New Type of Clock Keeps Time by Weighing Atoms

New Type of Clock Keeps Time by Weighing Atoms: The researchers start with a puff of cesium atoms that falls through space toward a detector. Along the way, the atoms encounter pulses of two opposing lasers with slightly different frequencies that gently nudge the atoms without making their inner structure change. The pulses split the cloud in two, and one half of the cloud falls as normal. The other gets pushed up away from the first half and then gets pushed back toward it to catch up.

Here's where the relativity enters. From the perspective of the un-nudged half of the cloud, the second half moves away and then moves back. Because that second half is moving at a few centimeters per second, its time should appear to slow down just a bit thanks to the weird time dilation predicted by Einstein's theory of special relativity. So the quantum wave for that half of the cloud oscillates slightly slower than the one for the first half of the cloud.

When the clouds recombine, that difference in oscillations affects how they overlap and "interfere." If the researchers tune the difference in the two lasers' frequency just right, the recombining waves will interfere "constructively" so that the cloud falls into the detector...

The real value of the approach may come in redefining the kilogram...