By Hilary Lamb Fri 23 Feb 2024
Collected at : https://eandt.theiet.org/2024/03/08/confidential-comms-quantum-satellites
With its Micius satellite, China proved it possible to use individual photons sent from space to communicate in almost total secrecy. Now, other countries are playing catch-up.
Time is running out for our encryption – although no one knows quite when that will be. This threat is posed by quantum computers, which are ideally suited to cracking classical encryption schemes like RSA through sheer brute force. Any data being sent place to place is vulnerable – from military secrets to the financial and medical records of private citizens.
“If you’re interested in the long-term security of data that has been transmitted … you should probably be concerned about the future emergence of quantum computing technology, because of the ‘harvest now, decrypt later’ attack that could be present on any encrypted data,” says Professor Tim Spiller, director of the UK’s Quantum Communications Hub. “That’s the looming threat.” Intelligence agencies are already collecting and storing vast troves of data, ready for the technological breakthrough that will make those secrets secret no more (‘Q Day’). This is no abstract threat. It is recognised by the world’s most influential people and powers – in December 2022, US President Joe Biden signed the bipartisan Quantum Computing Cybersecurity Preparedness Act, requiring government agencies to keep inventories of technology vulnerable to the quantum threat.
There are, broadly speaking, two complementary defences to the quantum threat – distinct in terms of expense, complexity and security. The more accessible is quantum-proof cryptography, which develops new generations of cryptographic algorithms more resilient to quantum computers. The other is quantum key distribution (QKD).
Quantum key distribution
QKD allows cryptographic keys to be shared in a manner that makes it impossible to eavesdrop without detection. Its security is rooted in fundamental laws of physics rather than the complexity of a mathematical problem, rendering it ‘quantum safe’ (safe from attack by quantum computers).
The sender, Alice, prepares photons in different states of polarisation (orientation) and randomly selects either a rectilinear (+) or diagonal (x) base to send them with. The receiver, Bob, randomly selects bases to measure the polarisation of the photons as they arrive. Alice and Bob then use a classical communication channel to compare the bases they used for each photon – although not the polarisations – and use the sequence of photons measured with matching bases to generate a unique secret key, with the polarisations corresponding to 0s and 1s. Because measuring a quantum system disturbs it, an eavesdropper cannot intercept the photons without being detected.
QKD is not a silver bullet. It requires specialised hardware, making it unsuitable for day-to-day consumer devices, and its security is not absolute (for instance, if Alice cannot verify if Bob is really Bob or an enemy agent). However, it provides an unmatched level of security appropriate for transmitting the most sensitive data, and is thus important to develop alongside quantum-proof cryptography.
A Sputnik moment
For many years, efforts to implement QKD were desultory. Little attention was given to satellite QKD in particular: costly, complex and unglamorous compared with other space ambitions. All this changed in 2017, when Chinese scientists announced that they had used a satellite to implement QKD.
The Micius satellite, named after the ancient Chinese philosopher, was launched in 2016 under the management of Professor Pan Jianwei of the University of Science and Technology of China. Pan, a brilliant and energetic physicist characterised as the ‘father of quantum’, spent much of his early career in Europe. The 2017 breakthrough was a collaboration between Pan’s team in China and a team led by his former doctoral supervisor, 2022 Nobel laureate in physics Professor Anton Zeilinger, in Austria. It could be considered a triumph of collaborative science, a product of a very different geopolitical atmosphere.
But quantum technologies – not only cryptography, but also sensors and computers – are critical for national security. The Micius breakthrough was perceived by much of the rest of the world as a challenge from China – a Sputnik moment. “Sometimes there are these moments. That’s probably similar to the space race; you see your adversary is displaying a technological advancement that you weren’t expecting, or is showing prowess in a technological area where you hadn’t been paying that much attention,” says Dr Alice Pannier, an expert in geopolitics and technology at the French Institute of International Relations. “China’s progress in that realm encouraged the US – not only the US, [but] other countries as well – to invest in quantum.”
The Biden administation has enacted laws requiring inventories of quantum-vulnerable technology
Air, ground, space
QKD has been on the path to commercialisation for many years. It has been performed all over the world with transmission of photons through free space and through fibre-optic cables. In the UK, the Quantum Communications Hub is overseeing a quantum fibre network spanning much of southern England – stretching from one metro network in Bristol to another in Cambridge – and has also carried out trials using track-side railway fibre for quantum communications in the north of England. BT, Toshiba and EY are trialling a commercial quantum network in London.
But these approaches are limited. Free-space transmission requires a direct line-of-sight path, making it impractical for many real-world applications. In fibre, the degradation of the signal limits individual spans to around 100km. QKD cannot be carried out over longer distances without compromising its absolute security – the very quality it is being pursued for.
In a classical fibre-optic network, this is easily handled by inserting nodes to relay the signal down further spans of fibre. At present, quantum fibre networks rely on equivalent ‘trusted nodes’, which generate a new classical signal, convert it back into a quantum signal, then send it on. But introducing that classical signal makes the nodes weak points, with no quantum security – so they must be made absolutely physically secure. Secure quantum versions of these nodes (known as quantum amplifiers or quantum repeaters) have not yet moved beyond the laboratory. Physicist Dr Rebecca Harwin summarises the problem: “When you send your QKD signal, you’re relying on it being secure and you’re relying on it not being intercepted by anyone. Unfortunately, the way that regular repeaters in a fibre-optic network work is they read your signal and retransmit it – so if you want something that’s definitely secure, you don’t want a point in your network where your signal has to be read and retransmitted.”
