From quantum gyroscopes to quantum memories: Harnessing noise to protect quantum information.

The quantum Zeno effect predicts that frequent measurements will stabilize a quantum state. Here we show that a similar effect can be obtained by adding a strong noise that leaves a quantum state unchanged, decoupling it from other noise components that otherwise would lead to decoherence. For instance, strong longitudinal Z noise can protect the population states of a single physical qubit from a transverse noise X, provided some conditions on their respective amplitude and statistics. We discuss the implications of this protection on the dynamics of a quantum gyroscope based on the use of nuclear spin population states as an application. While such strong noise can be used to protect trivial classical states, Z^⊕2 noise can be used to protect the coherences between a logical basis of 2 physical qubits in a Decoherence-Free Subspace (DFS) that is invariant under such collective noise, from non-symmetrical noises that drive leakage outside of the DFS. We discuss the nature of this competition in light of its connection to the quantum Zeno effect, generalizations to N-qubit systems, and practical challenges and approaches to realize such protection. Our work paves the way for using environmental noise to protect quantum information rather than a source of decoherence, enabling applications such as long-lived quantum memories.