  Amplifiers are crucial in every experiment carrying out a very sensitive
measurement. However, they always degrade the information by adding noise.
Quantum mechanics puts a limit on how small this degradation can be.
Theoretically, the minimum noise energy added by a phase preserving amplifier
to the signal it processes amounts at least to half a photon at the signal
frequency. In this article, we show that we can build a practical microwave
device that fulfills the minimal requirements to reach the quantum limit. This
is of importance for the readout of solid state qubits, and more generally, for
the measurement of very weak signals in various areas of science. We also
discuss how this device can be the basic building block for a variety of
practical applications such as amplification, noiseless frequency conversion,
dynamic cooling and production of entangled signal pairs.
