Researchers observe quantum entanglement formation in real time ⚛️

Electrons in atoms sometimes behave in surprising ways.

Thanks to very precise simulations, researchers from the universities of Vienna and China managed to observe in detail how two electrons become quantumly entangled in just a few attoseconds (one attosecond is 1 x 10⁻¹⁸ seconds), an ultra-short time scale where entanglement between particles appears.


This quantum entanglement creates such an intimate connection between two particles that they can no longer be described separately. This phenomenon is essential for technologies such as quantum computers and quantum cryptography. In this study, the researchers aimed to understand how this entanglement forms from the very first fractions of a second, by observing interactions between a laser and atoms.

To conduct this research, the scientists used a very high-frequency laser to eject an electron from a helium atom, a process that can excite a second electron in the atom. The second electron remains bound to the nucleus but in a different energy state. This phenomenon creates a link between the two electrons: they are now entangled, meaning that by studying one of them, information about the other can be deduced.

The researchers were able to show that the "birth moment" of the ejected electron—that is, the very instant it leaves the atom under the laser's impulse—is intimately tied to the state of the electron that remains in the atom. In quantum terms, this moment does not have a well-defined existence: it is a superposition of several possible instants.

This superposition indicates that the moment the electron leaves the atom depends on the energy state of the remaining electron. If the latter is in a higher energy state, it is more likely that the ejected electron left earlier. Conversely, a lower energy suggests a later departure, on average around 232 attoseconds, or a billionth of a billionth of a second.

This delay is incredibly short, but it allows researchers to precisely measure the link that forms between the two electrons during their separation. This temporal aspect is critical: the ejection of the electron occurs progressively, in the form of a wave that "flows" out of the atom, and it is during this phase that entanglement between the electrons happens.

The researchers now hope to reproduce these observations in the laboratory with other teams to validate the model. This work opens up new frontiers in quantum physics, revealing that phenomena once believed to be instantaneous are in fact far more complex and structured.