The trick that could put an optical clock on a chip

[...]in 2007, researchers succeeded in generating an optical frequency comb on a chip, by confining light to a circular structure called a microresonator6. The comb enables the translation of this highly precise frequency into a microwave frequency that is suitable for transmitting timing information electronically. [...]although microcombs themselves have undergone tremendous improvements, from increased spectral stability10 to self-initiated operation11, the challenges of tuning and locking them to the reference laser frequency have persisted. The authors' innovation was to inject the reference laser directly into the microresonator itself, thus automatically synchronizing the laser with the frequency comb (Fig. 1b).