Light switched on and off with few photons

CORNELL (US) — The passage of a light beam through an optical fiber can be controlled by just a few photons of another light beam, new research demonstrates.

All-optical control is the idea behind photonics, where beams of light replace electric currents in circuits, yielding faster and more efficient consumption. Just as a transistor can switch an electric current on or off, photonic circuits need a way for one light beam to switch another.

Researchers say the new technology, reported in Physical Review Letters, is one step closer to the holy grail of single-photon switching and could have applications in quantum computing where single photons act as qubits—the quantum equivalent of ones and zeroes.

Rubidium atoms will absorb photons only if two photons of specific wavelengths arrive at the same time. This allows one stream of photons to turn another on or off. (Credit: Gaeta Group)


Light consists of small packets of energy called photons, that under the right conditions, can be absorbed by an atom.

Alexander Gaeta, professor of applied and engineering physics, and colleagues at Cornell University exploited the unusual property of the element rubidium, which can absorb photons only if two photons of certain wavelengths arrive at the same time. They filled a hollow-core optical fiber with rubidium vapor and fired a continuous infrared light signal at a wavelength of 776 nanometers (nm) in one end and an intermittent “control” signal at 780.2 nm in the other.

In the narrow tube, light interacts strongly with the rubidium atoms. When the control beam is on, rubidium atoms absorb both wavelengths, and the signal is cut off; when the control is off the signal passes through.

Gaeta’s team observed the effect with less than 20 control photons at timescales as fast as five-billionths of a second, allowing modulation at frequencies up to 50MHz, the researchers say, referring to the rate of transmission of on and off pulses of light representing digital ones and zeroes in fiber-optic communication.

The research was funded by the National Science Foundation and the Defense Advanced Research Projects Agency.

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