研究人员利用光子创造出“光子猫”

Researchers at the National Institute of Standards and Technology (NIST) have created "quantum cats" made of photons（光子） (particles of light), boosting prospects¹ for manipulating light in new ways to enhance precision measurements（精密测量） as well as computing² and communications based on quantum physics. The NIST experiments, described in a forthcoming paper, repeatedly produced light pulses（脉冲） that each possessed³ two exactly opposite properties—specifically, opposite phases, as if the peaks of the light waves were superimposed（叠加的，重叠的） on the troughs（水槽） . Physicists⁵ call this an optical Schrödinger's cat. NIST's quantum cat is the first to be made by detecting three photons at once and is one of the largest and most well-defined cat states ever made from light. (Larger cat states have been created in different systems by other research groups, including one at NIST.)

A "cat state" is a curiosity of the quantum world, where particles can exist in "superpositions" of two opposite properties simultaneously⁶. Cat state is a reference to German physicist⁴ Erwin Schrödinger's famed 1935 theoretical notion of a cat that is both alive and dead simultaneously.

"This is a new state of light, predicted in quantum optics（量子光学） for a long time," says NIST research associate Thomas Gerrits, lead author of the paper. "The technologies that enable us to get these really good results are ultrafast lasers, knowledge of the type of light needed to create the cat state, and photon detectors⁸ that can actually count individual photons."

The NIST team created their optical cat state by using an ultrafast laser pulse to excite special crystals to create a form of light known as a squeezed vacuum（真空） , which contains only even numbers of photons. A specific number of photons were subtracted from the squeezed vacuum using a device called a beam splitter. The photons were identified with a NIST sensor⁹ that efficiently¹⁰ detects and counts individual photons (see "NIST Detector⁷ Counts Photons With 99 Percent Efficiency," NIST Tech Beat, Apr. 13, 2010, at www.nist.gov/eeel/optoelectronics/detector_041310.cfm.) Depending on the number of subtracted photons, the remaining light is in a state that is a good approximation（近似值） of a quantum cat says Gerrits—the best that can be achieved because nobody has been able to create a "real" one, by, for instance, the quantum equivalent to superimposing two weak laser beams with opposite phases.