Architectural Considerations in the Design of a Superconducting Quantum Annealing Processor

We have developed a quantum annealing processor, based on an array of tunable coupled rf-SQUID flux qubits, fabricated in a superconducting integrated circuit process. Implementing this type of processor at a scale of 512 qubits and 1472 programmable interqubit couplers and operating at similar to 20 mK has required attention to a number of considerations that one may ignore at the smaller scale of a few dozen or so devices. Here, we discuss some of these considerations, and the delicate balance necessary for the construction of a practical processor that respects the demanding physical requirements imposed by a quantum algorithm. In particular, we will review some of the design tradeoffs at play in the floor planning of the physical layout, driven by the desire to have an algorithmically useful set of interqubit couplers, and the simultaneous need to embed programmable control circuitry into the processor fabric. In this context, we have developed a new ultralow-power embedded superconducting digital-to-analog flux converter (DAC) used to program the processor with zero static power dissipation, optimized to achieve maximum flux storage density per unit area. The 512 single-stage, 3520 two-stage, and 512 three-stage flux DACs are controlled with an XYZ addressing scheme requiring 56 wires. Our estimate of on-chip dissipated energy for worst-case reprogramming of the whole processor is similar to 65 fJ. Several chips based on this architecture have been fabricated and operated successfully at our facility, as well as two outside facilities (see, for example, the recent reporting by Jones).