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