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Quasiparticle-induced linewidth of the Josephson radiation

Page Web française. PhD thesis may follow.
Published on 14 September 2021
Josephson junctions are made of two superconductors separated by a tunnel barrier. Thanks to the coherent transfer of Cooper pairs across the barrier, these junctions realize a unique non-linear and non-dissipative dipole for electrical circuits. These properties also make Josephson junctions a useful block for quantum technological applications. An emblematic example are the superconducting qubits, which are basic components for building complex quantum machines.

A major reason for the reduced dissipation is the superconducting gap, which renders the equilibrium concentration of quasiparticles exponentially suppressed at low temperature. Recent progress has been made in order to characterize and quantify the detrimental effect of quasiparticles in a variety of Josephson devices [1]. A striking result of these studies was to show that many devices are affected by an excess concentration of quasiparticles, which may originate from electromagnetic, cosmic, or nuclear radiation. Because of their unclear origin and adverse effect, quasiparticles in superconductors remain a “hot” topic of research.

The aim of the Master 2 internship will be to investigate theoretically the role of quasiparticles on the linewidth of the Josephson radiation, that is, the generation of an AC current by a DC voltage biased Josephson junction. While the role of the junction’s electromagnetic environment was much discussed, little is known on the role of such quasiparticles. To make progress, the candidate will use a combination of quantum field theories (scattering formalism, Keldysh Green functions) for quantum transport.

A possible extension for a PhD project could be to address the yet unknown role of quasiparticles on the coherence properties of a recently investigated family of superconducting quantum bits, known as "cat codes", which have been shown to display an intrinsic protection against decoherence induced by photon losses.

[1] Bogoliubov quasiparticles in superconducting qubits
L. I. Glazman and G. Catelani, SciPost Phys. Lect. Notes 31 (2021).

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