A Viewpoint on : Microscopic model of electric-field-noise heating in ion traps

Considering how it affects science and technology, surprisingly little is known about the electric field noise generated near the surface of metals. Now, in a theoretical paper appearing in Physical Review A, Arghavan SafaviNaini at the Massachusetts Institute of Technology in Cambridge and colleagues have uncovered a source of noise from the surfaces of metals that has been a major headache for ion trapping experiments. They argue that electrical dipoles, formed by impurity atoms adsorbed on the metal electrodes of an ion trap, cause noise of just the right strength and characteristics to explain the noise that has been observed in ion trap experiments [1]. Their work provides a guideline for attacking electric field noise and could impact many fields, including surface and materials science, modern electronics, quantum information technology, and precision tests of fundamental physics. Scientists in all of these disciplines strive to have an electrically quiet environment for their measurements. Is there, however, a fundamental lower limit for electric noise generated by an ordinary conductor, such as a piece of gold? Although electric field (or voltage) noise from the thermal motion of the freely moving electrons inside a conductor is well understood, this so-called JohnsonNyquist noise is typically not the sole source. In practice, nonequilibrium noise mechanisms, such as flicker and shot noise, often dominate device performance. Controlling noise is particularly important for modern applications that use ion traps, such as ion-trap quantum computing. In ion traps, electrodes, usually metallic, generate electric fields that confine the ions to a small volume close by. When researchers started laser-cooling ions to the ground state of the trap, they expected that the ions would stay cold for many minutes. However, electric field noise from the trap electrodes heated up the ions within milliseconds, several orders of magnitude faster than what was expected from Johnson-Nyquist noise [2, 3]. The intensity of this unexpected noise appeared to drop with the ion’s distance d from the metal as 1/d4 and the noise appeared to be thermally activated [4]. Trying to piece the puzzle together, researchers tried a host of different metals and even tested traps with semiconducting and superconducting electrodes. Still, no clear picture of what was causing the excessive noise emerged and even initially promising models could not explain the size of the field noise [3]. Early on, researchers guessed that the 1/d4 scaling could be the result of a large number of uncorrelated electrical-dipole-type noise sources sitting on the metal surfaces. For example, in the so-called patch-potential model, dipolelike fields are caused by patches on the surface of a metal in which the electrons have a different work function compared to other regions on the surface. In this model, the noise arises because the patches fluctuate as impurities adsorbed on the trap electrodes diffuse around the surface, similar to the mechanism causing flicker noise in field-emission tips [3, 5]. The model seemed plausible, since even the electrode surfaces in an ultrahigh vacuum ion trap apparatus can be strongly contaminated [5]. It turned out, however, that the surface patch model predicted noise that was much lower in spectral densities than what was observed [3]. Researchers tried to think of mechanisms other than diffusing impurities that would lead to patches and increased noise. Grain boundaries, material interfaces, and the bulk of the material itself were suspected to contribute to the noise, but a clear mechanism to explain it was lacking. One reason researchers were reluctant to abandon the patch model was that scientists in other fields were also encountering problems related to patches, although in different frequency and distance regimes. For example, scanning probe microscopy [6], the detection of Casimir forces [7], measurements of free fall of charged particles [8], and tests of general relativity [9] all encounter patch-potential effects. In all of this work, one important aspect might have been underestimated: Impurity atoms adsorb on the surfaces and the individual adsorbed atoms form electrical dipoles (Fig. 1). The thermal motion of these adsorbed