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Journal of Physics B Atomic Molecular and Optical Physics

A superconducting Fabry–Perot cavity for trapping cold molecules

Fatemeh S. Tahsildaran, Manish Vashishta, Amir Hossein Farahbod, Rasoul Malekfar, Pavle Djuricanin, Katsunari Enomoto, Takamasa Momose

Published Nov 18, 2020• 1 citation
Publisher

Abstract

Abstract A superconducting Fabry–Perot microwave cavity for a molecular trap was designed, constructed and characterized. The cavity was designed to create intense microwave fields that are sufficient to trap cold polar molecules by the AC Stark force. By coating the mirror surfaces with a superconducting material, an unloaded quality factor of up to 1.1 × 106 at 24.087 GHz was achieved at a temperature of 2.24 K. The field strength of 1.06 MV m−1 obtained for a TEM02 transverse mode with an input power of 10 W is sufficiently intense to trap ammonia molecules at a temperature of 85 mK, which is achievable by a conventional Stark molecular decelerator. We have also implemented an ion optics assembly for sensitive detection of molecules inside the microwave cavity by resonance enhanced multi-photon ionization.

DOI
10.1088/1361-6455/abcb51

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Abstract A superconducting Fabry–Perot microwave cavity for a molecular trap was designed, constructed and characterized. The cavity was designed to create intense microwave fields that are sufficient to trap cold polar molecules by the AC Stark force. By coating the mirror surfaces with a superconducting material, an unloaded quality factor of up to 1.1 × 106 at 24.087 GHz was achieved at a temperature of 2.24 K. The field strength of 1.06 MV m−1 obtained for a TEM02 transverse mode with an input power of 10 W is sufficiently intense to trap ammonia molecules at a temperature of 85 mK, which is achievable by a conventional Stark molecular decelerator. We have also implemented an ion optics assembly for sensitive detection of molecules inside the microwave cavity by resonance enhanced multi-photon ionization.
In this experimental study, the trapping of the translational motion of a supersonic beam of polar ammonia (NH₃) molecules in the rotational ground state (J = 0) of the para species using a microwave field is demonstrated. A cold beam of ammonia molecules seeded in argon gas passes through a superconducting Fabry–Perot resonator, where a standing-wave microwave field in the TEM₀₂ transverse mode is generated near the molecular transition frequency of 23.7 GHz. The mixed gas trapped at the center of the vacuum chamber is cooled to a temperature of 2.3 mK and ionized using the Resonance-Enhanced Multiphoton Ionization (REMPI) technique with the 16101 cm⁻¹ line of a Rhodamine dye laser. The emission spectrum is then recorded with the microwave field switched on and off. The change in the angular momentum of the polar ammonia molecule without the required excitation energy being supplied indicates a quantum tunneling effect induced by the Stark shift. This article presents the first report of the experimental results of this research conducted at the University of British Columbia.
We characterize a Fabry–Pérot microwave cavity designed for trapping atoms and molecules at the antinode of a microwave field. The cavity is fed from a waveguide through a small coupling hole. Focussing on the compact resonant modes of the cavity, we measure how the electric field profile, the cavity quality factor, and the coupling efficiency, depend on the radius of the coupling hole. We measure how the quality factor depends on the temperature of the mirrors in the range from 77 to 293 K. The presence of the coupling hole slightly changes the profile of the mode, leading to increased diffraction losses around the edges of the mirrors and a small reduction in quality factor. We find the hole size that maximizes the intra-cavity electric field. We develop an analytical theory of the aperture-coupled cavity that agrees well with our measurements, with small deviations due to enhanced diffraction losses. We find excellent agreement between our measurements and finite-difference time-domain simulations of the cavity.
