I have sort of a passing interest in time measurement and wanted to share a portion of an e-mail from the director of the time division at NIST (Dr. Tom O'brian):
That is an order of magnitude better than than before. It used to be you could tell which floor in a building you were on. Now, you can tell how many steps there are.
John
You may be interested in some very recent results using two new kinds of atomic clocks,
each with an accuracy better than 1 second in 1 billion years:
http://www.nist.gov/public_affairs/releases/logic_clock/logic_clock.html
Each of the two clocks described -- a clock based on a single ion of
mercury, and a clock using quantum computing principles with an ion
of aluminum -- has an uncertainty of about 2 x 10^-17. That level of
uncertainty is equivalent to the gravitational shift in moving up or
down by about 20 cm (about 8 inches).
That is an order of magnitude better than than before. It used to be you could tell which floor in a building you were on. Now, you can tell how many steps there are.
John