We revisit the problem of using a mechanical resonator to perform the transfer of a quantum state between two electromagnetic cavities (e.g., optical and microwave). We show that this system possesses an effective mechanically dark mode which is immune to mechanical dissipation; utilizing this feature allows highly efficient transfer of intracavity states, as well as of itinerant photon states. We provide simple analytic expressions for the fidelity for transferring both gaussian and non-gaussian states.

We show how strong steady-state entanglement can be achieved in a three-mode optomechanical system (or other parametrically-coupled bosonic system) by effectively laser-cooling a delocalized Bogoliubov mode. This approach allows one to surpass the bound on the maximum stationary intracavity entanglement possible with a coherent two-mode squeezing interaction. In particular, we find that optimizing the relative ratio of optomechanical couplings, rather than simply increasing their magnitudes, is essential for achieving strong entanglement. Unlike typical dissipative entanglement schemes, our results cannot be described by treating the effects of the entangling reservoir via a Linblad master equation. Introduction-The study of highly entangled quantum states is of interest both for fundamental reasons and for a myriad of applications to quantum information processing and quantum communication. Of particular fundamental interest is the possibility to entangle distinct macroscopic objects, a task made difficult by the unavoidable decoherence and dissipation associated with such systems. Equally interesting would be the ability to entangle photons of very different frequencies, e.g. microwave and optical photons.

In a recent publication (Wang and Clerk 2012 Phys. Rev. Lett. 108 153603), we demonstrated that one can use interference to significantly increase the fidelity of state transfer between two electromagnetic cavities coupled to a common mechanical resonator over a naive sequential-transfer scheme based on two swap operations. This involved making use of a delocalized electromagnetic mode which is decoupled from the mechanical resonator, a so-called 'mechanically dark' mode. Here, we demonstrate the existence of a new 'hybrid' state transfer scheme that incorporates the best elements of the dark-mode scheme (protection against mechanical dissipation) and the doubleswap scheme (fast operation time). Importantly, this new scheme also does not require the mechanical resonator to be prepared initially in its ground state. We also provide additional details of the previously described interferenceenhanced transfer schemes, and provide an enhanced discussion of how the interference physics here is intimately related to the optomechanical analogue of electromagnetically induced transparency. We also compare the various transfer schemes over a wide range of relevant experimental parameters, producing a 'phase diagram' showing the optimal transfer scheme for different points in parameter space.

We present an ultrafast feasible scheme for ground state cooling of a mechanical resonator via repeated random time-interval measurements on an auxiliary flux qubit. We find that the ground state cooling can be achieved with several such measurements. The cooling efficiency hardly depends on the time-intervals between any two consecutive measurements. The scheme is also robust against environmental noises.