  Quantum information technology based on solid state qubits has created much
interest in converting quantum states from the microwave to the optical domain.
Optical photons, unlike microwave photons, can be transmitted by fiber, making
them suitable for long distance quantum communication. Moreover, the optical
domain offers access to a large set of very well developed quantum optical
tools, such as highly efficient single-photon detectors and long-lived quantum
memories. For a high fidelity microwave to optical transducer, efficient
conversion at single photon level and low added noise is needed. Currently, the
most promising approaches to build such systems are based on second order
nonlinear phenomena such as optomechanical and electro-optic interactions.
Alternative approaches, although not yet as efficient, include magneto-optical
coupling and schemes based on isolated quantum systems like atoms, ions or
quantum dots. In this Progress Report, we provide the necessary theoretical
foundations for the most important microwave-to-optical conversion experiments,
describe their implementations and discuss current limitations and future
prospects.
