Small logical qubit architecture based on strong interactions and many-body dynamical decoupling

TL;DR

This paper introduces the Cold Echo Qubit, a superconducting logical qubit architecture that uses strong interactions and many-body dynamical decoupling to significantly extend quantum coherence times without measurement or feedback.

Contribution

It presents a novel superconducting logical qubit design that enhances coherence and gate fidelity through autonomous protection mechanisms, demonstrated with simple fluxonium-based implementations.

Findings

Predicted order-of-magnitude improvements in lifetime and fidelity.

Simple fluxonium implementation can surpass component coherence times.

More complex circuits can further double coherence times.

Abstract

We propose 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 relatively 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. Here, we consider the simplest possible implementation of the CEQ, using a pair of fluxonium qubits shunted through a shared mutual inductance. While not necessarily the best possible implementation, it 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 presented; it is expected to roughly double the coherence of its two-fluxonium counterpart.