What is entanglement?

Entanglement describes the inseparability of two (or more) particles or waves being subsystems of a greater quantum mechanical system. Note that the inseparability is not based on the exchange of information or on forces acting between the subsystems. The mutual distance between the subsystems therefore does not play a role. Measurements on the individual subsystems reveal a correlation in the measurement statistic, which is ‘classically’ not possible. Thus, entanglement is said to show a correlation stronger than any classical correlation. A necessary condition is, that the measurement statistic incorporates different quantities (such as position and momentum). For every measurement a new, identical entangled system is required, because the entanglement is destroyed in the course of the first measurement. The existence of entanglement is at the heart of quantum physics. Even if the entangled systems are perfectly identical, measurements on their subsystems will produce different, in fact, perfectly random results. But finally, a comparison of the measurement statistic shows that the random results fluctuate in a correlated fashion. Entangled subsystems belong to a well-defined overall state but don’t have individual properties. If one measures the individual properties anyway, these properties will become reality by chance. Entanglement is useful for quantum teleportation and quantum cryptography. A common way of producing two light fields with strongly entangled amplitudes and phases is by overlapping two squeezed beams on a semi-transparent beam splitter. The two output fields do neither have a defined amplitude nor phase. Only through a measurement one or the other quantity becomes reality (for the measuring time interval chosen).

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