In the quantum world, particles behave as if they are in two places at once. It turns out this bizarre property allows two people to send each other one bit of information using a single particle 鈥� something that is impossible with classical communications.
Say Alice and Bob each want to send the other a piece of information in the form of a 0 or a 1. In classical communications, each has to send the other a particle encoded with that information. So, you need two particles.
Now, at the University of Vienna in Austria and his colleagues have shown that quantum mechanics can help flout this limitation. Their experiment involves a staple of quantum labs: something called the Mach-Zehnder interferometer.
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Once a particle enters the device, it is sent into a beamsplitter, which either transmits or reflects the particle. Because a quantum particle can be in two places at once, it can be thought of as both being transmitted and reflected by the beamsplitter simultaneously. It is then in a superposition, taking two different paths. After a while, these paths meet at a second beamsplitter. If the two paths are of equal length, the particle always exits the second beamsplitter via route A. If you change the length of one of the paths precisely, the particle exits the beamsplitter via route B.
Exchanging bits
To use this interferometer to get Alice and Bob to exchange one bit of information, Walther and colleagues locate Alice midway alongside one arm of the interferometer and Bob midway alongside the other. Each can choose to change the length of their part of the device a smidgen, altering the length of the paths before the second beamsplitter.
When a particle enters the interferometer, if both Alice and Bob do nothing, the particle exits the interferometer via route A and goes to Alice, not to Bob. If both of them change the lengths of their respective paths, the particle again goes to Alice. But if only one of them takes an action, the particle exits the device via route B and goes to Bob, not to Alice.
This set-up can be used to exchange two bits of information using only one particle. For example, if Alice receives the particle and she did not change the path length, she knows that Bob also did nothing and hence transmitted a 0. Bob, who did not receive the particle, also simultaneously knows that Alice transmitted a 0.
Or, if Alice does not receive the particle and did not change her side of the interferometer, she knows that Bob must have, and hence he transmitted a 1. At the same, Bob receives the particle, and figures out that Alice transmitted a 0.
Secret messages
鈥淭he magic is in the quantum notion of superposition,鈥� says at the University of Oxford, who was not part of the study. 鈥淭his work is part of a broader trend, in which we are exploring how the quantum superposition of different configurations in space and time can be used to boost communication.鈥�
This magic can be used to exchange a secret message, says Walther. Say Alice wants to send a sequence of ones and zeroes. For every particle that goes through the interferometer, she either changes the path length or not. Meanwhile, Bob generates a random sequence of bits, and uses them to decide whether to modify the interferometer or not. Bob has access to his random sequence, so based on whether or not he receives each particle, he can tell what Alice transmitted. But an eavesdropper only has access to the interferometer鈥檚 output, and has no means of telling who sent the message, let alone how to decode it.
Scaling this up to large distances will require building stable interferometers, whose paths can be recombined after traveling for kilometres, says Walther.
Reference: arXiv,
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