Entanglement & Bell states
Two qubits that share a single fate — the phenomenon Einstein called 'spooky'.
Beyond individual qubits
Put H on qubit 0, then a CNOT from qubit 0 to qubit 1. The result is the Bell state (|00⟩ + |11⟩)/√2: a 50% chance of both qubits reading 0, a 50% chance of both reading 1 — and zero chance of them ever disagreeing.
Here's the strange part: neither qubit has a state of its own anymore. Ask 'what is qubit 0 doing?' and the honest answer is 'it's perfectly random — but perfectly correlated with qubit 1.' The information lives in the relationship, not the parts.
The shrinking Bloch arrow
Load the Bell State preset and look at the Bloch spheres: both arrows collapse to the center of their spheres. A centered arrow means the individual qubit is maximally mixed — pure coin-flip randomness on its own.
This is the visual signature of entanglement in our simulator: individual arrows shrink as qubits give up individual identity. Step through the circuit column by column to watch it happen at the CNOT.
What entanglement is (and isn't)
Measuring one qubit of a Bell pair instantly tells you what the other will read, no matter the distance. Experiments have confirmed these correlations are stronger than any classical explanation allows (the 2022 Nobel Prize honored exactly this work).
But entanglement does not let you send messages faster than light — each side alone sees pure randomness. It's a resource for correlation, not communication. Teleportation, superdense coding and quantum error correction all spend entanglement as fuel.
- ◆H + CNOT creates the Bell state (|00⟩ + |11⟩)/√2.
- ◆Entangled qubits have no individual state — their Bloch arrows shrink to the center.
- ◆Entanglement enables powerful correlations but not faster-than-light messaging.