Traditional optical atomic clocks are limited in their performance by laser frequency noise

and the intrinsic quantum noise of uncorrelated atoms. In this thesis, we advance the field of

optical clocks on both of these fronts. By developing the next generation of ultrastable laser technology,

we enable clock comparisons that have approached the quantum projection noise limit. To

go beyond this limit, we build and operate an optical clock with the capability of spin-squeezing.

Employing conditional spin squeezing via quantum nondemolition measurements based on cavity

QED, we produce a spin squeezed state that yields a spectroscopic enhancement of 1.7 dB beyond

the standard quantum limit. We then run a clock comparison between two spin squeezed

clock ensembles, making use of a movable optical lattice to individually squeeze and readout the

ensembles with cavity QED. This differential comparison between the two squeezed clocks directly

verifies enhanced clock stability of 1.9 dB beyond the quantum projection noise limit, and reaches

a measurement precision level of 10^{-17}. This constitutes the first direct demonstration of quantum

enhanced measurement in an operational optical atomic clock.

Traditional optical atomic clocks are limited in their performance by laser frequency noise

and the intrinsic quantum noise of uncorrelated atoms. In this thesis, we advance the field of

optical clocks on both of these fronts. By developing the next generation of ultrastable laser technology,

we enable clock comparisons that have approached the quantum projection noise limit. To

go beyond this limit, we build and operate an optical clock with the capability of spin-squeezing.

Employing conditional spin squeezing via quantum nondemolition measurements based on cavity

QED, we produce a spin squeezed state that yields a spectroscopic enhancement of 1.7 dB beyond

the standard quantum limit. We then run a clock comparison between two spin squeezed

clock ensembles, making use of a movable optical lattice to individually squeeze and readout the

ensembles with cavity QED. This differential comparison between the two squeezed clocks directly

verifies enhanced clock stability of 1.9 dB beyond the quantum projection noise limit, and reaches

a measurement precision level of 10^{-17}. This constitutes the first direct demonstration of quantum

enhanced measurement in an operational optical atomic clock.