(IBM.ResearchBlog) As of Aug 8, 2021, the topology of all active IBM Quantum devices will use the heavy-hex lattice, including the IBM Quantum System One’s Falcon processors installed in Germany and Japan.
The heavy-hex lattice represents the fourth iteration of the topology for IBM Quantum systems and is the basis for the Falcon and Hummingbird quantum processor architectures. Each unit cell of the lattice consists of a hexagonal arrangement of qubits, with an additional qubit on each edge.
The heavy-hex topology is a product of co-design between experiment, theory, and applications, that is scalable and offers reduced error-rates while affording the opportunity to explore error correcting codes. Based on lessons learned from earlier systems, the heavy-hex topology represents a slight reduction in qubit connectivity from previous generation systems, but, crucially, minimizes both qubit frequency collisions and spectator qubit errors that are detrimental to real-world quantum application performance.
IBM believes that the heavy hex lattice is superior to a square lattice in offering a clear path to quantum advantage, from enabling more accurate near-term experimentation to reaching the critical goal of demonstrating fault tolerant error correction. We demonstrate that the heavy-hex lattice is equivalent to the square lattice up to a constant overhead, and like other constant overheads such as choice of gate set, this cost is insignificant compared to the cost of mapping the problem itself.