The quest for a better theoretical understanding and experimental exploitation of many-body phenomena motivates us to develop and apply innovative control approaches as well as numerical simulation techniques such as tensor network algorithms.
Several platforms are nowadays available for realising so-called “Synthetic Quantum Matter”, the theoretical setting of which is the overall goal of our subdivision’s research activity. A paradigmatic example is the quantum optical shaping of cold atomic gases, but many others are also promising.
Achieving new phases “on-demand” is of primary interest not only for fundamental questions within the condensed matter and quantum information communities, but also in view of the emerging interest in applications for quantum technology (as affirmed by the EU-Flagship and the German Federal initiative for the upcoming five to ten years).
The central theme of our investigation is the combination of geometrical constraints, different types and ranges of interactions and (synthetic) gauge fields to access
- interacting topological states of matter and
- many-body effects in the transport properties of low-dimensional systems.
The goal is to formulate concrete experimental proposals for cold atomic gases, photonic waveguides, superconducting Josephson arrays, or artificially grown materials, just to mention a few set-ups.
In order to broaden our understanding, besides analytical mappings onto effective models, we routinely exploit numerical techniques inspired by quantum information, namely tensor network algorithms. This also leads to insights into the entanglement structure of correlated states. Numerical simulations could serve to tailor and validate the experimental setups before employing them to explore regimes that are classically hard to compute.
The fattest Schrödinger Cat state
Quantum entanglement involving coherent superposition of macroscopically distinct states is crucial for fundamental investigations in quantum physics as well as for applications in quantum information processing and other quantum technologies. Greenberger-Horne-Zeilinger (GHZ) states constitute an important class of genuine multipartite entangled states, which provide an essential resource for applications such as quantum metrology, quantum error correction, quantum information processing and many others. However, these states are extremely fragile since an individual error on any of the constitute qubits can destroy the entire coherent supposition.
A team of scientists from Harvard, Padova and Jülich has generated GHZ states with up to 20 qubits by using optimal control methods to suppress the detrimental effects in experiments based on Rydberg atoms that are individually trapped in an array of optical tweezers. The number of qubits for the prepared GHZ state has exceeded those of all previous efforts on various platforms ranging from optical photons, trapped ions and superconducting quantum circuits, thus establishing a new world record.
One essential ingredient in this success in preparing large-scale GHZ states is the use of the optimal control method known as RedCRAB, a version of the dressed Chopped Random Basis (dCRAB) optimal control which is accessible via a remote cloud server. The control fields of the experiments were adjusted in the optimization by adding modulation terms that were expanded into randomized Fourier functions. Then the coefficients of the Fourier functions were updated iteratively using the Nelder-Mead algorithm to improve their performance.
We have further demonstrated the entanglement manipulation by using the prepared GHZ states to distribute entanglement to distant sites in the array, establishing important ingredients for quantum information processing as well as quantum metrology, and providing a powerful toolbox for quantum simulation.
This work is supported by the DFG project SPP 1929 (GiRyd), in which we will exploit the giant interactions between Rydberg atoms to create high fidelity entangled many-body states to investigate fundamental physical questions in quantum science, and PASQuanS project, which as part of the EU Quantum Flagship projects is going to perform a decisive transformative step for quantum simulation, towards programmable analogue simulators addressing important problems in fundamental science, materials development, quantum chemistry, as well as real-world problems of paramount importance in industry.
Omran, A., Levine, H., Keesling, A., Semeghini, G., Wang, T. T., Ebadi, S., Bernien, H., Zibrov, A. S., Pichler, H., Choi, S., Cui, J., Rossignolo, M., Rembold, P., Montangero, S., Calarco, T., Endres, M., Greiner, M., Vuletić, V., Lukin, M. D. (2019).
Generation and manipulation of Schrödinger cat states in Rydberg atom arrays. https://doi.org/10.1126/science.aax9743