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arXiv:1902.00497 · Science

An optical tweezer array of ultracold molecules

Loïc Anderegg, Lawrence W. Cheuk, Yicheng Bao, Sean Burchesky, Wolfgang Ketterle, Kang-Kuen Ni, John M. Doyle

Published Sep 12, 2019• 312 citations
arXivHTMLPDF

Abstract

Tweezing cold molecules Arrays of optical tweezers have been used to trap atoms, but trapping and laser-cooling molecules in this setting is tricky. Such an approach would, however, be generalizable to many molecular species. Anderegg et al. created an optical tweezer array of calcium monofluoride molecules, which were laser cooled to their ground state (see the Perspective by Kotochigova). By distinguishing between single and multiple molecules in the tweezers, the researchers were able to observe molecular collisions. Boasting exquisite control over individual molecules, the optical tweezer array platform holds much promise for extending the applications of ultracold molecules. Science , this issue p. 1156 ; see also p. 1079

DOI
10.1126/science.aax1265

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Tweezing cold molecules Arrays of optical tweezers have been used to trap atoms, but trapping and laser-cooling molecules in this setting is tricky. Such an approach would, however, be generalizable to many molecular species. Anderegg et al. created an optical tweezer array of calcium monofluoride molecules, which were laser cooled to their ground state (see the Perspective by Kotochigova). By distinguishing between single and multiple molecules in the tweezers, the researchers were able to observe molecular collisions. Boasting exquisite control over individual molecules, the optical tweezer array platform holds much promise for extending the applications of ultracold molecules. Science , this issue p. 1156 ; see also p. 1079
Cold Molecules Arrays of optical tweezers have been used to trap atoms, but trapping and laser-cooling molecules in this setting is tricky. Such an approach would, however, be generalizable to many molecular species. Anderegg et al. created an optical tweezer array of calcium monofluoride molecules, which were laser cooled to their ground state (see the Perspective by Kotochigova). By distinguishing between single and multiple molecules in the tweezers, the researchers were able to observe molecular collisions. Boasting exquisite control over individual molecules, the optical tweezer array platform holds much promise for extending the applications of ultracold molecules. Science , this issue p. [1156][1]; see also p. [1079][2] [1]: /lookup/doi/10.1126/science.aax1265 [2]: /lookup/doi/10.1126/science.aay3989
Potential wide-ranging scientific applications, spanning fundamental physics to quantum engineering, has led to significant efforts in controlling molecules at the quantum level. The rich internal structure of molecules, that gives rise to these desirable properties also complicates the task of controlling such species. Over the past decade, our ability to produce, control, and detect molecules has advanced tremendously with direct laser-cooling and magneto-optical trapping of diatomic molecules realized by several groups around the world. In this thesis, we describe the creation of the first RF MOT of CaF, which at the time of the writing of this thesis, remains the largest and densest molecular MOT. New laser cooling techniques for molecules are demonstrated, allowing laser cooling of molecules to 4 uK, 50 times colder than the Doppler limit. Optical trapping of these molecules is also achieved, which, in combination with in-trap laser cooling increased the density of trapped molecules by five orders of magnitude compared to the density in the MOT. We developed methods to cool and detect single molecules with high fidelity, which aided us in the creation of an optical tweezer array of single, ultracold CaF molecules. The densities of molecules (10^12 cm^-3) attained inside the tweezer traps also enabled us to observe both light-assisted and ground state collisions of laser cooled molecules, for the first time. The phase space density within the merged tweezers reached 5x10^-4, 10 orders of magnitude larger than the initial MOT. We implemented internal quantum state control of the molecules and dynamical control of the tweezers to build a platform for exploring state-selective ultracold quantum chemistry.
Ultracold molecules are attractive candidates for various applications including ultracold chemistry, quantum information processing, quantum simulation and searches for physics beyond the standard model. Combined with optical tweezer techniques, an optical tweezer array of ultracold polar molecules holds promise for versatile quantum applications. Their long-lived molecular rotational states form robust qubits, while the long-range dipolar interaction between molecules provides quantum entanglement. In this thesis, we build a new generation experiment of optical tweezer array of ultracold CaF molecules. We optically transport the laser cooled and optically trapped molecules into a science glass cell using a hybrid method, where high numerical aperture optical access allows for tighter optical tweezers. In an array of these tight optical tweezers, we demonstrate dipolar spin-exchange interactions between single CaF molecules for the first time. We realize the spin-12\frac{1}{2}21​ quantum XY model by encoding an effective spin-12\frac{1}{2}21​ system into the rotational states of the molecules, and use it to generate a Bell state through an iSWAP operation. Conditioned on the verified existence of molecules in both tweezers at the end of the measurement, we obtain a Bell state fidelity of 0.89(6). Employing interleaved tweezer arrays, we demonstrate single site molecular addressability. To further improve the platform, cooling of the molecules to near the motional ground state is crucial for reducing various dephasings. We demonstrate Raman sideband cooling (RSC) of CaF molecules in optical tweezers to near their 3-D motional ground state, with a 3-D motional ground state probability of 54±18%54\pm18\%54±18% of the molecules that survive the RSC. This paves the way to increase molecular coherence times in optical tweezers for robust quantum information processing and simulation applications in the near future.
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.
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.
Ultracold Molecules Ultracold molecules hold promise for a wide range of exciting applications. However, such applications are currently hampered by the limited number of ultracold molecular ensembles that can be created and by their short lifetimes. Anderegg et al. used a microwave dressing field to tune the collisional properties of calcium monofluoride molecules trapped in optical tweezers. This approach allowed a sixfold suppression of inelastic trap-loss collisions. This scheme paves the way for the creation of a variety of long-lived ultracold molecular ensembles. Science , abg9502, this issue p. [779][1] [1]: /lookup/doi/10.1126/science.abg9502