A persistent-current-biased and current-actuated switch for superconducting circuits

We have realized an on-chip persistent flux bias for our flux based qubit by adding a switchable aluminum shunt to the niobium flux bias coils. This inductive shunt can create a persistent current loop by trapping the bias flux at the superconducting transition of the Al. When the Al is superconducting, the noise spectral density from the flux bias and the damping due to the flux bias should both be reduced by a factor of 100, since the inductance ratio,L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> /L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> , is 0.1. This reduces one of the sources of decoherence for the qubit. To permit changes to the persistent current in the superconducting loops at mK temperatures, a local heater has been coupled to the Al shunt. This makes it possible to briefly drive the shunt to its normal state to change to flux without heating the entire device.

In this work, the spectra in an Aharonov–Bohm quantum‐ring interferometer forming a Josephson junction between two topological superconductor (TSC) nanowires are investigated. The TSCs host Majorana bound states at their edges, and both the magnetic flux and the superconducting phase difference between the TSCs are used as control parameters. A tight‐binding approach is used to model the quantum ring coupled to both TSCs, described by the Kitaev effective Hamiltonian. The problem is solved by means of exact numerical diagonalization of the Bogoliubov‐de Gennes Hamiltonian and obtain the spectra for two sizes of the quantum ring as a function of the magnetic flux and the phase difference between the TSCs. Depending on the size of the quantum ring and the coupling, the spectra display several patterns. Those are denoted as line, point, and undulated nodes, together with flat bands, which are topologically protected. The first three patterns can be possibly detected by means of persistent and Josephson currents. Hence, the results could be useful to understand the spectra and their relation with the behavior of the current signals.

Studying the relationship between the Peierls instability, the magnetic flux, and the persistent current in the mesoscopic conducting polymer ring threaded by a magnetic flux, we found that the magnetic flux enhances the Peierls distortion. The persistent current increases slightly with increase of temperature due to the competition between the effects of the Peierls distortion and thermal fluctuation. In addition, the phase parity of the persistent current for the odd-even electron number will vanish for the electronic number ${N}_{e}$ in the range of $Nl{N}_{e}\ensuremath{\leqslant}2N$, where $N$ is the one-third number of sites in the ring. This phase locking effect originates from the flat energy band of the conducting polymer ring.

Persistent charge and spin currents in a quantum ring using Green׳s function technique: Interplay between magnetic flux and spin–orbit interactions

Edged topological disordered quantum ring in the presence of magnetic flux

We investigate the one-dimensional quantum ring constructed by the spin-orbit coupled material, in which the quantum spin-Hall Bernevig-Zhang (BZ) Hamiltonian and Rashba-Dirac (RD) type spin-orbit coupling are taken into account (called RD-BZ Hamiltonian in this paper). It is known that the curvature of the ring generates an out-of-plane effective magnetic field, acting as an internal Zeeman field. We find that only the BZ coupling can change the strength of the internal Zeeman field, which enables us to detect the effect of the internal Zeeman field. Furthermore, we find that the total angular momentum is conserved in the RD-BZ Hamiltonian, and the energy eigenvalue and wave function must be modified to fit the conserved quantity, which are ignored in the previous studies. The conductance without leads is discussed. Different from the previous results, we find that the conductance behaves like a beat phenomenon resulting from the interplay between the magnetic flux and Aharonov-Casher (AC) phase, and thus, it can oscillate without passing through the insulating state in some regimes of magnetic flux or AC phase. Importantly, we find that the conductance with integer and half-integer magnetic flux provides us a method to measure the AC phase. The cancellation of the internal Zeeman field due to the BZ coupling can be detected by using specific fractional magnetic flux. In the ring with nonvanishing RD and BZ couplings, the conductance can exhibit a quasiplateau near the small RD coupling. The increase in the strength of BZ coupling would result in wider quasiplateau in conductance, which implies that the ring could remain insulating state (or conducting state) regardless of the small change in RD coupling. The thermal average of spin and charge currents are calculated at low temperatures without impurities. Importantly, we find that the persistent spin and charge currents as a function of the magnetic flux exhibit nodelike lines, which amazingly are perpendicular to each other. As a consequence, the persistent spin current could be nonzero even when the charge current vanishes at nonzero magnetic flux. The result suggests a pure spin current in quantum rings, and its direction can be reversed by changing the magnetic flux. The increase in the BZ coupling would exhibit the plateaulike pure spin current.

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Tunneling currents in mesoscopic rings

The energy spectrum and the persistent currents are calculated for a finite-width mesoscopic annulus with radial potential barrier, threading a magnetic flux through the hole of the ring. Owing to the presence of tunneling barrier, the coupling effect leads to the splitting of each radial energy subband of individual concentrical rings into two one. Thus, total currents and currents carried by single high-lying eigenstate as a function of magnetic flux exhibit complicated patterns. However, periodicity and antisymmetry of current curves in the flux still preserve.

