Preparation of Arbitrary High‐Fidelity Decoherence‐Free States for a Four‐Qubit Hybrid System via Whispering‐Gallery‐Mode Microresonator

Abstract With the assistance of the practical nonlinear interaction of a nitrogen‐vacancy (NV) center coupled with whispering‐gallery‐mode (WGM) microresonator, a heralded preparation protocol of four‐qubit hybrid decoherence‐free states (logical qubits) is presented, where a single logical qubit encoded on the state of the hybrid system composed of two polarization photons and two NV centers is fully protected against collective decoherence. Compared with the three‐qubit decoherence‐free states, the four‐qubit decoherence‐free states have higher dimensions, stronger collective decoherence resistance, richer quantum coherence and higher fault tolerance. For an ideal photon‐NV interaction, the efficiencies and fidelities of the hybrid decoherence‐free states can, in principle, be close to unity. Otherwise, being directed against a nonideal interaction, the two error‐heralded quantum blocks of the preparation process make their fidelities near‐perfect at the expense of slight reductions of their efficiencies. Moreover, this hybrid decoherence‐free states work in a heralded way and may be experimentally feasible with current technique on regulating the NV‐WGM microresonator system.