Short answer: An air bridge is a small freestanding metal span that connects the ground planes of a superconducting coplanar waveguide, suppressing parasitic modes that cause loss. A 2014 Applied Physics Letters paper reports that aluminum air bridges add small loss at single-photon levels, a 2017 paper describes a way to build low-loss freestanding crossovers with a sacrificial SiO2 scaffold, and a 2018 Chinese Physics B paper reports a simpler fabrication of air bridges on niobium waveguides. These are fabrication and loss measurements.
Why air bridges
The 2018 abstract says coplanar waveguides can show spurious resonance, caused by discontinuities and asymmetries in the ground planes, and that this is a common source of decoherence in circuit quantum electrodynamics. The 2014 paper makes the same point in terms of parasitic slotline modes, which can lead to loss and decoherence. An air bridge ties the ground planes together to block them.
2014: the added loss is small
The 2014 paper, from the University of California, Santa Barbara, motivates air bridges as a reliable method for preventing the propagation of slotline modes. It describes fabricating aluminum air bridges on superconducting resonators and using the resonators to measure the loss from placing air bridges over coplanar lines. The authors find that the additional loss at single-photon levels is small, and that it decreases at higher drive powers.
2017: multilayer wiring with a scaffold
A 2017 preprint from the same group, with Google coauthors, starts from the observation that complex integrated circuits need multiple wiring layers. In standard CMOS processing, amorphous dielectrics separate the layers, but in superconducting circuits these dielectrics would dominate energy loss. The authors demonstrate a procedure that uses the structural benefits of inter-layer dielectrics during fabrication and then mitigates the added loss: they support multiple wiring layers with a SiO2 scaffold during fabrication, then remove it afterward. They say the technique is compatible with foundry-level processing and can be generalized. They use it to create freestanding aluminum vacuum-gap crossovers and characterize the added capacitive loss by connecting ground planes over quarter-wave coplanar waveguide resonators. The remainder of the abstract was cut off here, so its measured loss values are not reported.
2018: a simpler process
The Chinese Physics B paper fabricates superconducting aluminum air bridges on niobium coplanar waveguides to mitigate spurious resonance. The air bridges are approximately 3 micrometres high and up to 120 micrometres long. The authors say their fabrication procedure is simpler than other methods and that the air bridge can withstand strong ultrasound. The abstract does not report a loss measurement.
Limits
The three papers use different materials and processes, and only the 2014 paper reports a loss result in the part of the abstract that was readable. The statements that the 2017 technique is foundry-compatible and the 2018 process is simpler are the authors' own.
FAQ
What does an air bridge do?
It connects ground planes across a waveguide to suppress parasitic modes.
Do air bridges add loss?
The 2014 authors find the added loss at single-photon levels is small and decreases with drive power.
Why remove the SiO2 scaffold?
Amorphous dielectrics would dominate energy loss in superconducting circuits, per the 2017 abstract.
What is the size of the 2018 bridges?
About 3 micrometres high and up to 120 micrometres long.
Is this about qubits directly?
It is about microwave circuit components that qubits rely on.
All field notes