Perpetual emulation threshold of $PT$-symmetric Hamiltonians

We describe a technique to emulate the dynamics of two-level -symmetric spin Hamiltonians, replete with gain and loss, using the unitary dynamics of a larger quantum system. The two-level system in question is embedded in a subspace of a four-level Hamiltonian, with the exterior levels acting as reservoirs. The emulation time is normally finite, limited by the depletion of the reservoirs. We show that it is possible to emulate the desired behaviour of the -symmetric Hamiltonian without depleting the reservoir levels, by including an additional coupling between them. This extends the emulation time indefinitely, when in the unbroken symmetry phase of the non-unitary dynamics. We propose a realistic experimental implementation using dynamically decoupled magnetic sublevels of ultracold atoms. (paper)