For years, quantum entanglement was described as something that happens in an instant. Two particles interact, and suddenly they behave like a single shared system, no matter how far apart they are.
Now a team of physicists from TU Wien in Vienna and several Chinese universities has shown that this “instant” is not really instant at all. Entanglement appears to build up over an unimaginably short but still measurable time of only a few hundred attoseconds.
That small delay may sound like a technical detail. In practice, it opens a new window into how the quantum world really works and how future quantum technologies might be controlled.
What actually is entangling?
Quantum entanglement links particles so tightly that describing each one separately no longer makes sense. It is as if two coins stop being individual coins and instead become one shared “coin system” whose outcome is only decided when you look. Measure one, and you immediately learn something about the other, even if it is far away.
In many experiments, scientists focus on keeping this fragile connection alive for as long as possible because it underpins ideas like quantum encryption and quantum computers. The new work asks a different question. Not how long entanglement survives, but how it is born in the first place.
A two-electron shakeup
To explore that birth process, the researchers studied a simple atom, helium, with just two electrons. In their simulations, an intense pulse of extreme ultraviolet light slams into the atom. One electron absorbs enough energy to escape. The second electron, still bound to the nucleus, is kicked into a higher-energy orbit.
After this jolt, the two electrons are no longer independent. Calculations show that their properties are entangled. You cannot fully describe the outgoing electron without also accounting for the one that stayed behind.
Here comes the tricky part. The electron that flies away does not leave at a single sharp moment. In quantum terms it is a wave that gradually spills out of the atom.
During that spill, its “birth time” as a free electron becomes linked to the energy state of the electron that stayed. If the remaining electron is left with higher energy, the escape tends to have happened a bit earlier. If it ends in a lower-energy state, the average escape time shifts later by about 232 attoseconds.
Attosecond stopwatches and time zero
An attosecond is one billionth of a billionth of a second. If one second were stretched to the age of Earth, an attosecond would be less than a blink in comparison.
Yet modern “attosecond chronoscopy” can resolve changes on this scale using clever combinations of ultrafast laser pulses and timing techniques with names like streaking and RABBIT.
The new study shows that by tracking tiny time delays in the departing electron, physicists can monitor how interelectronic coherence and entanglement evolve in real time. The team solved the full time dependent Schrödinger equation for the two-electron system in strong light fields and found clear signatures of the atom and laser field interacting beyond simple linear response.
In plain language, they turned the “when-did-the-electron-really-leave” question into a sensitive probe of how strongly the electrons are correlated and how quickly their shared quantum state forms.
Why this matters outside the lab
Most of us will never see an attosecond laser up close. Still, the physics happening on these extreme time scales quietly shapes the technologies that fill our homes and offices.
Quantum communication schemes that promise ultra-secure links for financial data or power grid control depend on entanglement behaving in predictable ways. So do many designs for quantum sensors that could monitor tiny changes in magnetic fields or pollution levels.
By understanding how entanglement actually emerges, engineers may eventually learn to “tune” it instead of just hoping it appears and stays put. That could mean more reliable quantum devices and fewer wasted experiments, which in turn saves time, money, and the energy that runs supercomputers and laboratory facilities.
Will this immediately shrink the electric bill at a data center or make climate models greener? Probably not. But in the long run, more efficient quantum control may help future computing and communication systems do the same work with fewer resources.
Rethinking what “instant” really means
To a large extent, this research chips away at a comforting shortcut in how we talk about quantum events. Instead of picturing electrons that “jump” out of atoms in no time at all, the work paints a more nuanced picture.
The escaping electron is a spreading wave. During that brief spread, the two electrons knit themselves into an entangled pair whose properties are tightly correlated.
For the human brain, 232 attoseconds might as well be zero. For nature, it is long enough for quantum information to flow and for one of the fastest processes in the universe to leave a measurable imprint.
The next step will be to confirm these predictions in the lab with real attosecond measurements on helium and other atoms. If those experiments match the simulations, physicists will have a powerful new tool to watch entanglement switch on in real time.
The study was published in Physical Review Letters.
