Siddarth Koduru Joshi from the University of Bristol tells Michael Banks why countries are racing to build the first quantum network in space
How did you get started in quantum communication?
I first got into quantum physics during my bachelor’s degree at St Joseph’s College, Bangalore, where I started a project studying ion traps. Being able to manipulate a single ion was amazing and that’s when I became an experimentalist at heart. I then began working on photon sources during my PhD at the Centre for Quantum Technologies at the National University of Singapore before moving to Vienna to work on quantum key distribution (QKD) and other quantum technologies.
What excites you about quantum communication?
Many of the technologies that have been proposed or have been thought of for several years are now mature enough for applications. It’s a time when you can start engaging with industry and pushing this technology out into the real world. This is what gets me excited and I want to see happen in my lifetime. QKD is probably the oldest quantum technology that is now ready, but I think QKD is just the tip of the iceberg. There are many other aspects of quantum technologies, such as metrology and sensing, that are starting to become better than any other possible classical technology. This is all exciting.
Can you describe the basics of QKD?
QKD is about getting a message securely from one point to another. It is the protocol used to distribute a random key that is used to encode and decode a message. QKD comes in quite a few different flavours, all of which have different applications and different niches. But essentially QKD uses various features of quantum mechanics, such as entanglement, to distribute a key between two parties.
China did not spend billions of dollars on a QKD network that includes a dedicated satellite for nothing
Siddarth Koduru Joshi
Where are we currently with QKD?
QKD is being refined to fit certain applications. It’s relatively easy for two people to talk to each other – via pairs of links – using QKD. But this is like doing so with walkie-talkies and is really resource-wasteful. What you want instead is a mobile-phone-like network. QKD is now about going over longer distances and cutting the costs. As well as being used on Earth it is also being tested in space and with drones.
Did you say drones?
Yes. It’s not enough to be able to communicate with fixed, land-based ground stations. But maybe you also want to talk to a ship or a vehicle or a plane while it is moving? Using a drone is a nice testing platform for rapid-tracking QKD and it’s something that we’re working on at the moment.
Is anyone buying these systems yet?
The companies that build QKD systems are flourishing, so people are buying them. Of course, who is buying them is usually confidential. China did not spend billions of dollars on a QKD network that includes a dedicated satellite for nothing. They also see commercial applications and are connecting their banking centres to QKD networks. It is now just a question of how much are you willing to pay for what level of security? As things get cheaper, I expect that QKD will be more widely implemented.
What is it like doing QKD in space?
One of the biggest challenges is identifying the best protocol as there are a number of ways to do quantum communication. For example, you can encode a key on the ground and then send it up into space. But this means that you suffer early losses in the signal as it passes through the atmosphere. Alternatively, you can send a key from space to the ground where the losses would be after the signal is sent, but this requires more complex equipment up in space to produce the quantum signal. What we need is conclusive data showing which method is best for each case and to convince everyone else in the community that is the right direction to pursue.
Do we need to do QKD in space?
Yes. Losses of more than about 40 or 50 dB severely hinder QKD communication rates when sending signals through optical fibres. This limits us to around 400 km, which is not much on a global scale. It is possible to use quantum repeaters to boost the distance, but they are very complex pieces of technology that wouldn’t be practical to deploy widely. Yet if you’re using satellites then you may have some initial losses sending a signal through the atmosphere but after that you could transmit over gigantic distances in space given that it would be mostly loss-free.
Given that researchers in China demonstrated space-based QKD earlier this year, is there a quantum space race?
China has done an incredible amount of work in this field and made huge progress. The goal is to have a fully functional constellation of satellites, not just one. While China has a head start, I think other countries are starting to catch up. Especially when the technology miniaturizes enough so that it can be put on cubesats, which would cost around a million to launch instead of a billion for a dedicated satellite.
Physicists propose space mission to test quantum gravity
You also have a proposal to study quantum gravity using the International Space Station (ISS). What’s that all about?
The idea is to see whether there are any differences between an entangled system and a non-entangled classical system as they make their way to the ISS. We make the test by sending up entangled systems into space and measuring the entanglement and simultaneously sending up a classical system under the same conditions and then comparing the two. If there is no difference, then it will be an important baseline measurement where we can place limits on the effect of gravity on a quantum system. But if we see some difference then it could turn our understanding of physics upside down.
