Proton transport from the antimatter factory of CERN

Precision measurements in low-energy quantum systems are used to explore the limits of the Standard Model of particle physics (SM)¹⁵, providing an opportunity to understand the nature of dark matter¹⁶ and the baryon asymmetry in the universe¹⁷. This approach motivates tests that compare conjugate matter–antimatter systems to examine the fundamental symmetries of the SM. Results of precision measurements on antiprotonic systems at low energies, including antiprotonic helium, antihydrogen and single trapped antiprotons, have been obtained recently, providing stringent tests of CPT invariance^3,4,6,8. Moreover, the recent release of antihydrogen atoms from a vertical magnetic trap has provided direct and model-independent constraints on an antimatter-based test of the weak equivalence principle¹⁸. All these measurements are, without exception, performed inside the AMF hall of CERN. At the current level of measurement resolution, no deviations from established theory have been observed. However, these deviations could emerge with more sensitive measurements, highlighting the need for improved measurement precision.

Once a certain level of precision has been achieved in a quantum system, it becomes necessary to reduce the influence of external disturbance. Our measurements of spin-precession ν_L = gqB₀/(4πm) and cyclotron frequencies ν_c = qB₀/(2πm), carried out with single protons and antiprotons in cryogenic Penning-trap systems, give access to the charge-to-mass ratios q/m (ref. ⁸) and the g-factors of the particles^6,7, with g defining their magnetic moments. These measurements are extremely sensitive to external magnetic field noise because the measured frequencies in the artificial exotic geonium atom—formed by the trap and the particle—scale directly with the magnetic field B₀. Our recent comparison of the proton–antiproton charge-to-mass ratio at a fractional precision of 1.6 × 10⁻¹¹ constitutes the most precise test of CPT invariance in the baryon sector. However, it is limited by magnetic field fluctuations imposed by the antiproton decelerator and ELENA-synchrotron operated in the AMF hall. Advancing the shielding of our superconducting magnet operated at a field of B₀ = 1.945 T further is also challenging¹⁹.

Inspired by these limitations, and to markedly advance the limits of precision antiproton measurements, we propose a strategy that can lead to a new era of precision antimatter spectroscopy. Transporting antiprotons and their applications have been predicted and discussed earlier^20,21,22,23, but only one study reported the relocation of electrons in a superconducting magnet in a closed trap chamber²⁴. Our approach involves relocating antiprotons from the AMF of CERN using the transportable Penning-trap BASE-STEP to dedicated high-precision offline laboratories, in which advanced spectroscopy can be performed under disturbance-free conditions. For the next generation of antiproton precision measurements, we require that external magnetic field fluctuations contribute less than 1 nT uncertainty in frequency ratio measurements. In a 1-T magnetic field with a shielding factor of 100 (ref. ¹⁹) and averaging the centre to 10% of the linewidth, antiproton-based CPT tests with statistical uncertainty of the order of 10⁻¹² become possible, corresponding to an improvement by more than one order of magnitude compared with the present state of the art.

In this study, we present a milestone towards the realization of antiproton transportation in the next years. Using the open Penning-trap system BASE-STEP, which can transfer the trapped particles into another experiment, we successfully transported a cloud of around 100 trapped protons out of the AMF of CERN and demonstrated lossless particle transport on a truck across the Meyrin campus of CERN. Within our 4-h transport campaign, the persistent superconducting magnet system operated autonomously, based on battery supplies, cryopumping and cooling by a liquid helium (LHe) reservoir. Following the transport, we returned the trap to its original location and continued the experimental operation. Subsequently, we demonstrated the ability to manipulate the transported particles, separating them into fractions and ejecting them from the trap. We thereby validated the feasibility of the concept to conduct antiproton precision studies offline. This crucial step towards improved measurements of the fundamental properties of the antiproton is also a demonstration of the movement of other accelerator-production-based charged particles into a cleaner measurement environment, for example, the exotic highly charged ions Pb⁸¹⁺ and U⁹¹⁺ (refs. ^13,14) and other charged antimatter systems that may become available in the future, such as the antihydrogen ion \({\bar{{\rm{H}}}}^{+}\) (ref. ¹¹) and the antihydrogen molecular ion \({\bar{{\rm{H}}}}_{2}^{-}\) (ref. ¹²).

