Constraints on Gaussian Error Channels and Measurements for Quantum Communication

Joint Gaussian measurements of two quantum systems are important for quantum communication between remote parties and are often used in continuous-variable teleportation or entanglement-swapping protocols. Many of the errors in real-world implementations can be modeled by independent Gaussian error channels acting prior to measurement. In this work we study independent single-mode Gaussian error channels on two modes A and B that take place prior to a joint Gaussian measurement. We determine the set of pairs of such channels that render all Gaussian measurements separable, and therefore unsuitable for entanglement swapping or teleportation of arbitrary input states. For example, if the error channels are loss with parameters $l_A,l_B$ followed by added noise with parameters $n_A,n_B$ then all Gaussian measurements are separable if and only if $l_A+l_B+n_A+n_B\ge 1$.