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arXiv:2610.36061

Parity-doublet spin interactions in ultracold polyatomic molecules

Junheng Tao, Paige Robichaud, Christian Hallas, Giseok Lee, John M. Doyle

physics.atom-phquant-phSubmitted Oct 26, 2026• Announced Oct 28, 2026• 0 citations
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Abstract

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 $\ell$-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.

AI Summary

Can interactions be controlled directly within the parity-doublet manifold used for molecular precision sensing? This experiment encodes spin-$1/2$ states in opposite-parity components of the CaOH bending mode and probes resonant electric-dipolar exchange at zero electric field. Approximately 90 molecules at $60\,\mu\mathrm{K}$ occupy a crossed optical trap; the transition dipole is $0.73\,\mathrm{D}$, with peak densities reaching roughly $3\times10^{10}\,\mathrm{cm}^{-3}$. Two measurements characterize the resulting itinerant dipolar XY model: Ramsey contrast decays faster with increasing density, and an initialized population imbalance produces mean-field precession proportional to density and spin projection, reversing sign when the imbalance reverses. Both observables agree with discrete truncated Wigner simulations using independently calibrated experimental parameters, with approximately 25% density uncertainty. XY16 dynamical decoupling extends low-density coherence in the crossed trap from $2.5(4)\,\mathrm{ms}$ to $250(30)\,\mathrm{ms}$, exceeding the approximately $50\,\mathrm{ms}$ interaction timescale. Global frame rotations Floquet-engineer an XXZ Hamiltonian with anisotropy tunable from $\Delta=0$ to $2$. Density-dependent dephasing is strongly suppressed near the SU(2)-symmetric Heisenberg point, $\Delta=1$; accompanying precession suppression and reversal support the Hamiltonian tuning. Spin squeezing, entanglement, and improved sensing sensitivity are not demonstrated. The simulations neglect trajectory backaction, spin-motion entanglement, and spin-changing collisions. A separate scattering calculation gives similar dephasing and overlaps with the simulated long-range interaction contribution, so the rates are not additive. Deviations on the Ising side may involve collisions or higher-order Floquet corrections. Extension to polarized, high-electric-field sensing remains open.

