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Description
We demonstrate coherent quantum state manipulation and precision spectroscopy of a CaH+ molecular ion based on quantum-logic spectroscopy [1-6]. Similar to atomic ions, nowadays single molecular ions can be initialized and nondestructively detected the in a pure quantum state, albeit in a probabilistic but heralded fashion [2-6]. Numerous terahertz transitions between CaH+ states with different principal rotational quantum number J are directly probed with an optical frequency comb (OFC) [2, 3, 7], recently attaining sub-10 Hz spectroscopic linewidths and sub-Hz statistical uncertainty [8]. The effect of trap radio-frequency (RF) electric field on the rotational transitions is characterized and exploited to measure the dipole moment of CaH+ [8]. Coherent Rabi flopping is observed between different rotational states [7]. The initial and final states of the transitions, separated by Delta J = 2, can both be nondestructively detected [4-6], which facilitates unambiguous assignment of the observed signal to the corresponding rotational transitions.
We have also demonstrated entanglement of a molecular ion with an atomic ion [9], with possible applications in quantum information science. To further expand and improve quantum state control over CaH+, a 285-GHz millimeter-wave source is incorporated in the experiment to complement the OFC in coherent manipulation of CaH+ rotational states. Aiming at minimizing the adverse effects of blackbody radiation and background gas collision on molecular states, a new trap apparatus compatible to cryogenic operation is under development. Different molecular species could be introduced to the new trap via a molecular beam source attached to the set-up. Our methods are designed with the prospects of investigating and exploiting coherent rotational-vibrational transitions of a large class of diatomic and polyatomic molecules in the optical and infrared domains with high precision.
[1] P. O. Schmidt et al., Science 309, 749 (2005).
[2] D. Leibfried, New J. Phys. 14, 023029 (2012).
[3] S. Ding and D. N. Matsukevich, New J. Phys. 14, 023028 (2012).
[4] F. Wolf et al., Nature 530, 457 (2016).
[5] C. W. Chou et al., Nature 545, 203 (2017).
[6] M. Sinhal et al., Science 367, 1213 (2020).
[7] C. W. Chou et al., Science 367, 1458 (2020).
[8] A. Collopy et al., in preparation.
[9] Y. Lin, D. R. Leibrandt, D. Leibfried, C. W. Chou, Nature 581, 273 (2020).
