actually a fairly straightforward technique for any half decent quantum optics lab. Indeed, a company called ID Quantique sells an off-the-shelf system that has begun to attract banks and other organisations interested in perfect security.
