  Entangled states of light exhibit measurable correlations between light
detections at separated locations. These correlations are exploited in
entangled-state quantum key distribution. To do so involves setting up and
maintaining a rhythm of communication among clocks at separated locations.
Here, we try to disentangle our thinking about clocks as used in actual
experiments from theories of time, such as special relativity or general
relativity, which already differ between each other. Special relativity
intertwines the concept of time with a particular definition of the
synchronization of clocks, which precludes synchronizing every clock to every
other clock. General relativity imposes additional barriers to synchronization,
barriers that invite seeking an alternative depending on any global concept of
time. To this end, we focus on how clocks are actually used in some
experimental situations. We show how working with clocks without worrying about
time makes it possible to generalize some designs for quantum key distribution
and also clarifies the need for alternatives to the special-relativistic
definition of synchronization.
