Quantum physics

Spin flips with a single proton

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For the first time, spin flips of a single trapped proton in free space have been observed. This is a major step towards a million-fold improved test of matter–antimatter symmetry using a nuclear magnetic moment.

The world, as we know and feel it every day, consists of matter. Whereas the Big Bang is considered to have created both matter and antimatter in equal quantities, the present-day Universe clearly seems to display a huge asymmetry: antimatter is rarely observed, and if it is, it's only in highly exotic environments or in some radioactive reactions. Writing in Physical Review Letters, Ulmer et al.1 describe an experiment that paves the way for a high-precision test of the theoretically expected matter–antimatter symmetry.

Matter–antimatter symmetry is the most fundamental symmetry in the standard model of elementary particle physics. According to this symmetry, under a CPT transformation — which is a simultaneous inversion of the particle properties charge (C) and parity (P), and a reversal of time (T) — an antiparticle behaves exactly like its mirror-image particle. Ultra-high-precision tests of the CPT symmetry have been performed in different physical systems (for example, with mesons, leptons and baryons) by comparing the properties of particles and their antiparticles. Yet, until now a violation of this symmetry has never been observed.

One of the most fundamental precision tests of CPT — the comparison of the magnetic moment of a single proton and that of its antiparticle, the antiproton — has yet to be performed. Such a test is extremely challenging because it requires ultra-high-precision spectroscopy of, at best, single and unperturbed protons and antiprotons. And this is where Ulmer and colleagues' study comes in. Their experiment allowed them to flip the spin of a single proton, and this will enable a precision measurement of the proton's magnetic moment to be made. What's more, the experimental set-up can be readily applied to antiprotons, and will eventually provide a precision CPT test.

To observe the spin flips, Ulmer et al.1 stored a single proton in a Penning electromagnetic trap at cryogenic temperatures. The trapped proton oscillates with three main frequencies, one of which — the axial frequency, which is associated with the proton's oscillation along the direction of the Penning trap's magnetic field — is actually measured in the experiment. To obtain a dependence of the axial frequency on the proton's magnetic moment, the authors added a magnetic-field inhomogeneity to the otherwise homogenous magnetic field of the Penning trap (Fig. 1). In this 'magnetic bottle', the spin direction of the proton shifts the axial frequency, which can be subsequently measured with high sensitivity by means of a superconducting detection system.

Figure 1: Penning trap used by Ulmer et al.1 to measure proton spin flips.

A strong magnetic-field inhomogeneity (green) couples the proton's spin direction (arrow) to the frequency with which the proton (red) oscillates along the direction of the trap's magnetic field. This frequency is measured to monitor the single-proton spin flips, which are induced by an external radiofrequency field (not shown). Gold electrodes are held in place with insulating sapphire structures (blue).

The spin-dependent frequency shift observable in the proton's axial oscillation in the (inhomogeneous) Penning trap is due to the 'continuous Stern–Gerlach effect', which was introduced by Nobel laureate Hans Georg Dehmelt2. The shift allows the detection of the spin direction of a single trapped charged particle. This method was previously used3,4,5,6 with great success for precision measurements of the magnetic moment of the electron, of the positron and of bound electrons. Now, for the first time, the technique has been successfully applied to a single proton, whose magnetic moment is almost a thousand-fold smaller than that of the electron. The proton's extremely small magnetic moment had defied its measurement with this method for several decades.

Trapping a single proton in a cryogenic Penning trap allows the particle to be stored for months, and is ideally suited for a high-precision experiment. In the authors' experiment1, the large magnetic-field inhomogeneity changed the homogeneous magnetic field by 1 tesla within a distance of about 1 millimetre, and a spin flip changed the axial frequency of the single trapped proton by less than one part in a million — that is, 200 millihertz out of 680 kilohertz. The demanding detection of this tiny frequency shift, which is the signature of the proton's spin direction, represents a real experimental feat.

To drive the spin flips, Ulmer et al. used the magnetic component of a radiofrequency signal. If the signal is resonant with the energy difference between the two orientations of the magnetic moment, the spin-flip probability has a maximum of 50%; it decreases if the driving-signal frequency is detuned off-resonant. By measuring the spin-flip probability as a function of the driving-signal frequency, the authors were able to derive the value of the proton magnetic moment. To achieve better measurement precision, they will use a high-precision section of their trap arrangement together with methods that have already been tested7. This will allow for a million-fold improved test of the matter–antimatter symmetry using a trapped (anti)proton.

In addition to the exciting prospect of a new high-precision test of the matter–antimatter symmetry, the method can also be applied to directly measure magnetic moments of light atomic nuclei. Together with spectroscopic data, such measurements will contribute to a deeper understanding of nuclear size effects (on atomic spectra) and of the distribution of magnetic moments in atomic nuclei.


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Correspondence to Rainer Blatt.

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Blatt, R. Spin flips with a single proton. Nature 475, 298–299 (2011) doi:10.1038/475298a

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