Unveiling the role of Coulomb interactions in high-energy electron collisions

A group of three research teams, in which IFISC (UIB-CSIC) researcher Sungguen Ryu participated, have conducted groundbreaking experiments demonstrating that high-energy electrons repel each other upon collision in a beam splitter due to strong Coulomb interactions. These results clearly show the importance of Coulomb interaction when manipulating few electron excitations, in contrast to the photons. 

The experiments, published in Nature nanotechnology, fall within the field of electronic quantum optics and focus on high-frequency quantum transport. By utilizing fast voltage pulses in the gigahertz range, researchers can emit single electrons in nanoscale electrical circuits and manipulate them using electronic components similar to optical elements. 

Such electron excitations enable an electronic analog of the Hong-Ou-Mandel experiment. In the original version of photons (see Fig. a), the photons tend to bunch together due to the bosonic nature. In the electronic equivalent (see Fig. b), the electrons behave as fermions. If the colliding electron pair was low-energy excitations near the Fermi-sea, the screened Coulomb interaction is negligible and the pair would be splitted into different paths due to the Pauli-exclusion principal. However, if the Coulomb interaction effect is present, the fate of the colliding electron pair becomes more complicated.    

In separate experiments, the research teams employed various setups, yet they all reached the same conclusion. When high-energy electrons are emitted above the Fermi sea in an electric conductor and simultaneously arrive on each side of an electron beam splitter, they repel each other because of the strong Coulomb interactions, not because of the Pauli exclusion principle. The experiments involved using single electron pumps to emit charges in a semiconductor structure containing a two-dimensional electron gas. In the presence of a strong magnetic field, chiral edge states formed along the sample boundaries, acting as pathways for the emitted electrons. The electrons were guided to a central region, where they collided on either side of an electron beam splitter. 

These three experiments collectively demonstrate that Coulomb interactions play a dominant role in the antibunching effect observed when high-energy electrons collide in a beam splitter. This discovery has significant implications for quantum information processing with electronic qubits, as controlled interactions are vital for entanglement generation. Unlike photonic qubits, which typically exhibit weak or no interaction, electronic qubits can entangle through the Coulomb forces revealed in these experiments. This opens up possibilities for device architectures where spin or orbital degrees of freedom of electronic qubits intertwine through controlled Coulomb interactions. 

The experimental advancements represent a crucial step in the manipulation of single electrons in nanoscale conductors, enabling researchers to enter the nonlinear regime of electronic quantum optics. While challenges remain, these three experiments provide reasons to be optimistic about further advancements in the field of electronic quantum optics in the future.

Figure. Hong-Ou-Mandel-type collision of photons (a) and electrons (b).Extracted from Brange, F., Flindt, C. Interacting electrons collide at a beam splitter. Nat. Nanotechnol. (2023). 

Brange, F., Flindt, C. Interacting electrons collide at a beam splitter. Nat. Nanotechnol. (2023). https://doi.org/10.1038/s41565-023-01389-0 

Fletcher, J.D., Park, W., Ryu, S. et al. Time-resolved Coulomb collision of single electrons. Nat. Nanotechnol. (2023). https://doi.org/10.1038/s41565-023-01369-4