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Development of a Pulse-Tube-Cooled, Cryogenic, Permanent Magnet Trap for Laser Cooling at μTEx

P. Justus

Published Aug 1, 2025• 0 citations
OpenAlex

Abstract

A direct, high-precision measurement of the 3He2+ nuclear magnetic moment in a Penning trap will provide the most accurate calibration of 3He probes for nuclear magnetic resonance. The magnetic moment measurement at μTEx is carried out using the so-called continuous Stern-Gerlach effect, in which the spin state of the trapped ion is determined via axial frequency shifts in an inhomogeneous magnetic field. However, its mass and small magnetic moment lead to difficulties in determining the spin state, as the axial frequency shifts are on the order of background frequency fluctuations. These fluctuations can be reduced by lowering the energy of the trapped ion. Since traditional resistive cooling is not sufficient, and a bare helium nucleus cannot be laser cooled directly to reduce its energy, sympathetic cooling of 3He2+ is a well-suited technique. It is achieved by coupling the nucleus to a cloud of Doppler-cooled 9Be+ ions - produced by laser ablation from a solid target - in a coupling trap. To characterise laser cooling of 9Be+ at its 2S1/2 →2 P3/2 transition and to test the available coupling trap, a new cryogenic Penning-trap experiment at μTEx is currently being developed. The experiment uses a pair of Penning traps coupled together over their endcaps forming the coupling trap for sympathetic laser cooling of 3He2+ with 9Be+. The setup, developed and characterized within this thesis, utilises a pulse-tube cooler to reach cryogenic temperatures of 4 K and a permanent magnet capable of generating a magnetic field of 240 mT, with the B2 parameter between ±2 T/m2 over a distance of 5 mm, and 0.75 T/m2 at the trap centre. The choice of a pulse tube cooler and permanent magnet makes the experiment capable of supporting frequent cooling cycles and quick changes. Initially, the experiment is supposed to test the coupling traps and to characterise laser cooling of 9Be+ ions, and in future, offer an adjustable platform for further testing of new components related to high-precision experiments in Penning traps.

Related papers

Thermometry of 9Be+ ions in a cryogenic Penning trap

J. Mielke

High precision comparisons of the proton and antiproton g-factor enable stringent tests of CPT symmetry in the baryonic sector. However, current measurement routines suffer from long preparation times and high particle temperatures, which complicate the increase of the precision beyond the parts per billion regime. As a solution, new sympathetic cooling schemes based on co-trapped laser-cooled ions may be used to cool (anti-)protons to the ground state of motion and to significantly decrease particle preparation times. In addition, motional coupling to an auxiliary ion in a double-well potential can be used for the implementation of new readout schemes based on quantum logic spectroscopy for the spin-state of the (anti-)protons. This thesis describes a cryogenic Penning trap system for ground state cooling of single 9Be+ ions, which represents the latest development of the BASE experiment in Hannover towards the implementation of sympathetic cooling schemes and quantum logic spectroscopy of single (anti-)protons. Furthermore, the setup and implementation of lasers systems at 313nm for Doppler cooling, repumping and Raman interactions are described. To this end, an optical platform for laser beam delivery to the vacuum system and an optical phase-locked loop for the stabilization of the relative phase of two lasers at 313 nm are presented. In this work, the laser systems are used for measurements of the optical resonance frequencies for cooling, probing and repumping of 9Be+ ions. In addition, stimulated Raman transitions are demonstrated using two phase-locked lasers at 313nm. In the following, this technique is used for high-precision measurements of the qubit frequency and the magnetic field, as well as themometry measurements with 9Be+ ions. The temperature of the ions after Doppler cooling is measured as a function of the cooling laser parameters and a minimal axial mode temperature of 1.77(10) mK is obtained. The results of this thesis pave the way for sideband spectroscopy and ground state cooling of single 9Be+ ions in Penning traps, which is an essential prerequisite for sympathetic ground state cooling of single (anti-)protons and the application of quantum logic spectroscopy to (anti-)proton g-factor measurements in the future.

•Jan 1, 2021•0 citations

Characterisation of an experiment for sympathetic cooling and coupling of ions in a cryogenic Penning trap

