Quantum Information with Superconducting Circuits
Benjamin Huard (LPA)

Abstract

In this manuscript, I present my contribution to the rise of superconducting circuits as the basis of quantum information systems. The macroscopic variables of electrical circuits, such as voltages and currents, obey quantum mechanics as long as they are protected enough from their environment. Since the first qubits based on a superconducting circuit were realized 15 years ago, their coherence time has already increased by 5 orders of magnitude thanks to a better control of the electromagnetic environment of the Josephson junctions. We have performed experiments on these remarkable systems, which illustrate some of the most non-classical aspects of quantum information.
Quantum variables fluctuate even at zero temperature. These zero point fluctuations imply lower bounds on the noise added by a detector. We explore the quantum limits on amplification for propagating microwave signals, and show a concrete superconducting circuit, the Josephson mixer, able to reach this limit.
Contrarily to classical information, quantum information can be stored in a spatially delocalized manner thanks to entanglement. We demonstrate the first circuit able to entangle two propagating microwave modes at different frequencies and on separate transmission lines. We also present a device able to store a microwave field that is entangled with a propagating one.
Measurement of a quantum system results in an inherent back action, with no classical equivalent. We present how to correct for decoherence by measurement feedback on a superconducting qubit, in order to stabilize a desired quantum trajectory. This experiment makes clear the central role of measurement back action in quantum feedback, and thus in quantum error correction.
Weak measurements provide partial information on a system. Because quantum measurement have an inherent back action, one needs to modify the classical Bayes rule predicting the probability of finding a given outcome assuming some outcome in the future. We show an experiment where the fluorescence emitted by a qubit is recorded in time, which can be viewed as a weak measurement of the qubit. The influence of past and future information is explored.