The Swap Test

The swap test measures how similar two quantum states are - the similarity score behind the quantum-machine-learning kernels Kronos studies for plasma data.

The concept

An ancilla in |+> controls a SWAP between two registers and is measured in the X basis; the outcome probability gives the overlap ||^2 without revealing either state.

How Kronos uses it

Overlap is a kernel - a distance between data points. In Kronos's quantum-machine-learning research for classifying plasma states and diagnostics, the swap test is how a quantum kernel is evaluated, and how a prepared state is verified against a target.

Honest note

Quantum kernels are a research direction; classical ML runs Kronos's control today, with quantum advantage expected ~mid-2030s.