Coupling qubits on separate chips in a tileable superconducting circuit

Superconducting quantum circuits are a promising hardware platform for the simulation of lattice Hamiltonians, but circuits are typically defined on a planar surface, limiting connectivity to 2D.

Here, we present an approach to go beyond 2D that makes use of out-of-plane capacitive coupling, readout and control of qubits with coaxial symmetry.  Two layers of qubits can be fabricated on different substrates and kept separated by the sample holder by a macroscopic distance, meaning no fabrication complexity is added to achieve multiple qubit layers.

We present a single unit-cell demonstration comprising two out-of-plane coupled fixed-frequency transmon qubits.  We show the architecture is compatible with high coherence and high fidelity single-qubit operations, and demonstrate a conditional phase gate between the two qubits on separate substrates. Our proof-of-principle experiment employs fixed-frequency transmon qubits, but can be modified to use flux-tunable and multi-mode qubits to explore different kinds of lattice and interaction.