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Salha Jebari

The inelastic Cooper pair tunneling amplifier (ICTA)

Published on 26 June 2017

Thesis presented June 26, 2017

Josephson parametric amplifiers (JPA), have proven to be an indispensable tool for awide range of experiments on quantum devices in the microwave frequency regime, because they provide the lowest possible noise. However, JPAs remain much more difficult to use and optimize than conventional microwave amplifiers. Recent experiments with superconducting circuits consisting of a DC voltage-biased Josephson junction in series with a resonator, showed that a tunneling Cooper pair can emit one or several photons with a total energy of 2e times the applied voltage. In this thesis we show that such q circuit can be used to implement a new type of phase preserving microwave amplifier, which we call Inelastic Cooper pair Tunneling Amplifier (ICTA). It is powered by a simple DC bias and offers near quantum-limited noise performance.
We start this work by presenting a brief and simple picture of the basic ICTA operating principles. In analogy with the quantum theory of JPAs, we calculate the performances of this amplifier such as the gain, bandwidth and noise. Then, we present the first experimental proof that amplification close to the quantum limit is possible without microwave drive in an extremely simple setup. These measurements are made on a first generation of samples based on aluminum junctions. According to our theoretical and experimental results, we have designed microwave circuits presenting specific frequency-dependent impedances to the junction in order to optimize the performances of our amplifier. This second generation of ICTA samples is fabricated from niobium nitride and provide significantly lower noise and higher gain.
We expect that once fully optimized, such an amplifier, powered by simple DC voltages could then make measuring microwave signals at the single photon level much easier and allow to deploy many amplifiers on a chip. It could therefore be an important ingredient for qubit readout in large-scale quantum processors.

Quantum limit, Josephson junction, Microwave amplifiers, Dynamical Coulomb blockade, Superconducting quantum circuits, Quantum optics

On-line thesis.