  Development of robust quantum control has been challenging and there are
numerous obstacles to applying classical robust control to quantum system
including bilinearity, marginal stability, state preparation errors, nonlinear
figures of merit. The requirement of marginal stability, while not satisfied
for closed quantum systems, can be satisfied for open quantum systems where
Lindbladian behavior leads to non-unitary evolution, and allows for nonzero
classical stability margins, but it remains difficult to extract physical
insight when classical robust control tools are applied to these systems. We
consider a straightforward example of the entanglement between two qubits
dissipatively coupled to a lossy cavity and analyze it using the classical
stability margin and structured perturbations. We attempt, where possible, to
extract physical insight from these analyses. Our aim is to highlight where
classical robust control can assist in the analysis of quantum systems and
identify areas where more work needs to be done to develop specific methods for
quantum robust control.
