Josephson parametric amplifiers for quantum information processing

Abstract : Nowadays, it is possible to control and measure the quantum state of systems with a few degrees of freedom, whether they are microscopic objects like cold atoms and single photons, or macroscopic objects like superconducting circuits. Such delicate experiments require an interface which bridges the gap of orders of magnitudes in energy between the quantum object and the data acquisition system. This problem is solved by an active device: the amplifier. Although the amplifier is necessary to overcome the noise at the stage of data acquisition, it eventually alters the signal. We have developed an amplifier for microwave signals which adds only the minimum of noise allowed by quantum laws: the equivalent of half a photon of noise referred to the input. Our amplifier is based on a superconducting circuit of Josephson junctions in superconducting resonators. In this talk, I will review the principles of parametric amplification with superconducting circuits and show the quantum limits specific to the different types of amplifiers. I will present applications of quantum limited amplifiers for quantum information processing using superconducting qubits and pairs of twin microwave beams. In particular, I will show that the amplifier we developed allows a direct measurement of the quantity of entanglement between two beams.