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arXiv:2608.06138

A platform for nuclear symmetry-violation searches with laser-coolable molecules carrying spinful nuclei

Tatsam Garg, Jakob Weiß, Tesse Tiemens, Charly Beulenkamp, Andreas Schindewolf, Tim Langen

physics.atom-phnucl-exquant-phSubmitted Aug 6, 2026• Announced Aug 7, 2026• 0 citations
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Abstract

Cold heavy molecules are promising systems for exploring nuclear $\mathcal{P}$- and $\mathcal{CP}$-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 ${}^{137}$Ba${}^{19}$F 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 ${}^{137}$Ba nucleus within this molecule but also to the contributions from the lighter ${}^{19}$F 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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Probing P,TP,TP,T-Symmetry Violation with Optically Trapped Asymmetric Top Molecules
Searches for P,TP,TP,T-violating electromagnetic moments are among the most sensitive probes of physics beyond the Standard Model. Extending beyond current limits will benefit from molecules with fully controllable orientation at low electric fields, long coherence times, and laser coolability---all offered by asymmetric top molecules (ATMs). Exploiting the intrinsic rotational KKK-doubling in ATMs and the associated long-lived (T1≳10T_1 \gtrsim 10T1​≳10 s) parity doublets afforded by C2vC_{2v}C2v​ symmetry and nuclear-spin statistics, these species combine large electric polarizability with long coherence times in modest laboratory fields. We study alkaline-earth(-like) monoamides, M\mathcal{M}M--NH2_22​ (M\mathcal{M}M = Ca, Sr, Ba, Yb, Ra), which possess favorable electronic structure for laser cooling. We perform \textit{ab initio} calculations of fine and hyperfine constants, identifying the importance of relativistic effects in the spin-rotation tensor. An effective Hamiltonian then models the rotational and hyperfine structure of the vibronic ground state, quantifying electron electric dipole moment (EDM) sensitivities and identifying feasible measurement schemes. We compute the effect of external fields and identify engineered clock transitions that suppress sensitivity to external perturbations while retaining strong EDM sensitivity, and characterize the magic trapping conditions that null differential light shifts in an optical trap. Under these conditions we project a statistical electron-EDM sensitivity over an order of magnitude beyond current best experimental limits, with further gains available from increased molecule number and coherence time. Our results establish asymmetric top molecules as a tunable platform for sensitive symmetry-violation measurements with long coherence times.
Abstract Improved nuclear spin-dependent parity violation measurements will enable experimental determination of poorly known electroweak coupling parameters. Here, we investigate the suitability of optically trapped linear polyatomic molecules as probes of nuclear spin-dependent parity violation. The presence of closely spaced, opposite-parity ℓ\ellℓ ℓ -doublets is a general feature of such molecules, allowing parity-violation-sensitive pairs of levels to be brought to degeneracy in magnetic fields typically 100 times smaller than in diatomics. Assuming laser cooling and trapping of polyatomics at the current state-of-the-art for diatomics, we expect to measure nuclear spin-dependent parity-violating matrix elements iW with 70 times better sensitivity than the current best measurements. Our scheme should allow for 10% measurements of iW in nuclei as light as Be or as heavy as Yb, with averaging times on the order of 10 days and 1 s, respectively.
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Radioactive molecules have remained severely understudied relative to their stable counterparts, owing to difficulties associated with their radiological hazards and limited material availabilities. Despite this, their relevancy to many fields including nuclear science, chemistry, and astronomy make them attractive prospects for precision investigations. In particular, laser-coolable radioactive molecules containing octupole-deformed nuclei such as radium monohydroxide (RaOH) and radium monofluoride (RaF) promise dramatic sensitivity increases for experiments aiming to measure new physics originating from fundamental symmetry violations. Before they may be used for any of these purposes, however, new tools are needed that allow for their safe and precise study while making them accessible in a university (or similar) setting. In this thesis, we construct a tabletop cryogenic apparatus capable of studying radioactive molecules which we produce in-situ from trace quantities of starting material. We use this apparatus to perform the first spectroscopic observation of a radium-containing polyatomic molecule RaOH and its deuterated isotopologue RaOD, as well as high-resolution spectroscopy of the photon cycling transitions in RaF. To prepare for laser cooling, we develop a protocol to measure the vibrational branching ratios of these exotic species and use it to establish RaF to be among the most laser-coolable molecules known as of yet. We demonstrate the versatility of our approach by producing and detecting over twenty different atoms and molecules, ranging from transuranic radioisotopes to complex asymmetric top molecules. The species-agnostic methodology employed throughout this work allows its application to a wide class of previously ignored radioactive atoms and molecules, and makes them accessible to the many advanced experimental techniques already leveraged in stable systems.
Ultracold polyatomic molecules containing heavy nuclei, such as SrOH and YbOH, are sensitive to T- symmetry violating and P-symmetry violating physics beyond the Standard Model through the electron’s electric dipole moment. Furthermore, SrOH is sensitive to ultralight Dark Matter through measuring time variations of fundamental constants. However, in order to make full use of these molecules, they need to be cooled to ultracold temperatures and held in optical traps. We have realized a magneto-optical trap (MOT) of SrOH and used this as the essential element to achieve optical trap- ping in the microKelvin temperature regime. We observed MOT lifetimes approaching 200 ms with temperatures ∼ 1 mK and observed damped oscillations in the MOT. We further cooled the SrOH molecules to ∼ 40 μ K using Λ-enhanced gray molasses, and compressed the cloud to about 100 μm size using the conveyor belt MOT technique. Using this ultracold dense cloud of SrOH, we loaded an optical dipole trap (ODT), with a molecular lifetime of 1.2(1) seconds. In separate experiments with YbOH, which is heavier than SrOH, we demonstrated a novel deceleration method (Zeeman- Sisyphus deceleration) that can slow YbOH to under 20 m/s with only a few photons scattered utilizing high magnetic fields up to 2.5 T. By driving spin-flip transitions at the maxima and minima of the magnetic fields, slowing was achieved, which is a critical first step toward loading of a MOT for this species. We addressed the challenges associated with driving spin-flip transitions in molecules that have molecular perturbations among the electronic excited states and developed an understanding of the physics of such systems that is applicable to a wide range of molecules.
Precision measurements exploit internal molecular structure to search for physics beyond the Standard Model. Reaching the entanglement-enhanced regime requires preparing and preserving correlated molecular states within the same manifold used for sensing. Parity-doublet states are central to proposals for precision measurements with polyatomic molecules because they enable large molecular polarization for enhanced sensitivity, and provide internal state co-magnetometry to suppress systematic errors. Here we report the observation and control of resonant dipolar exchange interactions in a parity-doublet sensing manifold of ultracold polyatomic molecules. Using laser-cooled CaOH molecules in a crossed optical dipole trap, we encode a pseudospin in the ℓ\ellℓ-type parity-doublet states and observe two complementary signatures of interactions: a density-dependent Ramsey contrast decay and a spin-projection-dependent mean-field precession whose sign reverses with the initialized population imbalance. Both observables are well described by a common discrete truncated Wigner model using independently calibrated experimental parameters. We further Floquet-engineer the interaction anisotropy and observe suppressed many-body dephasing near the SU(2)-symmetric Heisenberg point, identifying a protected regime relevant for spin squeezing and precision measurement with dense molecular ensembles. Our results establish parity doublets as a coherent, interacting, and programmable molecular degree of freedom, connecting the polyatomic precision-measurement platform to dipolar many-body physics and opening a route to entanglement-enhanced tests of fundamental physics.