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Physics

Fine Control of Ultracold Polar Molecules

Charles Day

Published May 13, 2024• 0 citations
Publisher

Abstract

Fine Control of Ultracold Polar MoleculesThe ability to store molecules in reconfigurable optical traps could allow researchers to harness the rich physics of molecules in quantum applications.By Charles Day U ltracold atoms are a promising platform for quantum science.Molecules could be even better thanks to properties that atoms lack, such as polarity and the availability of rotational states.Now two independent groups have succeeded in filling one-dimensional arrays of optical traps with ultracold polar molecules assembled from ultracold atoms [1, 2].The groups also demonstrated that they could set, change, and read out the individual quantum states of the molecules.The feat may enable techniques developed for trapped atoms to be implemented with molecules, making the richer molecular physics available to new quantum applications.

DOI
10.1103/physics.17.s50

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Fine Control of Ultracold Polar MoleculesThe ability to store molecules in reconfigurable optical traps could allow researchers to harness the rich physics of molecules in quantum applications.By Charles Day U ltracold atoms are a promising platform for quantum science.Molecules could be even better thanks to properties that atoms lack, such as polarity and the availability of rotational states.Now two independent groups have succeeded in filling one-dimensional arrays of optical traps with ultracold polar molecules assembled from ultracold atoms [1, 2].The groups also demonstrated that they could set, change, and read out the individual quantum states of the molecules.The feat may enable techniques developed for trapped atoms to be implemented with molecules, making the richer molecular physics available to new quantum applications.
The ability to control particles in the quantum regime at the individual particle level is a challenge at the frontier of quantum science and engineering. Of the many quantum objects developed in pursuit of this goal, ultracold molecules host rich internal states and unique inter-particle interactions that could lend themselves useful to a broad range of applications including quantum computing and simulation, precision measurements and quantum chemistry. In this thesis we present a bottom-up approach to creating single molecules in individual optical tweezers. In particular, single atoms of two different species are first trapped and cooled in individual optical tweezers then brought together to form a single molecule coherently. Using this approach, we have successfully created an array of fully quantum-state-controlled polar molecules. The quantum state – including both internal and external states – of the resulting molecules are known to us, and is the first time single neutral molecules have been under full quantum state control. This platform offers exciting new opportunities to harness the rich properties of molecules in many quantum science applications.
Abstract Fully internal and motional state controlled and individually manipulable polar molecules are desirable for many quantum science applications leveraging the rich state space and intrinsic interactions of molecules. While prior efforts at assembling molecules from their constituent atoms individually trapped in optical tweezers achieved such a goal for exactly one molecule (Zhang J T et al 2020 Phys. Rev. Lett. 124 253401; Cairncross W B et al 2021 Phys. Rev. Lett. 126 123402; He X et al 2020 Science 370 331–5), here we extend the technique to an array of five molecules, unlocking the ability to study molecular interactions. We detail the technical challenges and solutions inherent in scaling this system up. With parallel preparation and control of multiple molecules in hand, this platform now serves as a starting point to harness the vast resources and long-range dipolar interactions of molecules.
Fully internal and motional state controlled and individually manipulable polar molecules are desirable for many quantum science applications leveraging the rich state space and intrinsic interactions of molecules. While prior efforts at assembling molecules from their constituent atoms individually trapped in optical tweezers achieved such a goal for exactly one molecule, here we extend the technique to an array of five molecules, unlocking the ability to study molecular interactions. We detail the technical challenges and solutions inherent in scaling this system up. With parallel preparation and control of multiple molecules in hand, this platform now serves as a starting point to harness the vast resources and long-range dipolar interactions of molecules.
Control over the quantum states of individual molecules is crucial in the quest to harness their rich internal structure and dipolar interactions for applications in quantum science. In this paper, we develop a toolbox of techniques for the control and readout of individually trapped polar molecules in an array of optical tweezers. Starting with arrays of up to eight Rb and eight Cs atoms, we assemble arrays of RbCs molecules in their rovibrational and hyperfine ground state with an overall efficiency of 48(2)%. We demonstrate global microwave control of multiple rotational states of the molecules and use an auxiliary tweezer array to implement site-resolved addressing and state control. We show how the rotational state of the molecule can be mapped onto the position of Rb atoms and use this capability to readout multiple rotational states in a single experimental run. Further, using a scheme for the midsequence detection of molecule formation errors, we perform rearrangement of assembled molecules to prepare small defect-free arrays. Finally, we discuss a feasible route to scaling to larger arrays of molecules. Published by the American Physical Society 2024
Chemical reactions can be surprisingly efficient at ultracold temperatures ( < 1mK) due to the wave nature of atoms and molecules. The study of reactions in the ultracold regime is a new research frontier enabled by cooling and trapping techniques developed in atomic and molecular physics. In addition, ultracold molecular gases that offer diverse molecular internal states and large electric dipolar interactions are sought after for studies of strongly interacting many-body quantum physics. Here we propose a new approach for producing ultracold molecules in the absolute internal and motional quantum ground state, where single molecules are assembled one by one from individual atoms. The scheme involves laser cooling, optical trapping, Raman sideband cooling, and coherent molecular state transfer. As a crucial initial step, we demonstrate quantum control of constituent atoms, including 3D ground-state cooling of a single Cs atom, in a simple apparatus. As laser technology advances to shorter wavelengths, additional atoms will be amenable to laser-cooling, allowing more diverse, and eventually more complex, molecules to be assembled with full quantum control.
Recent theoretical and experimental advances have led to better understanding and control of simple molecules, bringing them to the forefront of quantum science. Diatomic polar molecules, with their vibrational and rotational degrees of freedom and electric dipole moment, are excellent candidates for a scalable qubit, probes of beyond-Standard Model physics, and quantum simulation of topological and chiral many-body systems. These applications call for high-phase space density (PSD) of molecules in a single quantum state. To date, the highest molecular PSD is achieved by coherently associating degenerate quantum gases of atoms into molecules. However, this requires fine-tuning of experimental parameters to maximize spatial overlap of the atomic density distributions while minimizing three-body recombination of the atoms.\nIn this thesis, we propose a bottom-up solution to gain single particle control of ultracold molecules for the first time. We use optical tweezers to deterministically assemble a single NaCs molecule in a single quantum state from a pair of Na and Cs atoms. Added benefits of using optical tweezers include: tight confinement of isolated atoms enabling cooling of Na and Cs to their quantum ground state of motion in less than 100~ms, and dynamic reconfigurability of optical tweezer arrays which will allow scaling up to defect-free arrays of single molecules. In the process, we realize a conceptually simple platform for studying atomic collisions and molecular spectra which derives its strength from the ability to gather "before" and "after" images of single atoms.