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Deciding who keeps the car

November 30, 2011

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|

Wolfgang Tittel, professor in the Institute for
Quantum Information Science and the Department of Physics and Astronomy
co-authored a paper exploring the concept of coin flipping in the context
of quantum physics. Photo credit Riley
BrandtAlice 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.

http://www.nature.com/ncomms/

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