The work includes development of quantum memories for quantum repeaters for long-distance quantum cryptography. Our quantum hardware, rare-earth-ion-doped crystals, is kept at cryogenic temperatures (~2 K). Carefully crafted light pulses and specially designed laser–matter interaction schemes are used to manipulate and fully control the wave functions of these quantum mechanical systems.
For quantum gates and quantum processing schemes Qubits, or quantum bits, are prepared in well-defined initial states; qubit state-to-state transfer efficiency are >97%, and arbitrary single qubit operations can be carried out with fidelities >90%, as determined by full quantum state tomography. Developing the hardware required for studies of quantum information is a highly challenging and inspiring task. Controlling and mastering the phase of the wave function is key to utilizing the full power of quantum mechanical systems for applications in information science, as well as in other areas. This requires a laser system with a coherence time on par or better than that of the atomic medium. At the LLC, a dye-laser system has been stabilized towards a target line width ~10 Hz and a fully computerized light pulse amplitude and phase control system has been developed.
Ongoing work includes quantum memories, developing ensemble based two-qubit gates and implementing quantum processing schemes with improved scalability, particularly single ion detection schemes. These single ion schemes involve either Purcell enhancement of weak transitions using micro-cavities or dedicated readout ions (Ce) with closed transitions that can be cycled.