Using light to encrypt communications
Using light to encrypt
communications
by K.w. Wesselink Msc (Kees), University of Twente DECEMBER 20, 2019
Quantum supremacy
Recently, Google claimed in Nature an experimental proof of this
"quantum supremacy," although with a calculation that has no
practical use. Nevertheless, we can no longer exclude the possibility that
quantum computers will become so powerful that they break existing cryptography
since there are known quantum algorithms that break today's most used
cryptographic methods. Luckily, quantum technology also offers solutions. With
Quantum Key Distribution (QKD) one can securely build up secret keys between a
sender and a receiver. This is no science fiction. Commercial QKD systems are
available from several vendors and space-based versions are already deployed.
Enlarge the quantum alphabets
Standard QKD systems use single particles of light—photons—that
are in one of two possible states, for instance horizontally or vertically
polarized. This limits the transmission to one bit per photon. In a sense, the
photons are encoded in an alphabet of just two letters: a and b.
Researchers from the UT now increased this number with more than
a thousand letters. This increases the resistance against noise and potentially
increases the data rate. They achieved this by encoding the quantum information in 1024 possible locations of the used
photons. To make it hard for an attacker to see what was sent, they randomly
switch the encoding between two different alphabets.
Speaking Dutch in a Chinese
conference room
Pepijn Pinkse, who led the experiment, explains: "It is
like trying to guess what is spoken in two conference rooms. In one room the
conference language is Chinese and in the other Dutch, but you do not know
before entering. If a Dutch speaker picks the Chinese room, he does not
understand anything, although for a Chinese speaker the lectures are crystal
clear. In our method, the sender uses two languages and randomly switches
between them. Also the receiver switches between listening in one language or
the other. Only if the languages coincide, useful bits are conveyed. Listening
to both languages at the same time is forbidden by fundamental laws of
physics."
Employing this technique together with very weak light, a video
projector chip and modern single-photon detecting camera, the researchers
demonstrated that they could transmit up to seven secure bits per photon. Their results are published on December
18th in New Journal of Physics in
their paper titled "Large-alphabet quantum key distribution using spatially
encoded light."
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