Researchers at Tianjin University have achieved quantum interference without relying on magnets, a conventional necessity, by using a The team demonstrated a revealing that the effect is a fundamental property of superconductivity rather than a sign of unusual material states. This work introduces an all-electric technology that replaces magnetic fields for controlling supercurrent interference, establishing the physical equivalence between the supercurrent gauge field and a magnetic field. The findings open new avenues for exploring novel phases of matter and designing superconducting quantum architectures.
Supercurrent Gauge Field Enables Magnetic-Free Quantum Interference
Researchers at Tianjin University demonstrated a phase-sensitive technique to characterize finite-momentum superconductivity without relying on external magnetic fields, a feat previously unattainable. This advance stems from the creation and amplification of a “supercurrent gauge field” (SGF) using high-kinetic-inductance superconductors and specifically designed device architectures. The team’s work establishes a physical equivalence between the SGF and a magnetic field, effectively decoupling quantum interference from the need for magnetic flux.
This equivalence is not merely a substitution. The SGF offers capabilities exceeding those of conventional magnetic fields in certain applications. The foundation of this technology lies in the Ginzburg-Landau theory, which describes the behavior of superconductors, and the understanding that supercurrent and magnetic fields can be functionally interchangeable. According to the first Ginzburg-Landau equation, the local supercurrent density is determined by the vector potential of an external magnetic field.
By manipulating the supercurrent itself, the researchers generated an equivalent gauge field, allowing for precise control over the superconducting phase coherence. Calculations of the current flow and corresponding SGFs in asymmetric and symmetric configurations predicted a lateral shift in the supercurrent interference pattern (SIP) under varying SGF strengths. These predictions were then experimentally verified using planar Josephson junctions fabricated from Bi₂O₂Se and bilayer graphene.
Experimental results confirm the SGF’s ability to modulate the interference pattern of the Josephson junctions, demonstrating its effectiveness as a magnetic-free control mechanism. Specifically, measurements of the differential resistance (dV/dI) revealed a clear modulation of the SIP with changes in the applied SGF, even at a field strength of -1.5 Gauss.
The team also demonstrated a “universal Josephson diode effect” (JDE), where the JDE is an inherent property of the Josephson effect itself, not an anomaly. The implications of this work extend beyond simply eliminating magnets from quantum experiments. The ability to create a field-free JDE mechanism has broad implications for detecting broken-symmetry states, which are important for understanding exotic materials. This work allows for the creation of more compact, energy-efficient and versatile superconducting quantum systems.
Bi₂O₂Se & Bilayer Graphene Demonstrate Universal Josephson Diode Effect
This achievement moves beyond earlier attempts at field-free JDEs, which relied on trapped flux, asymmetric geometries, or exotic materials, by establishing a mechanism within a standard device geometry. The team’s approach redefines the Josephson junction itself as an intrinsic diode, offering a potentially scalable blueprint for supercurrent rectification independent of material properties. The researchers engineered high-density phase gradients within the superconducting leads of their junctions, observing nonreciprocal switching currents.
These currents manifest as a skewed envelope, a characteristic interference pattern, in the supercurrent, with the diode polarity reversing with the sign of the applied flux. This behavior is attributed to a “supercurrent gauge field” that induces a phase-winding effect, mimicking a physical magnetic field and breaking time-reversal symmetry to produce the diode response. The ability to achieve this effect in a standard configuration is a departure from previous designs that required specific material properties or complex fabrication techniques.
Further amplification of the supercurrent gauge field was achieved through a novel junction design, allowing for macroscopic quantum interference and the JDE to be demonstrated using only microampere-level electrical currents. “We reproduce both symmetric and skewed patterns using μA-scale electrical currents,” the team reports, demonstrating repeatable rectifying behavior in the absence of any external magnetic influence. Line cuts taken from these measurements at varying magnetic fields displayed clear modulation of the supercurrent interference pattern, providing direct evidence of the supercurrent gauge field’s effect.
The findings suggest that the Josephson diode effect is not necessarily indicative of broken symmetry or exotic material properties, but rather a universal consequence of superconducting phase frustration. The team’s results elucidate that the JDE can be understood as a universal superconducting phase frustration, one that requires no intrinsic broken symmetry, spin-orbit coupling, or topological properties within the weak-link material itself.
Supercurrent Gauge Field Mimics Magnetic Flux for Phase Control
The ability to manipulate supercurrents without magnetic fields now extends to replicating complex interference patterns previously achievable only with external magnetic flux. This equivalence allows for the creation of an intrinsic diode, offering a material-independent method for supercurrent rectification. The team’s approach hinges on understanding how the supercurrent density relates to gauge invariance, a fundamental principle in physics. The combined gauge field, encompassing both an external magnetic field and the SGF, dictates the spatial phase gradient within the superconducting system.
This is mathematically expressed by defining the vector potential for the SGF as = m^c/n_sq^2J_s, where represents the effective mass, c is the speed of light, is the Cooper-pair density, and is the supercurrent density. This formulation highlights that both magnetic fields and supercurrents can cooperatively interfere, and even replace each other, in controlling the behavior of Josephson junctions.
