  A long-standing question is whether it is possible to delegate computational
tasks securely. Recently, both a classical and a quantum solution to this
problem were found. Here, we study the interplay of classical and quantum
approaches and show how coherence can be used as a tool for secure delegated
classical computation. We show that a client with limited computational
capacity - restricted to an XOR gate - can perform universal classical
computation by manipulating information carriers that may occupy superpositions
of two states. Using single photonic qubits or coherent light, we
experimentally implement secure delegated classical computations between an
independent client and a server. The server has access to the light sources and
measurement devices, whereas the client may use only a restricted set of
passive optical devices to manipulate the light beams. Thus, our work
highlights how minimal quantum and classical resources can be combined and
exploited for classical computing.
