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arXiv:2008.01904 · Nature Communications

Precisely spun super rotors

Ivan O. Antonov, Patrick Stollenwerk, Sruthi Venkataramanababu, Ana Paula de Lima Batista, Antonio G. S. de Oliveira‐Filho, Brian Odom

Published Apr 13, 2021• 13 citations
arXivHTMLPDF

Abstract

Abstract Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO+ molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C2Π state. Significant centrifugal distortion of super rotors pumped up to N = 230 allowed probing electronic structure of SiO+ stretched far from its equilibrium bond length.

DOI
10.1038/s41467-021-22342-6

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Abstract Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO+ molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C2Π state. Significant centrifugal distortion of super rotors pumped up to N = 230 allowed probing electronic structure of SiO+ stretched far from its equilibrium bond length.
Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid relaxation rates. Although atoms are routinely laser cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational-vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here, we cool trapped AlH+ molecules to their ground rotational-vibrational quantum state using an electronically-exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH+. We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3.8(+0.9/-0.3) K, with the ground state population increasing from ~3% to 95.4(+1.3/-2.0) %. This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications, and precision tests of fundamental symmetries.
Abstract. Preparing molecules at rest and in a highly pure quantum state is a long standing dream in chemistry and physics, so far achieved only for a select set of molecules in dedicated experimental setups. Here, a quantum-limited combination of mass spectrometry and Raman spectroscopy is proposed that should be applicable to a wide range of molecular ions. Excitation of electrons in the molecule followed by uncontrolled decay and branching into several lower energy states is avoided. Instead, the molecule is always confined to rotational states within the electronic and vibrational ground-state manifold, while a co-trapped atomic ion provides efficient entropy removal and allows for extraction of information on the molecule. The outlined techniques might enable preparation, manipulation and measurement of a large multitude of molecular ion species with the same instrument, with applications including, but not limited to, precise determination of molecular properties and fundamental tests of physics. 1.
We demonstrate rotational cooling of the silicon monoxide cation via optical pumping by a spectrally filtered broadband laser. Compared with diatomic hydrides, SiO^{+} is more challenging to cool because of its smaller rotational interval. However, the rotational level spacing and the large dipole moment of SiO^{+} allows for direct manipulation by microwaves, and the absence of hyperfine structure in its dominant isotopologue greatly reduces demands for pure quantum state preparation. These features make ^{28}Si^{16}O^{+} a good candidate for future applications such as quantum information processing. Cooling to the ground rotational state is achieved on a 100 ms timescale and attains a population of 94(3)%, with an equivalent temperature T=0.53(6) K. We also describe a novel spectral-filtering approach to cool into arbitrary rotational states and use it to demonstrate a narrow rotational population distribution (N±1) around a selected state.
A scheme is proposed for making highly rotationally excited diatomic molecules (“super rotors”) in their ground vibrational and electronic state, e.g., 6Li2X 1Σg+ (v=0,J⩾115) where the rotational energy exceeds the bond strength (E(0,J)−E(0,0)⩾D00). Such levels, while strictly speaking quasibound, have very long tunneling lifetimes (>1011 s for J⩽130), and should have very interesting and unique collisional properties, especially at low temperature. The rotation of the molecules is “spun up” by sequential irradiation by R branch photons in the A 1Σu+–X 1Σg+ bands starting with cold molecules at low J. Spontaneous emission to other vibrational levels is overcome by using a pump laser and its multiple Raman sidebands as in previous work on “spinning down” diatomics.
Preparing molecules at rest and in a very pure quantum state is a long-standing aspiration in chemistry and physics, so far achieved only for a select set of molecules in dedicated experimental setups. Here, a quantum-limited combination of mass spectrometry and Raman spectroscopy is proposed that should be applicable to a wide range of molecular ions. Excitation of electrons in the molecule followed by uncontrolled decay and branching into several lower-energy states is avoided. Instead, the molecule is always confined to rotational states within the electronic and vibrational ground-state manifold, while a co-trapped atomic ion provides efficient entropy removal and allows for extraction of information about the molecule. The outlined techniques might enable the preparation, manipulation and measurement of a multitude of molecular ion species with the same instrument, with applications including, but not limited to, precise determination of molecular properties and fundamental tests of physics.
This thesis presents a scheme for preparation of trapped molecular ions with a high degree of internal state purity by optical pumping with a broadband pulse-shaped femtosecond laser; the internal structure of SiO+ permits fast stepwise pumping through the tens of rotational levels populated in a room-temperature distribution. Two analyses, which guided the experimental implementation, are presented: (1) a novel method of quantifying anharmonicity in the trapping potentials, which limits the number of ions that can be trapped, and (2) a rate-equation simulation of the quantum state evolution during pumping. Experimental implementation of pulse shaping and its characterization are discussed, as is the molecular spectroscopy used to reference this light to the rotational cooling transitions. Internal state analysis can be performed using resonance enhanced multiphoton dissociation.