By placing an interferometer at one end of a long optical fiber with a mirror at its other end, using a laser source precisely synced with a frequency reference, and configuring things so that the fiber is one of the interferometer arms, two faraway points (the two fiber ends) can be precisely synced in frequency -- and thus time -- by cancelling phase noise. This technique can be used, for example, in physics experiments that explore fundamental questions concerning general relativity.
Twenty years ago, a group at the National Institute of Standards and Technology (NIST) and JILA, a joint institute of NIST and the University of Colorado at Boulder, created a double-pass setup that allowed them to place the interferometer at a point along the fiber rather than at an end, which allowed them to time-sync two remote points.1
That syncing feeling
Now, Gesine Grosche of Physikalisch-Technische Bundesanstalt (Braunschweig, Germany) has come up with a way of "tapping" such a fiber-based setup at any point along the fiber, obtaining both the forward- and backward-traveling signals to get the precise time, with a frequency instability (10-19 for a three-hour integration time) better than the 10-18 instability of precise optical clocks.2
Using a narrow-linewidth Koheras fiber laser from NKT Photonics (Birkerød, Denmark) as a source, the idea was tested over a short 100 m distance but can easily be extended to kilometers-long fiber sections, as well as free-space applications. Grosche calls the technique "eavesdropping" time. She notes that the scheme can be used with other geometries as well, including rings of fiber, all allowing many measurement points along the length of fiber.
This means that one of the obstacles toward vastly more comprehensive tests of physics theories and fundamental constants has been removed. Physicists should certainly be excited -- I know I am.
REFERENCES:
1. Long-Sheng Ma et al., Optics Letters, Vol. 19, p. 1777 (1994).
2. Gesine Grosche, Optics Letters, Vol. 39, p. 2545 (2014).
