Two-Qubit Gate
An operation that makes one qubit's state depend on another, the step that creates entanglement and where most of a quantum computer's error budget goes.
A two-qubit gate is an operation that makes the state of one qubit depend on another, the step that creates entanglement, and the step where quantum computers earn their keep. Single-qubit rotations are easy and clean on every platform; two-qubit gates are where the error budget goes to die.
That's why 'two-qubit gate fidelity' is the spec the industry brags about. The gap between 99% and 99.9% sounds cosmetic but is the difference between an algorithm dying after a hundred operations or surviving a thousand. Trapped-ion machines (Quantinuum, IonQ) currently post the best two-qubit numbers; superconducting machines compensate with speed; neutral-atom platforms like Atom Computing's have been closing the gap via Rydberg interactions.