Quantum Steganography

Besides talking about environmental issues, I’d also like to use this blog to talk about my day job at the Centre for Quantum Technologies. I hope this isn’t too distracting…

I’d like to try live-blogging a talk here. Today there’s a talk by Bilal Shaw of the University of Southern California about a paper he wrote with Todd Brun on Quantum Steganography.

“Steganography” is the art of hiding information by embedding it in a seemingly innocent message. In case you’re wondering – and I’ve got the kind of mind that can’t help wondering – the word “steganography” actually is etymologically related to the word “stegosaurus”. They both go back to words meaning “cover” or “roof”. Some other words with the same root are “thatch”, “deck” -and even “detect”, which is like “de-deck”: to take the lid off something!

Steganography is an ancient art, still thriving today. For example, that Russian spy ring they just caught were embedding secret data in publicly visible websites. The advantage of steganography over ordinary cryptography is that if you do it right, it doesn’t draw attention to itself. See this picture?

Remove all but the two least significant bits of each color component and you’ll get a picture that’s almost black. But then make that picture 85 times brighter and here’s what you’ll see:

All this is purely classical, of course. But what fiendish tricks can we play using quantum mechanics? Can we hide Schrödinger’s cat in a seemingly innocent tree?

Bilal’s paper describes a few recipes for quantum steganography. Alas, I’m not good enough at cryptography and live-blogging to beautifully deliver an instant summary of how they work. But roughly, the idea is to fake the effects of mildly “depolarizing” channel, one that introduces some errors into the qubits you’re transmitting, pushing pure states closer to the center of the Bloch sphere, where pure noise lives. You can’t introduce too many errors, since this would make the error rate suspiciously high to someone spying on our transmissions. So, there’s a kind of tradeoff here…

I’d be happy for an expert to give a better description!

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