coplanar waveguide (CPW)

A coplanar waveguide is the everyday wiring shape that carries microwave signals across a superconducting quantum chip. It is just three flat metal strips lying side by side on the same surface: a thin center conductor in the middle, flanked by a wide ground plane on each side, with a narrow gap of bare substrate separating them. The signal travels along the center strip, while the two grounds give the field something to push against. Because everything sits on one face of the chip, you can pattern all of it in a single step out of the same superconducting film, which is a big part of why it became the workhorse layout.

What makes CPW useful is that its behavior is set by simple geometry. The width of the center strip and the size of the gaps to ground fix the line's characteristic impedance, and designers almost always tune those dimensions to hit about 50 ohms, the value the rest of the microwave world expects, so signals flow in and out without bouncing back. The same shape does double duty: cut a length of CPW to the right size and let it ring, and it becomes a resonator, the tuning-fork element used for qubit readout and filtering. So a single line style routes the control and readout signals and also forms many of the resonant circuits on the chip.

There are two honest catches. First, a CPW has two separate ground strips, and if they are left to drift apart they can host an unwanted ringing mode along the gap, called a slotline mode, that steals energy and leaks signal between qubits; designers suppress it by stitching the grounds together at intervals with little crossover bridges (airbridges). Second, the electric field crowds into the narrow gaps right at the metal edges and the exposed substrate surface, which is exactly where lossy defects and contaminants live, so energy leaks away there and feeds straight into qubit decoherence. Getting CPW right is therefore as much about clean edges and well-tied grounds as it is about hitting the target impedance.