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The comparison of the g-factors of the proton and the antiproton is a stringent test of CPT invariance. The state-of-the-art method for the determination of the (anti-)proton's g-factor is the application of the continuous Stern-Gerlach effect. With this method, precisions in the low parts per billion regime could be achieved. However, preparation times for cold enough particles are on the order of minutes to hours. The implementation of sympathetic cooling using a co-trapped atomic ion could lead to preparation times in the milliseconds regime. Furthermore, the applicability of sympathetic cooling would enable an alternative readout scheme using quantum logic methods. This thesis presents a Penning trap system that is designed for ground state cooling, adiabatic transport, and motional coupling of single 9Be+ ions. It has been built in a modular manner, giving the option of adapting it for motional coupling and sympathetic cooling of single (anti-)protons. In the course of this work, the prerequisites for motional coupling of 9Be+ ions have been fulfilled: A cloud of 9Be+ ions was cooled to a temperature of 1.7~mK applying Doppler cooling. The achieved temperature is about three times the Doppler limit which is 0.5~mK for beryllium ions. Furthermore, a scheme for reproducible loading of the trap and reducing the particle number was developed. Finally, first transport in our apparatus was demonstrated with clouds of beryllium ions. The method used for that is directly applicable to single ions. The results of this work pave the way towards adiabatic transport of single 9Be+ ions, which is crucial for sympathetic cooling of (anti-)protons in a double-well potential as well as for implementing quantum logic spectroscopy with single (anti-)protons.

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arXiv:2107.08433•CPT and Lorentz Symmetry•Apr 7, 2020•0 citations

Investigation of a Novel Method for Sympathetic Laser Cooling of Highly Charged Ions in a Penning Trap

F.J.W. Hahne

The Penning trap experiment Alphatrap tests quantum electrodynamics via g- factor measurements of bound electrons in highly charged ions (HCI). The precision of the measurements can be improved by cooling the HCI. This thesis describes a novel and versatile method to sympathetically cool HCIs with laser-cooled beryllium ions efficiently in high-precision Penning traps. The method is based on increasing the coupling of two ions stored in separate penning traps by adding an electrical tank circuit. In a proof of principle measurement this was demonstrated using a 84Kr23+ and 40Ar11+ ion: first, we saw an avoided crossing of the ion modes, and second, we showed a Rabi oscillation in the energy transfer. Our coupling technique enables active control of the coupling strength between the ions, so that a coupling time of τ = 0:856(16) s was achieved. We see this as a possible tool to efficiently cool a wide range of ion species. In addition, an active beam stabilization system was installed to reduce fluctuations of the cooling laser beam position and orientation, and to improve the alignment of the laser with respect to the beryllium ions. This system was calibrated and geometric losses of the laser transmission through our Penning trap setup were estimated.

