  Deterministic quantum interactions between single photons and single quantum
emitters are a vital building block towards the distribution of quantum
information between remote systems. Deterministic photon-atom state transfer
has been demonstrated by using protocols that include active feedback or
synchronized control pulses. Here we demonstrate a completely passive swap gate
between the states of a single photon and a single atom. The underlying
mechanism is single-photon Raman interaction (SPRINT) - an interference-based
effect in which a photonic qubit deterministically controls the state of a
material qubit encoded in the two ground states of a {\Lambda} system, and vice
versa. Using a nanofiber-coupled microsphere resonator coupled to single Rb
atoms we swap a photonic qubit into the atom and back, demonstrating
nonclassical fidelities in both directions. Requiring no control fields or
feedback protocol, the gate takes place automatically at the timescale of the
atom's cavity- enhanced spontaneous emission time. Applicable to any
waveguide-coupled {\Lambda} system, this scheme provides a versatile building
block for the modular scaling up of quantum information processing systems.
