Operation of Unshielded Kinetic-Inductance Traveling-Wave Parametric Amplifiers in Multi-Tesla Fields

In the quiet, freezing world of quantum physics, scientists rely on a special kind of electronic whisper to hear the faintest signals from the universe. To detect particles that might make up dark matter or to read the state of a quantum computer, researchers need to amplify tiny radio waves without adding any extra noise. For years, the best tools for this job have been delicate devices built from superconducting materials, which conduct electricity with zero resistance. However, these amplifiers have a fatal weakness: they are incredibly sensitive to magnetic fields. Even a tiny magnetic field can disrupt the superconducting currents inside them, causing them to stop working. This creates a major problem for experiments that require strong magnets, such as those searching for axions, a hypothetical particle that could explain the invisible mass of the universe. In these experiments, the axions are expected to turn into detectable radio waves only when bathed in a powerful magnetic field, but the very magnet needed to create the signal also destroys the amplifier meant to hear it.

To solve this, a team of researchers turned to a different type of amplifier that uses a property called kinetic inductance. Unlike the traditional devices that rely on tiny junctions which break easily in magnetic fields, kinetic inductance comes from the inertia of electron pairs moving through a superconductor. This mechanism is naturally much tougher against magnetic interference. The researchers, working at the National Institute of Standards and Technology and Pacific Northwest National Laboratory, set out to test whether these robust amplifiers could actually function inside the intense magnetic environments required for dark matter searches. They built two versions of the device and placed them inside a massive magnet capable of generating fields over one and a half Tesla, a strength roughly thirty thousand times stronger than the Earth's magnetic field.

The team discovered that the orientation of the amplifier relative to the magnetic field was the deciding factor. When they positioned the device so the magnetic field hit it from the side, perpendicular to its surface, the amplifier lost its ability to boost signals at a very low field strength of just 0.02 Tesla. However, when they rotated the device so the magnetic field ran parallel to its surface, the results were striking. In this alignment, the amplifier continued to work effectively even when the magnetic field was increased to over one Tesla. In fact, the researchers found that the device performed best, achieving its highest signal boost, when the magnetic field was between 0.25 and 0.5 Tesla. This suggests that the magnetic field does not simply break the device but actually changes its internal properties in a way that can be managed.

To understand why this happened, the scientists looked closely at how the magnetic field interacted with the superconducting material. They observed that the field caused tiny magnetic vortices, or swirls of magnetic flux, to enter the material. These vortices create a form of friction that dissipates energy, which is why the signal gets weaker. However, the team found that the superconducting state itself was not destroyed. Instead, the magnetic field slightly increased the kinetic inductance of the material. By adjusting the frequency of the pump signal used to drive the amplifier, the researchers could compensate for this change and restore the device's performance. This means that the amplifier does not need to be shielded from the magnet; it can operate directly within the field, provided it is oriented correctly and tuned to the right frequency.

The researchers also tested the durability of these devices by exposing them to multiple cycles of turning the magnetic field on and off, ramping the strength up to 2.5 Tesla. Even after these repeated, harsh treatments, the amplifiers retained their ability to function, showing no signs of permanent damage or the need for a special cooling cycle to reset them. This resilience is a significant step forward, as it proves that these amplifiers can survive the conditions of real-world dark matter experiments. The study confirms that kinetic inductance traveling-wave parametric amplifiers can provide strong signal amplification over a wide range of frequencies while sitting inside multi-Tesla magnetic fields. This capability opens the door for more sensitive searches for dark matter and other fundamental physics experiments that were previously limited by the fragility of their electronic sensors.