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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 Ata Imamoglu>, Pierre Petroff, Evelyn Hu and their colleagues at the University of California at Santa Barbara offers another optionnamely, 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 Bobthe two who communicate in secret in nearly all examples of cryptographyexchange 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."

To form the device, Petroff's postdoc Winston Schoenfeld first made a block of semiconducting materials using molecular beam epitaxy. The base contains gallium arsenide; the post, aluminum gallium arsenide; and the 200-nanometer-thick microdisk cap, quantum dots of indium arsenide embedded in gallium arsenide. Hu and graduate student Lidong Zhang then shaped the block by way of patterning and etching. Cutting the block into a mushroom shape, the researchers discovered, eliminated much of the material surrounding the quantum dotswhich can add contaminating background radiation. Imamoglu, postdocs Michler and Christopher Becher and graduate student Alper Kiraz next loaded the quantum dots with energy using laser pulses and observed a pattern of emission without peaking, indicating single photons. Adds Petroff, "It's like God saying, 'Let there be a photon,' and there is a photon."