  Long-distance two-qubit coupling, mediated by a superconducting resonator, is
a leading paradigm for performing entangling operations in a quantum computer
based on spins in semiconducting materials. Here, we demonstrate a novel,
controllable spin-photon coupling based on a longitudinal interaction between a
spin qubit and a resonator. We show that coupling a singlet-triplet qubit to a
high-impedance superconducting resonator can produce the desired longitudinal
coupling when the qubit is driven near the resonator's frequency. We measure
the energy splitting of the qubit as a function of the drive amplitude and
frequency of a microwave signal applied near the resonator antinode, revealing
pronounced effects close to the resonator frequency due to longitudinal
coupling. By tuning the amplitude of the drive, we reach a regime with
longitudinal coupling exceeding $1$ MHz. This demonstrates a new mechanism for
qubit-resonator coupling, and represents a stepping stone towards producing
high-fidelity two-qubit gates mediated by a superconducting resonator.
