For more than a century, the laws of thermodynamics have helped us understand how energy moves, how engines work, and why time seems to flow in one direction. Now, researchers have made a similarly powerful discovery, but in the strange world of quantum physics.
Scientists have shown for the first time that entanglement, the mysterious link between quantum particles, can be reversibly manipulated just like heat or energy in a perfect thermodynamic cycle.
The researchers support their findings using a novel concept called an entanglement battery, which allows entanglement to flow in and out of quantum systems without being lost, much like a regular battery stores and supplies energy.
This achievement resolves a long-standing puzzle in quantum information science and could significantly impact the design of future quantum computers, secure communication systems, and powerful quantum networks.
Reversibility in entanglement has remained out of reach
Entanglement is one of quantum physics’s most puzzling and powerful features. It connects particles so deeply that the state of one instantly tells you something about the other, no matter how far apart they are.
“It also happens to be the key resource in quantum information theory, allowing quantum teleportation and quantum cryptography, and offering significant advantages in quantum computing, communication, and precision measurements,” the study authors note. However, using and reusing entanglement efficiently has been a huge challenge.
A long-standing question in quantum science was whether one entangled state could be transformed into another and then back again, without any loss, much like how ideal heat engines can convert energy back and forth with no waste.
For decades, the answer seemed to be no. Most studies looked at scenarios where two parties (commonly named Alice and Bob) are only allowed to manipulate their local systems and send each other classical messages.
Under these rules, called local operations and classical communication (LOCC), it’s known that transformations typically reduce the amount of entanglement. That meant perfect reversibility (which is the core concept of the second law), as seen in classical thermodynamics, appeared impossible in the quantum world.
However, the authors of the current study made the impossible possible using a clever trick.
The magic of an entanglement battery
The researchers proposed using an extra quantum system called an entanglement battery. This device acts as a storehouse for entanglement. It can give or take entanglement during transformations, so long as the total amount stored in the battery isn’t reduced.
By carefully accounting for the flow of entanglement into and out of the battery, the researchers showed that even the most complicated (or mixed) entangled states could be converted into other states and then brought back without any loss.
Their results apply in the asymptotic limit, meaning when large numbers of identical entangled states are used. In this ideal setting, the transformation rate between states can be calculated as a simple ratio of how much entanglement each state contains.
The framework also allows different ways of measuring entanglement, each giving rise to its own transformation rules. This mirrors how energy and entropy behave in thermodynamics. This same idea could be extended to other quantum resources, like coherence or free energy, by designing batteries that preserve those properties instead.
“We can have a battery that is supposed to preserve coherence or free energy, and then we can formulate a reversible framework in this setting where, instead of entanglement, we reversibly manipulate that particular resource of our system,” Alexander Streltsov, one of the study authors and a researcher at the Institute of Fundamental Technological Research in Poland, said.
Using this framework, the researchers theoretically demonstrated that when the entanglement battery works in association with a standard LOCC operation, it can make any complex entanglement transformation reversible.
“Proving that entanglement manipulations across all quantum states are reversible is expected to lead to a family of second laws for entanglement manipulation,” the study authors added.
The next challenge is to develop a real version
The second law brings a new level of precision and control to quantum information science. It offers a roadmap for designing systems that use entanglement more effectively.
However, the work is still theoretical. The entanglement battery is a concept, not yet a physical tool. Real quantum systems face noise, imperfections, and size limitations, which make perfect reversibility difficult to achieve in practice.
The study authors now plan to explore how their theory might hold up in real-world conditions and whether smaller or simplified versions of the battery can be created in labs.
The study is published in the journal Physical Review Letters.