[Paper Review] Insensitivity of Ion Motional Heating Rate to Trap Material over a Large Temperature Range

John Chiaverini, Jeremy Sage|arXiv (Cornell University)|Oct 16, 2013

Quantum Information and CryptographyComputer Science28 references76 citations

TL;DR

This study measures trapped-ion motional heating rates in niobium and gold surface-electrode ion traps across temperatures from 4 K to room temperature using sideband-ratio spectroscopy after ground-state cooling. It finds heating rates decrease by over two orders of magnitude from room temperature to 4 K, with no significant difference between materials, indicating that the dominant source of anomalous heating is non-material-specific surface contaminants rather than electrode composition.

ABSTRACT

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.

Motivation & Objective

• . To measure temperature-dependent ion motional heating rates in niobium and gold surface-electrode ion traps over a broad range (4 K to 300 K).
• To determine whether the anomalous heating process is material-dependent or governed by universal surface effects.
• To rule out background gas collisions as the source of apparent heating by characterizing elastic and inelastic collision rates.
• To identify conditions under which cryogenic background gas can mimic steady-state heating due to rare elastic collisions.
• To provide high-precision data across materials and temperatures to guide the development of materials or processing solutions that reduce heating and improve fault-tolerant quantum computing.

Proposed method

• . Uses a cryogenic ultra-high vacuum (UHV) apparatus with a vibration-isolated cryocooler to control trap-electrode temperature from 3.5 K to 300 K.
• Loads single 88Sr+ ions into segmented, surface-electrode Paul traps fabricated from sputtered niobium (2 µm) or thermally evaporated gold (500 nm) on sapphire.
• Performs pulsed resolved-sideband cooling to prepare the ion in the motional ground state (n=0) with >99% fidelity.
• Measures heating rates via the sideband-ratio technique: extracts red-to-blue sideband amplitude ratios to determine average phonon number ⟨n⟩ as a function of delay time.
• Applies low-pass RC and multi-stage filters to suppress RF and low-frequency electric field noise near the trap frequency.
• Controls background gas density via cryopumping and measures inelastic collision rates using the Langevin model to estimate ion loss rates.

Experimental results

Research questions

• RQ1 . Does the anomalous ion motional heating rate depend on the trap electrode material, such as niobium versus gold?
• RQ2 . How does the heating rate vary with temperature across the range from 4 K to room temperature in both niobium and gold traps?
• RQ3 . Can elastic collisions with background gas molecules produce an apparent steady heating rate despite their low frequency?
• RQ4 . Is the dominant source of electric field noise at the trap frequency material-specific or governed by surface contaminants independent of electrode composition?
• RQ5 . Can cryoadsorption of background gas at low temperatures mimic heating effects, and if so, under what conditions?

Key findings

• . The ion motional heating rate decreases by more than two orders of magnitude from room temperature (300 K) to 4 K, with values below 10−3 Hz at 4 K.
• . Heating rates in niobium and gold traps are nearly identical across the entire temperature range, indicating no significant material dependence.
• . The lowest measured heating rates in both materials are comparable to the lowest ever reported in similarly sized cryogenic traps, reaching < 10−3 Hz at 4 K.
• . At 16 K, background gas collisions produce an apparent steady heating rate due to elastic collisions, but the inelastic collision rate (estimated at 0.6–1.6 s−1) is the dominant cause of ion loss.
• . The measured electric field noise is far below the expected Johnson noise for both materials, indicating that the anomalous heating is not due to resistive losses in the electrodes.
• . The temperature dependence of heating is consistent with a thermally activated process, and the lack of material dependence strongly implicates surface contaminants as the primary source of the anomalous heating.