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Quantum Cryptography Is Safe Again

In theory, so-called quantum cryptography provides a totally secure way of sending information. In practice, maybe not. But now physicists have demonstrated how to close a technological loophole that could have left secrets open to eavesdroppers.

Suppose Alice wants to send Bob a secret message. In ordinary cryptography, she can convert the message to binary numbers—i.e., a string of 0s and 1s—and then scramble it by combining it mathematically with another string of random 0s and 1s, which serves as the key. Bob then uses this key to undo the scrambling and read the message. Of course, to make the scheme work, Alice must pass the key to Bob without letting it be intercepted by an eavesdropper, Eve.

Quantum cryptography introduces a twist—literally. Alice passes Bob the key by encoding it in single photons, which can be polarized horizontally to signal a 0 or vertically to signal a 1. If this were all there were to it, then Eve the eavesdropper could also read the key and then pass the photons to Bob. But Alice can also randomly rotate her transmitter to send photons polarized diagonally at plus or minus 45° some of the time. When her transmitter isn't aligned with Bob's receiver, the key transmission becomes ambiguous: For example, if Alice sends a photon polarized at 45° and Bob has his detector set to the horizontal-or-vertical orientation, then according to the rules of quantum mechanics, Bob will register a horizontal click with 50% probability or a vertical click with 50% probability. That's no problem, as after the transmission of stream of photons, Alice and Bob can tell each other for which photons their devices were aligned and use only those to define the key.

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