The mercury-in-glass thermometer has served us well for the past 270 years, but sometimes you need something smaller — say to find the temperature inside a single living cell. Researchers at Harvard have discovered a new technique using lasers and diamond nanocrystals to measure temperatures of microscopic structures, recording temperature fluctuations as small as 0.05 Kelvin (0.09ºF) in size.

Laser light bouncing out of a cool nanodiamond shows up as a different color

The technique relies on the quantum properties of the diamonds’ tasty centers. In a diamond crystals with a nitrogen vacancy in its center — a kind of defect — the center's electronic spin comes to depend on its temperature. Laser light bouncing out of one of these nanodiamonds shows up as a different color depending on the center's temperature. And using diamonds also adds some other benefits. Because they're highly chemically inert, changes in the surrounding chemistry don’t affect the outcome, and the method can be used over a broad range of temperatures, for the same reason. In one series of experiments (pictured above), the team implanted a human cell with a gold nanoparticle, used a laser to heat it up (thereby heating up the surrounding cell), and bounced a laser off a diamond implanted in the same cell to record the temperature difference. The results will be published in the August issue of Nature.

So why would you want to know the temperature inside a living cell? The team believes that the gold heating trick, precisely monitored with its diamond-and-laser nanothermometer, could make it possible to "engineer biological processes at the subcellular level," possibly helping to screen for cancer, or cooking the perfect steak, one cell at a time.