Entanglement

Entanglement is, alongside superposition, one of the two pillars of quantum computing. While superposition lets a single qubit hold a blend of possibilities, entanglement links multiple qubits together so that their possibilities become correlated — in ways that have no classical equivalent.

The EPR paradox and Einstein's objection

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper pointing out a strange consequence of quantum mechanics: two particles could be prepared so that measuring one instantly tells you the outcome of measuring the other, no matter how far apart they are. Einstein found this deeply troubling, famously calling it "spooky action at a distance." He believed quantum mechanics must be incomplete — that there had to be some "hidden variable" determining the outcomes in advance.

For decades, this was a philosophical debate. Then in 1964, physicist John Bell devised a way to test it experimentally. Bell's theorem showed that if hidden variables existed, certain statistical correlations between measurements could never exceed a specific limit. Quantum mechanics predicted those correlations would exceed that limit. Experiments — repeated many times with increasing precision — have consistently confirmed the quantum prediction. Entanglement is real, and there are no simple "hidden variables" explaining it away.

What entanglement is NOT

• It is not faster-than-light communication. Although measuring one entangled qubit instantly correlates with the other, you cannot use this to send a message. To compare results and notice the correlation, both parties still need to communicate through normal (slower-than-light) means.
• It is not "copying" information. A principle called the no-cloning theorem says you cannot create an identical copy of an unknown quantum state. Entanglement creates correlation, not duplication.
• It does not mean the particles are "communicating." There's no signal passing between them. The correlation was established when they became entangled, and it simply persists.

How entanglement is created

In a quantum computer, entanglement is typically created using a combination of a single-qubit gate (like the Hadamard gate, which creates superposition) followed by a two-qubit gate (like CNOT, which creates correlation based on another qubit's state).

The circuit shown above produces what's called a Bell state — one of the simplest and most-studied entangled states. After this circuit runs, measuring either qubit will always give a result perfectly correlated with measuring the other: if qubit 0 is measured as 0, qubit 1 will also be 0; if qubit 0 is measured as 1, qubit 1 will also be 1 — even though, before measurement, each individual qubit's outcome was completely random.

Real-world applications

Quantum Key Distribution (QKD)

Entanglement enables protocols like quantum key distribution, where two parties can generate a shared secret encryption key in a way that reveals any eavesdropping attempt — because measuring an entangled system disturbs it in a detectable way.

Quantum Teleportation

"Quantum teleportation" doesn't move matter — it transfers the state of one qubit to another distant qubit, using entanglement plus a classical communication channel. This is a key building block for future quantum networks.

Quantum Algorithms

Many quantum algorithms rely on entangling qubits as an intermediate step — it's part of how quantum computers represent and manipulate correlations between different parts of a computation that would be expensive to represent classically.

What's next?

Now that you understand the two foundational phenomena — superposition and entanglement — the next step is understanding the gates used to create and manipulate them.

Continue to: Quantum Gates →

Frequently Asked Questions

Can entanglement be broken?

Yes — interactions with the environment (a process called decoherence) can destroy entanglement. Keeping qubits entangled long enough to complete a computation is one of the major engineering challenges in building quantum computers.

How many qubits can be entangled at once?

In principle, any number — and creating large, stable entangled states (often called "GHZ states" for three or more qubits) is an active area of research, since it's both a useful resource and a benchmark of hardware quality.

Is entanglement the same as correlation in everyday life?

Not quite. Everyday correlations (like two gloves from the same pair, one left and one right) are determined in advance. Entangled particles have outcomes that are genuinely undetermined until measured, yet still end up correlated in ways that violate what's possible for any "determined in advance" explanation — as proven by Bell's theorem.