The ‘https’ for a website url is a protocol that provides an encrypted connection between you and that website, so that no one can watch when you enter your address at a webshop. 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 arrive at the Amsterdam research institute QuSoft, where Brassard has occupied the Turing rotating chair for the past six months, they both wear a kind of beige bucket hat. During the conversation it turns out that they are not only attuned to each other in terms of headgear. They complement each other effortlessly as they enthusiastically talk about quantum cryptography.
Photons are not meant to be stored, they are meant to travel
Gilles Brassard computer scientist
How did you come up with the idea of quantum cryptography?
CB: “It started with an idea from physicist Stephen Wiesner, who died last year. He invented “quantum money,” which was impossible to counterfeit because it consists of quantum information in the form of photons [lichtdeeltjes]. Quantum information changes when you try to read it. It is like the information in a dream – you cannot 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 a computer scientist’s point of view. That’s how our collaboration started.”
GB: “We realized that for quantum banknotes you have to store and store photons. That is very difficult. Photons are not meant to be stored, they are meant to travel. Therefore, it made more sense to use them for communication. Because I am a cryptographer, the idea for cryptography with quantum information arose.”
If two people want to communicate securely, they can create a secret key via cryptography
Charles Bennett physicist
How does BB84 work?
CB: “If two people – Alice and Bob – want to communicate securely, they can create a secret key via cryptography. Only the other person can decipher and read the messages. At BB84, Alice and Bob use quantum properties of photons to create such a key. They use the polarization of photons. This 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 and 135 degrees). Alice sends a series of randomly polarized photons in one of four directions – 0, 90, 45, and 135 degrees – to Bob. Bob randomly chooses per photon to measure the horizontal/vertical or diagonal polarization. Only if his reading method matches the method used by Alice to send that photon will they be sure to find the same result. To find out for which photons they measured the same information, Bob publicly explains which measurement method he used for which photon. Alice will then let you know which measurements do not match and can therefore be removed. The rest should then be the same, unless the signal was disrupted along the way or someone eavesdropped. That eavesdropping changes the quantum state of the photons, so that Alice and Bob no longer see the same thing, despite the fact that the methods match. To check this, they compare a number of measurement results. If there are many differences, everything is overheard and they start again. If there are only a few differences, the eavesdropper does not know everything and it is possible to correct the errors via a smart protocol and then make a smaller key from the resulting, partially secret key, of which the eavesdropper demonstrably knows nothing. They can use that key for secure communication.
In China, there is a quantum connection between Shanghai and Beijing. It is also used seriously
Gilles Brassard computer scientist
Is such quantum cryptography already being used?
GB: “Yes, especially in China. A quantum connection has been built there between Shanghai and Beijing. It is also used seriously. And they have a satellite to send quantum information over greater distances than is possible with fiber optics. In 2017, the first video call secured with quantum cryptography took place between Beijing and Vienna. In Europe, too, work is underway on a test with a quantum communication infrastructure. And there are companies that sell quantum cryptography systems. This allows everyone to exchange a quantum key with each other to send quantum-secure information.”
Is that already 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 that someone is keeping it a secret.”
CB: “For example, to loot bank accounts.”
GB: “But even if there is no quantum computer yet, you now have to encrypt information that has to remain secret for a long time using quantum cryptography. Someone can now store all encrypted information sent. As soon as there is a quantum computer, they can decipher and view everything. For example, we know from the https protocol that a quantum computer can decipher it.”
It is an incredibly complicated and important field
Charles Bennett physicist
CB: “There are security methods that it has not yet been proven that a quantum computer can crack. But it has also not been proven that it is impossible to crack them, with a quantum computer or with an ordinary computer.”
GB: “The best thing you can do if you want to be sure that your information is and remains safe is quantum cryptography. If the laws of quantum mechanics are correct, then it is provably safe. There can be loopholes arise if the required quantum equipment is not perfect. Therefore, in practice, it may be best to combine ordinary and quantum cryptography to create a super-secure key.”
What do you think of the current developments in quantum technology?
CB: “I think it’s great that so much research is being done into quantum hardware and software. It is an incredibly complicated and important field. Quantum cryptography is only a fraction of what is possible.”
GB: “The Netherlands is leading, both in terms of quantum software – at QuSoft, and in terms of quantum hardware – at QuTech. It could well be the first country with a national quantum internet. China is also working on that.” Laughing: “But the Netherlands can be earlier, simply because it is a much smaller country.”