The BASE-STEP trap system⁹ is shown in Fig. 1a. It is located in the horizontal cold bore (4 K) of the transportable superconducting magnet, which is shown in Fig. 1b. This system is connected to an antiproton transfer line with its beamline connection port. For the injection and ejection of particles, this open trap system has a transfer channel towards the connection port with a diameter of 6 mm and a length of 135 mm, acting at the same time as a pumping barrier. A cryogenic, rotatable trap electrode, a rotatable degrader stage²⁵, and an inlet valve⁹ protect the trap vacuum from molecular background flow. Simulations indicate that at 4 K, with a closed inlet valve, background pressures of the order of 10⁻¹⁶ mbar can be reached⁹.

a, Overview of the trap system components of the BASE-STEP apparatus used in this study. Parts of the full trap system⁹ have been omitted, and the field emission point was moved closer to the proton cloud for better visibility. A cloud of protons is created by an electron beam from the field emission point and stored inside the cylindrical electrodes of the trap. The image-current detector (which is used for non-destructively monitoring the trap content), the rotatable degrader stage and the transfer channel (which is used for ejecting into another trap system) are shown. b, A model of the transport frame containing the superconducting magnet with the trap system inside. The frame contains the UPS batteries, a control PC, frequency generators for particle manipulation and detection, voltage supplies for trap biasing, amplifiers and heaters.

The trap consists of a stack of cylindrical, gold-plated electrodes, made from oxygen-free high conductivity copper, engineered in compensated and orthogonal design²⁶. Voltages applied to the electrodes provide an electrostatic quadrupole potential that stores the particles along the trap axis, which is parallel to the magnetic field lines, leading to an axial oscillation with a frequency of ν_z ≈ 383.3 kHz at a central ring electrode voltage of −2.950 V. A superconducting image-current detection system for the non-destructive detection of the trapped protons and measurements of their axial frequency ν_z is connected to one of the trap electrodes²⁷. Apart from the axial mode, two circular modes exist in the plane perpendicular to the magnetic field lines. With a magnetic field B₀ between 136 mT and 1.005 T, the modified cyclotron frequency ν₊ is between 2.02 MHz and 15.14 MHz, and the magnetron frequency ν₋ is between 40 kHz and 5 kHz, respectively.

The transportable trap system is integrated into the frame as shown in Fig. 1b. It weighs between 850 kg and 900 kg and contains all devices necessary for transportation. The entire system is designed for transport on public roads, which includes a carbon-steel vacuum chamber for magnetic stray field shielding and a support structure for the cold mass of the magnet and the transport frame to handle acceleration forces apart from gravity of up to 1.0g in all directions⁹. When connected to the power grid, the cooling to cryogenic temperatures to operate the system is provided by a pulse-tube cooler, whereas in autonomous transport mode, the system is cooled by an internal 30 l LHe tank. The precision voltage supply necessary for biasing the trap electrodes, as well as frequency generators for particle manipulation, a spectrum analyser for non-destructive detection and a sensor array for transport monitoring are powered by an uninterruptible power supply (UPS) with two battery units, also mounted on the frame. Although this design keeps the transport system on a compact footprint of 2 m × 0.85 m, it requires the disconnection and reconnection of the system to the pulse-tube compressor to be handled within a limited timeframe. Before moving the system, we verified, in several test runs, that the LHe and the UPS battery capacity are sufficient to run autonomously for 4 h and that restarting the pulse-tube cooler does not quench the persistent current that is loaded in the superconducting magnet.

For the transport, protons are loaded into the Penning-trap system using an in-trap cryogenic field emission point that is operated with electron currents up to 150 nA. The current evaporates any adsorbates hitting the surfaces of the trap system and creates ions of a few species (p, \({{\rm{H}}}_{2}^{+}\), …) in the centre of the trap (Fig. 1a). Using the procedures described in ref. ⁵, we prepare a clean cloud of protons using stored-wave-inverse-Fourier-transform radiofrequency drives to remove contaminant ions. Then, the radial modes of the proton cloud are cooled using sideband cooling²⁸. The number N of trapped particles prepared in this loading and cleaning sequence can be determined non-destructively, using the dip signature that is created by thermal-equilibrium particle–detector interaction²⁹. The width of the dip Δν is given as

Here, R_p ≈ 80 MΩ is the effective parallel resistance of the detector, and D ≈ 0.0133 m is a trap-specific length. The amount of loaded particles can be adjusted based on the loading time and the applied field emission current. Depending on the experiments that are being conducted, we usually prepare 10–200 particles (Methods). For the transport that is reported here, we loaded N = 105(2) protons into the trap.