Calculations and experimental results confirm this interchangeability, revealing identical interference behaviors between magnetic-field-driven and SGF-driven supercurrents. The researchers report, referencing their observations of the supercurrent interference pattern (SIP) under purely electrical control. Extended Data Fig. S6 provides further evidence of these identical interference behaviors in symmetric configurations. Beyond simply mimicking magnetic fields, the SGF offers unique capabilities, while the team’s findings also help to understand the nature of the Josephson diode effect (JDE).
This frustration can be induced by either a magnetic flux or, importantly, by the SGF. The ability to control the JDE through current configuration further emphasises this point. By adjusting the current flow, the researchers demonstrated that the existence of the JDE can be actively controlled, solidifying the understanding of the JDE as a fundamental property of the Josephson effect. This control opens avenues for designing more efficient and versatile superconducting devices, free from the limitations imposed by external magnetic fields.
Fraunhofer Pattern Skew Reveals Supercurrent-Driven Time-Reversal Breaking
The characteristic interference pattern generated by supercurrents, typically observed with the aid of magnetic fields, now exhibits a skew directly linked to time-reversal symmetry breaking, according to new work. Researchers demonstrated this effect by manipulating supercurrent flow itself, rather than relying on external magnetic fields to distort the usual pattern, the visual representation of supercurrent interference. This induced field drives a phase-winding effect, effectively breaking time-reversal symmetry and creating a universal diode response, where supercurrent preferentially flows in one direction.
The observed skew in the interference pattern, with lifted nodes, directly correlates with this induced asymmetry, demonstrating a current-dependent total wavenumber that shifts oppositely for positive and negative bias currents. Measurements in an external field confirmed that the artificially generated interference pattern and the magnitude of the Josephson diode effect closely resemble those produced by a conventional magnetic field, validating the supercurrent gauge field as a viable replacement.
The observed Fresnel diffraction pattern, a departure from the typical diffraction, arises from the linear gradient of supercurrent density along each lead within the four-terminal Josephson junction. This transformation of the interference pattern provides further evidence of the supercurrent gauge field’s influence on the phase profiles of the superconducting current.
SGF Amplification Achieves Field-Free Interference & Diode Function
The team’s approach requires neither, representing a fundamental shift in how supercurrent interference can be controlled and measured. The decoupling of quantum interference from magnetic fields is therefore a significant advancement in fundamental physics and a transformative leap for magnetic-free quantum technologies. The implications extend to the very definition of the Josephson junction itself, which the researchers now redefine as an intrinsic diode.
This recharacterization offers a material-agnostic blueprint for supercurrent rectification, meaning the principle should be applicable across a broad range of superconducting materials and device configurations. The team’s success in replicating the interference patterns and JDE magnitude without an external magnetic field confirms the supercurrent gauge field’s capacity to fully substitute for conventional magnetic control.
This equivalence is demonstrated through detailed measurements of the supercurrent interference pattern, revealing a close resemblance between the patterns generated with and without an applied field. This manipulation of symmetry, previously reliant on external fields, is now achievable through purely electrical means.
Finite-Momentum Superconductivity Probed via Supercurrent Gauge Field
The ability to manipulate supercurrents without relying on external magnetic fields has yielded a method for characterizing finite-momentum superconductivity, a state of matter where Cooper pairs possess a non-zero crystal momentum. Researchers demonstrated this capability by generating a “supercurrent gauge field”, an all-electric alternative to magnetic flux, and using it to construct and analyze this elusive superconducting phase.
This technique bypasses the limitations inherent in magnetic control, offering a new pathway for exploring exotic quantum states and building advanced superconducting devices. Detailed measurements of supercurrent interference patterns revealed that the induced phase frustration, typically achieved with magnetic flux, can be replicated solely through electrical means.
By defining a vector potential for the supercurrent gauge field, the combined gauge field captures the total phase-driving potential of the system. This formulation highlights the interchangeability of magnetic and electrical control, allowing for interference and replacement of the two. Experimental data, including calculated evolutions of critical current with increasing supercurrent gauge field, demonstrates the lateral shift of the supercurrent interference pattern, confirming the induced phase winding effect.
The team replicated standard patterns, the characteristic interference fringes observed in superconducting devices, using microampere-scale electrical currents, demonstrating the fidelity of the all-electric control. Further experiments with a symmetric current configuration suppressed the universal Josephson diode effect, allowing for focused analysis of the interference patterns.
These identical interference behaviors reveal that the fundamental role of magnetic flux is to induce phase frustration between the two sides of a Josephson junction, a frustration that can be achieved equally well with a supercurrent gauge field. By manipulating the current configuration, the researchers were able to generate phase-sensitive measurements of finite-momentum superconductivity. Differential resistance measurements, presented as supercurrent interference patterns, showed clear modulation of the interference pattern with control current, confirming the ability to construct and characterize this state without magnetic fields.
👉 More information
🗞 Magnetic-field-free quantum interference and universal Josephson diode effect driven by superfluid momentum
✍️ Haowei Ye et al.
🧠 DOI: http://link.aps.org/doi/10.1103/s8yg-g2jd
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