The initial speed of quantum entanglement has been measured in attoseconds.

The phenomenon where two or more particles remain so interconnected that the mathematical description of one cannot be made without considering the parameters of the other, even if they are located at great distances from each other, is referred to as quantum entanglement.
It is believed that particles become entangled instantaneously, meaning so quickly that the speed of this process is hardly perceptible—let alone measurable.

Nevertheless, an international research team from the Vienna University of Technology (Austria) has made progress in understanding how quantum entanglement begins. The team, led by Joachim Burgdörfer and Iva Březinová, developed new theoretical models and solved the Schrödinger equation for the helium atom, applying computer modeling to expose the atoms to an extremely intense, high-frequency laser pulse.

As a result, the physicists achieved a scenario where one electron was ejected from the atom and flew away, while the second altered its orbit around the nucleus and could transition to a different energy state. This allowed the researchers to demonstrate that the two electrons became quantum entangled: measuring one provided information about the state of the other.

"This means that the moment the electron is ejected is fundamentally undefined. One could say that the electron itself does not know when it left the atom and is in a superposition state, meaning it exits the atom at both an earlier and a later moment in time," explained the authors of the scientific paper.

It is worth noting that electrons in an atom move around the nucleus in orbitals: those located farther from the nucleus possess higher energy, while those closer have lower energy.

Although it is impossible to determine the exact time of the first electron's ejection, it is related to the state of the second: if the remaining electron has higher energy, then the first likely left the atom earlier. Conversely, if the second electron has lower energy, the first departed later, with a difference of approximately 232 attoseconds (to obtain one attosecond, one must divide a second by a million three times in succession).

Thus, the results of the research, presented in the journal Physical Review Letters, indicated that to fully understand quantum effects, it is insufficient to consider them instantaneous, as significant correlations only manifest at attosecond time scales. However, in the future, these processes can be modeled, calculated, and even measured experimentally.

The article has already attracted the attention of the scientific community, and its authors are collaborating with research groups aiming to experimentally confirm the findings described in the paper—though such confirmation has yet to occur. In any case, scientists are gradually gaining a better understanding of previously inaccessible (due to technological limitations) fundamental processes in quantum mechanics and are moving closer to developing innovative quantum technologies.