Frequency stabilization of an atomic clock against variations of the C-field


The frequency of an atomic clock may be stabilized against C-field variation by applying a radio-frequency (RF) magnetic field perpendicular to the C-field. The elegant solution to the problem of C-field variation can be implemented in atomic clocks by including a coil in the clock generating a magnetic field perpendicular to the C-field, and providing an electronic circuit to send RF signals to the coil. By Zeeman splitting the atoms to send a small number over the trajectory in states that are highly affected by the C-field, but in an opposite way as the majority of the atoms, nominal variations in the C-field are cancelled. Atomic clocks that particularly benefit from this technology use the Ramsey technique of time-separated electromagnetic fields before and after the trajectory to derive their rate from an atomic transition which has no first-order dependence on the magnetic field. In other words, most modern Cs and Ru atomic clocks.

Technology transfer

This technology is available for licensing or for further development through a collaborative research agreement with NRC. NRC is pursuing further tests and development of the technology.  The business opportunity may be referred to by its NRC ID: 12382.

Market applications

The atomic clock market is well established. The present invention is the next great advance in the art. By stabilizing clocks against C-field inhomogeneity, the reliability of the clock frequency is improved and the clock may operate over a longer life and in a more challenging environment.

How it works

Atomic clocks maintain a stable rate by locking the frequency of their local oscillator to the frequency of a selected atomic transition. Since the frequency of an atomic transition is stable, reproducible and universal, atomic clocks are used as standards for frequency and time.

For these atomic clocks to function properly, a magnetic field (the C-field) must be applied on the atoms. A problem with presently available atomic clocks is that the C-field can change with time, causing changes in the clock rate. The C-field can change for several different reasons including aging and hysteresis of the shield material, environmental temperature fluctuations, variations in the external magnetic field, magnetization of the shields, mechanical shocks, and aging of the electronic circuits. Another problem arises because the C-field is not perfectly spatially uniform, causing issues in evaluating the exact rate of the clock. Spatial non-uniformities of the C-field can be caused by non-uniformities of the shield material, necessary compromises in the design such as space restrictions and access holes, and the presence of other magnetic materials.

Because the frequency inaccuracy of the atomic clock is strongly dependent on the C-field, some methods have been proposed to reduce the magnetic field sensitivity. These methods can be classified by the type of mitigation used to reduce the magnetic field sensitivity and there are known deficiencies with each.

The technology being offered relates to a method of stabilizing the frequency of an atomic clock against variations and non-uniformity of C-field. The method comprises applying a RF magnetic field perpendicular to the C-field to cause a coherent population transfer between Zeeman states, that compensates exactly for quadratic frequency shift of the clock transition over a region defined by the trajectory of atoms between their interaction with two RF excitations. It is a feed-forward method in which 100% of the dependence of the frequency on the C-field can be compensated.


The technology stabilizes the frequency of the atomic clocks against variations and non-uniformities of the C-field. As such, it minimizes design restrictions and prolongs the life of atomic clocks.


NRC file 12382:

Patent granted in Great Britain and patent allowed in the US.


To inquire about this technology, please contact:

Martin Rutter, Portfolio Business Advisor
Telephone: 613-990-6439