Soon there will be national networks of quantum-secured fibre linking major cities, says Andrew Shields from his lab in the Cambridge Science Park. But for now the furthest that quantum-protected data can travel over fibre is 150km – or 175km at a push.
“We do have customers that are interested in long distances,” the head of Toshiba Europe’s Quantum Technology Division says. He tells me that he and his colleagues are working on how to send quantum-secured data over long distances. And then drops his bombshell: “One of the ways we’re studying is teleportation.”
The physicist means what you think he means: transferring data from one physical point to another without travelling through the space between the two locations.
Unlike most forms of communication technology, which effectively deliver a copy of data (such as your voice or an image) to another location, Shields says that using entangled photons to transmit data “destroys the information at the first location and recreates it in the second. You send it over a long distance, but you can’t make a duplicate.”
How far can data be sent using this method? “Ultimately, over any arbitrary distance,” Shields says.
If you find Shields’ theory spooky, you are in good company. “Spooky” was how Albert Einstein described quantum entanglement, the science that Shields is using to develop the future of secure communication.
Plain text
The reason scientists are working on using quantum physics to power encryption is that quantum computing promises to completely undermine current data communication security.
Today’s networks use public key cryptography (PKC) to encrypt messages so only the recipient can read them. PKC is used when you send your credit card number to Amazon or do online banking, and secures messaging systems such as WhatsApp and Telegram. When PKC was invented 50 years ago, the time needed to decrypt a message was regarded as impossibly long, which is still true (just). But this does not apply to quantum computers. They will be fast.
Shields worries about “harvest now, decrypt later” activity. This is a term I’ve heard from others, including Laura Thomas, a former CIA staffer working for ColdQuanta in the US. It refers to organisations intercepting and storing vast quantities of encrypted data ahead of the time when quantum computing will enable it to be decrypted and read.
When I ask Shields if this is really happening, he replies, “All communications is recorded these days.” Oh!
The physicist says he asks people how long are they happy for their emails to be kept secure: A few days? A year? Five years? Ten? “Everything is effectively going to be in plain text,” he warns.
But while quantum physics is threatening to break communication security, it is also promising to revive it.
Toshiba and BT have been running a trial of quantum key distribution (QKD) since April 2022. The accountancy firm EY is the first named tester. “There are others,” says Shields, “but I’m not allowed to tell you.” The trial has seen EY run quantum-secured links between two of its offices in London over ordinary BT or Openreach fibre.
QKD has encountered its share of problems. But Shields says the “maximum distance that the fibre can go over a single link” – the 175km limit mentioned earlier – is its biggest limiting factor.
Filtering the quantum
One of the issues that Toshiba’s QKD trial brought up was that data sent along a quantum-secured link kept drowning out the quantum keys sent along the same fibre which were supposed to secure it.
“The [main] data is 10 million times bigger than the separate quantum signal,” says Shields. This means that when both forms of data were carried along the same fibre, noise and interference from the main data’s optical signal would overwhelm that of the quantum data – unsurprising, given we’re talking about a few photons here. To solve this, Toshiba developed a multiplexing technique to separate the signals.
“Now we’ve developed a filtering technique, so we can work on the same fibre,” Shields says. “It means we can transmit the quantum information with the traffic.”
This has enabled dense wavelength division multiplexing (DWDM) fibre networks to be used for quantum-encrypted data traffic, says Shields, “so we can develop networks for much lower cost”. He adds that “the trial in London uses the existing fibre with a multiplexing system”.
Toshiba is also working with vendors such as Adva and Ciena, which “have implemented the QKD interface”, says Shields.
Shields says another “one of the challenges” quantum communications faces are standards. But standards for quantum technology are being developed, and Shields had a hand in them, as he led the European Telecommunications Standards Institute (ETSI) group on quantum communications standards for seven years. When he stepped down in 2020, one of his colleagues, Martin Ward, took over.
But Shields says the biggest limitation quantum communications faces is the 175km limit on how far a quantum-secured signal can travel. Which brings us closer to the spooky stuff.
Shields has not cracked teleportation yet, but he told me that “the technology has got to an exciting point” where it is possible to build a mesh of links secured with QKD that run between nodes installed in secure locations.
“These keys can be used to encrypt a global key,” he says. “We’re at a stage now where we can build national connectivity. People are in the process of building out a network.”
Shields says that the European Union’s project to build a bloc-wide quantum-secured network is at an advanced stage. The European Quantum Communication Infrastructure (EuroQCI) was introduced in 2019, and aims to finish building out a network by 2027.
“Hopefully the UK will do something similar,” he adds, pointing out that Japan, South Korea and China already are.
Across the oceans
While Shields sees a future where QKD-protected data is be sent across the globe, how does he expect such data to be sent across the oceans that separate Europe, the Americas and Asia, if it can only travel, at the very most, 175km?
“Distance has always been a problem,” Shields acknowledges. “Single photons get scattered.”
To solve this issue, he says that he and his colleagues are “working on a quantum repeater”. This device is similar to the optical repeaters used in subsea cables, except a quantum repeater uses entangled photos. And here is where things got spooky, as our discussion moved into teleportation.
“[Teleportation] is very difficult to create in spatially distant locations,” says Shields. “The technology is still in the laboratory.” But in 2021, Toshiba achieved what Shields calls “twin field QKD” across 600km of fibre. “It’s a very significant advance over simple point-to-point.”
Shields adds that Toshiba and others are working on a less weird solution: using a low Earth orbit satellite as a key distribution device. “You make a key and as you go round the globe it goes to a second ground station,” says Shields.
However, Shields says for this to work such satellites would not need to maintain direct contact with each end of the data connection.
“You form these keys once between the satellite and the different ground stations, and the keys are stored,” Shields explains. “When you have the [key] information, it’s instantaneous. This is viable today. It’s just an engineering problem.”
Toshiba is collaborating with the UK-based and US-funded Arqit Quantum on this development, Shields says.
Shields believes this technique will lead to worldwide commercial quantum networks being formed by the end of the decade.
“Maybe in five years we can expect that there’ll be a global network for quantum communications,” he says. “There are lots of plays for launching quantum satellites.”
Commercial service
On the operator side, Toshiba is working on QKD technology with carriers from around the world. Shields says these include KT Corporation in South Korea (Toshiba and KT are collaborating to build two QKD pilot projects); Orange and Telefónica (which are fellow partners in the EU-funded OpenQKD project); and Japan’s NTT (Toshiba co-founded the Quantum Strategic Industry Alliance for Revolution with NTT).
Shields says that Toshiba sees its role as being a service provider, supplying QKD keys to the industry.
“Eventually, this technology might be applied to everything,” he says. “In the nearer term, there’ll be applications that are high value and where users rely on long-term security.”
Shields got into the security world after receiving his PhD from Imperial College London for examining the storage limits of semiconductors when they get so small that quantum effects occur.
“Toshiba was worried that semiconductors were approaching the quantum limit,” Shields says, adding that this could limit development of devices.
But he changed his focus to the practical aspects of quantum physics. “We decided we could make something useful,” he says.
Since then, Shields has seen a complete about-turn in telecoms. Ten years ago, the Optical Fiber Communications (OFC) conference was sceptical of proposals involving quantum technology. “Now quantum technology is a big part of OFC,” Shields says.
If Shields is right, it will soon become a big part of future communications infrastructure as well.
