There seems to be a good deal of activity in quantum computing implementations using qubits based on spin states of one or two electrons. Alternative implementations involve photons, ions, or several other possibilities. So far, spin qubit implementations have been lagging somewhat. This is even though there are advantages of using spin qubits, since the implementation can involve solid-state semiconductor technology, which of course is now very sophisticated.
New research now seems to be advancing spin qubit technology. Among the earlier challenges were reading out single qubit states and addressing individual qubits among others in a group. Solutions to these issues has made possible single and double qubit operations. So the next stage is being able to deal with more than two spin qubits at a time in a solid-state system.
Over the last decade, the experimental emphasis for spin qubits has been on demonstrating the required criteria for a viable scheme for quantum computing. This has been driven primarily by groups at Harvard and Delft universities. The next stage is to go to higher numbers of coupled qubits and demonstrate more complex quantum gate operations and algorithms. The paper by Brunner et al. is a necessary step forward. It is clear that the spin qubit system currently lags behind other quantum computer implementation schemes. Solid-state based schemes, especially semiconductor ones, have always held the promise, however, that the enormous progress from decades of device integration technology development could one day lead to scalability not feasible with other schemes. To achieve this, however, we need parallel work on spin qubits in different materials to optimize coherence times, device designs, and architectures and to explore hybrid technology based on exploiting the most useful properties of different schemes.