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A new experiment bridges the old quantum trick of entanglement—the strange faster-than-light communication between particles—with the much newer technique of halting light dead in its tracks. Researchers report in Nature that they have successfully sent a pair of entangled states of light into separate corners of an ultracold atomic cloud, stored them there briefly, and then sent them back on their separate ways without completely destroying the quantum link in the process.
Although the distance of just one millimeter between the two storage points was tiny, the group says the demonstration opens the door for entangling two distinct atomic clouds and using quantum teleportation to flash the quantum state of a particle from one of the clouds to the other. In principle, such clouds could be strung together for thousands of miles, making a quantum telecommunications grid capable of sending potentially unbreakable coded messages from coast to coast.
The stopped light trick was first demonstrated in 2001 by Harvard University physicist Lene Hau and her research group. To achieve it, researchers fired a pulse of light into a cloud of atoms chilled to near absolute zero (which is –459.67 degrees Fahrenheit, or –273.15 degrees Celsius) and illuminated it with a continuous beam of laser light called the control beam. The pulse slowed dramatically inside the cloud and, when the control beam was switched off, froze as a quantum state of the atoms. When the beam was switched back on, the light pulse reformed and continued on its merry way.
In the new experiment, physicists led by H. Jeff Kimble at the California Institute of Technology in Pasadena used a semireflective surface called a beam splitter to cleave a single photon into a type of entangled pair. By toggling the control beam, they stored the two states one millimeter apart in a cesium cloud that was chilled to a temperature of 125 millionths of a kelvin above absolute zero (0 kelvin). When they converted the pair back into light, 20 percent of their original entanglement remained.
The method's efficiency is still low, but it improves on prior attempts to store entanglement that sometimes severed the quantum link, which would hamper attempts to design larger quantum communication networks outside of the lab, says study author Kyung Soo Choi, a physics PhD candidate at Caltech. "If we can generate entanglement every time we push a button, we can scale entanglement" to larger scales.
Whether that will ever happen is unclear, Harvard's Hau wrote in an accompanying editorial. But she added that a century after quantum mechanics was born, "the possibilities it offers continue to boggle our minds."