The rise of quantum entanglement is recognized as one of the quickest activities in nature. Researchers have demonstrated that with special techniques, this phenomenon can be studied on an attosecond timescale. They have successfully examined ultrafast processes that were previously thought to happen instantaneously: When a laser pulse strikes an atom with two electrons, one electron can be ejected from the atom while the other remains close to the nucleus. These two electrons can become entangled such that the moment the electron was ejected becomes uncertain and is influenced by the state of the other electron.
Quantum theory explains occurrences that transpire in extremely brief periods. Historically, such events were viewed as either ‘momentary’ or ‘instantaneous’: An electron travels around an atom’s nucleus and, in an instant, is forcibly ejected by a burst of light. Two particles collide and, in the next moment, become ‘quantum entangled’ out of nowhere.
However, we can now examine the temporal progression of these nearly ‘instantaneous’ occurrences. Collaborating with researchers from China, the team at TU Wien (Vienna) has crafted computer simulations to replicate ultrafast processes, allowing insights into how quantum entanglement develops in attoseconds. Their findings have been published in the journal ‘Physical Review Letters.’
Two particles – a singular quantum entity
When two particles experience quantum entanglement, it becomes irrelevant to describe them independently. Knowing the complete state of the system with the two particles does not allow you to define the state of either particle clearly. “It can be said that the particles lack distinct individual characteristics; they share common traits. Mathematically, they are intrinsically linked, even if they’re located at vastly different places,” explains Prof. Joachim Burgdörfer from the Institute of Theoretical Physics at TU Wien.
In tests with entangled quantum particles, the goal is typically to maintain this entanglement as long as possible, for instance, for applications in quantum cryptography or quantum computing. “In our case, however, we focus on understanding the emergence of this entanglement and determining which physical processes are at play during these extremely brief intervals,” states Prof. Iva BÅ™ezinová, one of the authors of the study.
One electron flees, one remains near the atom
The team examined atoms struck by an extremely strong, high-frequency laser pulse. One electron is ejected from the atom and escapes. If the laser’s intensity is sufficiently high, a second electron may also be influenced: it can be elevated to a higher energy state, orbiting the atomic nucleus on a different path.
After the laser pulse, one electron escapes while one stays nearby with an unknown energy level. “We can demonstrate that these two electrons are now quantum entangled,” states Joachim Burgdörfer. “They must be studied together; measuring one implies gaining insights into the other simultaneously.”
The electron itself is unaware of its ‘birth’
The research team has successfully demonstrated, through a specific measurement method utilizing two distinct laser beams, that it’s feasible to create a scenario where the ‘birth time’ of the escaping electron—when it left the atom—is connected to the state of the remaining electron. These two properties exhibit quantum entanglement.
“This implies that the escape time of the fleeing electron is fundamentally unknown. You might say that the electron doesn’t realize when it departed from the atom,” remarks Joachim Burgdörfer. “It exists in a quantum superposition of various states, having exited the atom at both earlier and later moments in time.”
Determining the ‘true’ moment of departure isn’t feasible—the ‘actual’ answer to this query simply does not exist in quantum physics. Yet, the answer is quantum-mechanically connected to the unknown state of the electron that remains with the atom: If the remaining electron holds higher energy, then the escaping electron likely was ejected earlier; if it’s at a lower energy state, then its ‘exit time’ was probably later—on average, around 232 attoseconds.
This timeframe is exceedingly short: an attosecond represents a billionth of a billionth of a second. “Surprisingly, these differences can not only be theoretically computed but also experimentally measured,” explains Joachim Burgdörfer. “We’re already collaborating with other teams keen to validate such ultrafast entanglements.”
The timing structure of ‘instantaneous’ events
This research illustrates that regarding quantum phenomena as ‘instantaneous’ is insufficient: Significant correlations only come to light through resolution of the ultra-brief durations of these occurrences. “The electron does not merely instantaneously exit the atom; it behaves as a wave that gradually disperses from the atom—it takes a certain period,” notes Iva BÅ™ezinová. “It is during this transition that entanglement arises, which can subsequently be accurately observed by monitoring the two electrons.”