Although the particle loading takes place at B₀ = 1 T, for the transport, the magnetic field is lowered to 136 mT. The reduction in magnetic field strength reduces the energy stored in the magnet and decreases the strength of the eddy currents induced during transportation. The low B₀ also increases the critical temperature of NbTi by about 250 mK (ref. ³⁰). Therefore, lowering the magnetic field reduces the risk and consequences of quenching the magnet, which is particularly important, because we encounter temperature spikes when reconnecting and restarting the pulse-tube cooler⁹ (Methods).

As the next step, we switch all instruments to UPS operation, stop the pulse-tube cooler and disconnect the compressor from the pulse-tube unit. This marks the starting point to move the trap system within the AMF hall, as indicated by the red arrows and numbers in Fig. 2a. To transport the experiment along these paths, we use both overhead cranes in the AMF hall, moving BASE-STEP from points 1 to 2 and 3 to 4. Between 2 and 3, an industrial four-wheel flatbed trailer is used to transfer the instrument between the hooking points of the two cranes. At point 4, the trap system is lowered with the overhead crane onto a truck, towed and checked for radiation contamination before leaving the radiation-controlled area of the AMF hall. Then, the system is moved along the roads of the Meyrin site of CERN. In the last step, the trap system is craned back to the experiment area (point 4 to point 1), and the pulse-tube cooler is reconnected and restarted to continue the non-destructive operation of the experiment.

a, The route for the first transport demonstration through the AMF hall. Point 1 is the experiment zone from which an overhead crane moved the transport frame to point 2; at point 2, the transport frame was loaded onto a trailer and moved to point 3, where it then got picked up by the second overhead crane. Point 4 is the loading bay with the truck. b, Road map of the Meyrin site of CERN and the GPS position data recorded during transportation. Map reproduced from https://www.openstreetmaps.org. c–e, Magnet temperature (c), total acceleration (d) and liquid helium level (e) measured during transport. The zero on the horizontal axis marks the shutdown time of the pulse-tube cooler. For details, see the text and the Methods. The green bars enclose the time interval corresponding to the truck transport of the trap. The red bars indicate the start and stop times of the crane transportation.

The GPS data recorded during the transport are shown in Fig. 2b, covering a distance of 3.72 km. In our attempt, a maximum velocity of 42.2 km h⁻¹ was reached. The development of the magnet temperature, acceleration and LHe level recorded during the transport are shown in Fig. 2c–e. The two temperature spikes that occur when reconnecting the pulse-tube cooler and when its operation restarts pose a particular challenge. We have reduced the amplitudes of these temperature spikes by cooling the first stage of the pulse-tube cooler through a heat exchanger of the LHe tank exhaust and by tuning the power of a heater located on the LHe tank (Methods). Under these conditions, we observe 7.1 K and 6.4 K as peak values for the temperature spikes and maintain the persistent current in the superconducting magnet. In the future, the first spike could be eliminated by integrating the compressor of the cooler into the transport frame at the cost of increasing the size and weight of the system. Tilting the magnet frame also creates turbulence in the helium tank, as shown by the helium-level meter reading, resulting in a temporary cooling effect of the magnet. The acceleration data show its largest value of 7.7 m s⁻² during the towing to the truck and up to 6 m s⁻² while the truck was moving.

