Dark Spin-Cat States as Biased Qubits

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 Fg≫1 to an excited Zeeman spin manifold of size Fe=Fg−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 Fg, 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 Fg relative to the dephasing rate. We provide an analysis of dark spin cats and their robustness to noise, and we discuss bias-preserving single qubit and entangling gates, exemplified on a Rydberg tweezer platform.