Alice and Bob have broken up and have moved far away from each other. But they still have to sort out who gets to keep the car. Flipping a coin over the phone won’t work because neither believes each other’s result.
A paper published in Nature Communications by researchers from Canada and Switzerland explores the concept of coin flipping in the context of quantum physics that uses light particles, so-called photons, to allow communication tasks in a manner that outperforms standard communication schemes.
Take an analogy involving a safe. Bob flips a coin and sends the result hidden in a safe. Upon receiving it, Alice sends the result of her own flip to Bob. Then Bob sends the key to Alice who unlocks the safe. Now, Alice and Bob both know each other’s coin flip and, according to a previous agreement, who drives away with the car.
In a world made out of electronic bits, used for communication instead of safes, the physical safe is replaced by encryption over email. This procedure is believed to be hard to break. Unfortunately, no one knows if this is truly a good safe. This would allow Alice to cheat by unlocking Bob’s “safe” without awaiting his key, reading the secret, and choosing the result of her own flip in a way that ensures that she will keep the car.
But it’s a different story with quantum communications.
“What we have shown here is the first implementation of quantum coin flipping in which a cheater can not take advantage of the fact that photons may get lost during transmission between Alice and Bob. All previous quantum communication schemes could be broken by a cheater,” says co-author Wolfgang Tittel, professor in the Institute for Quantum Information Science and the Department of Physics and Astronomy. The other co-authors are from the University of Calgary, Université de Montréal, École Polytechnique de Montréal and Université de Genève, in Switzerland.
The quantum coins in this study were tossed with one player on campus and the other about 5 km away.
“The exchange of quantum bits instead of electronic bits prevents unrecognized cheating because we base our protocol on properties of nature, such as the impossibility to perfectly determine the quantum state of a single photon” say Félix Bussières, who did parts of his PhD studies on campus, before moving to Switzerland to take up a postdoctoral fellowship.
"Unfortunately, even in the quantum world, coin flipping at a distance is not perfect either - it is still possible to cheat, at least to some extent. However, no party can fix the final outcome with certainty, and, if trying to cheat, risks being caught cheating."
Nevertheless, this demonstration has made the decision of who gets the car fairer.