A supply-chain device is paired once with a secret reference qubit state |ψref⟩, pictured as a point (Bloch vector n) on a sphere — drawn here as a 2D projection you can drag to rotate. To re-verify the device later, the checkpoint sends a batch of qubits prepared in that same state through the channel; the device measures them and the checkpoint compares the outcome statistics to the expected reference:
Fidelity: F(ψ1,ψ2) = |⟨ψ1|ψ2⟩|² = (1 + n1·n2) / 2
QBER ≈ 1 − F (average per-qubit disturbance)
An intercept-resend attacker (Eve) measures each intercepted qubit in a randomly chosen basis and re-prepares it before forwarding — exactly as in BB84 eavesdropping. Because her guessed basis only matches the true one half the time, this collapses the Bloch vector toward a random direction and drags the average fidelity down, exactly like tampering with a physical shipment leaves a detectable trace. Ordinary channel decoherence (heat, vibration, fiber loss over distance) instead nudges the vector by a small random amount without fully randomizing it.
- Intercept-resend attack rate — fraction of the N qubits an attacker tampers with this round.
- Channel decoherence — untampered qubits still pick up this much random drift from the physical channel.
- Qubits per round — batch size N averaged to estimate fidelity; larger N gives a steadier, more trustworthy readout.
- Authentication threshold — the minimum average fidelity the checkpoint will accept as "this is still the same device, untampered."
- Drag the view — click-drag the circles to rotate your viewpoint around both Bloch spheres, exactly like orbiting a 3D scene; it changes only what you see, never the physics.
Real-world relevance: this is the core idea behind proposed quantum-fingerprint device authentication for supply chains — any physical tampering or interception that touches the quantum channel necessarily disturbs the state, so the checkpoint can detect it statistically without ever trusting the device's own claims.