For as long as humans have had to pass secrets to each other, the cat-and-mouse game pitting Alice and Bob-as cryptographers call them-against eavesdropping Eve has raged. And for just as long, makers of codes have have sought to outdo eachother.
Codes called one-time pads are indeed unbreakable. But as their name suggests, each code can only be used once, so parties must pass fresh codes to each other constantly. Couriers can be bribed; or they might secretly copy the codes en route. But cryptographers now have a potentially unbreakable cipher on hand-and that’s where Schrödinger’s cat comes in.
In Erwin Schrödinger’s 1935 thought experiment, a cat is placed in a box, with a poison gas canister attached. An odd timing mechanism governs the box: if a certain unstable atom breaks apart during the next hour-and the odds are 50-50-the canister gas opens, killing the cat.
Until an observation of the system is made, quantum physicists say the system is in a superposition of states. In the unopened box, that is, the cat is both dead and alive at the same time. The limbo persists until a measurement is made: the box is opened to reveal the feline’s fate. But while this oddness does not apply to macroscopic objects, like ourselves, it certainly applies to microscopic photons-the stuff light is made of.
The simple act of observing a photon destroys it, and this quantum quirk makes unbreakable codes possible. Alice uses a laser to fire a sequence of random photons at Bob. Photons are of four kinds: they come in two different orientations and two “flavours.” If the orientation of Bob’s photon detector matches that of a photon sent by Alice, he registers a “hit,” and measures the photon’s flavour. Bob then tells Alice the sequence of hits he registered, and their orientation. He keeps each hit’s flavour secret though-and this is information only he and Alice know. This information forms the key used to encode messages between them. And a fresh key is produced every split-second.
If a photon is intercepted by Eve on the way, her act of measuring (or observing) it destroys the photon. Nothing gets through to Bob. When they compare answers, Alice’s and Bob’s will not agree. If this happens consistently, it indicates Eve may be plying her crafts.
This system is still not fool-proof, though. Lasers often produce several photons, or multiple copies, of each message. If Eve soaks up enough of the extra photons, she has a good crack at getting the key, when Alice and Bob compare results publicly.
Last year, physics and engineering researchers, led by Dr. Hoi-Kwong Lo, proposed a fix for this, using so-called decoy states. Their system uses two laser beams for communication. Each laser produces a different number of photons, and Alice randomly varies the laser uses when sending the key-making photons to Bob, adding another degree of complexity.
In a paper soon to appear in the journal Physical Review Letters, they now report sending signals using decoy states over 15 kilometres of fibre-optic cable-the quantum-encrypted transmission record 120 kilometres.
Two companies are already commercializing quantum cryptography, Lo pointed out, and most components they used in their experiment are off-the-shelf. Customers include spooks and financial institutions-today’s keenest keepers of secrets.
