Quasiparticles in superconducting quantum circuits

Iličin, Teodor (author), Žitko, Rok (mentor)

The density of quasiparticles (QPs) in superconducting devices is often significantly higher than predicted by thermal equilibrium, leading to quantitatively important effects such as decoherence. Previous theoretical considerations relied on first obtaining the effective description of circuits in the superconducting ground state and incorporating QPs afterwards. By projecting the microscopic Hamiltonian onto relevant subspaces of the Hilbert space, we obtain an effective description of circuits, with quasiparticles explicitly taken into account throughout the derivation. We also consider the role of QPs in a specific circuit element, the quantum dot Josephson junction. We present a variational Ansatz and solve the variational equations. The solutions are in good agreement with independent results obtained by the numerical renormalization group (NRG) approach.

• Language: English
• Keywords: quasiparticles, qubits, quantum circuits, superconductivity, quantum dot, Josephson junctions, circuit quantization
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2025
• PID: 20.500.12556/RUL-172980
• COBISS.SI-ID: 248793347
• Publication date in RUL: 12.09.2025

Secondary language

• Language: Slovenian
• Title: Kvazidelci v superprevodnih kvantnih vezjih

Abstract

Gostota kvazidelcev v superprevodnih napravah je pogosto bistveno višja, kot jo napoveduje termično ravnovesje, kar vodi do kvantitativno pomembnih učinkov, kot je dekoherenca. Prejšnje teoretične obravnave so temeljile na efektivnem opisu vezij ob predpostavki superprevodnika v osnovnem stanju in naknadnem upoštevanju kvazidelcev. S projekcijo mikroskopskega Hamiltoniana na ustrezne podprostore Hilbertovega prostora dobimo efektivni opis vezja, pri čemer so kvazidelci eksplicitno upoštevani skozi celoten potek izpeljave. Obravnavamo tudi vlogo kvazidelcev v specifičnem elementu vezij, Josephsonovem stiku z vgrajeno kvantno piko. Predstavimo variacijski nastavek in rešimo dobljene variacijske enačbe. Rešitve se dobro ujemajo z referenčnimi rezultati, izračunanimi s pomočjo metode numerične renormalizacijske grupe (NRG).

• Keywords: kvazidelci, kubiti, kvantna vezja, superprevodnost, kvantna pika, Josephsonov stik, kvantizacija vezij