Field-controlled breaking and restoration of parity-time symmetry in Josephson interference

Symmetry is the hidden grammar of the physical world, a set of rules that dictates how matter behaves and how energy moves. In the realm of quantum physics, where particles act more like waves than solid objects, these rules are especially strict. Scientists often look for ways to break these symmetries to create new states of matter or to build devices that behave in unusual ways. One of the most fascinating areas of study involves superconductors, materials that conduct electricity with zero resistance. When two superconductors are separated by a thin barrier, they can still "talk" to each other, allowing a current to flow across the gap without a battery. This flow, known as a supercurrent, is incredibly sensitive to magnetic fields. By applying a magnetic field, researchers can make this current interfere with itself, creating a pattern of peaks and valleys similar to the ripples formed when two stones are dropped into a pond. Understanding how to control these patterns allows scientists to engineer quantum devices with specific, useful properties.

A team of researchers has now demonstrated a way to switch these interference patterns on and off, and to change their shape, simply by rotating a magnetic field. Working with a device made from a thin flake of a material called platinum telluride, sandwiched between two superconducting electrodes, they discovered that the direction of the magnetic field relative to the electric current acts as a master switch. When the magnetic field is applied in a specific direction, the interference pattern becomes lopsided, with one side of the ripple pattern much larger than the other. However, if they rotate the field by ninety degrees, the pattern snaps back into a perfect, symmetrical shape, even though the magnetic field is still strong. This ability to toggle between a broken symmetry and a restored one offers a new way to control quantum transport, turning the geometry of the magnetic field into a tool for reconfiguring the device's behavior.

The experiment took place in a laboratory cooled to a temperature just above absolute zero, roughly 50 millikelvin, to ensure the delicate quantum effects remained visible. The researchers built their device using a flake of platinum telluride that was between 20 and 40 nanometers thick. They attached superconducting electrodes made of niobium-titanium to either side of the flake, creating a junction where the supercurrent could flow. To probe the system, they applied a magnetic field that had two components: one pointing straight up through the device and another lying flat within the plane of the material. By changing the angle of this flat, in-plane field relative to the direction of the current, they could watch how the interference pattern evolved.

When there was no flat magnetic field, the device behaved as expected, producing a classic, symmetrical pattern known as a Fraunhofer pattern. This pattern features a large central peak with smaller, evenly spaced peaks on either side, indicating that the supercurrent was flowing uniformly across the junction. But the moment they introduced a flat magnetic field parallel to the current, the symmetry vanished. The pattern became heavily distorted, with the peaks on one side of the central maximum shrinking while the peaks on the other side grew. This asymmetry was so pronounced that it looked like a mirror image of itself had been flipped. The researchers found that this effect was not random; it depended entirely on the angle between the current and the magnetic field. When the field was parallel to the current, the distortion was at its worst. When they rotated the field to be perpendicular to the current, the pattern instantly returned to its original, symmetrical state.

To understand why this was happening, the team developed a microscopic model of what was occurring inside the device. They realized that the superconducting electrodes were not just passive walls; they actively reshaped the magnetic field lines passing through them. Because of a property called the Meissner effect, which causes superconductors to expel magnetic fields, the flat magnetic field lines were forced to bend as they approached the edges of the electrodes. This bending created small, localized regions where the magnetic field pointed up in one spot and down in another, effectively forming a magnetic dipole right at the interface between the superconductor and the normal material.

These tiny magnetic dipoles acted like invisible hands, pushing and pulling on the paths taken by the electrons flowing through the junction. In a perfect, flawless material, these pushes would cancel out. However, real materials always contain some disorder—tiny imperfections in the atomic structure that scatter electrons. Because of this disorder, the electrons took different, winding paths through the junction. Some paths sampled the "up" part of the magnetic dipole, while others sampled the "down" part. When the researchers reversed the direction of the main magnetic field, the electrons on these different paths experienced a different net push, causing the interference pattern to shift and become asymmetric. The strength of this effect depended on how the flat magnetic field was oriented. When the field was parallel to the current, the bending of the field lines was maximized, creating strong dipoles that scrambled the interference pattern. When the field was perpendicular, the bending was minimized, the dipoles disappeared, and the symmetry was restored.

The researchers were careful to rule out other possible explanations. They considered whether the material itself had an intrinsic property that favored one direction over another, but their experiments showed that the effect rotated with the device, not with the crystal structure of the material. They also tested the device with different lengths and orientations, and the behavior remained consistent, confirming that the phenomenon was driven by the geometry of the field and current, not by a specific flaw in the sample. Furthermore, they used computer simulations to model the system, incorporating the effects of disorder and the magnetic field focusing. These simulations reproduced the experimental results perfectly, showing that the interplay between the bent magnetic field lines and the disordered paths of the electrons was indeed the cause.

This discovery establishes a new type of controllable quantum device. By simply rotating a magnetic field, scientists can now switch a superconducting junction between a state where symmetry is broken and a state where it is restored. This is not just a theoretical curiosity; it provides a practical method for engineering the symmetry of quantum transport. The ability to select which symmetry operation is being probed by changing the field-current geometry opens the door to new kinds of sensors and switches. For instance, the device could be used to detect the orientation of a magnetic field with high precision or to image the paths that supercurrents take through a material. The work highlights how disorder, often seen as a nuisance in quantum systems, can be harnessed as a feature when combined with the right magnetic environment.

The findings suggest that the behavior of these devices is governed by a delicate balance between the magnetic field's ability to focus flux and the random nature of the material's internal structure. The researchers noted that this effect persists up to magnetic field strengths of about 70 millitesla, a range that is substantial for such delicate quantum phenomena. The restoration of symmetry at the perpendicular angle is particularly robust, holding true even as the field strength increases. This suggests that the mechanism is stable and reliable, making it a promising candidate for future applications in quantum technology.

In the broader context of physics, this work adds to the growing understanding of how to manipulate quantum states using external fields. It demonstrates that symmetry is not just a fixed property of a material but a dynamic degree of freedom that can be tuned in real time. The researchers have shown that by controlling the relative orientation of the current and the magnetic field, one can effectively write and erase the rules of interference for superconducting electrons. This level of control is a significant step forward in the quest to build more sophisticated quantum circuits, where the ability to reconfigure the behavior of a device on the fly is essential. The study provides a clear, experimentally verified path to achieving this, grounded in the fundamental physics of how magnetic fields interact with superconductors and disordered materials.