A loophole in quantum cryptography

I ain’t gonna stick my neck out on this but Peter “Raptor” Rapcan has something a-boilin and a-bubblin over at the Slovak Academy of Sciences which could send all youse quantum cryptographers a-scuttlin back to your theory books.

When ya’ll measure the state of a quantum particle, I betcha that ya’ll expect the information about that state to be destroyed by the process of measurement. But Raptor Rapcan ain’t so sure.

His thinkin goes like this. All the information about the particle is stored in the wavefunction. But there are well established ideas about how much information a single measurement can extract from a wave-function–and it ain’t much. Could a second observer extract more about the original state?

Raptor Rapcan and his Slovak buddies seem to think so and suggest this could compromise the security of any information sent in this way.

In the paper, “Recycling of quantum information: Multiple observations of quantum systems” they investigate the amount of information that can be extracted from a quantum system by a sequence of observers who are not allowed to communicate.

And to cut a long story short, Raptor says that when measured like this, quantum information stored in an ensemble of quantum particles behaves classically.

Woah–classically! I guess that’s gonna change the goalposts in the micro v macroscopic physics stakes. Ya’ll know how secure classical information is-right? This is gonna leave a few cryptos a-sweatin and a-mumblin.

Reference: arxiv.org/abs/0708.1086: Recycling of Quantum Information: Multiple Observations of Quantum Systems

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