Electron microscope photo of a cross-section of photonic bandgap fiber. Tiny glass tubes surrounding the core bend light waves in such a way that they interfere and cancel out, focusing all the energy of a beam into the hollow core.
Quantum computing, where bits of information, or "qubits," are represented by the state of single atomic particles or photons of light, won't be of much use unless we can read the results. Cornell researchers have taken a step in that direction with a device that can measure the presence of just a few photons without disturbing them. The experiment mixes a strong beam of light with a very weak "signal" beam consisting of fewer than 20 photons in such a way that the phase - a measure of the timing of a wave - of the strong beam changes in proportion to the number of photons in the signal. "Ideally what people want is to be able to measure the presence of a single photon, without destroying it," said Alex Gaeta, professor and director of applied and engineering physics. "Nevertheless, there are interesting quantum information algorithms you can do with just a few photons." Switching a light beam with a single photon would be the equivalent of a "gate" in a conventional electronic computer, where a 1 or 0 input switches the output between 1 and 0. In future applications this could communicate the state of a qubit in a quantum computer, or the photons themselves might be the qubits. The device created by Gaeta's research group makes use of a new type of optical fiber known as photonic bandgap fiber, which consists of a hollow core surrounded by a honeycomb of tiny glass tubes.
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