Polyatomic molecules provide complex internal structures that are ideal for applications in quantum information science, quantum simulation, and precision searches for physics beyond the Standard Model. A key feature of polyatomic molecules is the presence of parity-doublet states. These structures, which generically arise from the rotational and vibrational degrees of freedom afforded by polyatomic molecules, are a powerful feature to pursue these diverse quantum science applications. Linear triatomic molecules contain $\ell$-type parity doublet states, which are predicted to exhibit robust coherence properties. We optically trap CaOH molecules, prepare them in $\ell$-type parity-doublet states, and realize a bare qubit coherence time of $T_2^* = 0.8(2)$ s. We suppress differential Stark shifts by employing molecular spectroscopy to cancel ambient electric fields, and characterize parity-dependent trap shifts, which are found to limit the coherence time. The parity-doublet coherence times achieved in this work are a defining milestone for the use of polyatomic molecules in quantum science.
Ultracold molecules are powerful tools for quantum science due to their structural complexity. Large intrinsic dipole moments and rich Hilbert spaces enable novel and robust schemes for quantum simulation [1][2][3], quantum information processing [4][5][6][7], and precision searches for physics beyond the Standard Model [8][9][10][11][12]. Progress towards these goals with direct laser cooling of molecules [13][14][15][16][17] and indirect assembly of molecules [18][19][20][21][22][23][24][25] has matured the platform to several realizations of functional optical tweezer arrays of ultracold molecules [25][26][27][28][29], where single quantum state control [30,31] and entanglement [32][33][34][35] have been demonstrated. The rotational and vibrational energy structure of molecules harbors a selection of strongly interacting and long-lived states that make versatile qubits [5,36,37]. In several species of diatomic molecules, long coherence times have been achieved in hyperfine storage qubits [30,38] and interacting rotational qubits [31,39,40]. These results took advantage of a "magic" trapping light condition, where differential light shifts between qubit states are tuned close to zero with the polarization [31,38,39,41,42], intensity [43], or wavelength of the light [40,44,45]. Enabled by long rotational coherence times, high-fidelity dipole-dipole interactions, including entangling iSWAP gate operations, have been recently demonstrated in diatomic molecules [32][33][34][35].
Polyatomic molecules possess additional, nuanced structural features compared to diatomics. One key advantage of polyatomic molecules is the presence of pairs of long-lived, near-degenerate states of opposite parity called “parity-doublets.” These states have been proposed for a number of novel applications in quantum science and precision measurements of fundamental physics. For example, parity-doublet states fully mix with modest applied electric fields, resulting in states with large, * paigerobichaud@g.harvard.edu saturated lab-frame electric dipole moments along with states having zero dipole moment. This structure enables fast and strong switchable electric dipole interactions for novel quantum computing schemes [6,7] and is a natural platform to explore models of quantum magnetism, including integer spin systems [1,46,47]. The Stark structure also allows for internal co-magnetometer states robust to systematic effects and advantageous to searches for physics beyond the Standard Model (BSM) [8,10,48,49]. At zero electric field, dipole-dipole interactions between parity-doublet state molecules can be used to construct iSWAP two-qubit gates [32][33][34][35].
The above applications require robust coherence properties. Parity-doublet states promise long coherence times because they share all quantum numbers-except for parity-and therefore suppress dephasing effects from environmental perturbations. In linear triatomic molecules, parity doublets arise from a degeneracy of the projection of vibrational angular momentum onto the internuclear axis, ℓ, as shown in Figure 1. These ℓ-type parity doublets possess, to high order, identical magnetic field sensitivities due to their common electron spin, nuclear spin, and rotational angular momentum. Similarly, differential light shifts in ℓ-type parity doublets are reduced to only parity-dependent effects. At low electric fields, the Stark sensitivity of parity-doublets is a quadratic differential shift, making them robust to dephasing from fluctuations in the environmental electric field. Motivated by the advantages of parity-doublet states, among other properties, polyatomic molecules have been laser cooled [50,51], trapped [52][53][54][55][56][57][58], loaded into optical tweezer arrays [59], and characterized for quantum science applications [60,61].
In this work, we measure the coherence time of paritydoublet states in optically trapped, ultracold polyatomic molecules. Two pairs of fully-stretched parity doublets in CaOH are examined, with each doublet belonging to either the N = 1 or N = 2 rotational state in an excited bending vibrational state, X(010), shown in Figure 1. To mitigate decoherence arising from differential quadratic Stark shifts, we cancel ambient electric fields using Hzlevel spectroscopy of the parity-doublet transition. We measure and compare the rotational dependence of the Stark sensitivity in the parity-doublet states. The coherence time of both doublets is found to be limited by light shifts from the optical trap, which are reduced with a magic polarization angle. We observe a bare coherence time of T * 2 = 0.8(2) s in the N = 1 parity-doublet states. Our experiment starts with loading laser cooled CaOH molecules into a 1064 nm optical dipole trap using singlefrequency gray molasses cooling following blue-detuned magneto-optical trapping [62]. The molecules are then optically pumped into the vibrational bending mode and prepared in a single hyperfine parity-dou
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