Strontium CIrcular Rydberg Quantum Simulator – SCiRQ

Quantum simulation is one of the most advanced quantum technologies. Out of the setup developed during the last few years, Rydberg atoms arrays are one of the most (if not the most) advanced platform. Systems based on alkali Rydberg atoms array can emulate arbitrary spin Hamiltonian with up to 200 particles, well beyond the ability of a classical computer. However, the duration of the simulation is currently restricted to a few microseconds by the lifetime of the laser-accessible low angular momentum Rydberg states. Switching to circular Rydberg state, which have a much longer lifetime, would overcome this limit. However, alkali circular atoms are difficult to manipulate with optical light.
The long-term goal of the SCiRQ project is to develop a quantum simulator based on circular states of strontium. Rydberg states of alkaline earth elements have an optically active ionic core that can be used to manipulate the atom. The proposed system will offer many advantages as compared to the alkali atom existing platforms. The second valence electron will allow us to trap the atom with simple Gaussian beams using the polarizability of the ionic core. Scattering photons on the ionic core transition will make it possible to detect the Rydberg atoms by direct fluorescence recorded by a camera whereas, in most simulators, the Rydberg states are detected as a loss of atom trapped in the ground state that leads to a dark spot on the final image. The second electron will also provide a way to cool down the motion of the circular Rydberg atom, limiting the heating of the atom array occurring during the simulation. The residual electrostatic coupling between the two valence electrons can be used to manipulate the state of the Rydberg electron using laser light interacting with the ionic core, performing local operations at a given site of the array with a micrometer spatial resolution. The coupling between the two electrons, combined with the shelving techniques of the ion trap tool box, also opens the way to encoding the state of the Rydberg atom into one of the metastable states of the core in order to perform state-selective fluorescence of the circular Rydberg atom.
In the framework of the SCiRQ project, the two partners LKB and LIDYL will pair up the experimental expertise in alkaline earth circular Rydberg atoms of LKB with the deep theoretical knowledge of doubly excited two-electron atoms of LIDYL in order to prepare an array of circular Rydberg atoms of strontium trapped in optical tweezers inside a cryostat. Within the next four years, we plan to demonstrate a set of four key features (trapping, cooling, individual addressing, state-selective spatially-resolved detection) that will show the potential of a quantum simulator encoding spin arrays onto two circular states of strontium.
Each tweezer will trap a Rydberg electron in a circular state holding in its center a Sr+ ion, to which it is coupled through the electrostatic interaction. The simulator will benefit both from the flexibility of the Rydberg/neutral atom manipulation and from the richness of the ion trap toolbox, merging in a single system the advantages of two of the most advanced platforms that exist today, and that will be ready to study the long-term dynamics of arbitrary spin Hamiltonians. The project will also have impact beyond the LKB-LIDYL collaboration. The expertise developed for the preparation of circular states of strontium will benefit to many fields of quantum technology. The coupling of the Rydberg electron to the ionic core makes it possible to use the clock transition of the Sr+ ion as a quantum memory to store the state of Rydberg qubit. It also opens the way to using circular Rydberg atoms for the coherent conversion of microwave to optical photons. Room-temperature adaptations of the experiment offer interesting perspectives for quantum metrology.