Cesium Magnetometry

Besides many other systematic effects, there are influences on the nEDM measurement by possible drifts and any gradients of the magnetic fields. So it becomes necessary to carefully investigate the magnetic field in the UCN storage chamber. For that purpose, full optical Cs atomic magnetometers will be used in the Munich nEDM experiment. These magnetometers will be placed in an array in the empty online access tube. The most important advantage of using Cs atomic magnetometers is that no electrical cables for the transmission of a signal are necessary and thus no additional sources of magnetic fields are added to the UCN storage chamber since only a polarized laser beam is used to interact with the magnetometer. The entire analysis is done outside the magnetically shielded chamber. Furthermore Cs based magnetometers don’t require a lot of maintenance and they don’t have to be cooled with liquid He down to 4 K such SQUIDs. The sensitivity of Cs magnetometers can even surpass the sensitivity of SQUIDs. Finally no crosstalk related noise is expected if light scattering between the magnetometers is avoided. Since an entire array of Cs magnetometers will be set up, the multipolarity of field perturbations can be resolved.

Cesium Magnetometry Team

Head of Cesium

Philipp Rößner, B.Sc.

ILL Contact

Inaktiv: Der Benutzer ist in TUMonline nicht mehr aktiv!
Inactive: The user is not active in TUMonline anymore!

Senior Cesium Consultants

Inaktiv: Der Benutzer ist in TUMonline nicht mehr aktiv!
Inactive: The user is not active in TUMonline anymore!


A new generation neutron EDM experiment will be setup at the FRM-II reactor in Munich, seeking for sources of CP violation while providing a new sensitivity limit on theneutron EDM of 10−28 e·cm. According to the Russian physicist Andrei Sacharov,who postulated three necessary conditions for explaining the asymmetry, one of them requires a certain amount of CP violation. The existence of an neutron EDM would fulfillthis criteria. The sensitivity improvement by a factor 100, with respect to the previous generation nEDM limit by the Sussex collaboration, requires a comprehensive understanding of systematics and influences due to magnetic fields on the measurementprinciple (Ramsey’s method of separated oscillatory fields). An all-optical magnetometerarray with up to 32 Cesium based full optical magnetometer heads will be part of the nEDMexperiment, avoiding electrical crosstalk or magnetic contaminants (metal parts e.g. wires)and providing uncertainties in the magnetic field detection of ∼ 40 fT/√Hz as measured during an initial characterization attempt at PTB Berlin. Field fluctuations, E ~ ×~v-effects aswell as geometric phases are influences on the measurement which might mimic an EDM and thus need to be detected precisely.


The most famous isotopes for operating an alkali metal atomic magnetometer are 87Rb and133Cs. The idea behind is, that both are stable isotopes with an unpaired electron in the 5s1 orbital for Rb and in the 6s1 orbital for Cs which spin state can be affected by theinteraction of unpaired electrons with photons. In principle, any alkaliatom could be used for magnetometry even non-alkali atoms with a non-zero ground state spin level. The reason why the discussion isfocused on alkali atoms is due to the fact that their level structure is easier (less relaxation channels), they usually form a monoatomic vapor at reasonable temperatures and can mostlybe addressed by using rather inexpensive laser systems. The Munich experiment, Cs is usedas an alkali metal vapor, contained in paraffin/silane coated pyrex cells, in order to measuremagnetic fields. Cs will be used as it just possesses one stable isotope, which makes it moreaffordable as no enriched 87Rb is required and it doesn’t need any heating as it comes with ahigher vapor density. This gives rise to focus on the Cs atom only in the following sections, but it’s worth to mention that the basic quantum mechanical concepts are analogous validfor Rb.

Publications and Theses

The most stable magnetic field measurement to date (18.01.2022): https://arxiv.org/abs/2201.06936