Found it. http://en.wikipedia.org/wiki/Parametron
This also might be of interest http://spectrum.ieee.org/computing/hardware/dudley-bucks-for... http://en.wikipedia.org/wiki/Cryotron
For completeness there was a great talk the UW EE department a couple days ago, https://www.youtube.com/watch?v=cwl6fORHNqs on using electron spin to store information.
https://www.ee.washington.edu/cgi-bin/research/colloquium/di...
EDIT: Add Reference (albeit a poor one)
http://www.theregister.co.uk/2013/06/28/wilkes_centenary_mer...
I wonder what the minimum spacing of such waves would be, how this would compare with current estimates on the upper bounds of density for silicon and if material fatigue factors into this somehow. I can't imagine such a system to be very stable mechanically.
They borrowed this trick from radar engineers. The engineers needed a way to compare what the radar is seeing now to what the radar saw on its last rotation, so that they could detect moving objects. They did this by sending the information down a mercury delay tube of suitable length.
Relatedly, if you ever had an old school analog sound mixer with sound effects, it would have a spring delay line for delay and echo effects. If you were clumsy like me and ever dropped one, you could here it boinging around.
BTW, Mercury Line Memory was not the first form of memory used in programmable computers. The "oldest" or "first" is debatable, but the earliest were switches/relays and vacuum tubes (AFAIK). I'd bet jgrahamc would know...