Little sign of AI-generated text

Error found: The reported 25% density uncertainty omits contributions that bring independent propagation of the stated number, temperature, and trap-frequency uncertainties to approximately 33%, without documenting an alternative uncertainty combination. The mean-field precession formula omits ℏ and has the opposite sign to the evolution of arg(Sx+iSy) under the stated Hamiltonian, leaving the frequency units and phase convention inconsistent.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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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.
Can interactions be controlled directly within the parity-doublet manifold used for molecular precision sensing? This experiment encodes spin-1/21/21/2 states in opposite-parity components of the CaOH bending mode and probes resonant electric-dipolar exchange at zero electric field. Approximately 90 molecules at 60 μK60\,\mu\mathrm{K}60μK occupy a crossed optical trap; the transition dipole is 0.73 D0.73\,\mathrm{D}0.73D, with peak densities reaching roughly 3×1010 cm−33\times10^{10}\,\mathrm{cm}^{-3}3×1010cm−3.
Two measurements characterize the resulting itinerant dipolar XY model: Ramsey contrast decays faster with increasing density, and an initialized population imbalance produces mean-field precession proportional to density and spin projection, reversing sign when the imbalance reverses. Both observables agree with discrete truncated Wigner simulations using independently calibrated experimental parameters, with approximately 25% density uncertainty. XY16 dynamical decoupling extends low-density coherence in the crossed trap from 2.5(4) ms2.5(4)\,\mathrm{ms}2.5(4)ms to 250(30) ms250(30)\,\mathrm{ms}250(30)ms, exceeding the approximately 50 ms50\,\mathrm{ms}50ms interaction timescale.
Global frame rotations Floquet-engineer an XXZ Hamiltonian with anisotropy tunable from Δ=0\Delta=0Δ=0 to 222. Density-dependent dephasing is strongly suppressed near the SU(2)-symmetric Heisenberg point, Δ=1\Delta=1Δ=1; accompanying precession suppression and reversal support the Hamiltonian tuning.
Spin squeezing, entanglement, and improved sensing sensitivity are not demonstrated. The simulations neglect trajectory backaction, spin-motion entanglement, and spin-changing collisions. A separate scattering calculation gives similar dephasing and overlaps with the simulated long-range interaction contribution, so the rates are not additive. Deviations on the Ising side may involve collisions or higher-order Floquet corrections. Extension to polarized, high-electric-field sensing remains open.
Polar molecules are a promising platform for quantum-enhanced sensing and precision tests of fundamental physics, owing to their strong long-range dipolar interactions, broad sensitivity to electromagnetic fields, and sensitivity to potential physics beyond the Standard Model. However, the creation of metrologically useful entangled states in molecular systems has remained elusive. Here, we report the first observation of a class of metrologically useful entangled states - spin-squeezed states - in polar CaF molecules trapped in an optical tweezer array. The spin degree of freedom is encoded in rotational levels which are directly coupled by dipolar exchange interactions. By harnessing appropriate dynamical decoupling schemes we observe up to 3.0(3)dB of metrological gain, (2.2(3)dB without measurement correction) from direct exchange interactions. Using Floquet engineering, we further realize richer Hamiltonians that preserve spin squeezing while enabling the development of longer-range quantum correlations. Using site- and spin-resolved measurements we demonstrate that these entangled states enhance sensitivity to both homogeneous and spatially varying fields, and reveal strong non-classical correlations, including bipartite entanglement and Einstein-Podolsky-Rosen steering. Finally, we transfer the spin-squeezed states into long-lived and non-interacting hyperfine states, where the metrological enhancement persists for up to 100ms. Our results establish molecular optical tweezer arrays as a scalable platform for generating, controlling, characterizing, and storing entangled states of molecules, opening new opportunities for quantum-enhanced sensing and precision tests of fundamental physics.
Ultracold polyatomic molecules are promising candidates for experiments in quantum science and precision searches for physics beyond the Standard Model. A key requirement is the ability to achieve full quantum control over the internal structure of the molecules. In this work, we established coherent control of individual quantum states in calcium monohydroxide (CaOH) and demonstrated a method for searching for the electron electric dipole moment (eEDM). Optically trapped, ultracold CaOH molecules were prepared in a single quantum state, polarized in an electric field, and coherently transferred into an eEDM-sensitive state where an electron spin precession measurement was performed. To extend the coherence time, we used eEDM-sensitive states with tunable, near-zero magnetic field sensitivity. Our results establish a path for eEDM searches with trapped polyatomic molecules.
We study a bulk fermionic dipolar molecular gas in the quantum degenerate regime confined in a two-dimensional geometry. Using two rotational states of the molecules, we encode a spin 1/2 degree of freedom. To describe the many-body spin dynamics of the molecules, we derive a long-range interacting XXZ model valid in the regime where motional degrees of freedom are frozen. Because of the spatially extended nature of the harmonic oscillator modes, the interactions in the spin model are very long ranged, and the system behaves close to the collective limit, resulting in robust dynamics and generation of entanglement in the form of spin squeezing even at finite temperature and in the presence of dephasing and chemical reactions. We discuss how the internal state structure can be exploited to realize time reversal and enhanced metrological sensing protocols.
