  In the context of variational compiling of a continuous-variable (CV) unitary
operation, the architecture and parameter space of the Xanadu X8 processor
constrain both the set of feasible compiling problems and the allowed cost
functions. In this paper, we motivate a faithful cost function for variational
compiling of a two-mode, continuous-variable beamsplitter gate with real matrix
elements (i.e., a "zero-phase" beamsplitter) that complies with the constraints
of the X8 processor. This cost function is then computed on the X8. Despite the
noise in the processor, we find that the cost function exhibits optimum
parameter resilience and, therefore, that this variational compiling problem is
feasible on the X8. The intent of the paper is partly to report a
proof-of-principle cost function calculation on near-term CV hardware, and
partly to present methods that may be relevant for CV variational compiling
problems in more complicated, large scale settings.
