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Communications Physics

Two-dimensional cooling without repump laser beams through ion motional heating

Yue Xiao, Yongxu Peng, Linfeng Chen, Chunhui Li, Zongao Song, Xin Wang, Tao Wang, Yurun Xie, Bin Zhao, Tiangang Yang

Published Dec 27, 2024• 4 citations
Publisher

Abstract

Laser cooling typically requires one or more repump lasers to clear dark states, which complicates experimental setups, especially for systems with multiple repumping frequencies. Here, we demonstrate cooling of Be+ ions using a single laser beam, enabled by micromotion-induced one-dimensional heating. By manipulating the displacement of Be+ ions from the trap’s nodal line, we precisely control the ion micromotion velocity, eliminating the necessity of a 1.25 GHz offset repump laser while keeping ions cold in the direction perpendicular to the micromotion. We use two equivalent schemes, cooling laser detuning and ion trajectory imaging to measure the speed of the Be+ ions, with results accurately reproduced by molecular dynamics simulations based on a machine learned time-dependent electric field inside the trap. This work provides a robust method to control micromotion velocity of ions and demonstrates the potential of micromotion-assisted laser cooling to simplify setups for systems requiring multiple repumping frequencies. Reducing the number of lasers in laser cooling experiments is beneficial for simplifying systems requiring multiple repumping frequencies. This work demonstrates micromotion-assisted cooling of Be+ ions with a single laser, eliminating the need for a 1.25 GHz offset repump laser, with results rigorously validated through molecular dynamics simulations.

