The correlation that has no classical explanation. Two entangled qubits share a fate — measure one and you instantly know the other, no matter the distance. This is quantum computing's most powerful resource.
Two classical coins can both show heads — but that's just because each was heads independently. Entangled qubits are different: neither one has a definite value until measured, yet their outcomes are perfectly correlated. Einstein called this “spooky action at a distance.”
Correlated coins
Two coins in separate boxes. You peek at one — it's heads. The other might be heads or tails. Each coin had a definite state all along. Boring.
Entangled qubits
Neither qubit is 0 or 1 until measured. But when you measure one, the other's outcome is instantly determined. Not because it was predetermined — because they share a single quantum state.
Two dice that always sum to 7 — but neither die has a value until you look. And it doesn't matter if they're on opposite sides of the universe. The correlation isn't hidden information — it's the fundamental nature of the quantum state.
A Bell state is the simplest entangled state — two qubits, maximally correlated. It takes just two gates: a Hadamard (H) to create superposition, then a CNOT to entangle.
Puts q0 into superposition: equal chance of 0 or 1.
Flips q1 if q0 is 1. Now their fates are linked.
Always get 00 or 11. Never 01 or 10. That's entanglement.
There are exactly four maximally entangled two-qubit states. They differ in which outcomes are correlated and whether there's a relative phase. Click to explore each one.
Both qubits always agree
If q0 is 0, q1 is 0. If q0 is 1, q1 is 1.
Circuit: H on q0, then CNOT
Entanglement isn't binary — states can be partially entangled. Drag the slider to smoothly transition from a separable product state |00\u27E9 to a maximally entangled Bell state |\u03A6+\u27E9. Watch the concurrence climb from 0 to 1.
1.000
0 = separable, 1 = max entangled1.000
0 = pure subsystem, 1 = maximally mixedWith three or more qubits, entanglement comes in fundamentally different flavors. GHZ and W states can't be converted into each other, even with local operations. They represent two distinct classes of quantum correlation.
| Property | GHZ | W |
|---|---|---|
| State | (|000⟩+|111⟩)/√2 | (|001⟩+|010⟩+|100⟩)/√3 |
| Entanglement | All-or-nothing | Distributed |
| Lose 1 qubit | All entanglement gone | 2/3 entanglement survives |
| Use case | Quantum error detection | Quantum networks |
| Fragility | Extremely fragile | Robust to qubit loss |
| Our data | GHZ-3: 88.9% (Tuna-9) | Not yet tested on hardware |
A chain vs a web — GHZ entanglement is a chain: break any link and the whole thing fails. W entanglement is a web: cut a strand and the rest holds. Both are useful for different things.
Bell state fidelity — the probability of getting the correct correlated outcome — is the simplest measure of how well a quantum processor can create entanglement. We tested it across three processors, sweeping qubit pairs and circuit sizes.
As you entangle more qubits, fidelity drops. At 50 qubits, IBM Torino achieves only 8.5% — barely above random noise. This decay rate is a key metric for quantum error correction readiness.
IBM Torino
IQM Garnet (96.3% mean)
8.5% fidelity (IBM)
Entanglement isn't just a curiosity — it's the resource that makes quantum computing more powerful than classical computing. Every quantum advantage relies on it.
Simulating a highly entangled state of n qubits requires 2^n classical resources. At ~50 qubits, no supercomputer can keep up. Entanglement IS the exponential.
VQE & ChemistryElectron correlation in molecules is entanglement by another name. The CNOT in our H₂ circuit creates the entanglement that captures correlation energy. Without it, we get Hartree-Fock — no chemical accuracy.
Quantum TeleportationA Bell pair is a quantum communication channel. Consume one Bell pair + 2 classical bits to teleport any qubit state. The entanglement is used up in the process.
Error CorrectionLogical qubits are encoded across many entangled physical qubits. The entanglement lets you detect and correct errors without measuring (and destroying) the data. Our [[4,2,2]] experiments encode 2 logical qubits in 4 entangled physicals.
One of four maximally entangled two-qubit states (Φ+, Φ−, Ψ+, Ψ−). The simplest entangled states.
A measure of two-qubit entanglement from 0 (separable) to 1 (maximally entangled). For pure states: C = |sin(2θ)|.
Entropy of the reduced density matrix. S = 0 for a pure (unentangled) subsystem, S = 1 for maximally mixed (maximally entangled).
(|000...0⟩+|111...1⟩)/√2. All-or-nothing entanglement. Fragile: losing one qubit destroys all entanglement.
Equal superposition of single-excitation states. Robust: entanglement partially survives qubit loss. Concurrence 2/3 after tracing out one qubit.
A state that can be written as a product |a⟩⊗|b⟩. No entanglement. Local measurements are independent.
ρ = |ψ⟩⟨ψ|. Encodes both probabilities (diagonal) and coherence (off-diagonal). Partial trace reveals entanglement.
Classical correlations are bounded by Bell's inequality. Quantum entanglement violates it — proving correlations have no classical explanation.
Entanglement is monogamous: if A is maximally entangled with B, A cannot be entangled with C at all. Constrains multi-party entanglement.
Local Operations and Classical Communication. The operations you can do without sharing quantum states. GHZ and W can't be converted under LOCC.
[1]Einstein, Podolsky & Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" Phys. Rev. 47, 777 (1935)
[2]Bell, "On the Einstein Podolsky Rosen Paradox," Physics 1, 195 (1964)
[3]Aspect, Dalibard & Roger, "Experimental Realization of Bell's Inequalities," Phys. Rev. Lett. 49, 1804 (1982)
[4]Greenberger, Horne & Zeilinger, "Going Beyond Bell's Theorem," in Bell's Theorem, Quantum Theory, and Conceptions of the Universe (1989)
[5]Dür, Vidal & Cirac, "Three qubits can be entangled in two inequivalent ways," Phys. Rev. A 62, 062314 (2000)
[6]Our experimental data: Bell and GHZ fidelity across IBM Torino, QI Tuna-9, and IQM Garnet (2025-2026)
Entanglement is a quantum correlation with no classical analogue. When two qubits are entangled, measuring one instantly determines the other — regardless of distance. The four Bell states are the simplest entangled states, each producing perfectly correlated (or anti-correlated) measurement outcomes.
GHZ and W states extend entanglement to three or more qubits with different properties. A GHZ state is maximally entangled but fragile — losing one qubit destroys all entanglement. A W state is more robust: losing one qubit still leaves the remaining qubits partially entangled.
This explorer includes real hardware fidelity data from IBM Quantum and Quantum Inspire (Tuna-9), showing how well current processors can prepare and maintain entangled states.