  The realization of quantum computers and quantum Internet requires not only
quantum gates and quantum memories, but also transistors at single-photon
levels to control the flow of information encoded on single photons.
Single-photon transistor (SPT) is an optical transistor in the quantum limit,
which uses a single photon to open or block a photonic channel. In sharp
contrast to all previous SPT proposals which are based on single-photon
nonlinearities, here I present a novel design for a high-gain and high-speed
(up to THz) SPT based on a linear optical effect - giant circular birefringence
(GCB) induced by a single spin in a double-sided optical microcavity. A gate
photon sets the spin state via projective measurement and controls the light
propagation in the optical channel. This spin-cavity transistor can be directly
configured as diodes, routers, DRAM units, switches, modulators, etc. Due to
the duality as quantum gate and transistor, the spin-cavity unit provides a
solid-state platform ideal for future Internet - a mixture of all-optical
Internet with quantum Internet.
