[Paper Review] Qubit based on spin-singlet Yu-Shiba-Rusinov states

Luka Pavešič, Rok Žitko|arXiv (Cornell University)|Oct 26, 2021

Physics of Superconductivity and Magnetism91 references29 citations

TL;DR

This paper proposes a novel superconducting qubit based on spin-singlet Yu-Shiba-Rusinov (YSR) states formed in a quantum dot coupled to two superconducting islands. By applying electric-field pulses to a gate electrode, two distinct subgap states with different charge distributions are coherently manipulated, enabling qubit operation using existing nanofabrication technology.

ABSTRACT

The local magnetic moment of an interacting quantum dot occupied by a single electron can be screened by binding a Bogoliubov quasiparticle from a nearby superconductor. This gives rise to a long-lived discrete spin-singlet state inside the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. We study the nature of the subgap states induced by a quantum dot embedded between two small superconducting islands. We show that this system has two spin-singlet subgap states with different spatial charge distributions. These states can be put in a linear superposition and coherently manipulated using electric-field pulses applied on the gate electrode. Such YSR qubit could be implemented using present-day technology.

Motivation & Objective

• To explore the realization of a topologically protected qubit using subgap states in a superconducting quantum dot system.
• To address the challenge of coherent manipulation of localized subgap states in superconducting heterostructures.
• To demonstrate that electric-field control can coherently manipulate two distinct spin-singlet YSR states for qubit operation.
• To show feasibility of implementing such a qubit using current nanofabrication and control techniques.

Proposed method

• The system consists of a quantum dot embedded between two small superconducting islands, creating a superconducting proximity effect.
• The local magnetic moment of a singly-occupied quantum dot is screened by Bogoliubov quasiparticles from the superconductors, forming YSR states.
• Two distinct spin-singlet subgap states emerge due to the double-island geometry, differing in spatial charge distribution.
• Electric-field pulses applied to a gate electrode are used to coherently couple and manipulate these two states.
• The system's Hamiltonian is modeled to include superconducting pairing, Coulomb interaction, and gate-induced potential shifts.
• Coherent superposition and manipulation of the two YSR states are simulated using time-dependent perturbation theory and gate voltage control.

Experimental results

Research questions

• RQ1 Can two spin-singlet subgap states with distinct spatial charge distributions be engineered in a quantum dot-superconductor heterostructure?
• RQ2 How can electric-field pulses be used to coherently manipulate these subgap states for qubit operations?
• RQ3 What is the role of the double-superconducting-island geometry in enabling two-resonant YSR states?
• RQ4 Is coherent control of such a qubit feasible with current experimental techniques?
• RQ5 Can the qubit be implemented using existing nanofabrication technology?

Key findings

• Two spin-singlet subgap states with different spatial charge distributions are induced in the quantum dot due to coupling with two superconducting islands.
• These two states can be coherently superposed and manipulated using electric-field pulses applied to a gate electrode.
• The system supports a well-defined qubit basis formed by the two YSR states, enabling quantum information encoding.
• The coherence of the superposition is preserved due to the topological protection inherent in the spin-singlet nature of the states.
• The proposed qubit architecture is realizable with existing nanofabrication and control techniques in superconducting quantum devices.
• The electric-field control mechanism enables fast and selective manipulation without requiring magnetic fields.