Entanglement is one of the properties of quantum mechanics which caused Einstein and others to dislike the theory. In 1935, Einstein, Podolsky, and Rosen formulated the EPR paradox, demonstrating that entanglement makes quantum mechanics a non-local theory. Einstein famously derided entanglement as "spooky action at a distance."

On the other hand, quantum mechanics was highly successful in producing correct experimental predictions, and the phenomenon of "spooky action" could in fact be observed. Some suggested the existence of unknown microscopic parameters, known as "hidden variables", that were deterministic and obeyed the locality principle, but gave rise to quantum mechanical behavior in the bulk. However, in 1964 Bell showed that the effects of quantum entanglement could be experimentally distinguished from the effects of a broad class of local hidden-variable theories. Subsequent experiments verified the quantum mechanical predictions, and entanglement has now become accepted as a *bona fide* physical phenomenon. The "Bell inequalities" are described in greater detail in the article EPR paradox.

Entanglement obeys the letter if not the spirit of relativity. Although two entangled systems can interact across large spatial separations, no useful information can be transmitted in this way, so causality cannot be violated through entanglement. This occurs for two subtle reasons: (i) quantum mechanical measurements yield probabilistic results, and (ii) the no cloning theorem forbids the statistical inspection of entangled quantum states.

Although no information can be transmitted through entanglement alone, it is possible to transmit information using a set of entangled states used in conjunction with a *classical* information channel. This process is known as quantum teleportation. Despite its name, quantum teleportation can not be used to transmit information faster than light, because a classical information channel is involved.

Though an area of active research, some of the essential properties of entanglement are now understood, and it is the basis for emerging technologies such as quantum computing and quantum cryptography. In the following article, we will briefly survey the mathematical formulation of entanglement.

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2 Entropy 3 Ensembles |

The following discussion builds on the theoretical framework developed in the articles bra-ket notation and mathematical formulation of quantum mechanics.

Consider two systems A and B, with respective Hilbert spaces *H _{A}* and

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Pick observables (and corresponding Hermitian operators) Ω_{A} acting on *H _{A}*, and Ω

- ,

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For example, given two basis vectors {|0⟩_{A}, |1⟩_{A}} of *H _{A}* and two basis vectors {|0⟩

- .

Now suppose Alice is an observer for system A, and Bob is an observer for system B. If Alice performs the measurement Ω_{A}, there are two possible outcomes, occurring with equal probability:

- Alice measures 0, and the state of the system collapses to |0⟩
_{A}|1⟩_{B} - Alice measures 1, and the state of the system collapses to |1⟩
_{A}|0⟩_{B}.

The outcome of Alice's measurement is random. Alice cannot decide which state to collapse the composite system into, and therefore cannot transmit information to Bob by acting on her system. (There is a possible loophole: if Bob could make multiple duplicate copies of the state he receives, he could obtain information by collecting statistics. This loophole is closed by the no cloning theorem, which forbids the creation of duplicate states.) Causality is thus preserved, as we claimed above.

Quantifying entanglement is an important step towards better understanding the phenomenon. The method of density matrices provides us with a formal measure of entanglement. Let the state of the composite system be |Ψ⟩. The projection operator for this state is denoted

- .

- .

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Given a general density matrix ρ, we can calculate the quantity

The entropy of any pure state is zero, which is unsurprising since there is no uncertainty about the state of the system. The entropy of any of the two subsystems of the entangled state discussed above is *k*ln 2 (which can be shown to be the maximum entropy for a one-level system). If the overall system is pure, the entropy of its subsystems can be used to measure its degree of entanglement with the other subsystems.

It can also be shown that unitary operators acting on a state (such as the time evolution operator obtained from the Schrödinger equation) leave the entropy unchanged. This associates the reversibility of a process with its resulting entropy change, which is a deep result linking quantum mechanics to information theory and thermodynamics.

The language of density matrices is also used to describe quantum ensembles, or a collection of identical quantum systems.

Consider a "black-box" apparatus that spits electrons towards an observer. The electrons' Hilbert spaces are identical. The apparatus might produce electrons that are all in the same state; in this case, the electrons received by the observer are then called a *pure ensemble*.

However, the apparatus could produce electrons in different states. For example, it could produce two populations of electrons: one with state |**z**+⟩ (spins aligned in the positive **z** direction), and the other with state |**y**-⟩ (spins aligned in the negative **y** direction.) Generally, there can be any number of populations, each corresponding to a different state. This is a *mixed ensemble*.

We can describe an ensemble as a collection of populations with weights w_{i} and corresponding states |α_{i}⟩. The density matrix of the ensemble is defined as

- .

The vacuum in quantum field theory, is hugely entangled, so entanglement isn't just about particles. See also Reeh-Schlieder theorem.