Session C05: Atomic and Molecular Clocks

First Deployments of NIST's Transportable Yb Optical Lattice Clock

We present the development of a transportable ytterbium optical lattice clock and report on its first deployments and testing. We report on comparisons with frequency standards at NIST demonstrating 1s stability at the mid 10^-16 level and averaging down to the low 10^-17 level. We highlight our most recent progress in which the clock has been successfully transported on a truck outside of its laboratory environment, showcasing its potential for practical applications such as relativistic geodesy and comparison with primary optical frequency standards towards redefinition of the SI second. We also discuss intranational and geodetic measurements that we hope to do in the upcoming future.   

Lattice light shift of a Wannier-Stark optical lattice clock

Lattice light shift is one of the most important systematic uncertainty sources in optical lattice clocks. As the technology continues to advance, the fractional frequency uncertainty is rapidly approaching the 19th digit, making a thorough understanding of the lattice light shift essential. We present a new evaluation of the lattice light shift in the JILA Wannier-Stark 1D optical lattice clock*. This work highlights the importance of understanding atomic motions and atom-light interaction in shallow lattices, as this understanding is key to reaching the goal of reducing the fractional frequency uncertainty to below 1E-18. Furthermore, we describe recent efforts to address other sources of systematic errors, such as the quadratic Zeeman shift and black body radiation shift. 

 

*arXiv:2210.16374 (2022).   

Temperature-shift suppression scheme for two-photon two-color rubidium vapor clocks

Compact optical clocks are at the heart of many important applications involving navigation, position and timing. Employing a vapor cell that is either slightly heated or cooled beyond room temperature to maintain a desirable vapor pressure, these compact clocks neither require a vacuum system nor laser cooling, and their size is typically best measured in liters, rather than cubic meters. However, such clocks only deliver a ‘poor’ fractional instabilities at the order of 10^-12 to 10^-15. This is mainly due to broad linewidths of clock transitions (e.g. a few 100 kHz for the 5S – 5D rubidium two photon transition), and more importantly the AC-Stark and the pressure-induced shifts driven by intensity fluctuations of the probe-laser and temperature variations of the vapor cell. Thus, reaching fractional frequency instabilities beyond 10^-15 impose technologically unrealistic long-term constraints on the laser power and the cell temperature. A scheme for rubidium vapor clocks was recently proposed to significantly reduce AC-Stark-induced shifts to the 5S_1/2 → 5D_5/2 two photon transition by using two lasers at 776 and 780 nm, instead of a single laser at 778 nm to probe the atoms. With an intermediate level closer to the 5P_3/2 level, this scheme allows a two-orders of magnitude increase in the signal to noise ratio that could allow operation of the clock at a lower cell temperature, where the atomic vapor becomes less dense. Unfortunately, we found that this two-color scheme adds a first-order contribution from the optical Doppler shift to the clock transition, which makes it slightly more sensitive to the cell temperature variations. Here, we propose to employ this residual Doppler shift against the frequency shifts induced by cell temperature variations and achieve a full cancellation for both of the shifts without using special gas mixtures. This could lead to a vapor clock that is insensitive to both, optical power fluctuations and temperature/pressure fluctuations of the atomic vapor, making liter-sized optical clocks with a stability of the previous-generation cold atomic clocks feasible.   

Compact cold atom microwave clock for GPS-denied environments

Microwave atomic clocks are near-term solutions for commercial GPS-like timing and holdover services. Cold atoms provide environmental decoupling and lasers eliminate bulky microwave cavities. Our objective is to miniaturize a cold rubidium microwave clock with rapid startup and a design optimized for manufacturability. A magneto-optical trap cools atoms that are probed using a laser-based coherent population trapping technique. To achieve our goal, we fabricated a miniaturized ultra-high vacuum cell, ion pump, atom source, optics, and an integrated control system. The entire device fits into a 25-liter rack mounted chassis. It is tolerant to orientation and acceleration. The electromagnetically induced transparency contrast approaches unity, producing strong Ramsey fringes with widths of tens of Hertz. Frequency stability, better than 10^-11 at one second, is comparable to commercial standards, but with reduced size and weight. This work has set the stage for our development of dramatically more precise optical standards. We demonstrate several two-photon rubidium schemes based on both thermal vapors and cold atoms. For example, our prototype thermal vapor approach yields sub-10^-12 performance at one second and long-term performance near 10^-15 with competitive size and weight characteristics.   

Towards Sr shelving spectroscopy based in microfabricated vapor cells

The 7 kHz Sr intercombination line at 689 nm could be used to implement an optical clock with a frequency instability below 10^-13 at 1s. Optical “shelving” detection provides a simple scheme to detect the 7 kHz transition in hot Sr vapors, making it possible to miniaturize the clock with the help of a microfabricated vapor cell. To perform shelving spectroscopy, a narrow linewidth laser at 689nm of the Sr intercombination line is needed. We achieved this by phase locking the 689 nm laser to a frequency comb. A 461 nm laser probing the ¹S₀ to ¹P₁ transition is frequency locked using saturation absorption spectroscopy. We will report on the progress of using the two lasers for shelving spectroscopy in a microfabricated Sr vapor cell.

