A year later, but ruling out that possibility, a pair of theoretical physicists from Japan and the Netherlands discovered that quantum entanglement has something fundamentally in common with the physics that powers steam engines, dries your socks, and can even keep the arrow of time pointing in one direction.
This universal property, if indeed it exists, as they suggest, would govern all transformations between entangled systems and give physicists a way to measure and compare entanglement beyond counting qubits—and to know their limits of manipulation of entangled pairs.
Quantum entanglement, the tendency for the quantum fuzziness of different objects to merge mathematically, is a fundamental part of quantum computing along with superposition. When particles, atoms, or molecules are entangled, knowing something about one tells us something about the other.
In their quest to realize these computing dreams, physicists have been primarily concerned with how to get two particles into an entangled state and keep them undisturbed so that they don’t fall apart and can reliably transmit information over long distances.
However, less thought has been given to whether entangled particles can be transformed from one quantum state to another, how difficult this would be, how many possible arrangements exist, and whether the entanglement process is ultimately reversible.
In thermodynamics, reversibility describes ideal processes that can be reversed in a way that leaves the system—and the universe—effectively unchanged. For example, turning water into steam with heat can drive a piston, while a piston pushing on steam can return it to a heated liquid state.
If entangled states can also be canceled out, even in theory, this could mean that other similarities with thermodynamics may point to a deeper truth in quantum mechanics.
“Our work serves as the first proof that reversibility is an achievable phenomenon in entanglement theory,” says quantum physicist Bartosz Regula of the RIKEN Center for Quantum Computing in Japan, who collaborated with Ludovico Lammy of the University of Amsterdam on the study.
“Not only does this have immediate and direct applications in the fundamentals of quantum theory, but it will also help understand the ultimate limits on our ability to effectively manipulate entanglement in practice,” adds Regula.
Reversible processes cannot actually happen, thanks to the second law of thermodynamics. Summarized in a concept known as entropy, it dictates that any new state in a closed system is unlikely to have the necessary energy to completely reverse itself after a change.
Do you want to turn this piston over? You’ll have to draw power from somewhere else. Since the universe is a closed system and cannot receive energy from elsewhere, its entropy will increase forever.
Given the strong connection between entropy and reversibility in thermodynamics, identifying a parallel in entanglement could have profound implications for understanding quantum transformations.
To establish entanglement “entropy”, Regula and Lammy had to show that entanglement transformations could indeed be reversible, just as work and heat could be converted in thermodynamics.
The suggestion that there is some kind of “entropy” to entanglement is a sudden twist from Regula and Lammy, who last year published a study in Natural physics who claims that “there is no second law for manipulating entanglement after all.”
They concluded that since entangled particles will always result in some loss of that entanglement, which can never be fully recovered, it would be impossible to transform one quantum state or resource into another and vice versa.
“We can conclude that no single quantity, such as entanglement entropy, can tell us all we need to know about the allowed transformations of entangled physical systems,” Lammy said at the time.
But these findings did not deter them. Rather, they thought it suggested that a unified theory of entanglement, if one existed, was much more complex than the classical laws of thermodynamics. So they kept counting.
Their latest proposal, using probabilistic entanglement transformations that only work for a while but provide more power, shows that a reversible entanglement framework might be possible.
But Regula admits that demonstrating how entangled particle transformations can work in practice, not just showing that it’s statistically possible, involves tackling mathematical problems “that have eluded all attempts to solve them so far.”
What’s more, the pair’s work is a departure from previous attempts to characterize certain quantum transformations in that it only considers transformations that can be achieved with some probability—however vanishingly small those chances may be. As a result, these probabilities may not be sufficient to demonstrate the existence of repeatable, reversible transformations of entangled states in practice.
“Thus, understanding the precise requirements for retaining reversibility remains a fascinating open problem,” says Regula.
The study was published in Nature Communications.
