Report on the DFG Project "Approaching Superconductor/Cold Atom Hybrid Quantum Circuits " (KL 930_16-1)

Abstract

The project was intended to realize superconductor-cold atom hybrid systems, whereby the atoms are captured and manipulated in the vicinity of a superconducting chip. The chip should operate in the millikelvin range, which enables both subsystems to operate in the quantum regime. The vision: In this geometry, the methods of solid-state physics and quantum optics can be combined to manipulate the atoms or solid-state circuits directly or via photons from microwave to optical frequencies. If successful, the hybrid system offers unique opportunities to study the coupling between macroscopic objects (superconducting qubits, resonators) and natural atoms. In the context of quantum information, one can imagine a hybrid in which the superconducting circuit acts as a processor and the atoms act as a quantum memory. Cold atoms coupled to superconducting resonators could also enable the realization of new quantum gates. At the start of the project, a cold atom/superconductor setup was already in operation, working at a bath temperature of 4.2 K. Measurement systems were also available that allow superconducting microwave structures to be examined in the absence of cold atoms. In these setups, concepts for atom-superconductor coupling could be developed and tested. On the atom side, the focus was on Rydberg atoms, which offer a variety of resonant transitions and enable strong electrical dipole coupling to the superconducting devices. On the superconducting side, chips have been developed that contain resonators optimized for coupling to Rydberg atoms and can be combined with structures to trap the atoms. Originally, magnetic trap structures were considered but, as the project progressed, the focus went to optical dipole traps. A UHV millikelvin system was also available at the start of the project and was designed to incorporate the superconductor-cold atom hybrid systems. In this cryostat, part of the transport path for capturing the atoms and transporting them magnetically to the millikelvin level was demonstrated as part of a previous project. The transport path was completely rebuilt in the project, including the necessary laser and microwave components. A suitable superconducting chip was also installed at the end of the transport path. Unfortunately, the construction of the coil system required for transport turned out to be considerably more difficult than expected and took until the end of the project. The system is now functional, although the actual experiments on the coupled superconductor-cold atom hybrid systems are reserved for further projects.