Stefan Ulmer Dissertation

Today we report in Nature on an improved measurement of the magnetic moment of the antiproton. The new measurement outperforms our old record measurement by a factor of 350 in experimental precision. Our updated value gpbar/2=2.792 847 3441 (42) is consistent with the magnetic moment of the proton gp=2.792 847 350 (9), and thus supports the combined charge, parity, and time-reversal (CPT) invariance, an important symmetry of the Standard Model of particle physics. Remarkably, this is the first time physicists have carried out a more precise measurement on antiprotons than on protons.  Together with the exciting new antihydrogen results, this milestone achievement is a demonstration of the immense progress made at CERN’s antiproton decelerator facility.

This extraordinary improvement in experimental accuracy was made possible by the invention of a novel two-particle multi-Penning-trap measurement method, which combines the non-destructive detection of the antiproton’s spin quantum state with particle-based high-resolution magnetic field measurements.

The determination of the magnetic moment of a single trapped particle is based on the measurement of two characteristic frequencies, the cyclotron frequency, which describes the particle’s revolutions per second in the magnetic field of the Penning trap, and the second, the precession frequency of the particle’s spin. Together, these allow us to access the particle’s magnetic moment. Previous antiproton measurements, such as those performed by the ATRAP collaboration in 2013 and later by BASE, used a single Penning trap with a superimposed magnetic bottle. This strong inhomogeneity in the magnetic field allows for non-destructive detection of the particle’s spin-quantum-state, a precursor to any determination of the Larmor frequency.  However, such a bottle broadens the particle’s resonance lines and limits the precision of the measurement, typically to the parts per million level.

To overcome this limitation, experimentalists apply a two trap method which separates the high-precision frequency measurements to a homogeneous precision trap and the spin state analysis to a trap with the superimposed magnetic inhomogeneity. While an elegant technique, this double trap method is very challenging to implement. It took seven years of research and development work until we were able to demonstrate this double-trap method with a single trapped proton, and later applied it in a measurement of the proton magnetic moment to nine significant figures.

In the measurement reported today, we have extended the double-trap technique to a three trap / two particle scheme, in which we use a “hot” particle with an effective temperature of 300K for magnetic field measurements and a cold particle at 0.12K for spin transition spectroscopy. By alternatingly shuttling the two particles to the precision trap we were able to quasi-simultaneously sample cyclotron and Larmor frequencies by a fast adiabatic particle exchange in the same ultra-homogeneous magnetic field. However, unlike the double trap method, the two particle technique avoids time consuming resistive cooling cycles to sub-thermal temperatures, and thus, enables measurements at drastically improved frequency sampling rate, which was the major breakthrough to accomplish the goal of measuring the antiproton magnetic moment with parts per billion precision. 

By combining the new 350-fold improved antiproton result with our previously measured proton result we obtain one of the most precise tests of CPT invariance in the baryon sector, which enables us to set drastically improved constraints on CPT-violating extensions of the Standard Model.



Max Planck Society:

Univ. Mainz:


Ulmer, Stefan

German Title: Erstbeobachtung von Spin Flips mit einem einzelnen, in einer kryogenen Penningfalle gespeicherten Proton

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In dieser Arbeit wird die erstmalige direkte Beobachtung von Spin-¨ Uberg¨angen eines einzelnen, in einer kryogenen Doppel-Penningfalle gespeicherten Protons pr¨asentiert. Der experimentelle Nachweis solcher Spin-¨ Uberg¨ange basiert auf der Anwendung des kontinuierlichen Stern-Gerlach Effekts. Hierbei wird der Spin-Freiheitsgrad ¨uber ein inhomogenes Magnetfeld an die nicht-destruktiv messbare axiale Eigenfrequenz des in der Penningfalle gespeicherten Protons gekoppelt. Eine ¨ Anderung der Spin-Quantenzahl macht sich so in einer Verschiebung der axialen Frequenz bemerkbar. Die besondere experimentelle Herausforderung beim Proton besteht in seinem winzigen magnetischen Moment. Um eine durch einen Spin-¨ Ubergang verursachte Verschiebung der axialen Frequenz beobachten zu k¨onnen, wurde das Proton in der st¨arksten, jemals einer Penningfalle ¨uberlagerten Magnetfeldinhomogenit ¨at gespeichert, und nicht-destruktiv nachgewiesen. Dazu wurden ultrahochempfindliche supraleitende Nachweissysteme entwickelt, welche die direkte Beobachtung des Protons, und die hochpr¨azise Messung seiner Eigenfrequenzen erm¨oglichen. Basierend auf neuartigen experimentellen Methoden, die im Rahmen dieser Arbeit entwickelt wurden, konnte die axiale Frequenz des Protons unter diesen extremen Magnetfeldbedingungen auf ein Niveau stabilisiert werden, das in Kombination mit der entwickelten Hochfrequenzelektronik die Beobachtung von Spin-¨ Uberg¨angen erm¨oglicht. Dieser experimentelle Erfolg stellt einen der wichtigsten Schritte zur direkten Messung des magnetischen Moments des freien Protons dar. Mit der Demonstration der erstmaligen nicht-destruktiven Beobachtung von Spin-¨ Uberg¨angen eines einzelnen Protons er¨offnet sich dar¨uberhinaus eine reizvolle Perspektive. Die im Rahmen dieser Arbeit entwickelten experimentellen Techniken k¨onnen auf das Antiproton angewandt werden. So r¨uckt die erstmalige Hochpr¨azisionsmessung des magnetischen Moments des Antiprotons in greifbare N¨ahe, was einen neuen hochpr¨azisen Test der Materie-Antimaterie-Symmetrie erm¨oglicht

Translation of abstract (English)

In this thesis the very first observation of spin transitions of a single proton stored in a cryogenic double-Penning trap is presented. The experimental observation of spin transitions is based on the continuous Stern-Gerlach effect, which couples the spin of the single trapped proton to its axial eigenfrequency, by means of an inhomogeneous magnetic field. A spin transition causes a change of the axial frequency, which can be measured non-destructively. Due to the tiny magnetic moment of the proton, the direct detection of proton spin-flips is an exceeding challenge. To achieve spin-flip resolution, the proton was stored in the largest magnetic field inhomogeneity, which has ever been superimposed to a Penning trap, and its axial frequency was detected non-destructively. Therefore, superconducting detection systems with ultrahigh-sensitivity were developed, allowing the direct observation of the single trapped proton, as well as the high-precision determination of its eigenfrequencies. Based on novel experimental methods, which were developed in the framework of this thesis, the axial frequency of the particle was stabilized to a level, where the observation of single-proton spin-flips is possible, which was demonstrated. This experimental success is one of the most important steps towards the high-precision determination of the magnetic moment of the free proton. With the very first observation of spin transitions with a single trapped proton, a highly exciting perspective opens. All experimental techniques which were developed in this thesis can be directly applied to the antiproton. Thus, the first high-precision measurement of the magnetic moment of the antiproton becomes possible. This will provide a new high-precision test of the matterantimatter symmetry.

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