One Photon per Cycle: A Light Conveyor Belt
On a factory line, a conveyor belt delivers parts one at a time per cycle. If the cycle is short and the defect rate low, the line runs fast. In quantum optics, such a conveyor belt is what a single-photon source does: it must emit exactly one quantum of light per cycle. Physicists from Denmark and Germany have built a conveyor that fires 515 million times per second, and more than half of the photons reach the end of the optical fiber.
The heart of the conveyor is a quantum dot, a tiny island of indium arsenide inside gallium arsenide. When a laser pulse hits it, the dot absorbs the energy and, 220 picoseconds (billionths of a second) later, emits a single photon. To keep the dot from idling, the laser pulses are carved at rates up to one gigahertz, meaning one pulse every nanosecond. This is done by passing a continuous beam through two crystals that open and close on command from a microchip.
The system's efficiency, the fraction of photons that make it into the fiber, stays above half at all three tested frequencies: 56.2%, 53.8%, and 51.5%. The purity of single photons ranged from 3.63% to 11.13%, meaning that out of a hundred pulses, only a few produce two photons at once. The most surprising part is the measurement method. The photon stream is so dense that its power can be measured with an ordinary industrial optical power meter. At gigahertz frequency, the power was 110 picowatts. That is nine million times weaker than a laser pointer beam, but the device handles it. Previously, counting photons required superconducting detectors that had to be cooled to near absolute zero. Now a standard off-the-shelf device does the job in seconds.
A conveyor that outputs one photon per cycle could already speed up secure data transmission over fiber optics or serve as a standard of faint light in metrology. The next step is to shorten the dot's lifetime to push the frequency above one gigahertz, or to split the stream into multiple channels. Such a source works at room temperature and does not require expensive cooled detectors, making it a candidate for the workhorse of future quantum networks.
An optical power of 110 pW is so tiny that an ordinary 1 mW laser pointer emits 9 million times more photons per second. Yet it can be measured with an off-the-shelf instrument in a few seconds.