Optimal control approach to dynamical suppression of decoherence of a qubit

We investigate optimal control of a single qubit coupled to an ohmic heat bath. For the weak bath coupling regime, we derive a Bloch-Redfield master equation describing the evolution of the qubit state parameterized by vectors in the Bloch sphere. By use of the optimal control methodology, we determine a field that generates a single-qubit rotation. We use techniques of automatic differentiation to compute the gradient for the cost functional.