Quantitative information is like a dream: you can’t make a perfect copy of it

The “https” of a website’s url is a protocol that provides an encrypted connection between you and that website, so that no one can see it when you enter your address into a webstore. Now that’s safe. “But anything secured with https technology can be broken with a quantum computer,” says computer scientist Gilles Brassard of the University of Montreal. This also applies to online banking transactions.

When they arrived at the QuSoft Research Institute in Amsterdam, where Brassard has occupied Turing’s swivel chair for the past six months, they were wearing a kind of beige bucket hat. During the conversation, it turns out that they are not only in tune with each other in terms of headgear. They complement each other effortlessly as they speak passionately about quantum cryptography.

Photons are not meant for storage, they are for travel

Jill Brassard Computer scientist

How did you come up with the idea of ​​quantum cryptography?

CB: “It started with an idea from physicist Stephen Weisner, who passed away last year. He invented “quantum money” that was impossible to counterfeit because it consisted of quantum information in the form of photons [lichtdeeltjes]. Quantum information changes when you try to read it. It’s like information in a dream – you can’t show it to others, and trying to describe it changes your memory of it. This makes it impossible to make a perfect copy of quantum information. I was excited about Wiesner’s idea and shared it with Gilles to hear the computer scientist’s point of view. This is how our collaboration began.

Gordon: “We realized that for quantum banknotes, you have to store and store photons. This is very difficult. Photons are not meant for storage, they are for travel. So, it made sense to use them for communication. Since I’m a cryptographer, the idea of ​​cryptography with quantum information arose “.

If two people want to communicate securely, they can generate a secret key via encryption

Charles Bennett physical

How does BB84 work?

CB: “If two people – Alice and Bob – want to communicate securely, they can generate a secret key via cryptography. Only the other person can decode and read the messages. In BB84, Alice and Bob use the quantum properties of photons to create such a key. They use the polarization of photons. This It has to do with the electromagnetic vibrations that make up light waves. Those vibrations can be polarized horizontally/vertically (0 and 90 degrees) or diagonally (45 degrees and 135 degrees). Alice sends out a series of photons that are randomly polarized in one of four directions – 0, 90, 45 and 135 degrees – to Bob. Bob randomly selects each photon to measure horizontal/vertical or diagonal polarization. Only if his method of reading matches the method Alice used to send that photon, they are sure to find the same result. To find out the photons they measured have the same information For her sake, Bob publicly explains the measurement method he used for the Photon. Alice will then tell you which measurements do not match and so can be removed. The rest should be the same, unless the signal is cut along the way or someone eavesdrops. This eavesdropping changes the truth. A quantum machine of photons, so that Bob and Alice do not see the same thing, despite the fact that the methods are identical. To verify this, they compared a number of measurement results. If there are many differences, everything is heard and starts all over again. If there are only a few differences, the eavesdropper does not know everything and it is possible to correct errors via an intelligent protocol and then make a smaller key than the resulting, partly secret, key that the eavesdropper obviously knows nothing about. They can use this key for secure communication.

In China, there is a quantitative link between Shanghai and Beijing. It is also used seriously

Jill Brassard Computer scientist

Is this quantum encryption already in use?

Gordon: “Yeah, especially in China. A quantum connection has been built there between Shanghai and Beijing. It’s also being used seriously. And they have a satellite to send quantum information over greater distances than is possible with optical fibers. In 2017, the first video call secured with quantum cryptography took place. Between Beijing and Vienna. Also in Europe, a test with quantum communication infrastructure is underway. There are companies selling quantum cryptographic systems. This allows everyone to exchange a quantum key with each other to send quantum-secured information.”

Is this really necessary? After all, there are no large quantum computers yet.

GB: “How do you know? It is possible that someone has succeeded in building a quantum computer, but someone is hiding it.”

CB: “For example, to loot bank accounts.”

GB: “But even if there is no quantum computer yet, you now have to encrypt information that should be kept secret for a long time using quantum cryptography. Anyone can now store all transmitted encrypted information. Once there is a quantum computer, they can decode and display everything. For example, we know from the https protocol that a quantum computer can decrypt it.”

It is an incredibly complex and important field

Charles Bennett physical

CB: “There are security methods that a quantum computer has not been proven to be able to crack. But it has also not been proven that it is impossible to crack with a quantum computer or with a regular computer.”

JP: “The best thing you can do if you want to make sure your information is safe and stays secure is quantum encryption. If the laws of quantum mechanics are right, then it sure is safe. There could be Gaps Arise if the required quantity equipment is not perfect. Therefore, in practice, it may be best to combine normal cryptography with quantum cryptography to create a super-secure key.”

What do you think of current developments in quantum technology?

CB: “I think it’s great that so much research is being done in quantum hardware and software. It’s an incredibly complex and important field. Quantum cryptography is only a fraction of what’s possible.”

GB: “The Netherlands is the leader, both in terms of quantum software – at QuSoft, and in terms of quantum devices – at QuTech. It could be the first country to have a national quantum internet. China is also working on it. He laughs: “But it could be the Netherlands earlier, simply because it is a much smaller country.”