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Entanglement of orbital angular momentum in non-sequential double ionization

AU researchers, Andrew Maxwell and Lars Bojer Madsen, with Maciej Lewenstein from ICFO, merge strong-field physics with quantum information.

Diagram of NSDI and the resulting entanglement of OAM
Diagram of NSDI and the resulting entanglement of OAM

Entanglement is a subtle combination of classical correlation and quantum superposition, which provides a computational resource, and the possibility of entanglement enhanced imaging processes. Attosecond physics, deals with processes in matter on the scale of attoseconds, i.e. 10-18s the time it takes for electron to move through atoms and molecule. This has led to the development of a host of ultrafast imaging procedures, known as attosecond imaging, which often exploit quantum phenomena. However, the potential for entanglement to optimize or improve attosecond imaging is relatively unexplored. There is growing interest in entanglement in attoscience, with a focus on entanglement between the electron and the ion but entanglement between two ionized electrons has received less attention. Furthermore, these studies almost always involve the entanglement of continuous quantities, which are challenging to compute and interpret, and often impossible to measure.

Recent rapid progress in the production and measurement of electron vortex states, which are free electrons with helical wavefront that may carry orbital angular momentum (OAM), offers a solution to the above problems. We exploit the discrete degree of freedom, the OAM—inherent to all free particles, to clearly demonstrate the manifestation of entanglement in non-sequential double ionization (NSDI), a highly correlated two-electron ionization process, see Figure 1. Through known conservation laws and the superposition of intermediate excited states, we demonstrate that the OAM of the two ionized electrons in NSDI is entangled.

The use of OAM in attosecond processes provides a new avenue, to further the aim of imaging and controlling matter on ultrafast times scales. Furthermore, the OAM entanglement of the photoelectrons demonstrates the fundamental non-classical nature of NSDI.


A S Maxwell, L B Madsen and M Lewenstein, Entanglement of Orbital Angular Momentum in Non-Sequential Double Ionization, Nature Communications 13, 4706 (2022)

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