DOI
10.1038/s42005-024-01920-2

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Laser cooling typically requires one or more repump lasers to clear dark states, which complicates experimental setups, especially for systems with multiple repumping frequencies. Here, we demonstrate cooling of Be+ ions using a single laser beam, enabled by micromotion-induced one-dimensional heating. By manipulating the displacement of Be+ ions from the trap’s nodal line, we precisely control the ion micromotion velocity, eliminating the necessity of a 1.25 GHz offset repump laser while keeping ions cold in the direction perpendicular to the micromotion. We use two equivalent schemes, cooling laser detuning and ion trajectory imaging to measure the speed of the Be+ ions, with results accurately reproduced by molecular dynamics simulations based on a machine learned time-dependent electric field inside the trap. This work provides a robust method to control micromotion velocity of ions and demonstrates the potential of micromotion-assisted laser cooling to simplify setups for systems requiring multiple repumping frequencies. Reducing the number of lasers in laser cooling experiments is beneficial for simplifying systems requiring multiple repumping frequencies. This work demonstrates micromotion-assisted cooling of Be+ ions with a single laser, eliminating the need for a 1.25 GHz offset repump laser, with results rigorously validated through molecular dynamics simulations.
Laser cooling typically requires one or more repump lasers to clear dark states and enable recycling transitions. Here, we have achieved cooling of Be+ ions using a single laser beam, facilitated by one-dimensional heating through micromotion. By manipulating the displacement from the trap's nodal line, we precisely controlled the ion micromotion direction and speed, reaching up to 3144 m/s, which corresponds to a 7.1 GHz Doppler frequency shift in our experiment. This approach eliminates the necessity of a 1.25 GHz offset repump laser while keeping the Be+ ions cold in the perpendicular direction. Measurements were taken using cooling laser detuning and imaging of ion trajectories. Molecular dynamics simulations, based on machine learned time-dependent electric field E(X, Y, Z, t) inside the trap, accurately reproduced the experimental observation, illuminating the relationship between the direction of micromotion and the trapping electric filed vector. This work not only provides a robust method for managing the micromotion velocity of ions but also sheds light on laser cooling complex systems that require multiple repumping lasers. Additionally, it offers a method for controlling energy in the context of ion-molecule collision investigations.
Single, rf-trapped ions find various applications ranging from metrology to quantum computation. High-resolution interrogation of an extremely weak transition under best observation conditions requires an ion almost at rest. To avoid line-broadening effects such as the second order Doppler effect or rf heating in the absence of laser cooling, excess micromotion has to be eliminated as far as possible. We have tested a novel method to probe the motional state of a confined three-level ion. Actually, dark resonances observed with two counterpropagating laser beams are very sensitive to the trapped ion's velocity. Their contrast increases with reduced micromotion and allows in this way to control the oscillation amplitude of the stored ion. The influence of different parameters such as laser linewidth and power has been investigated experimentally and numerically.
In this paper, direct observation of micromotion for multiple ions in a laser-cooled trapped ion crystal is discussed along with a measurement technique for micromotion amplitude. Micromotion is directly observed using a time-resolving, single-photon-sensitive camera that provides both fluorescence and position data for each ion on the nanosecond time scale. Micromotion amplitude and phase for each ion in the crystal are measured, allowing this method to be sensitive to tilts and shifts of the ion chain from the null of the radio-frequency quadrupole potential in the linear trap. Spatial resolution makes this micromotion detection technique suitable for complex ion configurations, including two-dimensional geometries. It does not require any additional equipment or laser beams, and the modulation of the cooling lasers or trap voltages is not necessary for detection as it is in other methods.
Abstract We implement three-dimensional polarization gradient cooling (PGC) of trapped ions. Counter-propagating laser beams near 393 nm impinge in lin ⊥ lin configuration, at a frequency below the S 1/2 to P 3/2 resonance in 40 Ca + . Our measurements demonstrate that cooling with laser beams detuned to lower frequencies from the resonance is robust against an elevated phonon occupation number and works continuously in the crossover from regular Doppler cooling to detunings of tens of linewidths. It is thus robust against heating events and also works well for an initial ion motion far out of the Lamb–Dicke regime. We show that PGC performance strongly depends on residual micromotion (MM) and find PGC working for a MM modulation index β ⩽ 0.1. Still, we find that the spectral impurity of the laser field affects both, cooling rates and cooling limits. Thus, a Fabry–Pérot cavity filter is employed to efficiently suppress amplified spontaneous emission of the diode laser. We demonstrate mean phonon numbers for a single ion of 5.4(4) at a trap frequency of 2 π × 285 kHz and 3.3(4) at 2 π × 480 kHz, in the axial and radial directions, respectively.
Abstract Reaching ultracold temperatures within hybrid atom–ion systems is a major limiting factor for control and exploration of the atom–ion interaction in the quantum regime. In this work, we present results on numerical simulations of trapped ion buffer gas cooling using an ultracold atomic gas in a large number of experimentally realistic scenarios. We explore the suppression of micromotion-induced heating effects through optimization of trap parameters for various radio-frequency (rf) traps and rf driving schemes including linear and octupole traps, digital Paul traps, rotating traps and hybrid optical/rf traps. We find that very similar ion energies can be reached in all of them even when considering experimental imperfections that cause so-called excess micromotion. Moreover we look into a quantum description of the system and show that quantum mechanics cannot save the ion from micromotion-induced heating in an atom–ion collision. The results suggest that buffer gas cooling can be used to reach close to the ion’s groundstate of motion and is even competitive when compared to some sub-Doppler cooling techniques such as Sisyphus cooling. Thus, buffer gas cooling is a viable alternative for ions that are not amenable to laser cooling, a result that may be of interest for studies into cold controlled quantum chemistry and charged impurity physics.
Laser cooling of a single trapped ion in a Paul trap is discussed theoretically in the Lamb-Dicke limit, with full consideration of the time dependence of the trapping potential. Resulting mean kinetic energies are defined as time averages over one period of the micromotion and are compared with final temperatures expected from the laser-cooling treatment with harmonic traps. For laser-atom detunings close to the micromotion frequency the results differ significantly from those expected for a harmonic trap potential. A physical interpretation is given and simple formulas are derived for the strong confinement case.