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arXiv:1708.03147 · Physical Review Letters

Measuring Anomalous Heating in a Planar Ion Trap with Variable Ion-Surface Separation

Ivan A. Boldin, Alexander Kraft, Christof Wunderlich

Published Jan 12, 2018• 60 citations
arXivHTMLPDF

Abstract

Cold ions trapped in the vicinity of conductive surfaces experience heating of their oscillatory motion. Typically, the rate of this heating is orders of magnitude larger than expected from electric field fluctuations due to thermal motion of electrons in the conductors. This effect, known as anomalous heating, is not fully understood. One of the open questions is the heating rate's dependence on the ion-electrode separation. We present a direct measurement of this dependence in an ion trap of simple planar geometry. The heating rates are determined by taking images of a single ^{172}Yb^{+} ion's resonance fluorescence after a variable heating time and deducing the trapped ion's temperature from measuring its average oscillation amplitude. Assuming a power law for the heating rate versus ion-surface separation dependence, an exponent of -3.79±0.12 is measured.

DOI
10.1103/physrevlett.120.023201

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Cold ions trapped in the vicinity of conductive surfaces experience heating of their oscillatory motion. Typically, the rate of this heating is orders of magnitude larger than expected from electric field fluctuations due to thermal motion of electrons in the conductors. This effect, known as anomalous heating, is not fully understood. One of the open questions is the heating rate's dependence on the ion-electrode separation. We present a direct measurement of this dependence in an ion trap of simple planar geometry. The heating rates are determined by taking images of a single ^{172}Yb^{+} ion's resonance fluorescence after a variable heating time and deducing the trapped ion's temperature from measuring its average oscillation amplitude. Assuming a power law for the heating rate versus ion-surface separation dependence, an exponent of -3.79±0.12 is measured.
Laser cleaning of the electrodes in a planar micro-fabricated ion trap has been attempted using ns pulses from a tripled Nd:YAG laser at 355 nm.The effect of the laser pulses at several energy density levels has been tested by measuring the heating rate of a single 40 Ca + trapped ion as a function of its secular frequency ω z .A reduction of the electric-field noise spectral density by ∼50% has been observed and a change in the frequency dependence also noticed.This is the first reported experiment where the 'anomalous heating' phenomenon has been reduced by removing the source as opposed to reducing its thermal driving by cryogenic cooling.This technique may open up the way to better control of the electrode surface quality in ion microtraps.The recent success of quantum information experiments based on trapped ions [1] triggered research on micro-fabricated radio-frequency (Paul) traps, which are in principle able to fulfill the scalability requirement of a quantum computer [2,3].In such traps, a set of micro-fabricated conducting electrodes generates oscillating and static electric fields that trap laser-cooled ions in a harmonic potential well at a sub-millimeter distance d from the substrate [4,5].However, the presence of uncontrolled fluctuating electric fields affects the ions' external motion and induces an 'anomalous heating' that limits the achievable fidelity of multi-ion quantum gates that rely on the coherent control of this motion [6].Experimental observations concerning this phenomenon 4
We present measurements of trapped-ion motional-state heating rates in niobium and gold surface-electrode ion traps over a range of trap-electrode temperatures from approximately 4 K to room temperature (295 K) in a single apparatus.Using the sideband-ratio technique after resolved-sideband cooling of single ions to the motional ground state, we find low-temperature heating rates more than two orders of magnitude below the room-temperature values and approximately equal to the lowest measured heating rates in similarly sized cryogenic traps. We find similar behavior in the two very different electrode materials, suggesting that the anomalous heating process is dominated by non-material-specific surface contaminants. Through precise control of the temperature of cryopumping surfaces, we also identify conditions under which elastic collisions with the background gas can lead to an apparent steady heating rate, despite rare collisions.
We measure and characterize anomalous motional heating of an atomic ion confined in the lowest quantum levels of a novel rf ion trap that features moveable electrodes. The scaling of heating with electrode proximity is measured, and when the electrodes are cooled from 300 to 150 K, the heating rate is suppressed by an order of magnitude. This provides direct evidence that anomalous motional heating of trapped ions stems from microscopic noisy potentials on the electrodes that are thermally driven. These observations are relevant to decoherence in quantum information processing schemes based on trapped ions and perhaps other charge-based quantum systems.
We model electric field noise from fluctuating patch potentials on conducting surfaces by taking into account the finite geometry of the ion trap electrodes to gain insight into the origin of anomalous heating in ion traps. The scaling of anomalous heating rates with surface distance ddd is obtained for several generic geometries of relevance to current ion trap designs, ranging from planar to spheroidal electrodes. The influence of patch size is studied both by solving Laplace's equation in terms of the appropriate Green's function as well as through an eigenfunction expansion. Scaling with surface distance is found to be highly dependent on the choice of geometry and the relative scale between the spatial extent of the electrode, the ion-electrode distance, and the patch size. Our model generally supports the ${d}^{\ensuremath{-}4}$ dependence currently found by most experiments and models, but also predicts geometry-driven deviations from this trend.
Electric field noise originating from metal surfaces is a hindrance for a variety of microengineered systems, including for ions in microtraps, but is not well understood at the microscopic level. For trapped ions, it is manifested as motional-state decoherence inexplicable by thermal noise of electrodes alone, but likely surface-dependent. Here, we investigate the role of surface properties in motional heating by creating an ion trap with a unique exterior. Using single trapped-ion probes, we characterize copper electrodes covered in monolayer graphene, a material free of surface charge and dangling bonds. Surprisingly, we measure an average heating rate of 1020 ± 30 quanta/s, which is ∼100 times higher than typical for an uncoated trap operated under similar conditions. This may be related to hydrocarbon deposits on the surface, which could be monitored on graphene to potentially elucidate the mechanisms of motional heating on the atomic scale.
Abstract. Laser-cleaning of the electrodes in a planar micro-fabricated ion trap has been attempted using ns pulses from a tripled Nd:YAG laser at 355nm. The effect of the laser pulses at several energy density levels has been tested by measuring the heating rate of a single 40Ca+ trapped ion as a function of its secular frequency ωz. A reduction of the electric-field noise spectral density by ∼ 50 % has been observed and a change in the frequency dependence also noticed. This is the first reported experiment where the “anomalous heating ” phenomenon has been reduced by removing the source as opposed to reducing its thermal driving by cryogenic cooling. This technique may open the way to better control of the electrode surface quality in ion microtraps. ar