Hybrid quantum systems: integrating atomic and solid-state qubits

Full text: For quantum information to emerge as a valuable technology, it is mandatory to pool the strengths of different systems to bridge their weaknesses. For example solid-state devices allow very fast processing and promise dense integration, but have very short coherence times. Atomic systems are slow, but exhibit very long coherence of quantum states if they are stored in hyperfine states. An ensemble of ultracold atoms would thus be an ideal quantum memory. Placing an ensemble of 106 ultra-cold atoms in the near field of a superconducting coplanar waveguide resonator (CPWR) with a quality-factor of Q = 106, one can achieve strong coupling between a single microwave photon in the CPWR and a collective hyperfine qubit state in the ensemble with an effective coupling rate larger than the cavity line width. Integrated on an AtomChip such a system constitutes a hybrid quantum device, which interconnects solid-state and atomic qubits. We present theoretical results concerning the coupling of a Bose-Einstein condensate (BEC) to a single photon stored in a CPWR on an AtomChip. Starting from the analytical expression for the quasi-TEM odd mode of the field generated by the photon in the CPWR, we construct the full quantum mechanical Jaynes-Cummings model of the system BEC + CPW photon. The Rabi-splitting of the resonance curve of the CPW-cavity is calculated analytically as a function of several parameters, like the number of atoms in the BEC, the position and the distance of the atom-cloud above the chip's surface, the geometry and the capacitive coupling of the CPWR, etc. We demonstrate the experimental accessibility of the strong coupling regime for the system BEC + CPWR with current available technologies. Further we present our ongoing experimental efforts on both the technology of CPW resonators as well as the experimental insertion/creation of a cloud of cold atoms in a dilution fridge at a temperature of 20 mK. (author)