For now, then, quantum fibre is not trusted to span large bodies of water and/or national borders (with the exception of borders of countries with common defence policies). To expand quantum communications networks, we must look to space. An orbiting satellite could generate photons and distribute cryptographic keys to Earth-based ground stations as it passes overhead, forming secure communication channels across the Earth. Future quantum networks would be likely to integrate free-space transmission for nearby connections, fibre for metro and national connections, and satellites for international connections.
SwitchingAlice and Bob
Quantum EncrYption and Science Satellite (QEYSSat) is part of Canada’s national quantum strategy. It is distinct from other satellite QKD technology demonstrator missions in that it aims to establish an Earth-to-space link rather than the conventional space-to-Earth link: “In cryptography, we often talk about Alice being the transmitter and Bob the receiver,” says Eric Gloutnay, a Canadian Space Agency engineer working on the mission. “Well, our Alice is our ground station, and Bob is the satellite.” This will allow for the testing of many different types of quantum source from the ground station, and opens up the possibility of other missions’ satellites sending quantum signals to theirs.
The QKD mission is strategically important for Canada. “The world has changed, right? Security-wise, it’s a big challenge,” says Gloutnay. “The goal for Canada is really about guaranteeing the future privacy of Canadian public, private, commercial [communication]. We think this technology is a way to get truly secure communication infrastructure in Canada.”
Taking to the sky
Since Micius made headlines, there has been a resurgence of global interest in satellite QKD. Several QKD satellite launches are scheduled for 2024, including Singapore’s SpeQtral-1, the EU’s Eagle-1, and two separate UK satellite demonstrators.
One of these is a UK-Singapore collaboration, SpeQtre. It aims to perform QKD with entangled photons between a satellite and ground station, SpeQtral having already demonstrated transmission of entangled photons within an orbiting satellite: “This is taking it one step further towards the infrastructure you’d need to make it into something commercially viable,” says Harwin, who is working on the optical payload for the mission at RAL Space.
Satellite QKD is a staggeringly complex operation. “You’re talking about sharing individual photons between space and the ground,” says Dr Rob Bedington, co-founder and CTO of SpeQtral. We’re generating photons at the individual level on the satellite, and you have to detect them on the ground and correlate: ‘Okay, that one we detected here corresponds to that one we generated up on the satellite at that time as the satellite flew over the ground station.’ It kind of sounds impossible.” The SpeQtral mission will also use a CubeSat platform, requiring the quantum source and supporting equipment to fit within a satellite no larger than a shoebox – in comparison, Micius was about the size of a car. The mission could be a first step towards a quantum satellite constellation, which would use hundreds or thousands of tiny satellites to distribute quantum keys all over the planet.
The other UK mission is being led by the Quantum Communications Hub. It is notable for using UK space and quantum tech, while SpeQtre is using UK space tech and Singaporean quantum tech. This mission will have two different types of quantum source on the satellite and a dual receiver at its ground station near Edinburgh. According to Spiller, the two missions are complementary. “Once we’ve hopefully got both of these experiments working, we might be able to mix and match and send quantum signals from one satellite to the other ground station and so on,” he says. “There is the potential, if we can get everything to work, that we might be able to share quantum keys, at least between Scotland and England, in the not too distant future.”
The world’s first quantum communication satellite, Micius, is named after an ancient Chinese philosopher
Newambitions
This feature reflects the opinions of the experts at the time of writing (summer 2023). The global picture has developed since then, most notably with China’s announcement of the ambitious next step in its quantum satellites programme. According to the Chinese Academy of Sciences, this will entail reaching beyond low Earth orbit to place a quantum satellite in an orbit similar to that of GPS satellites. This means the satellite will be visible to ground stations for much longer periods of time as it passes overhead.
The age of distrust
The sundry national efforts to master QKD, and quantum technologies more broadly, have inspired frequent comparisons to the space race. However, there is little ‘racing’ to speak of. China has already won. Other scientific powers are prodding away at their own technology demonstrator missions, each of which aims for something slightly different.
It is also a curious space race that does not involve the US. Although the US was initially the leader in QKD, what was perceived as a lack of interest led the centre of activity to shift to Europe. When Europe also lost interest, China took up the challenge and mastered it.
“The US has a difficult relationship with QKD,” says Bedington. “For a while, the US position has been that you don’t talk about QKD, you talk about quantum networks [complex networks that could utilise other quantum technologies alongside QKD]. Maybe it’s also a national pride thing with China – they’ve seen China take the lead five to six years ago and now they’re saying ‘Oh yeah, but we don’t want to do that, that’s too basic, that’s too simple, we want to go straight to this quantum network thing – QKD is silly.’”
Since Edward Snowden’s revelations of mass data gathering by US government agencies, the concept of technological sovereignty – which prioritises technology aligned with the country in which it is used – has taken hold.
The Micius mission was born from the Snowden exposé, Pan having explicitly credited it with prompting Chinese investment in quantum cryptography. Since 2017, further breakthroughs using the satellite have edged closer to absolute secrecy. In 2020, Micius sent entangled photons to two ground stations 1,200km apart, allowing them to establish a secure communication channel. The satellite was not part of that communication channel and did not ‘know’ the key – even if an American astronaut somehow captured and broke into the satellite, the two ground stations would still be able to communicate securely. In allowing a country to lock out eavesdroppers, QKD is ideally suited to this new age of distrust.
There are three powers considered technologically sovereign today: China, the US and the EU. The UK is in an awkward position, in self-inflicted isolation from the big technological projects critical for national security. It cannot, for instance, partake in EuroQCI – a bloc-wide fibre QKD network – which it might otherwise have joined via fibre laid through the Channel Tunnel.
Q Day is approaching. When it comes, how many countries will be able to say that their secrets are truly safe?
Leave a Reply