23-06-2025
Quantum embezzlement: An entanglement trick once thought impossible exists for real
In quantum physics, entanglement links particles across space in ways that defy logic. However, there's a lesser-known phenomenon that's even more intriguing than entanglement, and it's called quantum embezzlement.
It happens when one system quietly supplies entanglement to another system, helping the latter change its state, without being affected itself. It's a bit like quietly borrowing a few grains of sand from a vast beach to build a tiny sandcastle. The beach looks untouched, yet the sand is used.
For years, scientists thought such perfectly entangled systems existed only in theory. However, a new study from researchers at Leibniz University Hannover in Germany shows that embezzlement can occur naturally in a class of quantum materials called critical fermion chains.
Critical fermion chains are one-dimensional systems made of fermions (a type of subatomic particle) that sit at the transition point between two phases. At this point, they become highly sensitive and exhibit long-range quantum entanglement.
The discovery of embezzlement in a real physical system like critical fermion chains is of great importance as it can contribute to the development of robust technologies that rely on entanglement and involve large-scale quantum information transfer.
So what makes quantum embezzlement so strange? Well, normally, when you use a resource in quantum physics, like a highly entangled system, you expect its state to change. However, with embezzlement, it's like using a magic battery that powers a process without ever draining.
For this to work, the resource must be very entangled, so much so that many scientists doubted such a system could physically exist. It seemed too ideal to be real.
Moreover, the embezzlement here is universal, meaning that the resource system can help create any entangled state, not just specific ones. It works in many different situations, not just for one special case.
The study authors decided to investigate whether this universal embezzlement could actually occur in real, physical systems. They started by focusing on critical fermion chains, and instead of working with small, manageable systems, the researchers looked directly at infinite systems, using what physicists call the thermodynamic limit.
In this setup, they split the system into a left and right half and studied the entanglement between them. Surprisingly, they found that these half-chains satisfied the strict criteria for universal embezzlement that the team had defined in earlier work. In simple terms, the left and right sides were entangled enough to act like perfect embezzlers.
The researchers were able to assist in entangling other systems. Even more impressively, the team showed that this effect wasn't just an unusual property of infinite systems.
When they looked at large but finite fermion chains, systems that could potentially be built in a lab, scientists still found strong evidence of approximate embezzlement. That means this isn't just a theoretical phenomenon. Real materials might already be doing this under the right conditions.
"Finally, we demonstrate that the universal embezzlement property is not exclusive to the thermodynamic limit, but that it already emerges in large but finite fermion systems," Lauritz van Luijk, first author of the study, and a physicist at the Leibniz University Hannover, said.
This discovery rewrites the rules about how entanglement can behave in physical systems. It shows that quantum embezzlement is not a fragile or exotic phenomenon, but a robust property occurring in real systems.
In the future, this could help scientists discover new ways to transfer entanglement in quantum computers, improve methods of simulating quantum materials, or even new states of matter.
However, the work so far is entirely theoretical. "While our work shows that critical spin chains can embezzle entanglement, it does not provide a recipe for how to do so," Luijk said. The research doesn't offer a way to actually perform embezzlement in practice.
The study authors are now working on protocols using Gaussian operations, a type of quantum operation that's easier to implement with current technology. They hope these protocols might help them turn embezzlement from theory into something experimentally achievable.
The study is published in the journal Nature Physics.
