  Integrated quantum photonics provides a promising route towards scalable
solid-state implementations of quantum networks, quantum computers, and
ultra-low power opto-electronic devices. A key component for many of these
applications is the photonic quantum logic gate, where the quantum state of a
solid-state quantum bit (qubit) conditionally controls the state of a photonic
qubit. These gates are crucial for development of robust quantum networks,
non-destructive quantum measurements, and strong photon-photon interactions.
Here we experimentally realize a quantum logic gate between an optical photon
and a solid-state qubit. The qubit is composed of a quantum dot (QD) strongly
coupled to a nano-cavity, which acts as a coherently controllable qubit system
that conditionally flips the polarization of a photon on picosecond timescales,
implementing a controlled-NOT (cNOT) gate. Our results represent an important
step towards solid-state quantum networks and provide a versatile approach for
probing QD-photon interactions on ultra-fast timescales.