Electrons in pure nanorings or tubes possess eigenstates with circular currents. In the presence of disorder the eigenstates become standing waves suppressing the circular current. A magnetic field parallel to the axis of the tube/ring recreates propagating electron states with circular persistent current. Numerical calculations for disordered rings are performed in the tight-binding approximation for finite disorder and magnetic field. The application of a gauge transformation contributes to an intuitive understanding of the persistent current and its absence for a flux Φ equal to a multiple of half the flux quantum and leads to a formula for the current in the disordered ring with magnetic flux. The derivative of the eigenstate energies with respect to the magnetic flux Φ through the ring, dEn∕dΦ, yields an effective circular group velocity and represents an alternative method to calculate the circular current in disordered systems. Current and energy obey the Aharonov–Bohm effect. For large disorder a competition between persistent current and electron localization is observed. Despite the random disorder in the rings the dependence of the energy and the current on the flux is smooth and shows an interesting symmetry. For each state with negative energy there is a state with positive energy that carries exactly the opposite current.

We perform a numerical study of interacting one-dimensional Hubbard rings with a single impurity potential and pierced by a magnetic flux. Our calculations are carried out at the level of current lattice density functional theory (CLDFT) for the Hubbard model and compared to known results obtained in the thermodynamical limit from the Bethe ansatz. In particular, we investigate the effects of disorder and Coulomb interaction on the persistent current (PC) and the Drude weight. It is found that CLDFT is able to accurately describe qualitative and quantitative features of these ground state properties in the presence of disorder and electronic interaction. When the impurity potential is switched off, the CLDFT approach describes well the velocity of the Luttinger liquid excitations as a function of both interaction strength and electron filling. Then, when the impurity scattering potential is finite, we find the PC to vanish as for large L and independent on the strength of the scattering potential, in good agreement with Luttinger liquid theory.

We investigate the equilibrium property of a mesoscopic ring with a spin-orbit interaction. It is well known that, for a normal mesoscopic ring threaded by a magnetic flux, the electron acquires a Berry phase that induces the persistent (charge) current. Similarly, the spin of an electron acquires a spin Berry phase traversing a ring with a spin-orbit interaction. It is this spin Berry phase that induces a persistent spin current. To demonstrate its existence, we calculate the persistent spin current without an accompanying charge current in the normal region in a hybrid mesoscopic ring. We point out that this persistent spin current describes the real spin motion and can be observed experimentally.

Quantum computing holds transformative potential for solving problems beyond the reach of classical computers. As quantum systems demand unprecedented levels of control, coherence, and scalability, nanotechnology emerges as a critical enabler. This paper explores the convergence of nanofabrication techniques, quantum dot architectures, and nanomaterial-based qubits, emphasizing their role in enhancing quantum bit stability, integration density, and gate fidelity. We also discuss current challenges and anticipated advancements that could redefine the future landscape of quantum computation. The synergy between quantum information science and nanotechnology will likely fuel breakthroughs across cryptography, drug discovery, and complex system simulations.

We study the persistent current of noninteracting electrons subject to a pointlike magnetic flux in the simply connected chaotic Robnik-Berry quantum billiard and also in an annular analog thereof. For the simply connected billiard, we find a large diamagnetic contribution to the persistent current at small flux, which is independent of the flux and is proportional to the number of electrons (or equivalently the density since we keep the area fixed). The size of this diamagnetic contribution is much larger than mesoscopic fluctuations in the persistent current in the simply connected billiard. Moreover, it can ultimately be traced to the response of the angular-momentum $l=0$ levels (neglected in semiclassical expansions) on the unit disk to a pointlike flux at its center. The same behavior is observed for the annular billiard when the inner radius is much smaller than the outer one. The usual fluctuating persistent current and Anderson-like localization due to boundary scattering are seen when the annulus tends to a one-dimensional ring. We explore the conditions for the observability of this phenomenon.

We analyze fluctuations of persistent current (PC) produced by a charged quantum particle moving in a ring and interacting with a dissipative environment formed by diffusive electron gas. We demonstrate that in the presence of interactions such PC fluctuations persist down to zero temperature. In the case of weak interactions and/or sufficiently small values of the ring radius $R$ PC noise remains coherent and can be tuned by external magnetic flux ${\ensuremath{\Phi}}_{x}$ piercing the ring. In the opposite limit of strong interactions and/or large values of $R$ fluctuations in the electronic bath strongly suppress quantum coherence of the particle down to $T=0$ and induce incoherent ${\ensuremath{\Phi}}_{x}$-independent current noise in the ring, which persists even at ${\ensuremath{\Phi}}_{x}=0$ when the average PC is absent.