SPEQTRE: Protecting data using the ‘spooky’ power of quantum mechanics
Quantum technologies harness the quirks of the unimaginably small, so it’s only fitting that quantum cryptography is finding a home in tiny satellites.
A collaboration known as SPEQTRE, between Singapore and STFC’s RAL Space in the UK is working to create a swarm of miniature satellites armed with quantum technology that will provide a theoretically unhackable method of encrypting the world’s data. SPEQTRE will harness the ‘spooky’ weirdness of quantum mechanics to future-proof the security of all our online transactions and communications.
A brief history of cryptography
For almost as long as humans have used the written word to pass information, we have tried to find ways to keep that information out of the hands of others — whether that be to give us an advantage over a rival, or to prevent a rival gaining an advantage over us. At its most basic, this is what cryptography is: a method of protecting information, through the use of codes and ciphers, so that the only people who can read it are those for whom the information is intended.
The first known evidence of cryptography can be found in the tomb of Khnumohotep II in ancient Egypt where, almost 4,000 years ago, a cunning scribe replaced the standard hieroglyphic symbols with alternative pictographs in order to change the meaning of the inscriptions.
Fast forward to 100BC and we find Julius Caesar using something called a substitution cipher to keep military commands and intelligence away from the prying eyes of his enemies. In a substitution cipher, each character of the alphabet is substituted with another character from further up or down the alphabet — thus encrypting the message. In theory, only the person with the ‘key’ to the cipher can decrypt and read the message. So, if the key to the cipher is ‘3’, the recipient knows to replace each letter with one removed from the original by three — A becomes D, B becomes E, and so on.
Unfortunately such ciphers are relatively easy to crack — after all, the alphabet only has 26 letters so any would-be snooper only has to try a maximum of 26 combinations before they will stumble across the solution. Of course, once the enemy has the key, they can decode any message they intercept and you will have no idea whether your code has been cracked until you suddenly find a well-prepared force ready to take you by surprise instead of vice versa.
Although in the intervening centuries cryptography has become vastly more sophisticated, the same problem remains: no code is uncrackable and once it is cracked you have no idea whether your messages are being intercepted and potentially used against you — as the Nazis discovered when their uncrackable Enigma code was deciphered by a British team at Bletchley Park in 1941.
Spycraft, or witchcraft?
But what if there was a way to encrypt messages in such a way that, as soon as someone attempts to so much as look at the message, the message will ‘know’ it is being looked at and immediately become corrupted and undecipherable? Surely such a thing would be more witchcraft than spy craft? Welcome to the weird world of quantum cryptography.
Quantum Key Distribution (QKD) is a secure communications cryptographic protocol that will provide the first truly hack-proof method of encrypting all of the world’s data communications — making them resistant to all known computational attacks, including from future quantum computers. As an added bonus, QKD can be integrated into the network systems already in use.
Quantum Key Distribution is based on one of the weirder phenomenon of quantum mechanics (which has more than its fair share of weirdness) known as quantum entanglement.
Clouding around with probability
We are used to thinking of particles as being like miniature spheres — they might be almost unimaginably small, but they are as solid and real as you are (after all, you are made up from them) — but this is not always the case. In quantum physics, rather than existing as points of matter, they can exist as both a particle and a wave (think of the photon, which is a particle of light even though light is a wave). Particles can exist in a state called quantum superposition in which they exist as both a particle and wave; in one place and another; with one set of properties and with an opposing set of properties; all at the same time — they are effectively a cloud of possibility.
Only when an outside influence interrupts this cloud (such as when a photon interacts with the retina in your eye) is the particle ‘forced’ to settle on its final form. You can think of it as being like a coin spinning on a table top — it is a blur of both heads and tails that only settles on its final identity when friction, gravity and the table top forces it to stop spinning.
In quantum physics it is possible to create two particles that, while they are in this cloud-like state of quantum superposition, have their fates entwined to such a degree that the fate of one will affect the fate of the other — this is quantum entanglement. If you have two entangled particles, when you interfere with one of them and ‘force’ it to settle on an identity, you also ‘force’ its twin to immediately settle on its own identity (an identity that will always be the polar opposite of the other’s). If the particles were our two coins, when you stop one and force it to become ‘heads’, the other will automatically become ‘tails’.