Ultracold polar molecules provide a powerful platform for quantum science because they combine long-lived rotational states, rich internal structure, and strong electric dipole-dipole interactions. Optical tweezer arrays provide a pristine way of controlling these molecules at single-particle level, in which molecules can be individually trapped, rearranged into programmable geometries, coherently manipulated by optical and microwave fields, and entangled through their native dipolar interactions. This dissertation describes the development of geometric control as a resource for improving interaction coherence and high fidelity of quantum entanglement between directly laser-cooled calcium monofluoride (CaF) molecules in optical tweezers. A central challenge in molecular tweezer systems is that dipolar interactions are intrinsically sensitive to the relative position of the molecules. Thermal motion in the traps, especially along the weakly confined axial direction, causes molecules to sample a distribution of intermolecular separations, degrading dipolar coherence and limiting entanglement fidelity. We show in this work that the interaction itself can be made robust by controlling its geometry. The experimental work begins with improvements to the production and optical trapping of dense CaF samples. We realize a blue-detuned ``conveyor-belt" magneto-optical trap (MOT) that compresses laser-cooled CaF to a mean cloud radius of 64(5) μm64(5)~\mu\mathrm{m}64(5) μm and a peak density of 3.6(5)×1010 cm−33.6(5)\times10^{10}~\mathrm{cm}^{-3}3.6(5)×1010 cm−3, a 600-fold density increase over conventional red-detuned CaF MOTs. Loading from this compressed source into an optical dipole trap yields up to 2.6×1042.6\times10^42.6×104 trapped molecules at 14(2) μK14(2)~\mu\mathrm{K}14(2) μK, with a peak phase-space density of approximately 2.4×10−62.4\times10^{-6}2.4×10−6. This high-density source improves the transfer of laser-cooled molecules into conservative optical traps and provides the starting point for loading and manipulating single molecules in a hybrid SLM-AOD optical tweezer array. Using this platform, we prepare CaF molecules in selected rotational and hyperfine states to encode a rotational qubiyt. We show how to preserve the single-qubit coherence with a combination of techniques, including ``magic" polarization condition and microwave dynamical decoupling, as well as reduce motional quantum state of the trapped molecule with Raman sideband cooling. We then characterize coherent dipolar spin exchange between pairs of molecules and identify thermal motion and relative trap-center offsets as dominant sources of interaction dephasing. To suppress these effects, we engineer the three-dimensional interaction geometry between two individually trapped molecules. More specifically, we show that one can tune the \textit{static} geometric of the dipolar interaction to configurations where sensitivity to thermal fluctuations is strongly reduced while retaining a finite interaction strength. Furthermore, we show that a molecule can be moved during the interaction while preserving the coherence of ongoing entangling operations. We leverage programmable molecular motion to refocus dephasing arising from relative positional jitter of the optical tweezers. Combining static geometric insensitivity, programmable molecular motion, and partial Raman sideband cooling, we generate two-molecule entanglement with a Bell-state fidelity of F=0.976−0.011+0.008\mathcal{F}=0.976^{+0.008}_{-0.011}F=0.976−0.011+0.008​ in directly laser-cooled molecules. Crucially, the high-fidelity molecular entanglement here is performed without ground-state preparation. Rather than relying on stringent cooling or species-specific trapping conditions, as in prior work, the methods demonstrated here are instead based on dynamic and static geometric control of interacting molecules and are therefore broadly applicable across molecular platforms. These results extend moving optical tweezers beyond assembly and rearrangement, opening routes to robust two-qubit gates, many-body dynamics, and dynamically programmable networks of interaction.
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.
Ultracold molecules are a promising platform for a wide range of applications in quantum science, including searches for new particles (beyond the Standard Model), quantum computing and simulation, and studies of ultracold collisions and chemistry. Over the last decade, tremendous progress has been made in developing techniques to trap diatomic molecules (molecules containing only two atoms) and cool them to ultracold temperatures using laser cooling. Polyatomic molecules (molecules containing more than two atoms) provide new, unique scientific opportunities not afforded by their diatomic counterparts and represent the next frontier in laser cooling and quantum science with molecules. In this thesis, we present our work on extending laser cooling and quantum control to polyatomic molecules. We describe our demonstration of laser cooling and the creation of a magneto-optical trap (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). We further demonstrate loading of the trapped molecules into an optical dipole trap (ODT) and loading of individual CaOH molecules into the conservative potential an optical tweezer array. We also describe our realization of a new type of MOT developed for molecules, a "conveyor belt MOT", which provides increased compression compared to traditional molecular MOTs and was discovered using numerical simulations of the CaOH MOT that were developed as part of this thesis. The increased MOT density and ODT loading afforded by the conveyor belt MOT provides the density required to perform studies of collisions between ultracold CaOH molecules. We also describe our work on the laser cooling the symmetric top molecule CaOCH₃, opening the possibility for 3-D laser cooling of even larger, more complex molecules into the ultracold regime. With the ability to laser cool and trap CaOH molecules, our most recent work has focused on developing tools for quantum control and on initial explorations of some of the new features afforded by polyatomic molecules. We describe a novel protocol established to perform a measurement of the electron's electric dipole moment ("eEDM"). Looking towards quantum information processing with polyatomic molecules, we create a new type of qubit using parity-doublet states in the bending mode of CaOH and characterize its coherence properties. Finally, we provide a brief outlook towards the future of laser cooling of polyatomic molecules for quantum science and provide a few suggestions for new directions opened up by the work presented in this thesis.