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arXiv:2210.06309 · Science

On-demand entanglement of molecules in a reconfigurable optical tweezer array

Connor M. Holland, Yukai Lu, Lawrence W. Cheuk

Published Dec 7, 2023• 131 citations
arXivHTMLPDF

Abstract

Entanglement is crucial to many quantum applications, including quantum information processing, quantum simulation, and quantum-enhanced sensing. Because of their rich internal structure and interactions, molecules have been proposed as a promising platform for quantum science. Deterministic entanglement of individually controlled molecules has nevertheless been a long-standing experimental challenge. We demonstrate on-demand entanglement of individually prepared molecules. Using the electric dipolar interaction between pairs of molecules prepared by using a reconfigurable optical tweezer array, we deterministically created Bell pairs of molecules. Our results demonstrate the key building blocks needed for quantum applications and may advance quantum-enhanced fundamental physics tests that use trapped molecules.

DOI
10.1126/science.adf4272

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Abstract Realizing quantum control and entanglement of particles is crucial for advancing both quantum technologies and fundamental science. Substantial developments in this domain have been achieved in a variety of systems 1–5 . In this context, ultracold polar molecules offer new and unique opportunities because of their more complex internal structure associated with vibration and rotation, coupled with the existence of long-range interactions 6,7 . However, the same properties make molecules highly sensitive to their environment 8–10 , affecting their coherence and utility in some applications. Here we show that by engineering an exceptionally controlled environment using rotationally magic 11,12 optical tweezers, we can achieve long-lived entanglement between pairs of molecules using detectable hertz-scale interactions. We prepare two-molecule Bell states with fidelity $$0.92{4}_{-0.016}^{+0.013}$$ 0.92 4 − 0.016 + 0.013 , limited by detectable leakage errors. When correcting for these errors, the fidelity is $$0.97{6}_{-0.016}^{+0.014}$$ 0.97 6 − 0.016 + 0.014 . We show that the second-scale entanglement lifetimes are limited solely by these errors, providing opportunities for research in quantum-enhanced metrology 7,13 , ultracold chemistry 14 and the use of rotational states in quantum simulation, quantum computation and as quantum memories. The extension of precise quantum control to complex molecular systems will enable their additional degrees of freedom to be exploited across many domains of quantum science 15–17 .

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Entanglement is crucial to many quantum applications, including quantum information processing, quantum simulation, and quantum-enhanced sensing. Because of their rich internal structure and interactions, molecules have been proposed as a promising platform for quantum science. Deterministic entanglement of individually controlled molecules has nevertheless been a long-standing experimental challenge. We demonstrate on-demand entanglement of individually prepared molecules. Using the electric dipolar interaction between pairs of molecules prepared by using a reconfigurable optical tweezer array, we deterministically created Bell pairs of molecules. Our results demonstrate the key building blocks needed for quantum applications and may advance quantum-enhanced fundamental physics tests that use trapped 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.
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.
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.
Polar molecules are a promising platform for quantum-enhanced sensing and precision tests of fundamental physics, owing to their strong long-range dipolar interactions, broad sensitivity to electromagnetic fields, and sensitivity to potential physics beyond the Standard Model. However, the creation of metrologically useful entangled states in molecular systems has remained elusive. Here, we report the first observation of a class of metrologically useful entangled states - spin-squeezed states - in polar CaF molecules trapped in an optical tweezer array. The spin degree of freedom is encoded in rotational levels which are directly coupled by dipolar exchange interactions. By harnessing appropriate dynamical decoupling schemes we observe up to 3.0(3)dB of metrological gain, (2.2(3)dB without measurement correction) from direct exchange interactions. Using Floquet engineering, we further realize richer Hamiltonians that preserve spin squeezing while enabling the development of longer-range quantum correlations. Using site- and spin-resolved measurements we demonstrate that these entangled states enhance sensitivity to both homogeneous and spatially varying fields, and reveal strong non-classical correlations, including bipartite entanglement and Einstein-Podolsky-Rosen steering. Finally, we transfer the spin-squeezed states into long-lived and non-interacting hyperfine states, where the metrological enhancement persists for up to 100ms. Our results establish molecular optical tweezer arrays as a scalable platform for generating, controlling, characterizing, and storing entangled states of molecules, opening new opportunities for quantum-enhanced sensing and precision tests of fundamental physics.
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.
Abstract Realizing quantum control and entanglement of particles is crucial for advancing both quantum technologies and fundamental science. Substantial developments in this domain have been achieved in a variety of systems 1–5 . In this context, ultracold polar molecules offer new and unique opportunities because of their more complex internal structure associated with vibration and rotation, coupled with the existence of long-range interactions 6,7 . However, the same properties make molecules highly sensitive to their environment 8–10 , affecting their coherence and utility in some applications. Here we show that by engineering an exceptionally controlled environment using rotationally magic 11,12 optical tweezers, we can achieve long-lived entanglement between pairs of molecules using detectable hertz-scale interactions. We prepare two-molecule Bell states with fidelity 0.924−0.016+0.0130.92{4}_{-0.016}^{+0.013}0.924−0.016+0.013​ 0.92 4 − 0.016 + 0.013 , limited by detectable leakage errors. When correcting for these errors, the fidelity is 0.976−0.016+0.0140.97{6}_{-0.016}^{+0.014}0.976−0.016+0.014​ 0.97 6 − 0.016 + 0.014 . We show that the second-scale entanglement lifetimes are limited solely by these errors, providing opportunities for research in quantum-enhanced metrology 7,13 , ultracold chemistry 14 and the use of rotational states in quantum simulation, quantum computation and as quantum memories. The extension of precise quantum control to complex molecular systems will enable their additional degrees of freedom to be exploited across many domains of quantum science 15–17 .