Project C2: Optically Addressable Spins of Molecular Rare-Earth Ions for Quantum Information Processing

The aim of the project is to investigate and establish molecular lanthanide (Ln) or rare-earth ions (REIs) for quantum information processing (QIP) and networking applications. REI, so far mostly studied in solid state crystals, provide nuclear spin states with very long coherence times in which quantum states can be stored. They also offer 4f-4f optical transitions with exceptionally good coherence properties that can be used for direct optical readout and control of the spin states. In particular, they are prime candidates to realize long-lived quantum memories and qubits with the capability to perform quantum logic gates. So far, most work in this direction has been performed with macroscopic ensembles of REIs doped into inorganic oxide crystals (host lattices).
To realize quantum information elements such as multi-qubit quantum nodes, quantum repeaters, and quantum networks in a scalable manner, it is highly desirable to achieve control and readout of REIs in microscopic ensembles and eventually at the single-ion level. Due to the dipole-forbidden character of the coherent 4f-4f optical transitions, the absorption and emission rates are extremely low; therefore, addressing and readout of small ensembles or single ions are very challenging. To counteract this weak light-matter coupling, engineering of the photonic environment can be used to enhance the interaction.
In this context, molecular materials appear as a promising novel direction for the following reasons. First, chemical design of the ligand field can be used to maximally induce the transitions, enabling an efficient optical interface for qubit operations. Second, the shape and size variety of molecular crystals facilitate integration into photonic structures for further enhancement of light-matter interactions. Third, minimization of multiphonon relaxation through deuteration can be used to achieve high quantum efficiencies, i.e., here specifically a high radiative fraction of the Ln centered 4f-4f transition. Fourth, by using isotopically enriched REIs, hyperfine energetics can be controlled. Fifth, increase of the spin qubit Hilbert space by the use of qudits (discrete multinuclear REI complexes) can enable scalable quantum processing units. Alternatively, REI complexes can self-assemble in highly ordered architectures following molecular self-assembly strategies, addressing qubit scalability.
As an additional aspect, studies of stoichiometric REI crystals indicate that long optical coherence times and extraordinary large optical depths can be obtained in such crystals, rendering them particularly suitable for quantum memory applications. The prospect of following this approach with molecular REI complexes that allow for extended control – for example, over ion-ion distances – is promising considering the small doping concentration (0.001 % to 0.05 %) of REI-impurity centres typically studied in host lattices.
The objectives of the project are to realize molecular rare-earth complexes with tailored optical and spin properties and investigate optical spin readout towards the goal of single-ion detection and qubit control by coupling ions to optical microcavities. 

Project leaders: 
Prof. Dr. David Hunger (KIT) 
Prof. Dr. Mario Ruben (KIT) 
Prof. Dr. Michael Seitz (UTÜ)