Researchers from ICFO and Aarhus University report in Nature Communications a novel technique that uses orbital angular momentum to detect entanglement in the attosecond regime.
The Significance of Entanglement in Quantum Physics and Attosecond Physics
Entanglement is one of the sweet words we listen to lately when referring to quantum physics. It has provided staggering results in quantum simulations and computing, proving to optimize time in calculations, or even enhance imaging processes. In addition, attosecond physics is the field of physics that studies processes that occur in matter on the scale of attoseconds, i.e. 10-18s the time it takes for electrons to move through atoms and molecules. The combination of entanglement techniques and attoscience has led to a new field where attosecond imaging methods exploit quantum phenomena like interference, however, the role of entanglement remains unclear/unexplored.
Recent Advances in Studying Entanglement in Attoscience
Now, the potential for entanglement to optimize or improve attosecond imaging has been unexplored so far because, among other things, scientists never assumed that it would have such a leading role in the process. However, in recent years, there has been a growing interest in entanglement in attoscience, with the main focus on the entanglement between electrons and ions, revealing a connection between them that allows better understanding of coherence. Most of the focus has been on the connection between electrons and ions, but studies on the entanglement between two ionized electrons have received less attention. And those few studies that have tackled the issue have based their work on the entanglement of continuous quantities, which are challenging to compute and interpret, and often impossible to measure.
Demonstrating Entanglement through Electron Vortex States
In a recent study published in Nature Communications, ICFO researchers Andrew S. Maxwell, who then transferred to Aarhus carrying this study, and ICREA Prof. at ICFO Maciej Lewenstein, in collaboration with Lars Madsen from Aarhus University, have shown that the production and measurement of electron vortex states, which are free electrons with helical wavefront that may carry orbital angular momentum (OAM), offer a solution to the above problems.
Quantifying Entanglement and Developing Detection Methods
In their study, they 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. Through known conservation laws and the superposition of intermediate excited states, they demonstrate that the OAM of the two ionized electrons in NSDI is in fact entangled.
They then use the logarithmic negativity as a way to quantify the entanglement for a wide range of targets and parameters. They construct an entanglement witness, which provides a novel way for detecting the entanglement in an experiment in a much simpler way, avoiding full tomographic measurements. The interplay of intermediate excited states allows the photoelectrons to approach maximally entangled states. Furthermore, the entanglement is robust as it survives incoherent averaging over the focal volume of the laser.
Implications of OAM Entanglement for Attosecond Imaging
The use of this new technique involving OAM in attosecond processes provides a new pathway for improving imaging techniques and learning how to control matter on ultrafast times scales. Furthermore, the OAM entanglement of the photoelectrons demonstrates the fundamental non-classical nature of NSDI.
(a) NSDI process depicted for the EI and RESI mechanisms. Interaction via the field (e.g. tunnel ionization) is depicted by a dashed line, excitation in the singly charged ion is denoted by dotted lines, while the recollision and OAM sharing is denoted by the yellow spark. The two-electron states are defined in the paper (b) The excitation pathways in RESI, which lead to different final OAM states and an entangled superposition.
Links
Link to the research group led by ICREA Prof. at ICFO Maciej Lewenstein
