New Accuracy Mark for Optical Single-Ion Clock

Scientists at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, have demonstrated a single-ion optical clock with a reported systematic uncertainty of 3.2 × 10^–18—the most accurate value yet shown for a single-ion clock, according to the team (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.116.063001). The new clock could, in the view of the researchers, allow a previously elusive, high-sensitivity reference transition to be pressed into service on a number of questions in fundamental physics, including the search for dark matter.

Pros and cons of a single-ion clock

The state-of-the-art for atomic clocks at present relies on optically reading out electron state transitions in single trapped ions, or in ensembles of neutral atoms confined in an optical lattice (see “Optical Lattice Clocks,” OPN, January 2015). While ensemble-based approaches involving ⁸⁷Sr atoms have recently edged into the territory of 10^–18 accuracy, the ¹⁷¹Yb⁺ single-ion system has, according to the PTB team, some significant potential advantages in creating practical clocks.

One such advantage lies in the characteristics of a particular electronic transition in the Yb⁺ system (specifically, the E3 transition, from the ²S_1/2 to the ²F_7/2 state), which has an exceptionally narrow linewidth, for high potential accuracy, as well as low sensitivity to electric and magnetic fields. Another advantage lies in the wavelengths needed to excite that transition, which are in ranges easily attained by affordable semiconductor lasers. And the details of the relationship between E3 and another Yb⁺ transition, E2, have made this system look unusually promising for probing some central questions in physics, such as changes in certain fundamental constants over time.

The sticking point for using the narrow-linewidth E3 transition, however, lies in the high laser probe light intensity required to excite the transition. That intensity, in turn, introduces a “light shift” due to nonresonant coupling with higher energy levels, which complicates any attempt to access the E3 transition through conventional spectroscopy.

Hammering down the uncertainty

The PTB team got around that problem by cleverly playing two forms of spectroscopy off of one another. Their setup involved a single, laser-cooled ¹⁷¹Yb⁺ ion, confined in a radio-frequency Paul trap and excited to the E3 transition with a probe laser. For the basic frequency measurement for the clock, they used a highly precise form of Ramsey spectroscopy, which increases accuracy by taking its measurement over a finite evolution time. And they used a second technique, Rabi spectroscopy, to get a precise measurement of the light shift due to nonresonant coupling, and the consequent correction that needs to be applied.

Through a feedback loop between the two measurements, the team was able to iteratively tweak the light intensity to hammer down the frequency uncertainty related to light shift to 1.1 × 10^–18. Additional uncertainty tied to the residual thermal motion of the ion brought the clock’s total systematic uncertainty to 3.2 × 10^–18—“more than an order of magnitude smaller than previously published values,” according to the authors.

The PTB scientists see a number of applications for this accurate new frequency standard. For example, certain characteristics of the Yb⁺ system make it an excellent potential yardstick for investigating variations with time in the fine-structure constant and the proton-electron mass ratio. And the system also has strong predicted dependence on the details of interactions with certain hypothesized forms of ultralight dark matter—the search for which, according to the team, the new, ultra-sensitive clock could help advance.