Laser-induced charging of microfabricated ion traps

Abstract

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 $\mu$m above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 10$^{14}$ photons/cm$^2$/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.Comment: 8 pages, 8 figure

Technologies for trapped-ion quantum information systems

“…The strongest charging effect was measured following continuous illumination with a 405-nm laser (with power about 200 µW, focused to a 50 µm spot size) grazing the trap for 10 min, after which the induced electric field at the ion location was inferred to be 20 V/m. This field strength is comparable to that seen in metal traps without dielectric mirrors [80], suggesting that the close presence of dielectric mirrors would not significantly affect trapping in planar trap configurations. Basic functionality of the embedded mirror was confirmed in situ through observations of the ion and its mirror image at different focal points of the imaging system (see Fig.…”

“…But there are challenges to be overcome in putting a fiber into an ion trap: Transporting light efficiently to or from an ion requires accurate spatial overlap of the ion's optical cross section with the field mode of the fiber. Additionally, dielectric materials comprising the fiber may result in charging near the fiber tip (particularly as lasers pass through) which distorts the trapping potential [79,80]. Van Devender et al [86] were first to demonstrate an embedded optical fiber; they collected fluorescence from an ion trapped above the surface through an integrated multi-mode fiber.…”

“…3). However, short cavities, which place the ion very close to the mirrors, have been avoided because of concerns about charge buildup on the dielectric mirror surface [79,80]. This can be circumvented via collective coupling of multiple ions to a longer cavity [91], at the cost of greater experimental complexity (see Sect.…”

“…However, using ITO electrodes in an ion trap poses some difficulty because ITO has a resistivity about 1000 times higher than metals typically used for trap electrodes (at room temperature), and ITO is an oxide, making laser-induced charging of its surface a concern [79,80]. To construct a workable transparent ion trap, Eltony et al [44] sputtered 400 nm of ITO for all trap electrodes and then added 5 nm of evaporated gold on the RF electrodes to enhance their conductivity nearly 1000-fold while still maintaining about 60 % optical transparency at the fluorescence wavelength.…”

“…Integrated optics are not easily made reconfigurable, so their alignment to the ions must be partially built into the trapping architecture. The second problem is that dielectric surfaces are susceptible to light-induced charging, which results in strong and difficult-to-control forces on the ions, inducing micromotion, large displacements, or even making the ions untrappable [79][80][81]. Nonetheless, these challenges have started to be addressed in the last several years by a number of groups integrating various optical elements with ion traps, including microfabricated phase Fresnel lenses [82,83], embedded micromirrors [84,85] and fibers [45,86], transparent trap electrodes [44], nanophotonic dielectric waveguides [87], macroscopic optical cavities [46,[88][89][90][91], and microscopic, fiber-based cavities [92].…”