Just over 200 years after French engineer and physicist Sadi Carnot formulated the second law of thermodynamics, a world team of researchers has unveiled a similar law for the quantum world. This second law of entanglement manipulation proves that, similar to heat or energy in an idealized thermodynamics regime, entanglement may be reversibly manipulated, a press release which until now had been heavily contested. The brand new research – released on July 2, 2025 in Physical Review Letters – deepens understanding of entanglement’s basic properties and provides critical fundamental insight into the best way to efficiently manipulate entanglement and other quantum phenomena in practice.
Entanglement is arguably the central feature of quantum mechanics. If two microscopic particles are said to be entangled, then if someone measures a quantum property of one in all the particles after which repeats the measurement on its entangled partner, they’ll at all times find that the pair is correlated, even when the 2 particles are separated by vast distances. Due to this fact, knowing the state of 1 particle robotically provides information concerning the other. Entanglement was introduced about 90 years ago as proof of the absurdity of quantum theory if treated as an entire description of nature. Yet it shouldn’t be considered absurd today. After exhaustive proofs of entanglement’s authenticity in the actual world, it’s now the important thing resource in quantum information theory, allowing quantum teleportation and quantum cryptography, and offering significant benefits in quantum computing, communication and precision measurements.
Though entanglement still appears counterintuitive to our lived experience of the world, researchers have discovered striking parallels with something way more familiar: thermodynamics. In actual fact, many similarities have emerged between the theories of quantum entanglement and thermodynamics. For instance, ‘entanglement entropy’ is a characteristic of idealised, noiseless quantum systems that mimics the role of thermodynamical entropy.
Nevertheless, an akin to the second law of thermodynamics – which dictates that processes tend towards increasing disorder (the aforementioned entropy) and that perfect reversibility is an attainable though rare and highly efficient ideal – has remained stubbornly out of reach. Here, reversibility doesn’t discuss with time symmetry but the power of an external agent to govern the system into a special state after which manipulate it back to its initial state with none loss. “Finding a second law analogous to the second law of thermodynamics has been an open problem in quantum information science,” says study co-author Tulja Varun Kondra. “Solving this has been our primary motivation.”
Much work towards addressing this problem has focused on a scenario wherein two distant parties (often called Alice and Bob) wish to exchange quantum information, but are restricted to act locally on their quantum systems and communicate classically, by say phone or the web. This limitation to local operations and classical communication (LOCC) simplifies the situation, meaning whatever Alice and Bob do, they can’t affect the intrinsically nonlocal properties of entanglement between their quantum systems.
“It is thought that under LOCC operations on this scenario, entanglement is irreversible,” explains lead creator of the study Alexander Streltsov. “So the query is, can we one way or the other transcend LOCC in a meaningful way, and get better reversibility?” The team’s answer is ‘yes’, so long as Alice and Bob share an extra entangled system: an entanglement battery.
Just as an unusual battery stores energy which may be used to inject or store work within the context of thermodynamics, an entanglement battery injects and stores entanglement. The battery may be utilized in the state transformation process and the state of the battery itself may be modified to perform operations. There is simply one rule: whatever Alice and Bob do, they have to not decrease the extent of entanglement inside the battery.
And just as an everyday battery allows tasks to be performed that may be unattainable without one, so too does an entanglement battery. By assisting standard LOCC operations with their hypothetical entanglement battery, the team demonstrated that any mixed-state entanglement transformation may be made perfectly reversible.
This achievement is a big contribution to the controversy around whether entanglement manipulation is mostly reversible. But a more vital final result of this work is that the researchers have shown that the methods they’ve developed are applicable beyond mixed-state entanglement transformation, allowing them to leverage the entanglement battery to confirm reversibility in various scenarios. Proving that entanglement manipulations across all quantum states are reversible is anticipated to guide to a family of second laws for entanglement manipulation.
The entanglement battery may even find uses outside entanglement theory. For instance, the identical principles apply to systems involving greater than two entangled particles, paving the best way for understanding and manipulating complex quantum networks and maybe developing future, highly efficient quantum technologies.
As well as, generalising the concept of an entanglement battery to a resource battery – an extra quantum system that participates within the transformation process without reducing the resource in query – could allow the systematic demonstration of reversibility across quantum physics based on a minimal set of assumptions. “We are able to have a battery that’s speculated to preserve coherence or free energy, after which we will formulate a reversible framework on this setting where, as a substitute of entanglement, we reversibly manipulate that specific resource of our system,” says Streltsov. “Though a lot of these other principles of reversibility have already been confirmed via other approaches, our technique offers a unified proof framework based on well-established physical principles.”