Instantaneous key invalidation in response to a detected eavesdropper. A quantum computing system that includes a plurality of qubits and a quantum channel uses a quantum key distribution protocol to generate a key. The quantum computing system determines that an eavesdropper has eavesdropped on the quantum channel. In response to determining that the eavesdropper has eavesdropped on the quantum channel, the quantum computing system sends a key-revocation message to a designated destination.