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Physics

Elusive Clock Transition in Strontium Revealed

Marric Stephens

Published Jun 20, 2024• 0 citations
Publisher

Abstract

Researchers have measured a hard-to-observe electronic transition in strontium that was predicted six decades ago. By Marric StephensO ptical atomic clocks rely on the existence of precisely defined, narrow-linewidth electronic transitions of atoms and ions.Currently, the most precise of these clocks uses the transition of strontium-87 from its ground state to its so-called 3 P 0 excited state (see Viewpoint: A Boost in Precision for Optical Atomic Clocks).Last year, Sebastian Blatt at the Max Planck Institute for Quantum Optics in Germany and his colleagues observed a similar transition in strontium-88.Now that same team has demonstrated coherent control of this transition, which involves strontium-88's ground and 3 P 2 excited states [1].The researchers say that, in addition to its

DOI
10.1103/physics.17.s72

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Researchers have measured a hard-to-observe electronic transition in strontium that was predicted six decades ago. By Marric StephensO ptical atomic clocks rely on the existence of precisely defined, narrow-linewidth electronic transitions of atoms and ions.Currently, the most precise of these clocks uses the transition of strontium-87 from its ground state to its so-called 3 P 0 excited state (see Viewpoint: A Boost in Precision for Optical Atomic Clocks).Last year, Sebastian Blatt at the Max Planck Institute for Quantum Optics in Germany and his colleagues observed a similar transition in strontium-88.Now that same team has demonstrated coherent control of this transition, which involves strontium-88's ground and 3 P 2 excited states [1].The researchers say that, in addition to its
Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise
Optical lattice clocks of fermionic strontium offer a versatile platform for probing fundamental physics and developing quantum technologies. The bivalent electronic structure of strontium gives rise to a doubly forbidden atomic transition that is accessible due to hyperfine mixing in fermionic strontium-87, thus resulting in a submillihertz natural linewidth. Currently, the most accurate optical lattice clocks operate on this narrow transition by tightly trapping strontium-87 atoms in a optical lattice at 813 nm. Magic wavelengths occur where the Stark shifts of both the ground and excited states are equal, thus eliminating any position- and intensity-dependent broadening of the corresponding transition. Theoretical calculations of the electronic structure of strontium-87 have also predicted another magic wavelength of the clock transition at 497.01(57) nm. In this work, we experimentally measure the magic wavelength to be 497.4364(4) nm. Compared to the 813-nm magic wavelength, 497 nm is closer to the strong 461-nm dipolar transition of strontium, resulting in an order-of-magnitude increase in atomic polarizability and deeper traps with less optical power. The proximity to the 461-nm transition also leads to an enhanced sensitivity of 334(10) Hz/(nm E R ) at the magic wavelength.
Optical lattice clocks based on the narrow ( 5 s 2 ) S 0 1 − ( 5 s 5 p ) P 0 3 transition in neutral strontium (Sr) are among the most precise and accurate measurement devices in existence. Although this transition is completely forbidden by selection rules, state mixing from the hyperfine interaction in Sr 87 provides a weakly allowed transition that can be coherently driven with practical clock laser intensities. While the coherent interrogation times of optical clocks are typically set by the linewidth of the probe laser, this limitation can be overcome in synchronous differential comparisons between ensembles. In such measurements the natural lifetime of the S 0 1 – P 0 3 clock transition becomes the fundamental limiting factor to the duration of a single run of the experiment. However, a direct measurement of the decay rate of the clock excited state is quite challenging due to the competing effects of other loss channels such as Raman scattering, inelastic collisions, and atom loss due to background gas. In this work, we monitor the decay of Sr atoms trapped in an optical lattice and initialized in the P 0 3 state. By making measurements of high and low density ensembles of both Sr 87 and Sr 88 across varying lattice trap depths, we isolate radiative decay, which accounts for a significant fraction of the observed decays at low depths. We obtain a natural radiative decay lifetime of 167 − 40 + 79 s for the P 0 3 clock state in Sr 87 , a value that is consistent with previously reported measurements and theoretical predictions. We also introduce an additional measurement scheme that involves repeated measurements of the ground-state population within a single experimental sequence, validating our model and the consistency of the measured rates. We expect that the techniques introduced in this work are applicable to lifetime measurements of long-lived metastable states in other atoms and ions used for clocks and quantum computing. The results presented in this work inform performance estimates of proposed gravitational wave detectors and long-baseline atom interferometers making use of Sr 87 .
Strontium optical lattice clocks represent the current state of the art in precise measurement of time. By trapping atoms at the antinodes of a one dimensional interference pattern of light, frequency shifts associated with the absorption and re-emission of photons used to probe the narrow linewidth clock transition can be avoided. In such a periodic system atoms develop a motional bandstructure analogous to electrons in condensed matter systems. By studying the behavior of atoms in the optical lattice we can, by analogy, gain understanding into the behavior of electrons in the solid state. Indeed, we can perform experiments which cannot be done in the real bulk material by controlling parameters which are impossible to change in macroscopic solids. \n \nIn this work we report on modifications to the existing strontium optical lattice clock experiment which have allowed the study of spin-orbit coupling. By dynamically ramping the intensity of one lattice beam during an experimental cycle we can study a regime in which the inter-site hopping rate J is sufficiently large that an atoms internal state can become coupled to its quasimomentum. The design requirements of the changes and the preliminary results are discussed.
Techniques of modern quantum optics allows for the preparation of atoms in well controlled quantum states ideal for precision measurements and tests of fundamental laws of physics. We report on our recent progress with a highly stable and accurate optical atomic clock based on ultracold fermionic87Sr atoms confined in a one dimensional optical lattice. Currently, we have carried out a detailed evaluation of our clock at the 10−16level and can report stability at 2 x 10−15level at one second. At typical operating parameters for the clock we observe evidence of a density dependent clock shift. Operating the clock at a particular excitation ratio of ground state and excited clock state we observe a shift consistent with zero.