Reduction of trapped-ion anomalous heating by in situ surface plasma cleaning

Anomalous motional heating is a major obstacle to scalable quantum information processing with trapped ions. Although the source of this heating is not yet understood, several previous studies suggest that noise due to surface contaminants is the limiting heating mechanism in some instances. We demonstrate an improvement by a factor of 4 in the room-temperature heating rate of a niobium surface electrode trap by in situ plasma cleaning of the trap surface. This surface treatment was performed with a simple homebuilt coil assembly and commercially available matching network and is considerably gentler than other treatments, such as ion milling or laser cleaning, that have previously been shown to improve ion heating rates. We do not see an improvement in the heating rate when the trap is operated at cryogenic temperatures, pointing to a role of thermally activated surface contaminants in motional heating whose activity may freeze out at low temperatures.

Figure 1

Simplified schematic of the apparatus used for plasma cleaning studies. A surface-electrode ion trap on a temperature-controllable stage is enclosed within a 50-K radiation shield inside of a larger vacuum enclosure. A mixture of Ar, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$ gases with a total pressure of 700–800 mTorr is introduced into the system via the gas inlet, which can be valved off when the plasma system is not in use. The plasma is generated by rf power applied to a coil located near the ion trap. After plasma cleaning, a retractable linear stage allows the coil to be moved outside of the 50-K shield. Omitted from this simple schematic are many optical access ports as well as the source of neutral ${}^{88}\mathrm{Sr}$ atoms. Figure not to scale.

Figure 2

Ion heating rate in Trap A before (open red squares) and after (filled black circles) 20-min plasma cleaning as a function of trap frequency at trap chip temperatures of (a) $T=295\mathrm{K}$ and (b) $T=4\mathrm{K}$.

Figure 3

Time course of the Trap-A heating rate (for chip temperature of 295 K and trap frequency of 1.3 MHz). Vertical dashed lines show plasma cleanings (I and III for 20 min and IV for 75 min). The gray area (II) indicates the 72-h exposure to air to allow surface contaminants to re-adsorb to the trap.

Figure 4

Ion heating rate in Trap B before (open red squares) and after (filled black circles) 75-min plasma cleaning as a function of trap frequency at trap chip temperatures of (a) $T=295\mathrm{K}$ and (b) $T=4\mathrm{K}$.