    

Extending frequency metrology into the extreme-ultraviolet range with highly charged ions

Extending frequency metrology beyond the optical range will enhance the performance of optical clocks, which have become a key tool for novel fundamental physics studies using atomic systems. Their outstanding resolution, reproducibility, and accuracy make them in principle capable of sensing effects of all Standard Model interactions on the frequency of electronic transitions, such as those due to, e. g., a variation of the fine-structure constant. For disentangling the underlying source of any measurable perturbation of the electronic wave function is thereby crucial to carry out isotopic studies. By changing the neutron number as well as the overlap of the electronic wave function with that of the nucleus in a well-defined way, as in the generalized King-plot method [1], different contributions from Standard Model interactions can be potentially distinguished from hypothetical New Physics effects [2]. Isoelectronic and isonuclear sequences of highly charged ions (HCI) offer a plethora of possibilities in this regard [3], since they possess many different types of exceptionally long-lived metastable states up to x-ray energies, and photoionization coupling the bound electronic system to the continuum is excluded when the charge state is high enough. The recent demonstration of an optical clock based on HCI [4] shows how an extension of frequency metrology beyond the optical range is possible, and the benefits from such an approach. For this purpose, we are preparing an experiment combining an extreme-ultraviolet frequency comb based on high-harmonic-generation [5] with a superconducting radio-frequency trap [6].

[1] Berengut, J. C., et al., Rev. Res. 2, 043444 (2020)

[2] Rehbehn, N.-H., et al., Phys. Rev. A 103, L040801 (2021)

[3] Kozlov, M. G., et al., Rev. Mod. Phys. 90, 045005 (2018)

[4] King, S.A., et al., Nature 611, 43 (2022)

[5] Nauta, J., et al., Opt. Express 29, 2624 (2021)

[6] Stark J., et al., Rev. Sci. Instrum. 92, 083203 (2021)   

Quantum field theory and the mass defect for composite bosons

Atom interferometers and atomic clocks use the coupling of internal and center-of-mass degrees of freedom of composite systems, such as atoms, to electromagnetic fields. Unifying both concepts leads to quantum-clock interferometry, which makes use of the relativistic coupling between internal and center-of-mass degrees of freedom encoded in the mass defect. In this contribution, we derive such a coupling based on quantum field theory.

In many quantum-field-theoretical treatments of atoms, the description is often limited to point particles denying access to the internal structure and thus the mass defect. In contrast, in field-theoretical bound-state calculations the center-of-mass degrees of freedom, most relevant for atom interferometry, are usually neglected. We now combine both approaches to describe atoms as composite particles formed by two elementary fermions and derive an effective quantum field theory for an interacting ensemble of charged, composite particles. This way, the coupling between internal and center-of-mass degrees of freedom encoded in the mass defect naturally arises. Moreover, our field theory is of cobosonic (composite boson) nature rather than purely bosonic, first-order relativistic corrections are contained consistently for the energy of one coboson and the scattering of two cobosons, and the light-matter interaction is given in multipolar form. Hence, the Hamiltonian of our effective theory can directly be used for calculations regarding various aspects of matter-wave and quantum-clock interferometry.   

A new Yb+ optical clock system for testing fundamental physics

Several clock states in Yb+, especially those within the metastable F7/2 manifold, are unusually sensitive to anticipated effects of non-Standard-Model physics. For instance, pronounced clock shifts are predicted due to Lorentz asymmetry, variations of the fine structure constant, or coupling to dark matter. Here we propose a duplex Yb+ quantum sensor dedicated to precision tests of fundamental physics. Basic design aspects of the experimental apparatus are discussed.   

Internal clock interferometry for testing fundamental physics

Atomic clocks and atom interferometers are important platforms for tests of fundamental physics. Here we propose a clock comparison scheme relying on an internal atomic clock Ramsey interferometer comprising two interfering clock states within one atom. The atoms are prepared in a superposition of two clock states and one ground state. Over the coherent evolution of this three-level system, the differential phase accumulation between the two clock states leads to high-contrast interference fringes. The ground state population exhibits a corresponding fringe visibility modulation. Because the two clocks are located within the same particle, all clock shifts are fully correlated. With no spatial splitting or recombining being involved, the full intrinsic system coherence time can be exploited. Implemented in appropriate multi-clock state systems, e.g., Yb+, this scheme opens up the path to novel differential clock spectroscopy protocols with enhanced detection sensitivities for non-Standard Model physics.   

Pathways toward a second-generation molecular vibrational clock

Molecular clocks represent the cutting edge for precision measurements with molecules. We have completed a full systematic evaluation of a vibrational molecular clock at the 10^-14 level in the THz regime. Current limitations include scattering-limited lifetimes, and large lattice lightshift systematics in part due to hyperpolarizability. We discuss the path toward an improved molecular clock through increased coherence times and the mitigation of lattice light scattering and two-body collisional loss. Additionally, we discuss tests of ab initio quantum chemistry calculations and the potential for tests of fundamental physics using the molecular clock.