Partial decoherence in a qubit coupled to a control register subject to telegraph noise

A qubit (containing two quantum states, 1 and 2), is coupled to a control register (state 3), which is subject to telegraph noise. We study the time evolution of the density matrix $\rho$ of an electron which starts in some coherent state on the qubit. At infinite time, $\rho$ usually approaches the fully decoherent state, with ${\rho }_{nm}^{}={\delta }_{nm}^{}\mathrm{/}3$. However, when the Hamiltonian is symmetric under $1\leftrightarrow 2$, the element ${\rho }_{12}^{}$ approaches a non-zero real value, implying a partial coherence of the asymptotic state. The asymptotic density matrix depends only on ${\rm Re}[\rho^{}_{12}(t=0)]$. In several cases, the information stored on the qubit is protected from the noise.