What Is Quantum Entanglement?
- Basic
- Article
- Ages 12+
- 6 min read
Quantum entanglement is a form of quantum correlation between parts of a system. For a pure entangled pair, the joint state cannot be separated into a pure state for each particle. Measurement results are statistically related, but they need not always be identical, and entanglement alone cannot send a message.
Contents
- Source that prepares the pair
- First particle
- Second particle
- Entanglement link (dashed line)
What you will learn
- State what entanglement is and who named it.
- Describe how two particles become entangled and why their measurement results match.
- Tell that matching apart from sending a message from one particle to the other.
Before you start
It helps to read What Is Quantum Superposition? first. Other terms are explained as they appear.
Where does entanglement come from?
One way to create entanglement is to let systems interact while preparing a joint state. This is the situation Schrödinger described when naming “entanglement”, according to the Stanford Encyclopedia of Philosophy. It is a preparation method, not a definition requiring every pair of particles to meet directly.
In the famous thought experiment of Einstein, Podolsky and Rosen, two particles are prepared from one source in a “pure” state of the pair as a whole. That state cannot be rewritten as a separate state for each particle.
The important point is to describe the joint state. A past interaction by itself does not specify which entangled state was prepared or what measurement correlations to expect.
- Two systems interacting
- Moving apart
- The two systems after separating
- Remaining link (dashed line)
What does measuring one particle tell you about the other?
In the ideal EPR example described by Stanford, a position measurement on one particle lets us predict the other particle’s position result with certainty; momentum works similarly. This is a property of that state and those measurement choices, not a rule for every entangled state.
The two measurements exclude each other. You can pick only one each time, position or momentum. Once you measure one, the matching in the other disappears.
Apart from that detail, the Stanford source describes the matching as much like the classical correlation between two billiard balls after a collision. Classical correlations can be explained by a common cause.
The strange part lies elsewhere. Schrödinger observed that the best possible knowledge of a whole does not necessarily include the best possible knowledge of its parts, even when the parts are entirely separate.
- First detector
- Second detector
- Dial that selects the kind of measurement
- Result lamps
- Two parallel bars: the results match
Can entanglement send a message?
Entanglement cannot send a message, because the person measuring cannot choose the outcome. According to the Stanford source, the second system cannot be steered into a particular quantum state at the experimenter’s whim. With many entangled pairs, the experimenter can only restrict the set of states the second system may end up in.
Picture one person measuring a particle in Hanoi and another measuring a particle in Hue. The first cannot write a “yes” or a “no” into the second person’s result. Both see the matching only when they compare notes.
The next article in this topic goes deeper into that question. For now, remember that matching and messaging are two different things.
- First person's choice
- Barrier: the choice does not reach the other side
- Results at the second particle
- Entanglement link (dashed line)
Real-world example
A pair of gloves split between two boxes gives a feel for the “common cause” part. Whoever packed them knows that if one box holds the left glove, the other holds the right. Open one box and you know the other, and the reason for the match is the moment of packing.
The glove example describes pre-existing correlation. It does not distinguish entanglement from classical correlation. To see the difference, we need correlations across different measurement choices and Bell limits, as the Stanford entry explains. Matching results alone do not establish entanglement.
Entanglement is useful in practice too. According to the Stanford source, a pair of systems in an entangled state can serve as a quantum information channel for computing and cryptographic tasks that classical systems cannot perform.
Common misunderstandings
Two entangled particles are not joined by a hidden wire that carries a signal. The person measuring cannot steer the result on the other side.
Entanglement does not mean every measurement gives a matching result. In the classic example, position or momentum, only one kind matches each time.
Matching outcomes alone do not establish entanglement. We need to know the prepared state and how its correlations are tested.
Frequently asked questions
Who named quantum entanglement?
Schrödinger named it “entanglement”, the link between quantum systems that interacted and then separated. The word is Schrödinger’s own coinage.
Can entanglement send information instantly?
No. The person measuring one particle cannot choose the result of the other, so cannot place a message in it.
What is entanglement used for?
A pair of entangled systems can serve as a quantum information channel for some computing and cryptographic tasks that classical systems cannot do.
Are two entangled particles like two colliding billiard balls?
Both can have correlated results. But classical billiard-ball correlations do not explain correlations that violate Bell limits across different measurement choices.
Summary
- Entanglement is a link between quantum systems that interacted or were prepared together.
- Certain predictions require the appropriate state and measurement choices, as in the ideal EPR example.
- That matching does not let the person measuring choose the other result, so it cannot send a message.
Terms in this article
English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.
- entanglementrối lượng tử
- Quantum correlations between parts of a joint state, used as a resource in quantum information.
- correlationtương quan
- A statistical relationship between measurement outcomes, which need not be identical.
- common causenguyên nhân chung
- One cause behind both results, such as the moment someone packs a pair of gloves into two boxes.
- measurementphép đo
- An operation that takes one definite result out of a quantum system.
This article is not marked as viewed
Spotted something wrong or unsafe?
Tell the administrator by email. The message already has the article title and address.
More in this topic
BasicWhat Is Quantum Superposition?
Understand superposition, measurement and qubits through a thought experiment, without assuming every possible result can be read at once.
5 min read
BasicHow Does a Qubit Differ from a Bit?
Compare bits and qubits through states, measurement and use; avoid mistaking a qubit for a memory cell whose possible answers can all be read at once.
3 min read
BasicHow Does CNOT Link Control and Target Qubits?
CNOT is a two-qubit gate that flips or keeps the target by the control qubit. See why it is a building block and how it differs from a measurement.
6 min read
BasicWhy Does Teleportation Need a Classical Channel?
Quantum teleportation needs a shared entangled pair and two bits sent over an ordinary channel. See each step and why it fails without the message.
6 min read
Sources
These sources are written for older readers. If they feel hard, read them with an adult.
- Quantum Entanglement and Information(opens in a new tab)
Stanford Encyclopedia of PhilosophyTier A: government, standards or research bodiesRead on November 7, 2026
- Entanglement and correlations(opens in a new tab)
Microsoft LearnTier B: vendor documentationRead on November 7, 2026