Hybrid Quantum Devices
The goal of the research group of Johannes Majer (JM) is to bring together all the fantastic solid-state science developed recently for quantum information research: superconducting qubits and Circuit Cavity Quantum Electro Dynamics (C-CQED), with the science developed for atomic or molecular physics. Research of the JM group towards this ambitious goal will be establish by connecting the fast processing in a solid-state device to the exceptional long coherence times in an atomic/molecular system as a quantum memory. It will add essential features, presently missing in existing quantum information processing (QIP) and furthermore realize an interface between an optical photon (travelling qubit) and a microwave photon (stationary qubit). One of the aims of the JM group at realizing such a hybrid quantum devices is to couple a superconducting transmission line resonator to color centers in a diamond.
Recently a particular color center, the nitrogen-vacancy center has drawn a lot of interest in the context of quantum information. This center consists of a nitrogen atom replacing a carbon atom, combined with an adjacent vacant lattice site, hence nitrogen-vacancy (NV). Negatively charged, the ground state of this center is a spin 1 system. A crystal field splits up this ground state leading to a transition frequency of 2.88 GHz. These three levels are very weekly coupled to lattice vibrations, which leads to very long coherence times even at room temperature. In parallel, superconducting circuits have seen an enormous progress in the past years. In 2004 such a superconducting qubit has been coupled to a superconducting transmission line resonator, which lead the new field of circuit cavity quantum electrodynamics (circuit CQED). This allowed experiments being performed on a chip at the single quantum level previously only possible with atoms.
Recently the JM group has combined the two systems by placing a diamond with a high concentration of nitrogen–vacancies on top of a superconducting resonator. It was shown a strong coupling between the superconducting circuit and the ensemble of nitrogen vacancies. Additionally, a weak coupling to 13C nuclei was observed, which opens the possibility of a nuclear quantum memory with very long coherence time. In the context of Solids4Fun the aim of the group is to further develop this hybrid quantum system with the aim of building a long-lived quantum memory. Furthermore, the group will make use of the optical transitions of the NV system, which ultimately allows converting an optical photon into a microwave photon and vice versa.