  Atomic clocks have been at the leading edge of accuracy and precision since
their inception in the 1950s. However, typically the most capable of these
clocks have been confined to laboratories despite the fact that there are
compelling reasons to apply them in the field and/or while in motion. These
applications include synchronization of distributed critical infrastructure
(e.g. data servers, communications, electricity grids), scientific applications
(e.g. radio-astronomy) and to mitigate the effects of interruption to Global
Navigation Satellite Systems.
  Over the last 20 years, there has been a breakthrough in the performance of
atomic clocks by transitioning from an atomic reference based on a microwave
transition to an optical frequency transition. The $10^5$-fold increase in
reference frequency confers the potential for significantly higher performance.
However, this performance increase has come at the cost of size, complexity and
fragility which has continued the confinement of these clocks to the
laboratory.
  Here we report on a recent international collaboration where three emerging
optical clocks, each operating on different principles, were trialed at sea.
These clocks incorporate optical frequency combs so that their stable frequency
outputs can be used directly in electronic apparatus and were also automated so
that they do not require expert supervision. We present the frequency stability
and reliability of these three clocks over multiple weeks of unsupervised naval
trials, both in harbour and on the ocean. The performance of all three devices
was orders of magnitude superior to existing best-in-class commercial solutions
over short and medium timescales. This demonstrates that optical clocks are
ready to deliver advantages for real-world applications.
