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While the world is still waiting for a full-blown quantum communications setup, quantum key distribution – QKD – is already a contested product market. Now, an international collaboration has shown that QKD can be brought to the smartphone.

The project, carried out by the University of Bristol, Cambridge, Griffith University in Queensland and , Xi’an Jiaotong University in China, has published a paper on Arxiv outlining its work.

The researchers have, essentially, split the QKD problem into a client-server architecture, allowing most of the “heavy lifting” to be carried out server-side so that a resource-constrained client like a smartphone. It wouldn't work on any of today's smartphones, since there's still one somewhat exotic component needed at the client end, an on-chip polarisation rotator.

And the client device wouldn't be able to use QKD over the air, since it would need to tether to a fibre to receive the quanta from the far end.

Whereas most QKD kit on the market today has quantum optics equipment at both ends, the scheme proposed in the Arxiv paper would do most of the quantum work at one end only. “Alice” creates the photons and sends them down the fibre to “Bob”, who only needs the capability to change the photons' polarisation and send them back.

The protocol devised to make this work is called rfiQKD, “reference frame independent quantum key distribution”, and it works without needing to align Alice and Bob's equipment. As it's described at MIT's Arxiv Blog:

“Instead Alice and Bob make measurements in random directions and then publish the list of directions for anyone to see. Only those measurements that happened to be aligned contribute to the code.”

As the researchers note in their paper, “the results significantly broaden the operating potential for QKD outside of the laboratory and pave the way for quantum enhanced security for the general public with handheld mobile devices.”

And before readers poke fun at the idea of a smartphone containing quantum polarisers on-board, think of this: how many of us carried around accelerometers ten years ago? ®

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