  Beam-splitter operations are widely used to process information encoded in
bosonic modes. In hybrid quantum systems, however, it might be challenging to
implement a reliable beam-splitter operation between two distinct bosonic
modes. Without beam-splitters, some basic operations such as decoupling modes
and swapping states between modes can become highly non-trivial or not feasible
at all. In this work, we develop novel interference-based protocols for
decoupling and swapping selected modes of a multimode bosonic system without
requiring beam-splitters. Specifically, for a given generic coupler
characterized by a Gaussian unitary process, we show how to decouple a single
mode or swap any pair of modes with a constant depth sequence of operations,
while maintaining the coupling for the remaining system. These protocols
require only multiple uses of the given coupler, interleaved with single-mode
Gaussian unitary operations, and thus enable efficient construction of
operations crucial to quantum information science, such as high-fidelity
quantum transduction. Our results are directly derived from fundamental
physical properties of bosonic systems and are therefore broadly applicable to
various existing platforms.
