# Will quantum cryptography break classical encryption?

*Quantum cryptography comes with a promise to be unhackable. While quantum computers might be decades away, the technology of quantum cryptography is much more mature. At the same time, it poses a threat to break the encryption we are using now.*

That is why we should start preparing now, Eleni Diamanti, a CNRS researcher at Sorbonne University in Paris, told CyberNews. Her research focuses on experimental quantum cryptography and communication complexity and the development of photonic resources and protocols for quantum networks.

Quantum cryptography, or quantum encryption, applies the principle of quantum mechanics to encrypt communication. It uses photons to carry signals and is said to be unhackable. To send photons along, you need a rather large-scale and expensive technology - a quantum computer, which is still decades away.

Quantum encryption comes with a promise to be unbreakable, and it will provide us with a much more secure way to communicate. As Maria Korolov and Doug Drinkwater from CSO put it, it’s no silver bullet but could improve security.

Yet, it might disrupt classical encryption and may (or may not) break the cryptocurrencies.

We sat down with Diamanti, whom we first met during the MIT Tech Review Cyber Secure conference, to discuss this. We did not aim to dive into technical details about quantum technologies. Instead, we discussed the progress of quantum encryption and how far we are from having the first quantum computer.

**I’ve just read that a ****University of Tokyo research team developed a communication method that does not rely on photons****. While I’m not asking you to comment on this particular news, I wonder how quantum cryptography and quantum technology, in general, is progressing these days.**

There is a big difference between the maturity of the technology in quantum computing and quantum cryptography. For me, these fields are different in the sense that, to do a very large-scale quantum computer, you need technology that is not there yet. There are several experimental platforms, from photons to superconducting qubits, to ions, or spins in semiconductors. There are all sorts of different experimental platforms for quantum computing. They all have their advantages and disadvantages and are progressing to do more and more complex computations. But, I think several milestones will have to be reached for this technology to come to an actual maturity and be useful.

On the contrary, in quantum cryptography, to show that quantum technology can be useful and offer a higher level of security, you need a lot less interaction between quantum objects than for a quantum computer. In this sense, it is more straightforward to do, which means that the maturity of systems in quantum cryptography is a lot bigger, and so there are even commercial systems, and we are progressing towards a real deployment of quantum cryptographic techniques in real life. It’s not there yet, we are not using it in our daily life, but technology is advancing really fast. So I would say that technology in quantum computing and quantum cryptography is progressing very well, but there is a big difference in terms of what needs to be done.

**Why is quantum cryptography necessary?**

Quantum cryptography is necessary because quantum computing is coming over. There’s an advent of quantum computers, which is a threat for current cryptographic schemes, and we need quantum cryptography to counter this threat. Although it will take some time for it to come, one needs to prepare much earlier because it takes a long time to scrutinize cryptographic techniques, to put them in place, to be ready for when the threats by the quantum computer are going to be materialized. This is the way that it should be seen. And so, although quantum cryptography is more mature, we are waiting for quantum computers to become relevant. While we are waiting for it, it is important to advance even more in quantum cryptography.

**So you don’t necessarily need a quantum computer to work on quantum cryptography?**

You do not need a quantum computer to do a quantum cryptography system. And while for quantum computers, there are many candidates in terms of technological platforms, for quantum cryptography, we only use photons, photonic technologies.

**When are we going to have a quantum computer?**

That’s a million-dollar question. What now holds a lot of promise is called noisy intermediate-scale quantum computing (NISQ) devices. These are devices that are not full-scale devices. They are noisy and sufficiently large to do useful things. There’s a lot of hope for the next few years to show that such devices can be useful. But for big, large-scale quantum computers… 10-20 years, I don’t know. It’s extremely hard to tell.

**Will we need supercomputers once a quantum computer arrives?**

The current trend, in both quantum computing and quantum cryptography, is to say that we don’t want to eradicate or make disappear classical technologies. Neither supercomputers nor classical cryptographic schemes should disappear. The idea would be that we can use these new quantum technologies to complement and enhance the classical technologies.

**Will quantum cryptography disrupt the classical model?**

That’s why we need to prepare for it now. There will be changes in cryptographic systems. Quantum cryptography is very different from classical cryptography. Classical cryptography, whenever you have an encrypted thing, they rely on mathematical algorithms. Quantum cryptography relies on physics, so you need a physical object - an optical fiber or satellite link, or something like this for a cryptographic technique to kick in. Basically, this means that there’s a need for some infrastructure chains, and because this takes a lot of time, we should start preparing so that we are ready when it comes.

**Will quantum cryptography be able to break the cryptography that we are using now? There are fears that it will break cryptocurrencies (****some, or course, are quite sure it will not****).**

It’s a real, not an imaginable threat. It’s a real threat to cryptographic techniques that we are using everywhere - in currencies and other transactions.

**Is there a race for quantum computers as we see for supercomputers?**

There’s definitely a race to build a quantum computer. You see all the big technology giants like Google or IBM, and between nations as well - between the US, China, Europe. For quantum cryptography, it is not quite the same. Because of all these cybersecurity issues that come with quantum cryptography, there’s a lot of interest in sovereignty in being able to develop a technology with a controlled supply chain.

**Is quantum cryptography unhackable?**

Is it even possible? Some people say that you can never claim that it’s unhackable. Quantum cryptography, in principle, in theory, is totally unhackable. It is not the case for classical cryptography. In practice, there are side channels, which means that sometimes, while you try to implement the system, you unwittingly deviate from the security proof. It means that you potentially open the door to a malevolent party, and you use some property of your system to attack it. This is called a side-channel attack. It’s very much scrutinized for quantum cryptography as it is for classical systems, as well. It’s an issue that is interesting for everyone. I’m pretty optimistic with respect to this. There’s a lot of progress. I don’t think it can widely comprise the security of quantum cryptographic systems, but we should not forget this aspect.

**Is it prone to errors?**

One party is sending photons - this quantum and coded information. In any physical system, there are imperfections. Some photons are not going to arrive, and there are going to be some errors, things that are happening in a channel. What a security proof of quantum cryptographic system does is that it considers that all of these errors are due to a malevolent party, there are no innocent errors, and they all come from the actions of an eavesdropper, and despite this, it is still possible with quantum cryptography to extract a secret key, to prove the security of your cryptographic process. These are very powerful security proofs. They consider that whatever happens, it’s due to an eavesdropper. Therefore, from the moment you can characterize them, you measure them in your system, you can cover them with your security proof. Errors per se are not destroying the security of quantum cryptographic systems. What can do that, are things that you are not aware of, that you have not taken into account in your security proof. For example, there’s a leakage in one degree of your system, and you are not aware of it.

**What will the first quantum computer look like?**

The way you should see the very first quantum computer is when you look at these photos of the very first classical computers - they were these huge machines that were taking up the whole room. I think this is the way you have to see it. I think the first quantum computer is going to look a bit like this. It depends on the technology. The photonics people that are trying to build quantum computers are trying to make chips and they are saying it’s going to be immediately very small. The ones from Google, who are leading the race right now with superconducting qubits, you see these huge cryostats with a lot of devices so it looks more like a big room. We have to imagine it a bit like this.

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