Scalable device structure for large-scale superconducting quantum annealing machines

Practical superconducting quantum annealing machines are expected to have more than one million qubits. However, the number of qubits on a chip is limited to several tens of thousands, because miniaturization of flux-qubits which are the optimum qubit for annealing is restricted by the value of single flux quanta. We propose a multi-chip device structure called QUIP (Qubit-chip/ Interposer/ Package-substrate) in which qubit chips consisting of qubits and couplers are flip-chip connected to active interposers including readout and control circuits and a number of them are placed on a package substrate which has electrical signal lines and I/O pads. Circuits on the active interposer are connected to the signal lines by way of TSVs in the interposers and the adjacent interposers are connected with a bridge interposer. We think this 2.5D structure is the best one for the annealing devices, because QUIP can expand to horizontal direction with keeping same temperature of qubits. We estimated a quantum annealing device consisting of one million qubits was implemented on a 90 mm square package substrate by using the QUIP structure.