China is the benchmark for quantum cryptography applications--Interview with BB84 protocol proposer

The "https" in a website URL is a protocol that provides an encrypted connection between the user and the site, so that no one can see it when the user enters the address in the online store. Now this approach is secure.

"But any information protected with https technology can be cracked with a quantum computer," says Gilles Brassard, a computer scientist at the University of Montreal. This approach also applies to online banking transactions.

In 1984, Brassard and Charles Bennett, a physicist at IBM Research, developed a protocol to enable more secure communication through quantum mechanics. The protocol, called BB84, made up of Bennett and Brassard's initials and the year it was proposed, marks the birth of quantum cryptography. bennett and Brassard will receive the breakthrough prize in fundamental physics in 2023 for BB84. They are currently in the Netherlands.

The first meeting between physicist Charles Bennett (New York, 1943) and computer scientist Gilles Brassard (Montreal, 1955) took place on an island near Puerto Rico, where Bennett recognized Brassard and wanted to talk to him. "I was swimming quietly when he suddenly came up to me and started talking about quantum mechanics," says Brassard, "and I had no idea who he was. It was an unexpected and surprising meeting that produced life-changing results."

Photons are not used for storage, they are used for propagation.

-- Gilles Brassard

From left to right: Brassard and Bennett

How did you come up with the idea for the quantum code?

CB: "It started with an idea by Stephen Wiesner, a physicist who died last year. He invented 'quantum money,' which is impossible to forge because it consists of quantum information in the form of photons. When you try to read quantum information, it changes. It's like information in a dream - you can't show it to someone and try to describe it it will change 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 perspective of computer scientists. That's how our collaboration started.

We realized that for quantum money, you have to store photons. This is very difficult; photons are not used for storage, they are used for propagation. Therefore, it made more sense to use them for communication. Because I'm a cryptographer, the idea of using quantum information for encryption emerged."

If two people want to communicate securely, they can create a key through cryptography.

--Charles Bennett

How does BB84 work?

CB: "If two people, Alice and Bob, want to communicate securely, they can create a key through cryptography. Only other people can decipher and read the message. In BB84, Alice and Bob use the quantum properties of photons to create such a key. They use the polarization of photons, which is related to the 'electromagnetic vibrations' that make up light waves. These vibrations can be polarized horizontally/vertical (0 and 90 degrees) or diagonally (45 and 135 degrees.) Alice sends Bob a series of randomly polarized photons in one of four directions (0, 90, 45, 135 degrees).

Bob randomly selects each photon to measure horizontal/vertical or diagonal polarization. They are only sure to find the same result if his reading method matches the method Alice uses to send the photons. To find out which photons they measured identically, Bob publicly shows which measurement method he used for which quantum. Alice then shows which measurements do not match and can therefore be removed. The rest should be the same unless the signal was scrambled along the way or someone bugged it. The eavesdropping changes the quantum state of the photon, so Alice and Bob no longer see the same information despite the method match.

To verify this, they compared many measurements. If there are many differences, the eavesdropping is proven and the communication is then reconnected. If there are only a few differences, the eavesdropper does not know everything and can correct the error with an intelligent protocol that then generates a smaller key from the generated partial key, of which the eavesdropper apparently knows nothing. They can use that key for secure communication."

The establishment of a quantum connection between Shanghai and Beijing in China is being effectively applied.

——Gilles Brassard

Is this quantum cipher already being applied?

GB: "Yes, especially in China where a quantum connection was established between Shanghai and Beijing, being effectively applied. They have a satellite that can send quantum information over longer distances than optical fibers. 2017 saw the first video call using quantum cryptography protection between Beijing and Vienna. In Europe, testing of quantum communication infrastructure is also underway. There are also companies selling quantum cryptosystems, which allow everyone to exchange quantum keys with each other to send quantum-secure messages."

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

GB: "How can you be sure? It's possible that someone has already successfully built a quantum computer and just hasn't made it public. For example for stealing bank accounts.

But even if there is no quantum computer yet, quantum ciphers must now be used to encrypt information that must be kept secret for a long time. Now someone can store all the encrypted information sent, and as soon as a quantum computer comes along, they can decipher and view everything. For example, we know from the https protocol that a quantum computer can decipher it."

This is a very complex and important area.

——Charles Bennett

"There are security methods that have not been proven that quantum computers can break encrypted messages, but there is no proof that it is impossible to break them with quantum computers or classical computers."

"If you want to make sure your information is secure, the best thing you can do is a quantum cipher. If the laws of quantum mechanics are correct, then it is provably secure. If the required quantum devices are not perfect, there may be vulnerabilities. Therefore, in practice, it is best to combine ordinary and quantum ciphers to create a super-secure key."

How do you see the development of quantum technology?

CB: "I think it's fantastic that so much research is being done on quantum hardware and software. It's an extremely complex and important field. Quantum cryptography is only a small part of what is possible.

The Netherlands is leading the way in quantum software (QuSoft) and in quantum hardware (QuTech). It may well be the first country to have a national quantum Internet. China is also working on this, but the Netherlands may be even earlier because it is a much smaller country."