  The stability of an optical atomic clock is a critical figure of merit for
almost all clock applications. To this end, much optical atomic clock research
has focused on reducing clock instability by increasing the atom number,
lengthening the coherent interrogation times, and introducing entanglement to
push beyond the standard quantum limit. In this work, we experimentally
demonstrate an alternative approach to reducing clock instability using a phase
estimation approach based on individually controlled atomic ensembles in a
strontium (Sr) optical lattice clock. We first demonstrate joint Ramsey
interrogation of two spatially-resolved atom ensembles that are out of phase
with respect to each other, which we call "quadrature Ramsey spectroscopy,"
resulting in a factor of 1.36(5) reduction in absolute clock instability as
measured with interleaved self-comparisons. We then leverage the rich hyperfine
structure of ${}^{87}$Sr to realize independent coherent control over multiple
ensembles with only global laser addressing. Finally, we utilize this
independent control over 4 atom ensembles to implement a form of phase
estimation, achieving a factor of greater than 3 enhancement in coherent
interrogation time and a factor of 2.08(6) reduction in instability over an
otherwise identical single ensemble clock with the same local oscillator and
the same number of atoms. We expect that multi-ensemble protocols similar to
those demonstrated here will result in reduction in the instability of any
optical lattice clock with an interrogation time limited by the local
oscillator.
