Coherent oscillations in weakly anharmonic NbSe2 qubit

The quest to build machines that think and calculate in ways impossible for today's computers relies on a tiny, fragile unit of information called a qubit. Unlike the bits in a standard computer that are either zero or one, a qubit can exist in a delicate state of both at once, but only if it is kept perfectly isolated from the noisy, chaotic world around it. To achieve this, scientists often use superconducting circuits, which are electrical loops that carry current without any resistance, but only when chilled to temperatures near absolute zero. The heart of these circuits is a special junction that acts as a gatekeeper, allowing the qubit to behave like a quantum oscillator. For years, researchers have built these junctions using aluminum, a material with a well-understood manufacturing process. However, aluminum has limits; it is prone to microscopic defects and struggles to function in strong magnetic fields, which blocks scientists from using these powerful quantum tools for certain types of cosmic detective work, such as searching for invisible particles that might make up dark matter.

To overcome these barriers, a team of researchers has turned to a different class of materials known as van der Waals solids. Imagine these as stacks of ultra-thin sheets, like a deck of cards, where the layers can be peeled apart and stacked in new ways with atomic precision. These materials offer a cleaner, more perfect internal structure than aluminum, potentially shielding the qubit from the defects that cause it to lose its quantum state. While scientists have already managed to make qubits using graphene, a single layer of carbon, they have not yet succeeded in creating a working qubit from other promising candidates in this family. In a new study, researchers have successfully built and tested a qubit using niobium diselenide, a material that is not only a superconductor but also remarkably resistant to magnetic fields. This achievement marks the first time a qubit has been made from this specific material, opening a door to more robust quantum devices that could operate in environments where aluminum would fail.

The researchers constructed their device by taking two tiny flakes of niobium diselenide, each only about ten to fifteen nanometers thick, and carefully overlapping them to create a junction where the two layers meet. They placed this delicate junction inside a hollow aluminum box, which acts as a resonator to help control and read the state of the qubit. To ensure the junction worked correctly, they connected it to large aluminum pads and surrounded the entire setup with filters to block stray heat and radiation. The experiment took place in a specialized laboratory where the temperature was cooled to roughly twenty millikelvin, a fraction of a degree above absolute zero, to ensure the material remained superconducting. By sending microwave pulses into the box, the team could probe the behavior of the junction and see if it acted like a quantum oscillator.

What they found was a device that behaved exactly as a weakly anharmonic oscillator should, meaning it could hold quantum information while resisting the noise that usually destroys it. When the researchers increased the power of the signals sent into the device, the resonance of the cavity shifted in a way that indicated the material was behaving like a "softening" oscillator, a specific type of non-linear behavior. Remarkably, the device remained stable even when the researchers introduced a significant amount of thermal noise, equivalent to five to ten extra photons of heat energy inside the cavity. This level of robustness is far superior to what has been seen in other qubits made from similar layered materials. Furthermore, the team demonstrated that by adjusting the power of the input signal, they could flip the behavior of the oscillator from softening to "hardening," effectively changing the sign of its non-linearity. This ability to tune the device's fundamental properties suggests a high degree of control over the quantum system.

The most critical test for any qubit is how long it can hold its quantum state before losing coherence, a property measured by a value called the relaxation time. In this experiment, the niobium diselenide qubit maintained its state for 6.5 microseconds, a duration that is roughly one hundred times longer than the best results previously achieved with graphene-based qubits. This extended lifetime is likely due to the high crystalline quality of the niobium diselenide, which minimizes the internal defects that usually disrupt quantum states, combined with the material's ability to withstand higher currents. The team also observed clear, rhythmic oscillations in the qubit's state, known as Rabi oscillations, which confirmed that the device could be manipulated with precision. While they were unable to measure the full decoherence time due to the length of the control pulses required, the relaxation time alone represents a massive leap forward for this class of materials.

This work serves as a powerful demonstration that van der Waals materials can be successfully integrated into quantum technologies, offering a path toward devices that are more durable and versatile than current aluminum-based systems. The ability of niobium diselenide to function in strong magnetic fields is particularly significant, as it suggests these qubits could eventually be used in experiments to detect dark matter, a task that requires placing quantum sensors directly inside powerful magnets where traditional aluminum qubits would stop working. While the current device has a small amount of non-linearity, which limits how easily it can be used for certain types of calculations, the researchers note that this parameter can be fine-tuned in future designs by adjusting the size of the junction or using more precise manufacturing techniques. By proving that a qubit can be built from this material and can perform with such high coherence, the study provides a solid foundation for the next generation of quantum sensors and computers, bridging the gap between exotic materials and practical quantum applications.