The Cold Echo Qubit: a Novel Small Logical Qubit Architecture Inspired by AC-Driven Quantum Annealing*

We describe a novel superconducting logical qubit architecture, called the Cold Echo Qubit (CEQ), which is capable of preserving quantum information for much longer timescales than any of its component parts. The CEQ operates fully autonomously, requiring no measurement or feedback, and is compatible with strong interaction elements, allowing for fast, high fidelity logical gates between multiple CEQ's. Its quantum state is protected by a combination of strong interactions and microwave driving, which implements a form of many-body dynamical decoupling to suppress phase noise. Estimates based on careful theoretical analysis and numerical simulations predict improvements in lifetimes and gate fidelities by an order of magnitude or more compared to the current state of the art, assuming no improvements in base coherence. This talk focuses on the simplest possible implementation of the CEQ, using a pair of fluxonium qubits shunted through a shared mutual inductance. This version is the easiest to test experimentally and should display coherence well past breakeven (as compared to the limiting coherence times of its components). A more complex three-node circuit is also discussed; it is expected to roughly double the coherence of its two-fluxonium counterpart.