circuit QED (cQED)

Circuit QED is the toolbox for building qubits and the microwave "light" they talk to entirely out of electrical circuits printed on a chip. Instead of trapping a real atom and a real photon in a mirror box, engineers etch a superconducting circuit that behaves like an artificial atom, and a resonator next to it that behaves like a box of microwave photons. It is the chip-design version of an old physics setup called cavity QED.

The trick is that a superconducting LC circuit (an inductor plus a capacitor) naturally rings at one microwave frequency, like a tiny tuning fork. Add a Josephson junction and the circuit's energy rungs become unevenly spaced, so you can address just the bottom two as a qubit. Place a resonator nearby and the two share energy through their electric fields; that coupling is what lets you read the qubit out and wire qubits to each other, all in the gigahertz range where ordinary microwave electronics already work.

The honest part: this only works at a few hundredths of a degree above absolute zero, the qubits drift and lose coherence, and packing many of them onto one chip means their frequencies start to collide and their control lines get crowded. cQED is the dominant framework for superconducting chips, but it is one modality among several, and none has clearly won.