  Optical clock comparison via optical fibre links has been achieved over
continental scales, but has not yet been demonstrated intercontinentally. The
transfer of ultra-stable optical frequencies over transoceanic distances is a
challenging task, as the seafloor cable architecture prevents the use of
environmental noise suppression techniques currently employed on land-based
metrological links. As a fundamental first step towards devising suitable
frequency transfer techniques to enable future clock comparison on a global
scale, here we show the free-running frequency stability and phase noise
measurements of a transatlantic seafloor optical link between the UK and
Canada. To the best of our knowledge, these are the first ever measurements of
an intercontinental optical fibre link.
