  We have realized a microwave quantum-limited amplifier that is directional
and can therefore function without the front circulator needed in many quantum
measurements. The amplification takes place in only one direction between the
input and output ports. Directionality is achieved by multi-pump parametric
amplification combined with wave interference. We have verified the device
noise performances by using it to readout a superconducting qubit and observed
quantum jumps. With an improved version of this device, qubit and preamplifer
could be integrated on the same chip.
