The fundamental noise limit of a phase-preserving amplifier at frequency f is the standard quantum limit Tq ~ hf/2kB. In the microwave range, the best candidates have been amplifiers based on superconducting quantum interference devices (reaching the noise temperature Tn ~ 1.8Tq at 700 MHz), and non-degenerate parametric amplifiers (reaching noise levels close to the quantum limit Tn < Tq at 8 GHz).

Rapid development has taken place in the field of quantum amplifiers during the past years and experiments utilizing these techniques are becoming more and more frequent. We have investigated various approaches for realizing quantum limited amplifiers, trying to optimize noise, dynamic range, power tolerance, and band width. Furthermore, we have developed converters based on quantum mechanical principles as well as qubits for amplification of optomechanical coupling.