•Jan 1, 2020•0 citations
A direct, high-precision measurement of the 3He2+ nuclear magnetic moment in a Penning trap will provide the most accurate calibration of 3He probes for nuclear magnetic resonance. The magnetic moment measurement at μTEx is carried out using the so-called continuous Stern-Gerlach effect, in which the spin state of the trapped ion is determined via axial frequency shifts in an inhomogeneous magnetic field. However, its mass and small magnetic moment lead to difficulties in determining the spin state, as the axial frequency shifts are on the order of background frequency fluctuations. These fluctuations can be reduced by lowering the energy of the trapped ion. Since traditional resistive cooling is not sufficient, and a bare helium nucleus cannot be laser cooled directly to reduce its energy, sympathetic cooling of 3He2+ is a well-suited technique. It is achieved by coupling the nucleus to a cloud of Doppler-cooled 9Be+ ions - produced by laser ablation from a solid target - in a coupling trap. To characterise laser cooling of 9Be+ at its 2S1/2 →2 P3/2 transition and to test the available coupling trap, a new cryogenic Penning-trap experiment at μTEx is currently being developed. The experiment uses a pair of Penning traps coupled together over their endcaps forming the coupling trap for sympathetic laser cooling of 3He2+ with 9Be+. The setup, developed and characterized within this thesis, utilises a pulse-tube cooler to reach cryogenic temperatures of 4 K and a permanent magnet capable of generating a magnetic field of 240 mT, with the B2 parameter between ±2 T/m2 over a distance of 5 mm, and 0.75 T/m2 at the trap centre. The choice of a pulse tube cooler and permanent magnet makes the experiment capable of supporting frequent cooling cycles and quick changes. Initially, the experiment is supposed to test the coupling traps and to characterise laser cooling of 9Be+ ions, and in future, offer an adjustable platform for further testing of new components related to high-precision experiments in Penning traps.
High precision comparisons of the proton and antiproton g-factor enable stringent tests of CPT symmetry in the baryonic sector. However, current measurement routines suffer from long preparation times and high particle temperatures, which complicate the increase of the precision beyond the parts per billion regime. As a solution, new sympathetic cooling schemes based on co-trapped laser-cooled ions may be used to cool (anti-)protons to the ground state of motion and to significantly decrease particle preparation times. In addition, motional coupling to an auxiliary ion in a double-well potential can be used for the implementation of new readout schemes based on quantum logic spectroscopy for the spin-state of the (anti-)protons. This thesis describes a cryogenic Penning trap system for ground state cooling of single 9Be+ ions, which represents the latest development of the BASE experiment in Hannover towards the implementation of sympathetic cooling schemes and quantum logic spectroscopy of single (anti-)protons. Furthermore, the setup and implementation of lasers systems at 313nm for Doppler cooling, repumping and Raman interactions are described. To this end, an optical platform for laser beam delivery to the vacuum system and an optical phase-locked loop for the stabilization of the relative phase of two lasers at 313 nm are presented. In this work, the laser systems are used for measurements of the optical resonance frequencies for cooling, probing and repumping of 9Be+ ions. In addition, stimulated Raman transitions are demonstrated using two phase-locked lasers at 313nm. In the following, this technique is used for high-precision measurements of the qubit frequency and the magnetic field, as well as themometry measurements with 9Be+ ions. The temperature of the ions after Doppler cooling is measured as a function of the cooling laser parameters and a minimal axial mode temperature of 1.77(10) mK is obtained. The results of this thesis pave the way for sideband spectroscopy and ground state cooling of single 9Be+ ions in Penning traps, which is an essential prerequisite for sympathetic ground state cooling of single (anti-)protons and the application of quantum logic spectroscopy to (anti-)proton g-factor measurements in the future.
The comparison of the g-factors of the proton and the antiproton is a stringent test of CPT invariance. The state-of-the-art method for the determination of the (anti-)proton's g-factor is the application of the continuous Stern-Gerlach effect. With this method, precisions in the low parts per billion regime could be achieved. However, preparation times for cold enough particles are on the order of minutes to hours. The implementation of sympathetic cooling using a co-trapped atomic ion could lead to preparation times in the milliseconds regime. Furthermore, the applicability of sympathetic cooling would enable an alternative readout scheme using quantum logic methods. This thesis presents a Penning trap system that is designed for ground state cooling, adiabatic transport, and motional coupling of single 9Be+ ions. It has been built in a modular manner, giving the option of adapting it for motional coupling and sympathetic cooling of single (anti-)protons. In the course of this work, the prerequisites for motional coupling of 9Be+ ions have been fulfilled: A cloud of 9Be+ ions was cooled to a temperature of 1.7~mK applying Doppler cooling. The achieved temperature is about three times the Doppler limit which is 0.5~mK for beryllium ions. Furthermore, a scheme for reproducible loading of the trap and reducing the particle number was developed. Finally, first transport in our apparatus was demonstrated with clouds of beryllium ions. The method used for that is directly applicable to single ions. The results of this work pave the way towards adiabatic transport of single 9Be+ ions, which is crucial for sympathetic cooling of (anti-)protons in a double-well potential as well as for implementing quantum logic spectroscopy with single (anti-)protons.
Abstract Cooling and detection schemes using laser cooling and methods of quantum logic can contribute to high precision CPT symmetry tests in the baryonic sector. This work introduces an experiment to sympathetically cool protons and antiprotons using the Coulomb interaction with a ion trapped in a nearby but separate potential well. We have designed and set up an apparatus to show such coupling between two identical ions for the first time in a Penning trap. In this paper, we present evidence for successful loading and Doppler cooling of clouds and single ions. Our coupling scheme has applications in a range of high-precision measurements in Penning traps and has the potential to substantially improve motional control in these experiments.
Current precision experiments with single (anti)protons to test CPT symmetry progress at a rapid pace, but are complicated by the need to cool particles to sub-thermal energies. We describe a cryogenic Penning-trap setup for 9^99Be+^++ ions designed to allow coupling of single (anti)protons to laser-cooled atomic ions for sympathetic cooling and quantum logic spectroscopy. We report on trapping and laser cooling of clouds and single 9^99Be+^++ ions. We discuss prospects for a microfabricated trap to allow coupling of single (anti)protons to laser-cooled 9^99Be+^++ ions for sympathetic laser cooling to sub-mK temperatures on ms time scales.
The Penning trap experiment Alphatrap tests quantum electrodynamics via g- factor measurements of bound electrons in highly charged ions (HCI). The precision of the measurements can be improved by cooling the HCI. This thesis describes a novel and versatile method to sympathetically cool HCIs with laser-cooled beryllium ions efficiently in high-precision Penning traps. The method is based on increasing the coupling of two ions stored in separate penning traps by adding an electrical tank circuit. In a proof of principle measurement this was demonstrated using a 84Kr23+ and 40Ar11+ ion: first, we saw an avoided crossing of the ion modes, and second, we showed a Rabi oscillation in the energy transfer. Our coupling technique enables active control of the coupling strength between the ions, so that a coupling time of τ = 0:856(16) s was achieved. We see this as a possible tool to efficiently cool a wide range of ion species. In addition, an active beam stabilization system was installed to reduce fluctuations of the cooling laser beam position and orientation, and to improve the alignment of the laser with respect to the beryllium ions. This system was calibrated and geometric losses of the laser transmission through our Penning trap setup were estimated.