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.
Spectroscopy has nothing to do with it, but time is directly linked to light. The source emitted photons not one by one but in "bunches"—this is called bunching. Each photon had a coherence time of 180 nanoseconds, meaning its wave maintained order only briefly, like ripples on water from a tossed pebble. With such weak flickers, the two sides found a common rhythm.
The synchronization accuracy was 10 nanoseconds. That's like two people, not seeing each other, clapping their hands with a difference of less than one ten-millionth of a second. And this went on for 25 hours straight. Meanwhile, the light lost almost all its strength along the way: an attenuation of -102 dB means that out of a trillion photons, only one arrived.
To not miss the moment of coincidence, the authors built a model. It predicts the probability of finding the coherence peak under such a weak signal. The model proved more accurate than previous estimates, and now one can calculate in advance how long to wait for reliable synchronization.
This method will be useful where the signal must be as inconspicuous as possible: in secure communications, distributed sensors, or quantum networks. Instead of a bright beam, a barely perceptible flicker that carries precise time.
Losses of -102 dB mean that roughly one photon out of $10^{10}$ reaches the receiver. For comparison: light from a star visible to the naked eye is attenuated by the atmosphere only a few times.