New Molecular Resonances Could Enable Quantum Simulation of Many Body Systems

At the Max Planck Institute for Quantum Optics (MPQ), physicists have successfully created massive diatomic molecules and later detected them optically with the help of a high-resolution objective.

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Due to the microscopic size of traditional diatomic molecules in the sub-nanometer regime, direct optical resolution of their constituents is obstructed. Physicists at MPQ’s Quantum Many Body Division, headed by Professor Immanuel Bloch, successfully bound pairs of extremely excited atoms at 1 µm distance. Additionally, the large bond length—similar to tiny biological cells like the E. coli bacteria—enables a microscopic analysis of the underlying binding structure by directly and optically resolving the pair of bound atoms.

The interaction of all contributing electrons and the tiny size make it highly difficult to theoretically and experimentally examine the molecular bonds in an extremely comprehensive way. One cannot analytically calculate even the simple structure of atoms—the basic building blocks of chemical bonds. Only the hydrogen atom can be measured accurately. Within the periodic table, the hydrogen atom is the first and simplest element containing just a single electron and a single proton. The difficulty is further increased by the transition from atoms to molecules.

Since nearly all atoms on the Earth are bound in molecules, observing the molecular binding structure is important to figure out the material characteristics of the environment. Atoms containing only one electron in an extremely excited state, what is known as Rydberg atoms, transfer the basic structure of a hydrogen atom to more complex atoms because the one excited electron is in a remote distance from the other electrons and the nucleus. In addition, Rydberg atoms attracted a great deal of attention in the recent past because of their powerful interactions, which can be easily determined even at micron distance and are already utilized in the domain of quantum computation and quantum simulation. These interactions could now be used by the physicists, guided by Christian Groß and Immanuel Bloch, to bind a pair of Rydberg atoms with the help of laser light.

Due to the comparatively simple theory of Rydberg atoms, the spectroscopically resolved vibrational states of the resulting molecules are in quantitative agreement with the theoretically calculated energy levels. Furthermore, the large size allows for a direct microscopic access to the bond length and the orientation of the excited molecule.

Simon Hollerith, Study First Author and PhD Student, Quantum Many Body Systems Max Planck Institute for Quantum Optics

During the experiment, the team began with a two dimensional (2D) atom array with 0.53 µm interatomic distances, wherein each site of the array was originally occupied by precisely a single atom. Laser beams were interfered to produce the underlying optical lattice that pins the ground state atoms at the original position. Considering the fact that the related molecules were repelled from the optical lattice, the excitation of the molecules results in a couple of empty lattice sites divided by a bond length, which correlates to a distance of a lattice diagonal in the example of this work. Following an excitation pulse, the occupation of the lattice’s remaining atoms was determined with the help of a high-resolution objective, and molecules were eventually detected as correlated empty sites. Through this microscopic detection technique, the physicists could further demonstrate that the excited molecular orientation for varied molecular resonances was interchanging successively between perpendicular and parallel alignment in relation to the polarization of the excitation light. This is due to an interference effect on the basis of the electronic structure and also the molecule’s vibrational degree of freedom, which also predicted the theoretical anticipation.

In the days to come, the MPQ physicists intend to apply the novel molecular resonances for quantum simulation of several body systems. Additionally, the bound states of the pair of Rydberg atoms can be used for engineering huge interaction strengths at the distance of a bond length. This latest study has been published in Science on May 17^th, 2019 and funded by “Deutsche Forschungsgemeinschaft” and a number of EU projects, among them the EU Flagship for Quantum Technologies with the PASQUANS project