Foolproof Quantum Cryptography Image: A. Imamoglu
To date, when physicists have dreamed of quantum computing devices, they have most often envisioned encoding the data in terms of electron spin states. But a new gizmo developed by Ataç Imamoglu, Pierre Petroff, Evelyn Hu and their colleagues at the University of California at Santa Barbara offers another option—namely, using polarized light as qubits. The scientists describe in today's issue of Science a mushroom-shaped microdisk, combined with a quantum dot, capable of emitting single photons (see image).
Because the device produces single photons, it clears the path not only toward new ways of performing quantum computating but also toward entirely foolproof cryptography. Imagine that Alice and Bob—the two who communicate in secret in nearly all examples of cryptography—exchange encoded messages. They must also swap "keys," telling each other how to decode the messages. If a key travels in the form of a string of single photons, no one trying to listen in on their conversation can intercept it without leaving a trace. "Because measurements unavoidably modify the state of a single quantum system," the scientists write, "an eavesdropper cannot gather information about the secret key without being noticed, provided that the pulses used in transmission do not contain two or more photons."
