Trapped laser-cooled molecules: From quantum simulation to particle physics to chemistry

Daniel McCarron

Michael R. Tarbutt

Recently, laser cooling methods have been extended from atoms to molecules. The complex rotational and vibrational energy level structure of molecules makes laser cooling difficult, but these difficulties have been overcome and molecules have now been cooled to a few microkelvin and trapped for several seconds. This opens many possibilities for applications in quantum science and technology, controlled chemistry, and tests of fundamental physics. This article explains how molecules can be decelerated, cooled and trapped using laser light, reviews the progress made in recent years, and outlines some future applications.

Christian Hallas

Ultracold molecules are a promising platform for a wide range of applications in quantum science, including searches for new particles (beyond the Standard Model), quantum computing and simulation, and studies of ultracold collisions and chemistry. Over the last decade, tremendous progress has been made in developing techniques to trap diatomic molecules (molecules containing only two atoms) and cool them to ultracold temperatures using laser cooling. Polyatomic molecules (molecules containing more than two atoms) provide new, unique scientific opportunities not afforded by their diatomic counterparts and represent the next frontier in laser cooling and quantum science with molecules. In this thesis, we present our work on extending laser cooling and quantum control to polyatomic molecules. We describe our demonstration of laser cooling and the creation of a magneto-optical trap (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). We further demonstrate loading of the trapped molecules into an optical dipole trap (ODT) and loading of individual CaOH molecules into the conservative potential an optical tweezer array. We also describe our realization of a new type of MOT developed for molecules, a "conveyor belt MOT", which provides increased compression compared to traditional molecular MOTs and was discovered using numerical simulations of the CaOH MOT that were developed as part of this thesis. The increased MOT density and ODT loading afforded by the conveyor belt MOT provides the density required to perform studies of collisions between ultracold CaOH molecules. We also describe our work on the laser cooling the symmetric top molecule CaOCH₃, opening the possibility for 3-D laser cooling of even larger, more complex molecules into the ultracold regime. With the ability to laser cool and trap CaOH molecules, our most recent work has focused on developing tools for quantum control and on initial explorations of some of the new features afforded by polyatomic molecules. We describe a novel protocol established to perform a measurement of the electron's electric dipole moment ("eEDM"). Looking towards quantum information processing with polyatomic molecules, we create a new type of qubit using parity-doublet states in the bending mode of CaOH and characterize its coherence properties. Finally, we provide a brief outlook towards the future of laser cooling of polyatomic molecules for quantum science and provide a few suggestions for new directions opened up by the work presented in this thesis.

Allon Bartana, Ronnie Kosloff, David J. Tannor

Johanna L. Miller

Benjamin L. Augenbraun

Molecules controlled at the single-quantum-state level offer wide-ranging applications spanning many of the frontiers of modern physics. Within the past decade, simple diatomic molecules have been brought under full quantum control both via assembly from pre-cooled atoms and by direct laser cooling. Polyatomic molecules\footnote{Molecules that contain three or more atoms.} have attracted focus as novel quantum resources providing distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, nearly all polyatomic molecules have long-lived states that can be fully polarized at low applied electric fields. These and other features generic to polyatomic molecules can be applied to quantum simulation, quantum chemistry, and tests of fundamental physics beyond the Standard Model. While these structural features of polyatomic molecules are highly promising for applications, they complicate the process of bringing molecules under full quantum control. In this thesis, we describe a series of experimental and theoretical advances toward the production of ultracold complex polyatomic molecules. We demonstrate the first laser cooling of the polyatomic radical ytterbium monohydroxide (YbOH), a promising candidate to search for particles and interactions beyond the Standard Model of particle physics. We develop a novel deceleration technique (``Zeeman-Sisyphus deceleration") capable of slowing molecular beams to trappable velocities while scattering fewer than 10 optical photons. This deceleration method is demonstrated using CaOH molecules, a lighter analog of YbOH. In order to determine a viable pathway to full three-dimensional cooling and trapping of YbOH, we develop a spectroscopic technique to measure vibrational branching ratios with relative intensity sensitivity around 1 part in $10^5$, approximately two orders of magnitude beyond the previous state of the art. We also perform the first spectroscopic observations and characterizations of the related molecule ytterbium monomethoxide, YbOCH$_3$, a promising species to probe beyond-the-Standard-Model physics. In addition, we show theoretically that an entire class of asymmetric top molecules can be laser cooled using essentially the same techniques used for much simpler species. Measurements of the vibrational branching ratios for a few prototypical asymmetric to molecules (CaSH and CaNH$_2$) are presented to confirm these predictions.