Crystalline Layers Slash Microwave Loss in Emerging Quantum Computers

David A. Garcia-Wetten and colleagues at Northwestern University have developed a new materials platform for superconducting quantum circuits. They present a new approach using crystalline dielectrics integrated into a layered structure grown by pulsed laser deposition. The team created high-quality, single-crystal $γ$-Al$$2O$3$ with exceptionally low microwave loss, specifically, a two-level system loss of $(2.8 \pm 0.1) \times 10^{-5}$. This achievement offers a key advance for more compact and efficient quantum devices, including transmons and microwave kinetic inductance detectors.

Low-loss gamma-alumina dielectric enables enhanced superconducting qubit performance

A two-level system loss of (2.8 ±0.1) × 10⁻⁵ was achieved in the fabricated γ-Al2O3 dielectric, representing a two-order-of-magnitude improvement over amorphous aluminium oxide. This improvement surpasses a key threshold for viable superconducting qubit operation, as high dielectric loss had previously severely limited qubit coherence and scalability. Pulsed laser deposition, a precise heteroepitaxial growth technique, enabled the integration of crystalline γ-Al2O3 within a titanium nitride trilayer, constructing layers with atomic precision.

Correlative imaging and spectroscopy confirmed the high quality and chemical integrity of these layers, with minimal defects hindering performance. This new materials platform promises more compact and coherent designs for quantum circuits, including advanced qubit architectures like merged-element transmons and microwave kinetic inductance detectors. Microwave lumped-element resonators, used to measure dielectric properties, employed parallel-plate capacitors having an area of just 2.4 × 10⁻² mm², a sharp reduction in size compared to the 2-3 mm² typically occupied by existing transmon qubits per device.

Correlative high-resolution imaging, diffraction, and spectroscopy provided evidence of minimal defects and limited anion interdiffusion within the crystalline γ-Al2O3 layers. While these results demonstrate a strong improvement in material quality, current measurements focus on small-area devices and do not yet demonstrate sustained performance or scalability across larger, more complex quantum circuits. Atomic-level precision in layer construction achieved through the growth of these trilayers on α-Al2O3 substrates using the aforementioned technique.

Pulsed laser deposition of titanium nitride and aluminium oxide heterostructures

Pulsed laser deposition proved central to achieving these results, involving the vaporisation of a target material with a high-power laser and subsequent deposition onto a substrate. The method was chosen because it had previously yielded high-quality crystalline films of both titanium nitride and aluminium oxide, allowing consecutive growth of all three layers of the titanium nitride/aluminium oxide/titanium nitride structure without environmental exposure. This approach enables heteroepitaxial growth, similar to building with LEGO bricks where each layer is carefully aligned and bonded, creating a precise and ordered structure.

Titanium nitride/aluminium oxide/titanium nitride trilayers fabricated on sapphire substrates using this deposition method. The first sample comprised 62.2 nanometres of titanium nitride, 13.5 nanometres of aluminium oxide, and 53.8 nanometres of titanium nitride, while the second featured 63.0 nanometres, 58.3 nanometres, and 68.1 nanometres respectively. Measurements utilising techniques like X-ray diffraction confirmed the single-crystal quality and sharp interfaces between layers, with minimal material intermixing.

Low-loss aluminium oxide crystals offer potential for improved quantum coherence

Scientists are edging closer to building practical quantum computers, yet a fundamental obstacle remains: energy loss within the materials used to create them. This work offers a promising new crystalline dielectric, aluminium oxide, integrated into a layered structure, demonstrating sharply reduced loss compared to existing amorphous materials. However, the team’s measurements were conducted at very low temperatures, around 4.2 Kelvin, and it remains unclear whether this low loss will hold true under the warmer, more demanding conditions of a functioning quantum processor.

It is important to acknowledge that these encouraging results were obtained at extremely low temperatures, as practical quantum computers operate at much warmer levels. Nevertheless, demonstrating such low energy loss in a crystalline material represents a vital step forward in materials science for quantum computing. Reducing loss, even under these specific conditions, validates aluminium oxide as a promising dielectric, a key electrical insulator for building the delicate circuits within a quantum processor.

Successful integration of crystalline aluminium oxide within a titanium nitride structure achieved using a precise layer-by-layer technique. Direct measurement confirms exceptionally low microwave loss within the aluminium oxide, establishing a new benchmark for materials used in superconducting qubits, the fundamental building blocks of quantum computers. This advance establishes oxides on transition metal nitrides as a viable platform for building more compact and efficient quantum processors, paving the way for future designs incorporating these materials for enhanced performance.

Researchers demonstrated a low intrinsic two-level system loss of $(2.8 \pm 0.1) \times 10^{-5}$ in crystalline aluminium oxide integrated into a titanium nitride structure. This finding matters because energy loss within materials limits the performance of superconducting qubits, the core components of quantum computers. The results establish a new materials platform for these circuits, potentially enabling more compact device architectures. The team’s work confirms the viability of using heteroepitaxial oxides on transition metal nitrides for future quantum computing designs.