Free-Space Quantum Key Distribution

In our free-space quantum key distribution system, Alice's four attenuated lasers transmit polarized single photons to Bob's receiving telescope, which directs them to one of four detectors. In this example, Bob receives a single photon that Alice transmitted in the "horizontal/vertical" basis. The first beamsplitter randomly directs the photon either straight ahead or to the right. If the photon goes straight ahead, a second polarizing beamsplitter will direct it to the correct h/v basis detector, and it becomes a useful bit of key material. If the photon goes to the right, another polarizing beamsplitter will randomly send it to either diagonal basis detector—this randomness eliminates its usefulness as cryptographic key material. Bob then communicates with Alice over a public channel: revealing how he detected each photon (i.e., h/v or diagonally)—but not the result (a "1" or a "0"). Alice tells Bob which photons were correctly tested, and those bits form the "sifted" cryptographic key.

For more information about quantum key distribution, visit http://www.lanl.gov/orgs/p/rh03_hughes.shtml.