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Review of Scientific Instruments

A fast-switching high-critical-temperature superconducting magnetic trap for atoms and molecules

Julia Narevicius, Yair Segev, Martin Pitzer, Edvardas Narevicius

Published Mar 1, 2026• 0 citations
Publisher

Abstract

We present a 1 T deep high-critical-temperature (high-Tc) superconducting magnetic trap with magnetic field transients reaching 2000 T/s, spatial gradients of ∼0.3 T/mm, and arbitrarily tunable time evolution of the trapping field intensity for durations up to several minutes. In addition, our trap driver electronic circuit allows independent operation of the two superconducting coils with only a single high-current power supply and two cryogenic leads. By implementing the possibility to change polarity of the current pulse in one of the trap coils, we enable a compact design that facilitates the loading sequence of a high-gradient magnetic quadrupole trap.

DOI
10.1063/5.0309316

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We present a 1 T deep high-critical-temperature (high-Tc) superconducting magnetic trap with magnetic field transients reaching 2000 T/s, spatial gradients of ∼0.3 T/mm, and arbitrarily tunable time evolution of the trapping field intensity for durations up to several minutes. In addition, our trap driver electronic circuit allows independent operation of the two superconducting coils with only a single high-current power supply and two cryogenic leads. By implementing the possibility to change polarity of the current pulse in one of the trap coils, we enable a compact design that facilitates the loading sequence of a high-gradient magnetic quadrupole trap.
Cold atom experiments utilize magnetic fields, commonly generated by coils, as an essential tool to control and manipulate atomic samples. In these experiments, it is often necessary to rapidly switch the magnetic field between two values. However, typical power supplies have a limited switching time for the current flowing through the coil. We present a control scheme implemented as an electronic circuit that overcomes this limitation. A faster control is achieved by momentarily applying an on-demand high voltage when the control signal variation surpasses the limits that the conventional power supply can follow, allowing a faster current flow into the magnetic coil. In our specific application, a coil with an inductance of 491 μH and a resistance of 0.26 Ω, corresponding to a time constant of ∼1.9 ms, is driven so that its current faithfully follows the control signals from -1 to 1 A. It is capable of completing a full-scale transition in just ∼31 μs, with an effective bandwidth of 15.2 kHz. This corresponds to an improvement by a factor of more than 20 in both bandwidth and switching speed over a conventional power supply. By appropriately selecting the components of the circuit, both the bandwidth and switching time can be tuned to match specific needs within a wide range of inductive and power requirements.
We describe the design, construction and performance of three realizations of a high-field superconducting magnetic trap for neutral atoms and molecules. Each of these traps utilizes a pair of coaxial coils in the anti-Helmholtz geometry and achieves depths greater than 4 T, allowing it to capture magnetic atoms and molecules cooled in a cryogenic buffer gas. Achieving this depth requires that the repulsive force between the coils (which can exceed 30 metric tons) be contained. We also describe additional features of the traps, including the elimination of trapped fluxes from the coils and the integration of the coils into a cryogenic vacuum environment suitable for producing cold atoms and molecules.
We have continuously stopped thermal sodium atoms with laser light and loaded them into a 0.1-K deep superconducting magnetic trap. The trap is formed by a local minimum of the magnetic field produced by two superconducting coils and a radially confining octupole field. The atoms are slowed and stopped in two stages: The atoms are first slowed to ~200 m/s using a counterpropagating resonant laser beam and tapered magnetic field to compensate for the changing Doppler shift as the atoms are slowed.1 A second laser beam and tapered magnetic field, actually located within the trap, are used to stop the atoms in the trap. This continuous loading process has allowed us to accumulate up to 1 × 109 trapped atoms, and trapping times of several minutes have been achieved. Our trap has the feature of having a uniform magnetic field at its bottom, opening up the possibility of precision spectroscopy of the trapped atoms. The fluorescence from the trapped atoms has been studied, and they have been cooled to ~1 mK using 1-D optical molasses.
The design and implementation of a current driver for the quadrupole coil of a magneto-optical trap are presented. The control and the power stages of the driver, both based on a push-pull configuration with high-speed transistors, are separated, allowing for a protection of the control electronics. Moreover, the coil current can be set by an analog voltage, a feature that makes possible both fast and adiabatic switching of the quadrupole magnetic field. In order to compensate magnetic field transients induced by eddy currents, the driver allows a quick reversal of the quadrupole coil current providing in such a way the required magnetic flux compensation. From a fit to the measured magnetic field transients, we have extracted the decay constants which confirm the efficiency of the compensation. Furthermore, we measured the influence of eddy currents on a sample of cold caesium atoms, testing at the same time the ultimate performance of the driver for our applications. The results show that the implemented electronic circuit is able to reduce substantially the transient effects with switching times of the magnetic field below 100 microseconds.
A combination of current coils and ferromagnetic pole shoes is used to construct a magnetic quadrupole trap for ultra cold neutral atoms. Field gradients of up to 3000 G/cm have been achieved continuously with an electric power consumption of only 10 watts. A maximum field gradient of 1 T/cm is calculated for a current that can be applied for several seconds. The trap has been loaded with 8 × 10 6 lithium atoms ( 7 Li) from a magneto-optical trap and the energy distribution determined by stepwise reduction of the trapping potential. Finally, magnetic trapping of fermionic 6 Li has been demonstrated.
We present a moving-trap Zeeman decelerator (MTZD) for use in molecular beam manipulation and magnetic-trapping experiments of paramagnetic atoms and molecules. The 0.49 m MTZD consists of a combination of a 2D magnetic quadrupole guide and deceleration coils, which together produce an array of 3D magnetic traps. The design is based on that of Trimeche et al. with significant modifications. The 2D quadrupole is driven by fast rising and falling square pulses of current (up to 700 A) of arbitrary lengths of time. The traps can be made to move at velocities from ca. 370 m/s down to zero, which is done by driving through the deceleration coils a sinusoidal current with frequencies ranging from zero to ca. 9 kHz and peak currents up to 1000 A. The trap velocity is proportional to the current frequency. The MTZD manipulates the velocities of molecular beams by traveling initially at the same velocity as the beam. 3D guiding is achieved with a constant current frequency and deceleration is achieved with a downward chirp of the current frequency at a rate corresponding to the desired deceleration. We explain the technical design and operation principles of the MTZD and we detail the custom power electronics that drive the 2D quadrupole guide and the decelerator coils. We present extensive Monte-Carlo trajectory simulations to demonstrate the properties of the MTZD and we conclude that decelerations should be kept below 30000 m/s/s to maintain a good 6D phase-space acceptance. In proof-of-principle experiments, with the deceleration coils operating with a reduced current in the range 100-200 A, we demonstrate the 3D guiding of a beam of metastable argon atoms at 373 m/s and deceleration at 25000 m/s/s from 342 m/s to 304 m/s. The deceleration corresponds to the removal of 21% of the kinetic energy of the beam.