How quantum entanglement unfolds in attoseconds
by Robert Schreiber
Berlin, Germany (SPX) Oct 23, 2024

Quantum processes, once considered instantaneous, are now being understood in terms of time, thanks to advanced research and simulations. A team from TU Wien, in collaboration with researchers in China, has developed computer models capable of simulating the development of quantum entanglement at incredibly short time intervals, measured in attoseconds (one billionth of a billionth of a second). Their findings, published in *Physical Review Letters*, provide new insights into how quantum entanglement arises.

Entanglement and Temporal Evolution
When two particles are entangled, their individual properties cannot be separately described-they only have collective characteristics. As Prof. Joachim Burgdorfer from the Institute of Theoretical Physics at TU Wien explains, "From a mathematical point of view, they belong firmly together, even if they are in two completely different places." While much research focuses on preserving this entanglement, this study seeks to understand how it emerges in the first place, examining physical effects on extremely short time scales.

Exploring the Birth of Entanglement
In their experiments, the researchers used intense, high-frequency laser pulses to target atoms. When struck by the laser, one electron was ejected from the atom. In certain conditions, a second electron within the atom gained energy and moved to a different orbital path. The team discovered that these two electrons became quantum entangled-knowing the state of one provided information about the other, regardless of distance.

"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," said Burgdorfer. However, the exact moment the electron leaves the atom is uncertain, as quantum physics dictates that it exists in a superposition of different states. This means that the electron is both ejected earlier and later, depending on the energy state of the remaining electron.

Attosecond Timescales
The study revealed that on average, the electron is likely to be ejected around 232 attoseconds after the laser pulse hits. This measurement can be confirmed through experiments designed to probe these ultrafast processes. Prof. Burgdorfer added, "We are already in talks with research teams who want to prove such ultrafast entanglements."

Resolving Instantaneous Events
The findings challenge the notion that quantum events occur instantaneously. Instead, the research shows that quantum entanglement happens over a very short period, and this temporal development is key to understanding how entangled states evolve. "The electron doesn't just jump out of the atom. It is a wave that spills out of the atom, so to speak-and that takes a certain amount of time," said Prof. Iva Brezinova, one of the authors of the publication.

Research Report:Time Delays as Attosecond Probe of Interelectronic Coherence and Entanglement

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