Superconducting resonators at K-band frequencies have been developed for different applications in general physics. Niobium pillbox cavities have been built for the one-atom maser experiment by which the interaction of Rydberg atoms with single microwave photons has been investigated. At 21.5 GHz and 1.3 K, quality factors of up to 10/sup 11/ were obtained. Coating of the cavity with Nb/sub 3/Sn resulted in quality factors of 6*10/sup 8/ at 4.2 K and 6*10/sup 9/ at 2 K. A superconducting Fabry-Perot resonator consisting of two spherically curved niobium mirrors was also investigated. The quality factor of 1.8*10/sup 7/ measured at 25 GHz and 4.2 K was found to be two orders of magnitude higher than for a corresponding copper resonator. Fabry-Perot resonators can be used for detecting small position changes for one mirror with respect to another caused by gravitational forces. First experiments with copper Fabry-Perot mirrors suspended in a vacuum chamber provided a maximum sensitivity for a gravitational acceleration of one mirror of 4*10/sup -11/ m/s/sup 2/. These results are promising for a possible fifth force detector based on a superconducting Fabry-Perot resonator.>
Focusing of a molecular beam in a high-field seeking state by an intense microwave standing wave is demonstrated. An 18 GHz microwave field that is near resonant to a rotational transition deflects a molecular beam due to the first-order ac Stark shift. The intense standing wave is obtained with a 47 cm long superconducting microwave resonator with a high unloaded quality factor of about 10 6 . A cold molecular beam of PbO in the rotational ground state with a center velocity of 145 ms −1 is efficiently focused to have about a factor 10 enhancement of the flux density by the resonator with a microwave input power of about 1 W, despite the heaviness of the PbO molecule.
Cold polar molecules provide unique opportunities to test fundamental physics and chemistry. Their permanent electric dipole moments and rich internal structure arising from their vibrational and rotational motion, makes them sensitive probes for new physics. These features also make them ideal for studying ultracold chemistry, for simulating the behaviour of strongly interacting many-body quantum systems, and for quantum information science. This thesis describes a number of advances in cold molecule physics. The optimum method for producing an intense, pulsed, supersonic beam of cold CH molecules is investigated, resulting in a beam with 3.5 × 109 ground state CH molecules per steradian per shot. The beam has a translational temperature of 400 mK and a velocity that is tuneable between 400 and 1800 m s−1 . The lowest-lying Λ-doublet transitions of ground state CH, at 3.3 GHz and 0.7 GHz, are exceptionally sensitive to variations in the fine-structure constant α and the electron-to-proton mass ratio µ. Many modern theories predict that these constants may depend on time, position, or local matter density. Using a novel spectroscopic method, the frequencies of these microwave transitions are measured with accuracy down to 3 Hz. By comparing to radio astronomical observations, the hypothesis that fundamental constants may differ between the high and low-density environments of the Earth and the interstellar medium of the Milky Way is tested. These measurements find no variation and set upper limits of |∆α/α| < 2.1 × 10−7 and |∆µ/µ| < 4.3 × 10−7 . The frequency of the lowest millimetre-wave transition of CH, near 533 GHz, is also measured with an accuracy of 0.6 kHz. The development of a novel type of trap for ground-state polar molecules is presented. Trapping polar molecules is a necessary condition to cool them to ultracold temperatures of 1 µK and below. The trap uses a high intensity microwave field in a Fabry-P´erot resonator. Experimental and theoretical investigations are presented that explore the modes of the cavity, how to obtain the highest possible quality factor, and how to optimally couple the microwave power into the cavity. Finally, a cold supersonic beam of BH molecules is developed. This molecule appears to be particularly well-suited to direct laser cooling due to its favourable rotational structure and Franck-Condon factors. The laser cooling concept is described, and a first spectroscopic investigation of the relevant molecular structure is presented.
We here report on the implementation of a microwave lens for neutral polar molecules suitable to focus molecules in both low-field-seeking and high-field-seeking states. By using the TE{11m} modes of a 12 cm long cylindrically symmetric microwave resonator, Stark-decelerated ammonia molecules are transversally confined. We investigate the focusing properties of this microwave lens as a function of the molecules' velocity, the microwave power, and the frequency detuning Δ from the molecular resonance. Such a microwave lens can be seen as a first important step towards further microwave devices, such as decelerators and traps.