Quantum wall states for noise mitigation and eternal purity bounds

The present work analyzes state-stabilization techniques for decoupling a subsystem from environmental interactions. The proposed framework uses analytical and numerical tools to find an approximate decoherence-free subspace with improved passive noise isolation. Active state-stabilizing control on a subsystem mediating dominant environmental interactions, which we call the wall subsystem, creates an effective quantum wall state. The proposed method controls only the wall subsystem, leaving the logical subsystem untouched. This simplifies logic operations in the protected subsystem, and makes it suitable for integration with other quantum information protection techniques, such as dynamical decoupling (DD). We demonstrated its effectiveness in improving the performance of selective or complete DD. Under suitable conditions, our method maintains the purity of the system above a threshold for all times, achieving eternal purity preservation. Theoretical analysis links this behavior to the asymptotic spectrum of the Hamiltonian when the control gain grows unbounded.