# Synopsis: Cross-Country Time Keeping

A new distance record is set in the fiber transmission of stable frequency signals capable of synchronizing atomic clocks.

Next-generation atomic clocks are so precise they can’t be synchronized remotely with traditional communication pathways. Researchers are therefore investigating novel synchronization methods. A new milestone in this development is presented in Physical Review Letters, with the longest distance transmission of a highly stable optical frequency. The signal was sent back and forth across Germany on optical fibers, while keeping a fixed frequency to within a few parts in ${10}^{19}$.

Several applications, such as navigation and fundamental physics, require the comparison of clocks at large physical separation. In geodesy, for example, the time difference between two distant clocks can provide relative elevation measurements with centimeter precision. Currently, clock signals are relayed by satellite communication, but the frequency of these radio signals drifts over time by as much as a few parts per ${10}^{16}$. Higher stability is needed to compare recently developed optical atomic clocks that have precisions on the order of one part in ${10}^{17}$.

Several past experiments have shown that optical fibers can faithfully transmit a clock-synchronizing frequency signal over hundreds of kilometers. Stefan Droste of Max Planck Institute of Quantum Optics, Germany, and his colleagues have now sent a highly stable $194$ terahertz ($1542$ nanometer) frequency over a distance of $1840$ kilometers, doubling their previous record. The team achieved this result by equipping the dedicated optical fiber connecting two German research institutions with active stabilization to overcome frequency shifts from thermal noise and acoustic noise. The method might one day link together optical clocks around the world. – Michael Schirber

### Announcements

More Announcements »

## Previous Synopsis

Particles and Fields

## Next Synopsis

Atomic and Molecular Physics

## Related Articles

Quantum Physics

### Synopsis: Position Detector Approaches the Heisenberg Limit

The light field from a microcavity can be used to measure the displacement of a thin bar with an uncertainty that is close to the Heisenberg limit. Read More »

Atomic and Molecular Physics

### Synopsis: A Crystal of Light and Atoms

A predicted type of atom-light crystal could host phonon-like excitations, allowing for new ways to simulate the physics of solids.   Read More »

Condensed Matter Physics

### Viewpoint: An Arrested Implosion

The collapse of a trapped ultracold magnetic gas is arrested by quantum fluctuations, creating quantum droplets of superfluid atoms. Read More »