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arXiv:2505.07593

Gate modulation and interface engineering on Coulomb blockade in open superconducting islands

Huading Song, Dong Pan, Runan Shang, Zhaoyu Wang, Ke He, Jianhua Zhao, Zhang, Hao

Published May 12, 2025• 0 citations
arXivHTMLPDF

Abstract

Mesoscopic Coulomb blockade (MCB) is recognized as a phase-coherent variant of the conventional Coulomb blockade that arises in systems with open contacts. In open quantum dots, MCB is enhanced by a decrease in background conductance. This occurs because the reduction in coupling strength between the quantum dot and the outer reservoir renders the system more closed, thereby facilitating the emergence of conventional Coulomb blockade. In this work, we demonstrate that the MCB in open superconducting islands exhibits an different correlation with coupling strength compared to open quantum dots. Specifically, a decrease in background conductance may result in a weakening of the MCB. This observation indicates that the MCB in superconducting islands originates from the presence of superconducting-normal interfaces.

DOI
10.48550/arxiv.2505.07593

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Mesoscopic Coulomb blockade (MCB) is recognized as a phase-coherent variant of the conventional Coulomb blockade that arises in systems with open contacts. In open quantum dots, MCB is enhanced by a decrease in background conductance. This occurs because the reduction in coupling strength between the quantum dot and the outer reservoir renders the system more closed, thereby facilitating the emergence of conventional Coulomb blockade. In this work, we demonstrate that the MCB in open superconducting islands exhibits an different correlation with coupling strength compared to open quantum dots. Specifically, a decrease in background conductance may result in a weakening of the MCB. This observation indicates that the MCB in superconducting islands originates from the presence of superconducting-normal interfaces.
In this thesis I find an analytic expression for the conductance of a single electron transistor in the regime when temperature, level spacing, and charging energy of an island are all of the same order. I also study the correction to the spacing between Coulomb blockade peaks due to finite dot-lead tunnel couplings. I find analytic expressions for both correction to the spacing averaged over mesoscopic fluctuations and the rms of the correction fluctuations. In the second part of the thesis I discuss the feasibility of quantum dot based spin- and charge-qubits. Firstly, I study the effect of mesoscopic fluctuations on the magnitude of errors that can occur in exchange operations on quantum dot spin-qubits. Mid-size double quantum dots, with an odd number of electrons in the range of a few tens in each dot, are investigated through the constant interaction model using realistic parameters. It is found that the number of independent parameters per dot that one should tune depends on the configuration and ranges from one to four. Then, I study decoherence of a quantum dot charge qubit due to coupling to piezoelectric acoustic phonons in the Born-Markov approximation. After including appropriate form factors, I find that phonon decoherence rates are one to two orders of magnitude weaker than was previously predicted. My results suggest that mechanisms other than phonon decoherence play a more significant role in current experimental setups.
The manipulation of single charge quanta in solid state systems has been a field of active interest for several decades. At the basis of such structures lays the Coulombic repulsion between electrons in a small metallic island yielding the so-called Coulomb-blockade regime. Many quantum phenomena, such as the tunnelling effect or the Josephson coupling in superconducting junctions, have been exploited so far to control the charging state of a conducting island, and enable a number of applications ranging from on-chip cooling and current generation to single-photon detection. Yet, in all Coulombic systems the external control parameters play a crucial role, reaching sometimes a rather complex experimental configuration. Here we show that quantum interference established in a superconducting nanowire can provide a phase-tunable energy barrier which allows charge manipulation with enhanced functionalities. This additional degree of freedom for single electronics stems from the coupling between magnetic flux-dependent proximity effect and charging levels discretization present in a Coulomb-blockaded island. Our quantum interference nanostructure represents the first realization of a superconducting hybrid single-electron transistor called SQUISET in which the charging landscape is phase-coherently manipulated by an external magnetic flux. The interferometric nature of the transistor adds new perspectives to single electronics making it a promising alternative building block in quantum metrology, coherent caloritronics, and quantum information technology.
We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor-superconductor-normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.
We show that charge transfer through a small superconducting (S) island of a single-electron transistor is strongly affected by vorticity. This interplay of charge and rotational degrees of freedom in a mesoscopic superconductor occurs through the effect of vorticity on the quantum mechanical spectrum of electron-hole excitations. The subgap quasiparticle levels in vortices can host an extra electron, thus suppressing the so-called parity effect in the S island. We propose to measure the collective dynamics of vorticity and electric charge via the charge pumping effect caused by alternating vortex entry and exit controlled by a periodic magnetic field.
In the field of nanoelectronics, one seeks to combine quantum mechanical effects with macroscopic entities such as currents in order to find new ways of storing and manipulating information. One possibility to accomplish this is to use the quantum correlations found in superconductors. Teaming superconductors with normal metals and magnetic systems, one can obtain novel correlation and fluctuation effects. \n\nIn the first work of this thesis, we examine how charge correlations in a metallic dot are modified due to the presence of a capacitively coupled superconducting island. It is possible to overcome the Coulomb repulsion between electrons in a hybrid double-dot system consisting of a metallic island coupled capacitively to a Cooper-pair box. The superconducting island S acts as a dynamic gate on the metallic island N and in the strong coupling regime the electric charge of the Cooper pairs in S and their fluctuations are able to overscreen the repulsion between electrons in N. This produces an attractive interaction between two additional electrons in the metallic island and results in a non-monotonous charging curve of the metallic island. \n\nThe second part of the thesis considers how the dynamics of a precessing molecular magnetic moment affects the Josephson current. The precession gives rise to a time-dependent tunnel potential which not only creates different tunneling probabilities for spin-up and spin-down quasiparticles, but also introduces a time-dependent spin-flip term. In particular, we study the effects of the spin-flip term alone on the Josephson current between two spin-singlet superconductors as a function of precession frequency and junction transparency. The system displays a nonequilibrium steady-state solution which depends on the precession frequency of the classical spin.\n\n\n
We report measurements and quantum mechanical simulations of the behavior of superconducting two-junction Coulomb-blockade electrometers biased near the superconducting gap. Our quantum model allows coherent superpositions of states with different number of charges on the island. It predicts maximum current modulation /spl delta/I for devices with tunneling resistance R/sub /spl Sigma///spl ap/64/spl pi//spl Delta//spl planck//sup 2/C/sub /spl Sigma///sup 2//(e/sup 6//spl tau/), where /spl tau/ is the time for which the system maintains quantum coherence, C/sub /spl Sigma// is the total island capacitance and /spl Delta/ is the superconducting energy gap. For comparison, we built Al-AlO/sub x/-Al electrometers with R/sub /spl Sigma// from 61 k/spl Omega/ to 1.7 M/spl Omega/ and C/sub /spl Sigma// ranging from 0.9 fF to 8 fF. We find good agreement for /spl tau//spl ap/0.35 nsec.