Overview
A thorium-229 ($^{229}$Th) optical nuclear clock was implemented, utilizing a laser-accessible nuclear transition. This clock operates through the stabilization of a continuous-wave laser to the 148 nm nuclear transition, employing a rapid feedback mechanism based on continuous absorption spectroscopy. The nuclear clock’s performance was characterized by its fractional frequency instability, achieving $3\cdot 10^{-12} \sqrt{\tau/\text{s}}$, and approaching $10^{-15}$ over one day of continuous operation. The implemented nuclear clock was also used to set constraints on models of ultralight dark matter.
Research Context
The thorium-229 isotope's laser-accessible nuclear transition has been identified as a candidate for an optical nuclear clock. Such clocks are anticipated to potentially rival or exceed optical clocks relying on electron-shell transitions in atoms or ions. They are also expected to exhibit greater robustness against external perturbations and offer heightened sensitivity for clock-based tests of fundamental physical principles. The enhancement in sensitivity provided by the thorium-229 transition is of particular relevance for probing fundamental physics.
Current optical clocks primarily utilize electron-shell transitions. A nuclear clock, in contrast, would leverage transitions within the nucleus, which are generally less susceptible to external electromagnetic fields and other environmental disturbances, potentially leading to improved stability and accuracy for certain applications.
Approach
The implementation of the thorium-229 nuclear clock involved the stabilization of a continuous-wave laser to the 148 nm nuclear transition. This stabilization was achieved through a rapid feedback loop, which incorporated continuous absorption spectroscopy. The thorium-229 nuclei were embedded within a millimeter-sized calcium fluoride crystal, which operated at room temperature. A subharmonic of the 148 nm radiation generated by the nuclear clock was continuously compared against a Yb+ single-ion clock. This comparison facilitated the characterization of the nuclear clock's fractional frequency instability.
For constraining ultralight dark matter models, the nuclear clock was utilized to search for periodic fluctuations and slow drifts in the nuclear transition energy. These searches were conducted over timescales ranging from 20 seconds to one day. The enhanced sensitivity of the thorium-229 transition was leveraged in this application.
Findings
The thorium-229 nuclear clock demonstrated a simple shot-noise limited scaling of its fractional frequency instability. The quantified instability was $3\cdot 10^{-12} \sqrt{\tau/\text{s}}$, where $\tau$ represents the averaging time in seconds. Over a continuous operation period of one day, the clock approached instabilities of $10^{-15}$.
The nuclear clock was applied to constrain models of ultralight dark matter. By searching for periodic fluctuations and slow drifts in the nuclear transition energy between 20 seconds and 1 day, the constraints derived from this nuclear clock compete with the best atomic clocks regarding dark matter coupling to photons. Furthermore, these constraints extended beyond previous measurements concerning coupling to the strong force and quarks.
Why This Matters
The implementation and characterization of a thorium-229 optical nuclear clock with an observed instability of $3\cdot 10^{-12} \sqrt{\tau/\text{s}}$ indicates a potential advancement in timekeeping technology. The documented ability to approach $10^{-15}$ instabilities over one day of operation suggests a path toward more stable and robust timekeeping devices. The use of this nuclear clock to constrain ultralight dark matter models, with constraints competing with atomic clocks for photon coupling and exceeding prior measurements for strong force and quark coupling, highlights its utility for precision measurements in fundamental physics.
Potential Applications
Future solid-state nuclear clocks are projected to yield improvements in instability by several orders of magnitude. The inherent design of such a nuclear clock, expected to be more robust against external perturbations, suggests potential for enhanced stability in various applications. Its enhanced sensitivity could also provide benefits for clock-based tests of fundamental principles of physics.
Key Limitations Mentioned by Researchers
No specific limitations are explicitly mentioned by the researchers within the provided source material.