Unwanted coupling of signals between adjacent channels or components.Crosstalk is the unwanted coupling of signals between adjacent channels or components, where a signal in one circuit or transmission line leaks into another. In everyday terms, it is the 'bleed' you hear when a phone conversation picks up a faint second conversation, or the ghost image on a television screen caused by a reflected signal. In physics and engineering, crosstalk is a fundamental limitation in any system that packs many signal-carrying elements close together, from microchips to quantum processors.
The term originated in early telephone engineering in the late 19th century, when multiple conversations on adjacent wires would audibly interfere. Systematic study began with Oliver Heaviside's work on transmission lines in the 1880s.
Crosstalk arises through two main mechanisms: capacitive and inductive coupling. When two conductors run parallel, an alternating voltage on one creates an electric field that induces a charge on the other (capacitive coupling), while an alternating current creates a magnetic field that induces a current in the other (inductive coupling). The effect scales with frequency and proximity, so high-speed or high-frequency systems are especially vulnerable. In quantum devices, crosstalk can also occur through shared substrate vibrations or stray photons, causing unwanted interactions between qubits.
Crosstalk is a central design constraint in superconducting quantum chips, where it limits qubit coherence and gate fidelity; engineers use careful layout, shielding, and compensation pulses to minimize it.
In the 1950s, crosstalk in submarine telephone cables was so severe that engineers developed 'compandors'—compressors and expanders—that deliberately distorted the signal to mask the leak.
