Justin Yu Xiang Peh

An ordinary laser emits photons one by one, like a faucet dripping water. But what if you need several particles of light born at the same instant? Physicists have made a cloud of cold rubidium atoms emit quadruples of photons — as if the faucet suddenly released four drops at once, each falling into its own test tube.

Light from an ordinary bulb is a stream of individual photons, like drops from a faucet. But there is also light where photons come in pairs, like twins walking together. Such grouped light has long been desired for measuring distances and imaging objects hidden around corners. But sources of this light were either too dim, or their flashes too short for ordinary detectors to catch.

Two clocks in different rooms need to tick in unison. Usually, this is done by sending bright pulses or pairs of photons that arrive one right after the other. But the signal can be so weak that photons barely make it through. In this work, clocks were synchronized with light a billion times dimmer than usual.