The Search for the Optimum Temperature: Improving Superconducting Resonator Performance

Saturday - March 30 2024, 21:44 UTC - 1 year ago

Scientists have found that changing the deposition temperature affects the crystal structure of Nb on sapphire films, with higher temperatures resulting in lower losses in Nb qubits. An intermediate temperature of 550 K offers the best balance between crystal ordering and surface roughness. Understanding material structures is crucial in optimizing superconducting circuits for applications in quantum computing and other fields.

Superconducting circuits have opened up the possibility of creating large-scale quantum processors, bringing us one step closer to practical quantum computers. These circuits utilize materials with superconducting properties, such as niobium (Nb). To further optimize their performance, scientists have been studying the effect of temperature on the fabrication process.

One key finding is that Nb on sapphire, a common material platform for superconducting circuits, displays varying crystallographic structures depending on the deposition temperature. Films grown at room temperature have a poly-crystalline structure, while films grown at higher temperatures of up to 975 K exhibit a mono-crystalline character. This change in crystal structure also results in differences in surface roughness, which can affect the overall performance of the circuits.

Another important discovery is the correlation between substrate temperature and the losses in Nb qubits on sapphire. By increasing the substrate temperature by just 250 K, losses in the qubits can be reduced. This suggests that there may be an optimum temperature for other material systems as well, which requires further investigation. Additional optimization techniques such as HF etching, trench etching, encapsulation, and geometry optimization can also further reduce the loss rate.

Interestingly, the highest quality factors were observed in films grown at an intermediate temperature regime of 550 K. This temperature range offers a balance between preferential ordering of crystal domains and low surface roughness. By studying the temperature-dependent behavior of these resonators, researchers were able to gain insights into the effect of niobium crystal structure and grain boundaries on the density of quasiparticles in the Nb film. This further emphasizes the importance of understanding material structures in optimizing superconducting circuits.

It's not just quantum computing that can benefit from improved superconducting resonators. Other applications such as parametric amplifiers, quantum sensors, and studies of materials can also benefit from low levels of microwave dissipation. Thus, the search for the optimum temperature and other fabrication optimizations are crucial in the development of high-performance superconducting circuits.