Superconducting circuits host two-level systems with lifetimes exceeding one second

In the quest to build machines that can solve problems beyond the reach of today's computers, scientists are turning to a strange and delicate form of matter: superconducting circuits. These are loops of metal that, when cooled to temperatures near absolute zero, allow electricity to flow without any resistance. Within these circuits, researchers create artificial atoms called qubits, which serve as the basic units of information for quantum computers. Unlike the bits in a standard computer that are either zero or one, a qubit can exist in a combination of both states simultaneously. However, these qubits are incredibly fragile. They are constantly bombarded by their environment, and even the tiniest interaction can cause them to lose their information, a process known as decoherence. For years, the scientific community has worked tirelessly to shield these qubits from their surroundings, hoping to extend the time they can hold onto their quantum state. The prevailing belief was that as qubits became more stable, the noisy background of their environment would eventually become quiet enough to ignore.

A team of researchers at the Karlsruhe Institute of Technology in Germany has now challenged this assumption by discovering that the environment of a superconducting circuit is far more persistent than anyone expected. Using a specialized type of superconducting circuit known as a transmon, which is made from tantalum metal and cooled to extreme temperatures, the scientists looked for hidden sources of noise that might be interfering with their qubits. They were searching for "two-level systems," which are tiny defects in the material that can act like miniature switches, flipping between two states and stealing energy from the qubit. While these defects have been known for some time, the researchers found something extraordinary: a specific class of these defects that can hold onto energy for more than a second. This is a staggering amount of time in the quantum world, where qubits typically lose their state in a fraction of a millisecond.

To find these elusive defects, the researchers employed a technique called quantum-jump correlation spectroscopy. Imagine listening to a single note played on a violin in a quiet room. If the note suddenly stops, you might wonder where the sound went. In this experiment, the qubit is the violin, and the researchers watch for the moment it stops vibrating, or "jumps down" to a lower energy state. Usually, when a qubit loses energy, that energy disappears forever into the surrounding environment. However, the team observed that sometimes, after the qubit lost a photon of energy, that energy would return to the qubit after a short delay. This return of energy indicated that the environment had not simply absorbed the energy but had temporarily stored it in a long-lived state before giving it back. By carefully tracking these events, the researchers were able to isolate three specific defects that were holding onto energy for durations ranging from hundreds of milliseconds to over one second.

The researchers then designed a new method to measure exactly how long these defects could hold their energy. They used a "pump-probe" strategy, which involved first exciting the defect to fill it with energy and then waiting to see how long it took for that energy to leak out. By adjusting the frequency of the qubit to be slightly different from the frequency of the defect, they could control how strongly the two interacted. When the interaction was weak, they found that the defects held their energy for remarkably long periods. In fact, for two of the defects they studied, the energy remained stored for more than one second. This is orders of magnitude longer than the lifetime of the qubit itself, which typically lasts only about one hundred microseconds. The fact that these defects could store energy for so long suggests they are exceptionally isolated from the rest of the world, making them incredibly difficult to detect with standard measurement tools.

The study also provided clues about what these defects might actually be. The researchers compared their measurements to a standard model used to describe how atoms or molecules move between two positions in a solid material. This model suggests that defects which move very slowly and interact very weakly with their surroundings should have very long lifetimes. The data from the experiment matched this prediction perfectly, suggesting that these long-lived defects are likely tiny structural imperfections in the material that tunnel, or move, between two positions very slowly. While the researchers cannot yet say exactly what chemical or physical structure these defects are, the evidence points away from other possibilities, such as magnetic impurities or trapped particles, which would behave differently. The weak interaction between the qubit and these defects, measured in the range of a few thousand cycles per second, is consistent with the idea that they are positional defects described by this standard model.

This discovery changes how scientists view the environment of quantum computers. Previously, it was thought that as qubits became better, the remaining noise would be short-lived and easy to manage. Now, it appears that the most persistent noise comes from these long-lived defects that can store energy for seconds. This has important implications for the future of quantum error correction, which is the method used to fix mistakes in quantum calculations. If the environment remembers the state of the qubit for a long time, it can create complex patterns of errors that are harder to predict and correct. However, the researchers also see a potential upside. Because these defects can hold quantum information for such a long time, they might one day be used as a form of memory within a quantum circuit. The researchers suggest that as scientists continue to improve the quality of their qubits, they will likely encounter even more of these long-lived defects, much like a telescope with better lenses revealing fainter stars in the night sky.

The work presented in this paper establishes a new framework for understanding the microscopic world inside superconducting circuits. It proves that these circuits can act as sensitive probes for detecting microscopic degrees of freedom that last for seconds, a timescale that was previously thought impossible in this context. While the exact nature of these defects remains a subject for further investigation, the ability to measure their lifetimes and coupling strength provides a crucial step toward controlling them. Whether these long-lived systems are viewed as a nuisance that must be eliminated or a resource that can be harnessed, their existence fundamentally alters the landscape of quantum technology. The researchers have opened a door to a hidden layer of the quantum environment, one that is quiet, persistent, and full of potential for both challenge and discovery.