The precise control of micro-mechanical oscillators is fundamental to many contemporary techno­logies, from sensing and timing to radiofrequency filters in smartphones. Over the past decade, quantum control of mechanical systems has been firmly established with atoms, molecules, and ions in the first wave of development and super­conducting circuits in the second quantum revolution. This has been in particular catalyzed by cavity opto­mechanics. The field has allowed us to control mesoscopic mechanical objects with electromagnetic radiation pressure force. This has substantially improved our understanding of their quantum nature, which has enabled a host of advances including ground-state cooling, quantum squeezing, and remote ent­anglement of mechanical oscillators.

Pioneering theoretical studies have predicted that significantly richer physics and novel dynamics can be accessed in opto­mechanical lattices, including quantum collective dynamics and topological phenomena. But reproducing such devices experi­mentally under high control, as well as and building optomechanical lattices that can host multiple coupled optical and mechanical degrees of freedom has been a challenge. Researchers in the group of Tobias J. Kippen­berg at EPFL’s School of Basic Sciences have now built the first large-scale and configurable superconducting circuit optomechanical lattice that can overcome the scaling challenges of quantum optomechanical systems. The team realized an opto­mechanical strained graphene lattice and studied non-trivial topological edge states using novel measurement techniques. 

The key element, which is a part of the single site of the lattice, is a vacuum-gap drumhead capacitor, which is made of a thin aluminium film suspended over a trench in a silicon substrate. This constitutes the vibrating part of the device and, at the same time, forms a resonant microwave circuit with a spiral inductor. “We developed a novel nano­fabrication technique for super­conducting circuit opto­mechanical systems with high reproduci­bility and extremely tight tolerances on the parameters of the individual devices,” says Amir Youssefi, who led the project. “This enables us to engineer the different sites to be virtually identical, like in a natural lattice.”

​​​​​​The graphene lattice is well known to exhibit non-trivial topo­logical properties and localized edge states. The researchers observed such states in what they call an “optomechanical graphene flake”, consisting of twenty-four sites. “Thanks to the built-in opto­mechanical toolkit, we were able to directly and non-pertur­batively image the collective electro­magnetic mode shapes in such lattices,” says Andrea Bancora who contributed to the research. “This is a unique feature of this platform.” The team’s measurements closely match the theoretical predictions, showing that their new platform is a reliable testbed for studying topo­logical physics in one and two-dimensional lattices.

“By having access to both the energy levels and mode shapes of these collective exci­tations, we were able to reconstruct the full underlying Hamiltonian of the system, allowing for the full extraction of disorder and coupling strengths in a super­conducting lattice for the first time,” says Shingo Kono, another member of the research team. The demonstration of opto­mechanical lattices not only provides access to studying many-body physics in such realizations of condensed matter lattice models but will also provide a route towards novel hybrid quantum systems when combined with super­conducting qubits. (Source: EPFL)

Reference: A. Youssefi et al.: Topological lattices realized in superconducting circuit optomechanics, Nature 612, 666 (2022); DOI: 10.1038/s41586-022-05367-9

Link: Laboratory of Photonics and Quantum Measurements, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland