Yifan Li

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.

Water is the only common liquid that doesn't immediately contract when cooled. Ice melts, and water becomes denser, but only up to 4 degrees Celsius. Beyond that, with further cooling, it expands again. This quirk explains why fish survive in freezing ponds: the densest water sinks to the bottom, while a lighter cold layer stays on top and turns into ice.

A huge pool of clear liquid is hidden deep underground. Inside, there is nothing but a pristine 'broth' that flashes with faint light when an invisible particle flies through. This is the JUNO detector—20,000 tons of liquid scintillator built to hunt for neutrinos. The main enemy of such a hunt is not darkness but its own noise.

The instrument shines an infrared beam at a sample, and molecules respond with their own absorption pattern, like bells of different tones. This pattern, the spectrum, holds information about the chemical composition. But to read it, a specialist usually consults a library of reference records, which is slow and not always possible.