Entanglement is a phenomenon where two or more particles become linked in such a way that a measurement on one of the particles instantly influences the state of the other, no matter how far apart they are. It is a defining property of quantum mechanics, which is key to all quantum technologies and remains a serious challenge to realize in large systems.

However, a team of researchers from Sweden and Spain has recently made a large step forward in the field of ultrafast entanglement. Here, pairs of extreme ultraviolet pulses are used to exert quantum control on the attosecond timescale (a few quintillionths of a second).

Specifically, they studied ultrafast photoionisation. In this process, a high-energy light pulse hits an atom, ejecting an electron and leaving behind an ion.

This process can create entanglement between the electron and the ion in a controlled way. However, the entanglement is fragile and can be disrupted or transferred as the system evolves.

For instance, as the newly-created ion emits a photon to release energy, the entanglement shifts from the electron – ion pair to the electron–photon pair. This transfer process takes a considerable amount of time, on the scale of 10s of nanoseconds. This means that the ion-electron pair is macroscopically separated, on the centimetre scale.

The team found that during this transition, all three particles – electron, ion, and photon – are entangled together in a multipartite state.

They did this by using a mathematical tool called von Neumann entropy to track how much information is shared between all three particles.

Although this work was purely theoretical, they also proposed an experimental method to study entanglement transfer. The setup would use two synchronised free-electron laser pulses, with attosecond precision, to measure the electron’s energy and to detect if a photon was emitted. By measuring both particles in coincidence, entanglement can be detected.

The results could be generalised to other scenarios and will help us understand how quantum information can move between different particles.  This brings us one small step closer to future technologies like quantum communication and computing.