Quantum entanglement is not instantaneous?

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The research team has now been able to show, using a suitable measurement protocol that combines two different laser beams, that it is possible to achieve a situation in which the 'birth time' of the electron flying away, i.e., the moment it left the atom, is related to the state of the electron that remains behind. These two properties are quantum entangled.

"This means that the birth time of the electron that flies away is not known in principle. You could say that the electron itself doesn't know when it left the atom," says Burgdörfer. "It is in a quantum-physical superposition of different states. It has left the atom at both an earlier and a later point in time."

Which point in time it 'really' was cannot be answered—the 'actual' answer to this question simply does not exist in quantum physics. But the answer is quantum-physically linked to the—also undetermined—state of the electron remaining with the atom. If the remaining electron is in a state of higher energy, then the electron that flew away was more likely to have been torn out at an early point in time; if the remaining electron is in a state of lower energy, then the 'birth time' of the free electron that flew away was likely later—on average around 232 attoseconds.

This is an almost unimaginably short period of time: an attosecond is a billionth of a billionth of a second. "However, these differences can not only be calculated, but also measured in experiments," says Burgdörfer. "We are already in talks with research teams who want to prove such ultrafast entanglements."
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