High-connectivity quantum processor nodes using single-ion qubits in rare-earth-ion-doped crystals

We present two protocols for constructing quantum processor nodes in randomly doped rare-earth-ion crystals and analyze their properties. By varying the doping concentration and the accessible laser tunability, the processor nodes can contain anywhere from only a few tens to almost 1000 qubits. Furthermore, the average number of qubits each qubit can interact with, denoted by the connectivity, can be partly tailored to lie between just a few and roughly 100. We also study how a limited tunability of the laser affects the results and conclude that a tuning range of 100 GHz limits the results to roughly 100 qubits with around 50 connections per qubit on average. In order to construct an even larger processor, the vision is that several of these quantum processor nodes should be connected to each other in a multinode architecture via, e.g., optical interfaces or flying qubits in the form of light. Our results are encouraging for establishing the rare-earth-ion-based systems as a quantum computing platform with strong potential and can serve to focus the efforts within the field.