Throughout the entire transport, axial detector fast Fourier transformation (FFT) spectra are recorded while the axial frequency of the particles is tuned to the resonance frequency of the detector. A representative dip signal of the stored proton cloud is shown in Fig. 3a. The continuous monitoring of the protons during transportation is documented in Fig. 3b. Here the dip width Δν of the trapped protons is plotted as a function of time. During the transport on the truck, the particle and detector frequencies shifted because of a potential shift caused by the bad ground connection on one of the room-temperature filter boards for the electrode voltages and a movement of the semi-rigid detection wire in the trap, respectively. We needed about 15 min to match the axial frequency to the detector frequency and apply the magnetron sideband to exclude that the frequency shift is because of a sudden change in magnetron radius. This resulted in a lack of data close to the 2-h mark. This can be improved with improved soldering connections and additional fixtures on the trap wiring in the future. Furthermore, a reduction of the dip width is observed shortly after the 3-h mark, coinciding with the restart of the pulse-tube cooler. During this process, a temperature spike occurs, reducing the Q-value of the detector and consequently R_p, which leads to less particle damping and thus smaller Δν (equation (1)). However, given the recorded data, it can be concluded that the transportation was lossless within the uncertainty of the measurement, and we were not able to detect any indication of heating of the circular ν₊ and ν₋ modes within the resolution limits of the measurement.

a, FFT spectrum of the image-current signal showing the thermal noise of the superconducting LC circuit shorted by the trapped proton cloud at their axial frequency ν_z (ref. ²⁹). b, Plot of the fitted dip width of the proton cloud during transport. The blue line with the surrounding faint blue bar is the mean value of the proton cloud before the transport, with its 1σ uncertainty to visualize the efficiency of the transport. The vertical red and green bars in b indicate the transport on the crane and the truck, respectively.

In our future planned antiproton transport experiments, the last step in the transport campaign would be the extraction of a fraction of particles from the trapped antiproton reservoir, followed by the injection of the extracted fraction into a receiver experiment. Although we do not have such a receiver available yet, we have demonstrated particle separation and extraction after returning to the experiment zone. This is shown in Fig. 4, which shows the number of protons as a function of a separation cycle executed in a similar way as in ref. ³¹. We split the proton cloud by voltage ramps and move the separated particles into the separation position (Fig. 1a) of the trap stack. Then, we eject them from the trap to mimic the transfer procedure. This protocol mitigates the risk of losing the entire reservoir during the transfer into a receiving precision-trap system, which is essential for the offline operation of antiproton experiments. Although the number of stored particles was not pushed to the limit, the transported number would already be enough for our high-precision experiments to operate for several years³². As an example, the non-destructive high-precision measurements performed in BASE at CERN typically consume around six antiprotons per year.

Separation measurement to demonstrate that we can separate small fractions down to single protons from the stored cloud for the ejection procedure. The vertical lines indicate 1σ measurement uncertainties.

In summary, we have shown all key techniques to transport trapped antiprotons in the open Penning-trap system BASE-STEP to dedicated precision laboratories. Laboratories reachable within the 4-h autonomous operation time of BASE-STEP, provided by the LHe tank and the UPS units, can be supplied with the methods presented here. For long-distance transports, the compressor and a chiller needed for cooling the compressor will also be moved on the truck. This equipment requires 15 kW of power compared with the 150 W required to run the trap system. Therefore, this makes an onboard generator¹⁰ necessary as well. However, this setup ultimately enables the system to be continuously operated on roads across Europe without time restrictions.

Once the antiproton transport has been demonstrated, we are planning to deliver the particles to an offline state-of-the-art Penning-trap system now under construction at the Heinrich Heine University Düsseldorf, Germany, in which proton–antiproton CPT tests^33,34 with an at least 100-fold improved precision will be performed. After the successful demonstration, parallelized measurements in multiple laboratories^35,36,37 might become possible in the long term. Furthermore, an expansion of the antiproton offline program will require multiplication of transportable traps and a reduction of costs per device. Our transport demonstration at 136 mT implicitly shows the feasibility of particle injection after catching into transportable permanent-magnet traps, as developed in our laboratories at present. Established concepts for these devices³⁸ will be adapted to the extremely high vacuum conditions for antimatter storage and transportation. This concept will also eliminate the operational risks of quenching the superconducting magnet and will enable a cost-efficient multiplication of transportable traps. Applying this technology to charged antimatter systems that will become available in the future—such as the antihydrogen ion \({\bar{{\rm{H}}}}^{+}\) (ref. ³⁹) and the antihydrogen molecular ion \({\bar{{\rm{H}}}}_{2}^{-}\) (ref. ⁴⁰)—will ensure that the predicted measurement precision by leveraging advanced experimental concepts⁴¹ is unaffected by the accelerator-induced magnetic field ramps. Thus, the developments demonstrated in this paper have the potential to pave the way towards a new era of precision physics with charged antimatter systems.