Motional heating of spatially extended ion crystals

TL;DR

This study measures and analyzes motional heating rates in ion crystals within a linear rf trap, revealing how electric field noise, micromotion, and crystal symmetry influence heating and decoherence, with implications for quantum information processing.

Contribution

It provides precise measurements of ion and crystal motional heating rates, explores the effects of excess micromotion and rf circuit quality, and examines how crystal symmetry affects heating in extended ion systems.

Findings

Heating rates are stable over four years for single ions.

Excess micromotion causes quadratic increase in heating with displacement.

Crystal symmetry influences mode-specific heating rates.

Abstract

We study heating of motional modes of a single ion and of extended ion crystals trapped in a linear radio frequency (rf) Paul trap with a precision of $\Delta \dot{\bar{n}} \approx 0.2 $ phonons s$^{-1}$. Single-ion axial and radial heating rates are consistent and electric field noise has been stable over the course of four years. At a secular frequency of $\omega_\mathrm{sec}=2\pi\times620$ kHz, we measure $\dot{\bar{n}} = 0.56(6)$ phonons s$^{-1}$ per ion for the center-of-mass (com) mode of linear chains of up to eleven ions and observe no significant heating of the out-of-phase (oop) modes. By displacing the ions away from the nodal line, inducing excess micromotion, rf noise heats the com mode quadratically as a function of radial displacement $r$ by $\dot{\bar{n}}(r)/ r^2 = 0.89(4)$ phonons s$^{-1}$ $\mu$m$^{-2}$ per ion, while the oop modes are protected from rf-noise induced heating in linear chains. By changing the quality factor of the resonant rf circuit from $Q=542$ to $Q=204$, we observe an increase of rf noise by a factor of up to 3. We show that the rf-noise induced heating of motional modes of extended crystals also depends on the symmetry of the crystal and of the mode itself. As an example, we consider several 2D and 3D crystal configurations. Heating rates of up to 500 phonons s$^{-1}$ are observed for individual modes, giving rise to a total kinetic energy increase and thus a fractional time dilation shift of up to $-0.3\times 10^{-18}$ s$^{-1}$ of the total system. In addition, we detail on how the excitation probability of the individual ions is reduced and decoherence is increased due to the Debye-Waller effect.