Precision measurement

The magnetic proton

A record measurement of the proton's magnetism has been achieved by confining a single proton in a device called a double Penning trap. The result opens the way to exploring one of nature's fundamental symmetries. See Letter p.596

The achievement of an unprecedented degree of precision in the measurement of the proton's magnetic moment is described by Mooser et al.1 in this issue (page 596). This impressive feat, obtained by trapping and studying one proton at a time, sets the stage for a new test of a profound symmetry of nature and of Einstein's relativity.

Nature's secrets are encoded in fundamental laws of physics that govern the properties and behaviours of elementary particles. Ordinary matter consists of atoms formed from electrons, protons and neutrons. Unlike the electron, for which no evidence of substructure has been found, the proton and the neutron are each composed of elementary particles called quarks and gluons. It is impossible to isolate quarks or gluons because they interact too strongly with one another, so one way to determine their properties indirectly is to measure the properties of the proton.

Experimentally, the proton is found to be a stable particle with mass and electric charge. It also behaves like a tiny magnet, the strength of which is called the proton magnetic moment. Before Mooser and colleagues' work, the most precise measurement of the proton magnetic moment dated from more than 40 years ago2. This early study achieved a precision of about 10 parts per billion (p.p.b.) by means of spectroscopic studies of atomic hydrogen and input from separate theoretical calculations.

The trapping, storage and study of a single particle is an alternative and direct approach to precision measurements. For example, a device called a Penning trap uses a combination of magnetic and electric fields to keep a charged particle confined in a small region. This method has been used to measure the magnetic moment of the electron at an impressive precision of 0.0003 p.p.b. (ref. 3).

The application of this technique to the proton is much more challenging, because the proton magnetic moment is about 658 times smaller than that of the electron. It involves a magnetic field with a larger spatial inhomogeneity that, in turn, makes obtaining a precision measurement more difficult. Nonetheless, measurements of the proton magnetic moment using a Penning trap to confine and probe a single trapped proton have reached a sensitivity of about 2,500 p.p.b. (refs 4, 5).

To achieve the most precise measurement so far, Mooser et al. used a double Penning trap, in which two Penning traps are conjoined such that their midpoints are separated by about 5 centimetres. One trap has a magnetic field with a large spatial inhomogeneity and is used to analyse the quantum state of the proton; the other has a homogeneous field and is used to make precision measurements. In the experiment, the proton is shuttled back and forth between the traps and its quantum state manipulated until a measurement can be made, a process that takes about 2 hours. By analysing data taken over the course of 4 months, the authors were able to attain a record precision of 3.3 p.p.b. for the proton magnetic moment.

A primary motivation for making precision measurements of the proton magnetic moment is the prospect of comparing it to the magnetic moment of the proton's antiparticle, the antiproton. Antiprotons are observed experimentally to have the same mass as protons and an opposite charge, in accordance with a profound theoretical result from multi-particle quantum physics called the CPT theorem. The essence of this theorem is that performing a charge conjugation (C, which interchanges particles and antiparticles), a parity inversion (P, which involves a mirror reflection and a rotation) and a time reversal (T, which changes the direction of the flow of time) leaves physical laws unchanged, and so the combined CPT transformation represents a symmetry of nature. The theorem holds for realistic multi-particle quantum theories only if Einstein's relativity is exactly valid, so tiny deviations from CPT symmetry would be accompanied by tiny violations in the laws of relativity6. One consequence of the theorem is that the proton and antiproton magnetic moments must be equal in magnitude, so comparing these two quantities experimentally offers a sharp test of CPT symmetry, and therefore an opportunity to search for the tiny relativity violations predicted in some theories of nature.

In practice, any difference in the observed proton and antiproton magnetic moments would emerge from shifts in their quantum energies and would involve two factors, one constant and the other varying with time owing to the motion of the laboratory as Earth rotates on its axis and revolves around the Sun7. Disentangling these two effects requires a large data set, and a detailed experimental study with protons and antiprotons has not yet been published. Pioneering experiments of this type have used electrons and positrons (antielectrons) in Penning traps to constrain both constant8 and time-varying9 energy shifts violating the CPT theorem to about 2 parts in 1021 of the electron's rest energy (0.511 megaelectronvolts). Mooser and colleagues' new techniques offer one promising route by which to extend these tests to protons and antiprotons.

The prospects for future improvements in the measurement precision of the proton magnetic moment are excellent. For the double Penning trap, further reducing the field inhomogeneity and sharpening the experimental procedure are expected to increase precision by a factor of ten1. A different and ambitious scheme now under development involves a precision array with two Penning traps, one containing a proton or antiproton, and the other an atomic ion10. The ion would improve the control of the magnetic field and the measurement procedure, and the timescale required for a measurement would be reduced from about 2 hours to approximately a second. These and other future experiments on protons and antiprotons will stringently test and enhance our understanding of the fundamental laws of nature.

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Correspondence to V. Alan Kostelecký.

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Kostelecký, V. The magnetic proton. Nature 509, 571–572 (2014). https://doi.org/10.1038/509571a

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