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Controlling single polyatomic molecules in an optical array for quantum applications
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This is a summary of: Vilas, N. B. et al . An optical tweezer array of ultracold polyatomic molecules. Nature 628 , 282–286 (2024) .
The project .
Cooling atoms to ultracold temperatures of less than one millikelvin and controlling their internal energy states has led to the development of numerous technologies, including optical atomic clocks 1 and neutral-atom quantum computers 2 . Unlike atoms, molecules can rotate and vibrate, making them more challenging to control, but providing richer internal structures that might be harnessed for further quantum-science applications.
Optical tweezer arrays are a promising platform for realizing this sort of control. Each ‘tweezer’ is a tightly focused laser beam (about one micrometre in radius) that can hold on to a single atom or molecule, allowing it to be manipulated and placed in arbitrary positions. This approach has the added benefit of preventing unwanted collisions or chemical reactions, something that is particularly relevant for molecules.
Optical tweezers have previously been used with some success to control atoms and diatomic molecules 3 . However, polyatomic molecules have advantages for certain applications, meaning that a tweezer array for manipulating them is a desirable, albeit technically challenging, goal.
The solution .
We created an optical tweezer array with calcium hydroxide (CaOH) molecules, which we had previously cooled and trapped in a vacuum chamber at temperatures below 100 microkelvin using laser cooling 4 , 5 . We focused an array of six optical tweezer beams derived from a single laser into the vacuum chamber and loaded single CaOH molecules into the array, relying on a previously established technique in which molecules quickly collide and are ejected from the tweezer whenever there is more than one present.
The principal challenge of this work was to image the molecules and to determine whether a tweezer was loaded, without destroying the molecule in the process. This required illuminating a molecule with further lasers to excite an electronic transition, then collecting the resulting fluorescence before the molecule was either lost from the tweezer or stopped fluorescing owing to its complicated structure.
We found that a primary limitation to the imaging was the interaction of the tweezer laser beams with the complex structure of the molecules. By carefully tuning the wavelength of the tweezer laser, we were able to mitigate this effect and correctly detect a molecule about 90% of the time (Fig. 1), while keeping it in the tweezer array. After detecting the molecules in this way, we then pumped the molecules into a single internal quantum state and applied a combination of laser, microwave and radio-frequency fields to control their vibrational, rotational and electron- or nuclear-spin structure. We then imaged the molecules again to determine the outcome of these control operations. The experiments demonstrated the key tools needed for quantum-science applications with polyatomic molecules in tweezer arrays.
Figure 1 An optical tweezer array of polyatomic molecules. a , Image of calcium hydroxide molecules in the tweezer array. Each spot is a single tweezer containing one molecule, and the image is averaged over hundreds of iterations of the experiment. Scale bar, 5 ?m. b , Histograms of the number of fluorescence counts collected from the tweezers over thousands of experimental iterations. The bimodal distribution occurs because the tweezer can be either empty or loaded with a single molecule, and the two colours correspond to the loading probabilities per experimental shot of 31% (orange) and 13% (purple).
The implications .
The diverse structures available in polyatomic molecules could aid research in quantum computing, quantum simulation, quantum chemistry, ultracold molecular collisions and precision searches for particle-physics effects beyond the standard model. Each of these applications requires control of individual quantum states and would benefit from the versatility of optical tweezer arrays.
Although we have demonstrated the key techniques required to control polyatomic molecules in optical tweezers, improvements are still required to achieve many of the long-term goals outlined above. Existing optical-tweezer experiments can routinely image and control individual atoms with errors well below 1%, but the molecules in our demonstration were more frequently lost or misidentified during imaging, and the single-state control techniques also had sizeable infidelities. Many of these limitations could be overcome with technical upgrades to our experimental apparatus.
Although CaOH possesses many relevant characteristics of polyatomic molecules, including numerous rotational and vibrational degrees of freedom, certain experiments could require larger, more complex polyatomic molecules, which come with additional challenges. In the future, it might be possible to extend these methods to larger, more complex, laser-coolable polyatomic molecules, thereby making optical tweezer techniques even more versatile. — Nathaniel B. Vilas and John M. Doyle are at Harvard University, Cambridge, Massachusetts, USA.
Expert opinion .
These results represent an important and exceptional step towards the creation of cold samples of polyatomic molecules. They might have a substantial impact in several areas, such as quantum information, quantum chemistry and fundamental physics, where controlled samples of ultracold molecules are needed. — Rosario González Férez is at the University of Granada, Granada, Spain.
Behind the paper .
After implementing laser-cooling techniques for CaOH over the past few years, a major goal was to load the molecules into optical tweezers. A common refrain in the molecular laser-cooling community is that molecules behave similarly to atoms (for which cooling and trapping techniques are well established) once optically trapped. We therefore expected that loading and imaging a tweezer array would be relatively straightforward, aside from the anticipated technical challenges of creating the tightly focused tweezers and contending with the relatively low number of molecules that can be cooled in our experiment. It thus came as a surprise when we couldn’t image the molecules right away — prompting a detailed and, ultimately, fruitful exploration of how molecular complexity affected tweezer trapping and imaging. Although the project succeeded in the end, the molecules never allowed us to forget their rich underlying structure. — N.B.V. and J.M.D.
From the editor .
Quantum and atomic physicists have only recently mastered the ability to use light to control individual simple molecules to encode information or to work as a simulator. This work now shows how remarkably effective these control techniques can be in also handling polyatomic molecules, which were hitherto left largely untamed. — Federico Levi, Senior Editor and Team Manager, Nature .
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