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Register a new userPrecision frequency-comb terahertz spectroscopy on pure quantum states of a single molecular ion
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Spectroscopy is a powerful tool for studying molecules and is commonly performed on large thermal molecular ensembles that are perturbed by motional shifts and interactions with the environment and one another, resulting in convoluted spectra and limited resolution. Here, we use generally applicable quantum-logic techniques to prepare a trapped molecular ion in a single quantum state, drive terahertz rotational transitions with an optical frequency comb, and read out the final state non-destructively, leaving the molecule ready for further manipulation. We resolve rotational transitions to 11 significant digits and derive the rotational constant of CaH+ to be B_R = 142501777.9(1.7) kHz. Our approach suits a wide range of molecular ions, including polyatomics and species relevant for tests of fundamental physics, chemistry, and astrophysics.
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Laser cooling and trapping of atoms and atomic ions has led to numerous advances including the observation of exotic phases of matter, development of exquisite sensors and state-of-the-art atomic clocks. The same level of control in molecules could also lead to profound developments such as controlled chemical reactions and sensitive probes of fundamental theories, but the vibrational and rotational degrees of freedom in molecules pose a formidable challenge for controlling their quantum mechanical states. Here, we use quantum-logic spectroscopy (QLS) for preparation and nondestructive detection of quantum mechanical states in molecular ions. We develop a general technique to enable optical pumping and preparation of the molecule into a pure initial state. This allows for the observation of high-resolution spectra in a single ion (here CaH+) and coherent phenomena such as Rabi flopping and Ramsey fringes. The protocol requires a single, far-off resonant laser, which is not specific to the molecule, so that many other molecular ions, including polyatomic species, could be treated with the same methods in the same apparatus by changing the molecular source. Combined with long interrogation times afforded by ion traps, a broad range of molecular ions could be studied with unprecedented control and precision, representing a critical step towards proposed applications, such as precision molecular spectroscopy, stringent tests of fundamental physics, quantum computing, and precision control of molecular dynamics.
There is an increasing demand for precise molecular spectroscopy, in particular in the mid-infrared fingerprint window that hosts a considerable number of vibrational signatures, whether it be for modeling our atmosphere, interpreting astrophysical spectra or testing fundamental physics. We present a high-resolution mid-infrared spectrometer traceable to primary frequency standards. It combines a widely tunable ultra-narrow Quantum Cascade Laser (QCL), an optical frequency comb and a compact multipass cell. The QCL frequency is stabilized onto a comb controlled with a remote near-infrared ultra-stable laser, transferred through a fiber link. The resulting QCL frequency stability is below 10-15 from 0.1 to 10s and its frequency uncertainty of 4x10-14 is given by the remote frequency standards. Continuous tuning over ~400 MHz is reported. We use the apparatus to perform saturated absorption spectroscopy of methanol in the low-pressure multipass cell and demonstrate a statistical uncertainty at the kHz level on transition center frequencies, confirming its potential for driving the next generation technology required for precise spectroscopic measurements.
We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2^3S state atoms. We analyze schemes for measuring the two-photon $2^3S to 4^3S$ interval, and for resonant two-photon excitation to high Rydberg states, $2^3S to 3^3P to n^3S,D$. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1^1S or 2^1S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the $1^1S to 2^1S$ transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a ``triple magic wavelength at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1^1S, 2^1S and 2^3S states are all similar, small, and positive.
Direct frequency comb spectroscopy of trapped ions is demonstated for the first time. It is shown that the 4s^2S_(1/2)-4p^2P_(3/2) transition in calcium ions can be excited directly with a frequency comb laser that is upconverted to 393 nm. Detection of the transition is performed using a shelving scheme to suppress background signal from non-resonant comb modes. The measured transition frequency of f=761 905 012.7(0.5) MHz presents an improvement in accuracy of more than two orders of magnitude.
We describe the current status of high-precision ab initio calculations of the spectra of molecular hydrogen ions (H_2^+ and HD^+) and of two experiments for vibrational spectroscopy. The perspectives for a comparison between theory and experiment at a level of 1 ppb are considered.
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