Spooky action at a distance
Just in case that wasn’t weird enough, this link isn’t affected by distance — no matter how far apart the two particles become, the fate of one will instantaneously influence the other. You could carry our two spinning coins to opposite ends of the country (or to Alpha Centauri) and the moment you stop one, the other will also stop. Einstein was not a fan of this phenomenon when it was first proposed, calling it ‘spooky action at a distance’ because it seemed to violate the idea that nothing (not even information) can travel faster than the speed of light.
Einstein might not have liked it, but it is precisely this ‘spooky action at a distance’ that is at the heart of quantum cryptography. In QKD, long strings of entangled photons, shared between distant locations, can act as quantum messengers carrying secret keys that can securely encrypt data transmissions. Meanwhile, anyone attempting to intercept the keys will fall foul of quantum physics because the very act of trying to read the information would change the state of the photons and instantaneously tip off the recipients that the key had been compromised.
Unfortunately, entangled photons are extremely sensitive to even unintended interference so they cannot be transmitted very far through optical fibres or through the atmosphere before the signal is degraded too much to be read (the farthest anyone has managed is a few hundred kilometres). As such, the only way QKD can be effective as a global encryption method is to move to space. Any entangled photon stream sent from a satellite will suffer very little signal loss as it travels through the vacuum of space and will only have to contend with a relatively short trip through the atmosphere (where the ‘thick’ stuff only really resides in the last 16 kilometres or so).
There have already been success tests of satellite-based QKD systems, but these have employed large satellites that are expensive to build, launch and maintain in orbit. Where the UK/Singapore collaboration building SPEQTRE differs is that this is a step towards a space-based QKD network using swarms of tiny satellites known as CubeSats.
A swarm of quantum CubeSats
A CubeSat (unsurprisingly) is a cube that measures 10cmx10cmx10cm. Small and cheap to build, their most important feature is the fact that they are standardised. CubeSats give developers standard specifications for size, weight and basic construction, which enables parts to be built as a “one-size-fits-all” type of arrangement. Their tiny size and weight means that CubeSats can “piggyback” on the launch of bigger missions, which drastically reduces the cost of a launch — after all, that big mission was going up anyway.
Their standardised dimensions means that a launch carrier already knows exactly what they have to find room for within their payload. Furthermore, because they are cubes, multiple CubeSats can be stacked up like Lego bricks.
SPEQTRE will be made up of several standard Cubesat modules bolted together, but despite this it will still weigh in at around 10kg (a recent Chinese QKD satellite, for example, tipped the scales at 600kg). Fitting a quantum communications system into such a small package is extremely challenging however. The lack of internal real estate means there is very little room for redundant backup systems, should part of the system fail. The SPEQTRE team has been rigorously testing the system’s ability to survive the harsh conditions it will be subjected to during launch and operations in space by simulating launch vibrations and the extreme temperature swings of life in space.
Within the tiny satellite, along with the electronics, optical trackers, attitude adjustors, photon transmission systems, telescope (and much more), the designers have to squeeze in a system of lasers and crystals — to create a laser beam that is fired into a light-altering crystal that creates pairs of entangled photons ready to be transmitted down to Earth.
An alignment assignment
Sending and receiving those photons requires precise alignment between a telescope onboard the satellite and a telescope on Earth. For a tiny CubeSat, that means having to continuously track its position relative to the receiver and adjust its orientation in space as it races though its orbit. This is where the team at STFC’s RAL Space group comes in. They have been charged with developing both the CubeSat platform and the optical tracking system the satellite will need to accurately and precisely track ground stations at RAL Space’s Chilbolton Observatory and in Singapore.
The collaboration aims to build on both countries’ efforts to grow their space and quantum technologies sectors and allow them to stake a claim in the emerging QKD market, which is estimated to be worth up to £11.5 billion over the next ten years.
SPEQTRE will be followed by a separate mission led by the UK Quantum Communications Hub at the University of York. It is hoped that this mission will build on the success of SPEQTRE to extend the range of quantum secure communication and turn this experimental technology into a commercially viable solution to the problem of keeping sensitive information out of the wrong people’s hands.
