Dark spin-cats as biased qubits

TL;DR

This paper introduces a new type of atomic qubit called the 'dark spin-cat', which is robust against noise and can be implemented across various atomic platforms using light coupling of Zeeman levels.

Contribution

The paper proposes a novel 'dark spin-cat' qubit encoding that is immune to spontaneous emission and exhibits exponentially suppressed bit-flip errors with increasing ground state spin size.

Findings

Dark spin-cat states are decoupled from light and immune to spontaneous emission.

Bit-flip error rate decreases exponentially with ground state spin size.

Robustness of dark spin-cats to noise demonstrated and potential for bias-preserving gates discussed.

Abstract

We present a biased atomic qubit, universally implementable across all atomic platforms, encoded as a `spin-cat' within ground state Zeeman levels. The key characteristic of our configuration is the coupling of the ground state spin manifold of size $F_g \gg 1$ to an excited Zeeman spin manifold of size $F_e = F_g - 1$ using light. This coupling results in eigenstates of the driven atom that include exactly two dark states in the ground state manifold, which are decoupled from light and immune to spontaneous emission from the excited states. These dark states constitute the `spin-cat', leading to the designation `dark spin-cat'. We demonstrate that under strong Rabi drive and for large $F_g$, the `dark spin-cat' is autonomously stabilized against common noise sources and encodes a qubit with significantly biased noise. Specifically, the bit-flip error rate decreases exponentially with $F_g$ relative to the dephasing rate. We provide an analysis of dark spin-cats, their robustness to noise, and discuss bias-preserving single qubit and entangling gates, exemplified on a Rydberg tweezer platform.