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Physical Review Applied

Method for Estimating Locations of Laser-Induced Stray Charges on Surface-Electrode Ion Traps Using Secular Frequency Shift at Multiple Ion Positions

Changhyun Jung, Junho Jeong, Seungwoo Yoo, Taehyun Kim, Dong‐il Cho

Published Jul 17, 2023• 2 citations
Publisher

Abstract

This paper develops a method for estimating the locations of laser-induced stray charges on surface-electrode ion traps. How the electric potential at the trapped-ion location changes in the presence of laser-induced stray charges is theoretically modeled, and the magnitude of the shift in the secular frequency of a trapped ion is derived from the model. This model can be used inversely to find the locations of stray charges from the secular frequency shift under certain conditions. Typically, this inverse relationship is not unique. However, in surface-electrode ion traps, the secular frequency shift caused by the stray charges can be measured at multiple ion positions, and therefore, the required number of secular-frequency-shift measurements can be performed to determine the stray-charge locations uniquely. This model is experimentally evaluated by intentionally producing electric charges at a single location by irradiating a laser onto that spot and by measuring the resulting secular frequency shift. The location of produced charges is estimated from the measured result, and the estimated location is consistent with where the laser is irradiating. The developed method can be applied to characterize the laser-induced stray charges, which in turn can be used to design experiments to avoid or reduce the occurrence of stray charges.

DOI
10.1103/physrevapplied.20.014032

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This paper develops a method for estimating the locations of laser-induced stray charges on surface-electrode ion traps. How the electric potential at the trapped-ion location changes in the presence of laser-induced stray charges is theoretically modeled, and the magnitude of the shift in the secular frequency of a trapped ion is derived from the model. This model can be used inversely to find the locations of stray charges from the secular frequency shift under certain conditions. Typically, this inverse relationship is not unique. However, in surface-electrode ion traps, the secular frequency shift caused by the stray charges can be measured at multiple ion positions, and therefore, the required number of secular-frequency-shift measurements can be performed to determine the stray-charge locations uniquely. This model is experimentally evaluated by intentionally producing electric charges at a single location by irradiating a laser onto that spot and by measuring the resulting secular frequency shift. The location of produced charges is estimated from the measured result, and the estimated location is consistent with where the laser is irradiating. The developed method can be applied to characterize the laser-induced stray charges, which in turn can be used to design experiments to avoid or reduce the occurrence of stray charges.
The stray charges induced on ion trap surfaces can push trapped ions away from a pseudopotential null point. This can cause the micromotion of the trapped ions that in turn results in ion heating. One of the causes of stray charges is laser scattering on dielectric surfaces. The amount of laser-induced stray charges can be related to the area of the dielectric surface exposed to the ions. In this paper, stray charges are intentionally created at various dielectric surfaces on a microfabricated ion trap chip using a laser, and the resulting ion motion is measured. The ion movements are estimated by comparing ion positions before and after laser irradiation. When the laser irradiates a dielectric surface with an area of approximately 1,750 μm2exposed to the ion, the ion moves approximately −4.8 μm in the axial direction. When the laser irradiates another dielectric surface with an area of approximately 140 μm2exposed to the ion, the ion moves approximately −0.6 μm. The ratio between the ion movements corresponds to the ratio between the areas of each dielectric surface directly exposed to the ion normalized to the distance between the ion and the laser-induced stray charges. The results indicate the importance of reducing the area of dielectric surfaces that are directly in the line of laser paths.
The accurate characterization of the spatial electric field generated by electrodes in a surface electrode trap is of paramount importance. In this pursuit, we have identified a simple yet highly precise parametric expression to describe the spatial field of a rectangular-shaped electrode. Leveraging this expression, we introduced an optimization method designed to accurately characterize the axial electric field intensity produced by the powered electrode and the stray field. Distinct from the existing methods, our approach integrates a diverse array of experimental data, including the equilibrium positions of ions in a linear string, the equilibrium positions of single trapped ions, and trap frequencies, to effectively reduce the systematic errors. This approach provides considerable flexibility in voltage settings for data acquisition, making it especially advantageous for surface electrode traps where the trapping height of ion probes may vary with casual voltage settings. In our experimental demonstration, we successfully minimized the discrepancy between observations and model predictions to a remarkable degree. The relative errors of secular frequencies were contained within ±0.5%, and the positional error of ions was constrained to less than 1.2 μm, which surpasses the performance of current methodologies.
Accumulated electrostatic charges on the dielectric surfaces of ion traps are known to induce stray fields, leading to ion micromotions. In typical microfabricated ion-trap chips, metal electrodes are electrically isolated using thick dielectric pillars, which can accumulate stray charges on their sidewalls. This letter presents a new microfabrication method for ion-trap chips that reduces the exposure of dielectric surfaces to trapped ions. The dielectric pillars are fabricated with large T-shaped overhangs, and the sidewalls and top surfaces are coated with AlCu (1%) films. The bottom sides of the overhang parts provide electrical isolation. To prevent oxidation of the AlCu (1%) films, the electrode surfaces are coated with an additional Au film. The fabricated chips were implemented to trap174Yb+ions, and the laser-induced stray fields were measured. The results indicated that the trap chip fabricated by the newly developed method generates significantly smaller stray fields as compared with previous chips. [2017-0233]
The accurate characterization of the spatial potential generated by a planar electrode in a surface-type Paul trap is of great interest. To achieve this, we employ a simple yet highly precise parametric expression to describe the spatial field of a rectangular-shaped electrode. Based on this, an optimization method is introduced to precisely characterize the axial electric field intensity created by the powered electrode and the stray field. In contrast to existing methods, various types of experimental data, such as the equilibrium position of ions in a linear string, equilibrium positions of single trapped ions and trap frequencies, are utilized for potential estimation in order to mitigate systematic errors. This approach offers significant flexibility in voltage settings for data collection, making it particularly well-suited for surface electrode traps where ion probe trapping height may vary with casual voltage settings. In our demonstration, we successfully minimized the discrepancy between experimental observations and model predictions to an impressive extent. The relative errors of secular frequencies were suppressed within ±\pm± 0.5%\%%, and the positional error of ions was limited to less than 1.2 μμμm, all surpassing those achieved by existing methodologies.
Electrical charging of metal surfaces due to photoelectric generation of carriers is of concern in trapped ion quantum computation systems, due to the high sensitivity of the ions’ motional quantum states to deformation of the trapping potential. The charging induced by typical laser frequencies involved in Doppler cooling and quantum control is studied here, with microfabricated surface-electrode traps made of aluminum, copper, and gold, operated at 6 K with a single Sr+ ion trapped 100 μm above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 1014 photons/cm2/sec. Charging is detected by monitoring the ion’s micromotion signal, which is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: Lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate-equation approach.
Precise control of charged particles in radio-frequency (Paul) traps requires minimising excess micromotion induced by stray electric fields. We present a method to detect and compensate such fields through amplitude modulation of the radio-frequency trapping field. Modulation at frequencies close to the motional modes of the trapped particle excites coherent motion whose amplitude linearly depends on the stray field. In trapped-ion experiments, this motion can be detected by recording the arrival times of photons scattered during laser cooling. Only a single laser beam is required to resolve fields in multiple directions. In a demonstration using a 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ ion in a surface electrode trap, we achieve a sensitivity of 0.1 V m−1 / Hz0.1\, \mathrm{V}\, \mathrm{m}^{-1}\, /\, \sqrt{\mathrm{Hz}}0.1Vm−1/Hz​ and a minimal uncertainty of 0.015 V m−10.015\, \mathrm{V}\, \mathrm{m}^{-1}0.015Vm−1.