  High-speed switching with low loss would be a versatile tool for photonic
quantum technologies, with applications in state generation, multiplexing, and
the implementation of quantum gates. Phase modulation is one method of
achieving this switching, but existing optical phase modulators either achieve
high bandwidth or low loss, but not both. We demonstrate fast
($100\,\mathrm{MHz}$) bandwidth), low-loss ($74(2)\,\%$) transmission) phase
shifting ($\Delta\phi = (0.90(5))\pi$) in a signal field, induced by a control
field, and mediated by the two-photon $5S_{1/2} \rightarrow{} 5P_{3/2}
\rightarrow{} 5D_{5/2}$ transition in rubidium-87 vapour. We discuss routes to
enhance both performance and scalability for application to a range of quantum
and classical technologies.
