Simplified Superconducting Circuits Boost Performance with Single-Step Fabrication

A new architecture for connecting components in superconducting circuits simplifies manufacturing and reduces unwanted material interfaces. Prakiran Baidya from Friedrich-Alexander University Erlangen-Nuremberg and colleagues at Quint Computing GmbH replace traditional ‘bandages’ with solely airbridges for all electrical interconnects. The resulting high-yield, mechanically stable connections range from 0.5 to 4μm in width and 5 to 40μm in length. The resulting transmon qubits exhibit relaxation times exceeding 250μs, confirming the technique maintains high coherence while streamlining device fabrication. This provides a key step towards building more powerful quantum computers.

Airbridge fabrication extends qubit coherence beyond previous limitations

Relaxation times exceeding 250 microseconds represent a substantial improvement over previous superconducting qubit designs. These designs typically relied on both airbridges and bandages and struggled to surpass 100 microseconds. Longer coherence times allow for more operations before quantum information is lost, a crucial factor for performing complex quantum computations. Previously, maintaining coherence for this duration proved impossible due to material interfaces introduced by bandages. These interfaces create unwanted energy dissipation pathways, shortening the time a qubit can reliably maintain a superposition state, a fundamental requirement for quantum processing. The coherence time, specifically the relaxation time (T₁), dictates how long a qubit retains its excited state before decaying to the ground state; a longer T₁ directly translates to more computational steps possible before error correction is needed. Furthermore, the phase coherence time (T₂), a measure of how long a qubit maintains its phase relationship, is also critically impacted by material imperfections and interface states.

Baidya and colleagues at Quint Computing GmbH have established a new fabrication process, creating all electrical interconnects using solely airbridges and eliminating bandages and their associated limitations. A single-step gray-scale electron-beam lithography technique is employed in the fabrication process. This technique allows precise control over the deposition of the superconducting material, forming the airbridges with the desired dimensions. Electron-beam lithography uses a focused beam of electrons to pattern a resist layer, which is then developed to reveal the underlying substrate. The gray-scale capability allows for the creation of complex three-dimensional structures, crucial for forming robust and reliable airbridges. This contrasts with traditional binary lithography, which only allows for two states, exposed or unexposed, limiting the complexity of achievable structures. Airbridge dimensions ranging from 0.5 to 4 micrometers in width and 5 to 40 micrometers in length were achieved, providing flexibility in circuit design. Superconducting qubit devices have now achieved relaxation times exceeding 250 microseconds, marking a sharp leap forward in quantum computing hardware. This improvement is directly attributable to the elimination of bandage-induced material interfaces, reducing energy loss and preserving qubit coherence.

Measurements of coplanar waveguide resonators and transmon qubits, a type of superconducting circuit, reveal these airbridges introduce no measurable electrical loss. Coplanar waveguide resonators are used to characterise the properties of the superconducting material and interconnects, while transmon qubits are a leading candidate for building scalable quantum computers. The absence of measurable loss indicates the airbridges maintain the superconducting properties of the circuit, crucial for maintaining coherence. The process is also compatible with materials beyond aluminium, including niobium and tantalum, broadening its potential applications. Niobium and tantalum offer higher critical temperatures and improved performance characteristics compared to aluminium, potentially enabling the development of more robust and higher-performance quantum devices. However, these results do not yet demonstrate sustained performance across multiple qubit systems or address the challenges of scaling to the thousands of qubits needed for fault-tolerant quantum computation. Building a practical quantum computer requires not only high-performance qubits but also the ability to connect and control many of them, a significant engineering challenge.

Simplified fabrication enhances qubit performance and presents scaling challenges

Building more powerful quantum computers demands increasingly intricate superconducting circuits, and simplifying their manufacture is vital for progress. The complexity of these circuits arises from the need to precisely control the interactions between qubits, requiring a dense network of interconnects. While this new single-step airbridge fabrication process offers a clear advantage over existing methods, its scalability remains an open question. The technique’s adaptability to create the complex, three-dimensional architectures needed for truly large-scale quantum processors is yet to be fully determined. Scaling up the fabrication process to accommodate many qubits while maintaining high yield and uniformity presents significant challenges. Issues such as electron beam drift, resist contamination, and variations in deposition rates can all impact the quality of the airbridges and the overall performance of the circuit.

Deposition control and alignment precision may ultimately hinder its widespread adoption. Maintaining precise alignment between different layers of the circuit is crucial for ensuring proper connectivity and minimising crosstalk between qubits. Even acknowledging valid concerns about scaling up this fabrication process for vastly more complex quantum processors, its immediate benefits are substantial. Replacing traditional ‘bandages’, connections between circuit layers, with solely airbridges simplifies manufacturing and reduces unwanted electrical interference. This single-step airbridge technique maintains qubit coherence, important for reliable quantum computation, while accelerating production. Reducing the number of fabrication steps not only lowers costs but also reduces the likelihood of introducing defects, improving the overall yield and reliability of the devices.

Simplifying the fabrication of superconducting circuits represents a key step towards building more powerful quantum computers, fabricating all electrical connections within these circuits using only airbridges, tiny, arched metallic structures. Eliminating the need for ‘bandages’, a traditional component introducing material flaws and manufacturing complexity, is a significant advance. Bandages typically involve depositing dielectric materials and metals in multiple steps, creating interfaces that can trap electromagnetic noise and degrade qubit performance. Achieving high coherence, a measure of qubit stability essential for complex calculations, with relaxation times exceeding 250 microseconds confirms the viability of this streamlined approach, now prompting investigation into adapting the technique for fabricating the intricate, three-dimensional architectures required for large-scale quantum processors. Future research will focus on optimising the fabrication process for scalability, exploring alternative materials, and integrating this technique with other advanced fabrication methods to create even more powerful and reliable quantum computers.

The researchers successfully fabricated all electrical interconnects within superconducting circuits using only airbridges, ranging in size from 0.5μm to 40μm. This represents an improvement because it eliminates the need for ‘bandages’, which introduce unwanted material interfaces and complicate manufacturing. Maintaining high qubit coherence, with measured relaxation times exceeding 250μs, demonstrates the technique’s viability for reliable quantum computation. The authors intend to optimise this process for scalability and explore integration with other fabrication methods.