The first step is to prepare the photons. Entangled photons have properties, such as polarization, that are fundamentally linked. If two photons are entangled, then the measured polarization of one reveals the corresponding state of the other. The researchers used a technique in which they mixed together multiple wavelengths of light within a standard fiber to create entangled photon pairs.
The next step is to send one photon down the optical fiber to the switch, which changes the photon’s course. The researchers’ switch is made of only optical components, including a spool of 100 meters of optical fiber arranged in a loop. One photon of an entangled pair is sent through one end of the loop, and through a multiplexer, while a powerful laser sends pulses of light into the spool. The photon is shifted in such a way that at the other end of the loop it separates out along a separate path, while remaining entangled with its partner.
The end result is a switch that’s very fast, has low background noise, and most importantly, preserves the quantum information. Single photon detectors at the end of the fibers confirm that both photons maintained their entangled state, showing that the quantum information was preserved. The work is described in a recent issue of the journal Physical Review Letters.
“It’s an important development, because switching photons is really the main difference in going ahead in further progress in quantum computing using photons,” says Franson.