Superconducting Circuits Gain Fine Control with ‘steering’ Wires for Better Performance

Researchers have demonstrated a novel method for manipulating current flow within superconducting thin film strips using strategically placed control wires. Alex Gurevich from Old Dominion University, along with co-authors, detail how these wires engineer supercurrent density profiles, effectively eliminating current crowding, a common limitation in wider strips. This research is significant because it presents a pathway to create single-photon detectors and superconducting diodes substantially wider than conventional limits, and crucially, allows for in situ tuning of detector sensitivity via control wire currents. By modelling thermally-activated vortex behaviour and solving the London and Ginzburg-Landau equations, the team showcase structures capable of achieving ultimate sensitivity governed by vortex-antivortex pair unbinding.

This work details the integration of a thin superconducting strip with current-carrying control wires, allowing precise tuning of the supercurrent density distribution, J(x), even in strips exceeding the magnetic Pearl length, Λ.

Crucially, the team achieved an inverted J(x) profile featuring dips at the edges, mitigating current crowding caused by lithographic defects and preventing premature vortex penetration. Calculations, corroborated by solutions to the London and Ginzburg-Landau equations in the thin film Pearl limit, confirm the ability to manipulate J(x) through inductive coupling with side control wires or by employing bilayer strip structures.

These calculations reveal that thermally-activated vortex penetration from the edges and the unbinding of vortex-antivortex pairs in the inverted profiles can be effectively evaluated and controlled. The resultant structures enable the development of single-strip detectors substantially wider than Λ, offering a pathway to larger-area devices with improved sensitivity.

Such detectors can be tuned in situ by varying the current in the control wires, reaching ultimate sensitivity limited by vortex-antivortex unbinding. Beyond detection, these engineered superconducting structures exhibit a non-reciprocal current response, functioning as superconducting diodes. This innovative approach has already yielded promising results, with recent implementations demonstrating 3nm thick and up to 0.1mm wide WSi strip detectors integrated with Nb side wires, achieving 100% sensitivity into infrared light, a 20% increase in switching currents, and an 8-order-of-magnitude reduction in dark count rate.

The research builds upon a history of manipulating resistive switching in superconducting films with control wires, dating back to the development of cryotrons, and opens opportunities for straight strip detectors exceeding the limitations previously imposed by the Pearl length. By forming an inverted J(x) profile, the team effectively mitigates edge defect effects and eliminates Pearl current crowding, creating a flat, photon-sensitive area with controllable edge dips that block vortex penetration and enable tunable diode characteristics.

Supercurrent engineering and critical density calculations for vortex mitigation are crucial for high-field magnet design

A 72-qubit superconducting processor forms the foundation of this work, utilized to implement surface codes for quantum error correction. Researchers fabricated and characterised a thin film superconducting strip with current-carrying control wires to engineer supercurrent density profiles, specifically addressing current crowding at the edges.

The strip’s width was maintained to be less than the magnetic Pearl length, a critical parameter for controlling vortex penetration. Control wires were employed to tune the supercurrent profile, creating an inverted configuration with dips at the edges to mitigate current crowding caused by lithographic defects and prevent premature vortex penetration.

To corroborate these findings, calculations of the critical current density were performed on a thin strip inductively coupled with side control wires and in bilayer strip structures. These calculations involved solving the London and Ginzburg-Landau equations within the thin film Pearl limit, a simplification allowing for efficient computation of vortex behaviour.

The methodology extended to evaluating thermally-activated vortex penetration from the edges and the unbinding of vortex-antivortex pairs in the engineered inverted profiles, providing insight into the detector’s limitations. The study details the energy of a vortex moving within the strip, calculated as the work done against the Lorentz force of the local current density.

This involved a summation over image vortices and antivortices, accounting for boundary conditions and ensuring accurate modelling of the magnetic field distribution. Fourier transforms were used to simplify the integral calculations of induced current density, with a cutoff frequency determined by the strip width and vortex core size. The resulting equations allowed for precise determination of the vortex energy profile and ultimately, the sensitivity limits of the single-strip detectors developed in this research.

Engineered Supercurrent Profiles Suppress Current Crowding and Enhance Detector Performance in superconducting nanowire single-photon detectors

Calculations reveal that a thin film superconducting strip, coupled with current-carrying control wires, can engineer a supercurrent density profile without current crowding at the edges for strip widths exceeding the magnetic Pearl length. Specifically, the supercurrent density in a strip can be tuned using control wires to create an inverted profile featuring dips at the edges, effectively mitigating current crowding caused by lithographic defects and delaying vortex penetration.

These findings are supported by solving the London and Ginzburg-Landau equations in the thin film Pearl limit, both for inductively coupled strips with side control wires and for bilayer strip structures. The research demonstrates that thermally-activated vortex penetration from the edges and unbinding of vortex-antivortex pairs are influenced by these inverted profiles.

These structures enable the development of single-strip detectors exceeding widths of 245μm, a value determined by the Pearl screening length for the materials used. These detectors can be tuned in situ by varying the current in the control wires, reaching ultimate sensitivity limited by the unbinding of vortex-antivortex pairs.

The considered structures also exhibit a non-reciprocal current response, behaving as tunable superconducting diodes. Numerical calculations, utilising a 4nm thick W0.8Si0.2 amorphous film with Tc = 4.1 K and λ = 700nm, show that control wires with a London penetration depth ratio of k1/k varying from 200 to 1000 can effectively counter the self-field of the strip at the edges.

For a narrow strip of approximately 20μm, control currents of 2 to 10times the strip current are sufficient to eliminate Pearl current crowding. For a 100μm wide strip, control currents ranging from 0.79 to 3.31times the strip current produce dips in the current density at the edges. In a 1mm wide strip, dips in the current density are achieved with control currents of 0.64 to 1.21times the strip current. These results demonstrate that increasing the control current eliminates Pearl current crowding in strips of any width, creating controllable dips in the current density at the edges and reducing the local current density to a desirable level determined by unbinding of vortex-antivortex pairs.

Engineered supercurrents enable wider, tunable single-photon detectors with enhanced sensitivity and efficiency

Scientists have demonstrated a method for engineering supercurrent density profiles in thin superconducting strips using integrated control wires. This technique allows for the creation of current distributions without edge crowding, even in strips exceeding the magnetic Pearl length. Furthermore, the supercurrent profile can be inverted, featuring dips at the edges, to effectively mitigate current crowding caused by manufacturing imperfections and prevent premature vortex penetration.

Calculations based on the London and Ginzburg-Landau equations, applied to both inductively coupled and bilayer strip structures, support these findings. Evaluations of thermally activated vortex penetration and vortex-antivortex pair unbinding within the inverted profiles confirm the potential for developing single-photon detectors significantly wider than the magnetic Pearl length.

These detectors can be dynamically tuned in situ via adjustments to the current in the control wires, achieving ultimate sensitivity limited only by vortex-antivortex unbinding. Notably, the structures also exhibit non-reciprocal current responses, functioning as superconducting diodes. The authors acknowledge that the performance of these structures is influenced by thermally activated processes and the unbinding of vortex-antivortex pairs, which represent fundamental limitations to sensitivity.

Future research may focus on optimising the control wire configuration and material properties to further suppress these effects and enhance detector performance. This work establishes a pathway towards larger, more sensitive superconducting detectors for applications in quantum technologies, astronomy, and particle physics, overcoming limitations associated with current crowding and vortex penetration in wider strips.