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arXiv:2501.05578

Rotational-hyperfine cooling of $^{205}$TlF in a cryogenic beam

Olivier Grasdijk, David P. DeMille, Jakob Kastelic, D. Kawall, S. K. Lamoreaux, Oskari Timgren, Konrad Wenz, Tanya Zelevinsky

Published Jan 9, 2025• 0 citations
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Abstract

The aim of CeNTREX (Cold Molecule Nuclear Time-Reversal Experiment) is to search for time-reversal symmetry violation in the thallium nucleus, by measuring the Schiff moment of $^{205}$Tl in the polar molecule thallium fluoride (TlF). CeNTREX uses a cryogenic beam of TlF with a rotational temperature of 6.3(2) K. This results in population spread over dozens of rotational and hyperfine sublevels of TlF, while only a single level is useful for the Schiff moment measurement. Here we present a protocol for cooling the rotational and hyperfine degrees of freedom in the CeNTREX beam, transferring the majority of the Boltzmann distribution into a single rotational and hyperfine sublevel by using a single ultraviolet laser and a pair of microwave beams. We achieve a factor of $20.1(4)$ gain in the population of the $J=0$, $F=0$ hyperfine sublevel of the TlF ground state.

DOI
10.48550/arxiv.2501.05578

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Rotational-hyperfine cooling of 205^{205}205TlF in a cryogenic beam
The aim of CeNTREX (Cold Molecule Nuclear Time-Reversal Experiment) is to search for time-reversal symmetry violation in the thallium nucleus, by measuring the Schiff moment of 205^{205}205Tl in the polar molecule thallium fluoride (TlF). CeNTREX uses a cryogenic beam of TlF with a rotational temperature of 6.3(2) K. This results in population spread over dozens of rotational and hyperfine sublevels of TlF, while only a single level is useful for the Schiff moment measurement. Here we present a protocol for cooling the rotational and hyperfine degrees of freedom in the CeNTREX beam, transferring the majority of the Boltzmann distribution into a single rotational and hyperfine sublevel by using a single ultraviolet laser and a pair of microwave beams. We achieve a factor of 20.1(4)20.1(4)20.1(4) gain in the population of the J=0J=0J=0, F=0F=0F=0 hyperfine sublevel of the TlF ground state.
Adiabatic passage of 205^{205}205TlF with microwaves in a cryogenic beam
We present a hyperfine-resolved state preparation scheme for thallium fluoride (TlF) molecules based on microwave-driven adiabatic passage (AP) in a spatially varying electric field. This method enables efficient and robust population transfer between selected ∣J,mJ=0⟩\left|J,m_J=0\right\rangle∣J,mJ​=0⟩ hyperfine sublevels of the X 1Σ0+X\,^1Σ^+_0X1Σ0+​ ground state in a cryogenic molecular beam, a key requirement for the CeNTREX search for nuclear time-reversal symmetry violation. Two sequential stages of AP are implemented. The first transfers population from J=0J=0J=0 to J=1J=1J=1 at a local field of 173 V/cm173~\mathrm{V/cm}173 V/cm, and the second transfers from J=1J=1J=1 to J=2J=2J=2 at 110 V/cm110~\mathrm{V/cm}110 V/cm. Transfer efficiencies are quantified through laser-induced fluorescence, and accounting for residual population in excited rotational levels after a prior stage of rotational cooling. We achieve state transfer efficiencies of 0.92(6)0.92(6)0.92(6) and 1.05(5)1.05(5)1.05(5) for the first and second states of AP, respectively. This corresponds to a total efficiency of 0.97(8)0.97(8)0.97(8) for population transfer from J=0J=0J=0 to J=2J=2J=2. These results demonstrate robust and high-fidelity preparation of specific rotational/hyperfine states in TlF.
Cold heavy molecules are promising systems for exploring nuclear P\mathcal{P}P- and CP\mathcal{CP}CP-violating phenomena in search of new physics beyond the Standard Model. However, most proposed experimental strategies and their early realizations to date have been limited to proof-of-principle molecular species with effectively spin-zero nuclei that are not sensitive to nuclear symmetry-violating phenomena. Here, we introduce a comprehensive experimental toolbox that integrates cooling, trapping, coherent state manipulation, and a complete precision-measurement protocol that is applicable to molecules carrying relevant nuclear spins. Using 137{}^{137}137Ba19{}^{19}19F and nuclear-spin-dependent parity violation (NSD-PV) as representative species and benchmark application, respectively, our approach achieves a projected statistical sensitivity roughly two orders of magnitude beyond comparable molecular beams by combining techniques already demonstrated individually in current experiments. This level of precision could provide realistic experimental access not only to the enhanced NSD-PV signals arising from the heavy 137{}^{137}137Ba nucleus within this molecule but also to the contributions from the lighter 19{}^{19}19F nucleus, bringing direct benchmarks of nuclear \textit{ab initio} theory within reach. We further identify a candidate magic wavelength as a route to second-scale rotational coherence in future experiments. The techniques developed here can be transferred to measurements of nuclear Schiff and magnetic quadrupole moments in molecules containing deformed nuclei, establishing a general platform for laboratory searches for nuclear symmetry violations.
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Precision searches for time-reversal symmetry violating interactions in polar molecules are extremely sensitive probes of high energy physics beyond the standard model. To extend the reach of these probes into the PeV regime, long coherence times and large count rates are necessary. Recent advances in laser cooling of polar molecules offer one important tool-optical trapping. However, the types of molecules that have been laser cooled so far do not have the highly desirable combination of features for new physics searches, such as the ability to fully polarize and the existence of internal comagnetometer states. We show that by utilizing the internal degrees of freedom present only in molecules with at least three atoms, these features can be attained simultaneously with molecules that have simple structure and are amenable to laser cooling and trapping.
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