You do not bet against the universe. In any isolated system, entropy – the order of the system – can never decrease. In classical systems, this is as firm a law of reality as it is possible to be. It’s the second law of thermodynamics. But when it comes to quantum mechanics, things are not as firmly established. However, new work has shown that there is an entropy to a crucial quantum phenomenon: entanglement.
When a measurement is taken on a particle in quantum mechanics, it is said that the wavefunction collapses. Properties in quantum mechanics are probabilistic. If two particles are entangled, they are part of the same quantum state, and a measurement on one will also collapse the wavefunction of the other instantaneously, even if the two particles are the opposite ends of the universe.
This might seem counterintuitive or a violation of this law or that law of physics but it describes a way we can measure the world accurately. Quantum entanglement is extremely useful in cutting-edge quantum technology – but scientists weren’t sure how such a phenomenon plays ball with entropy.
To square quantum entanglement with the second law of thermodynamics, scientists needed to show that entanglement transformations are reversible. Work and heat are shown to be reversible in thermodynamical systems, but this is not straightforward for quantum entanglement.
The team used “probabilistic” entanglement transformations. They do not work every time, but this framework allows physicists to create the reversible transformations needed for the entropy calculations. And they were able to calculate the entropy of this entanglement.
“Our findings mark significant progress in understanding the basic properties of entanglement, revealing fundamental connections between entanglement and thermodynamics, and crucially, providing a major simplification in the understanding of entanglement conversion processes. This not only has immediate and direct applications in the foundations of quantum theory, but it will also help with understanding the ultimate limitations on our ability to efficiently manipulate entanglement in practice,” study author Bartosz Regula, from the RIKEN Center for Quantum Computing, said in a statement.
More work is needed – this is very much the first solid word on the entropy of quantum entanglement. The current solution and future expansion might provide novel insights into other problems that still limit our understanding of quantum physics.
“Our work serves as the very first evidence that reversibility is an achievable phenomenon in entanglement theory. However, even stronger forms of reversibility have been conjectured, and there is hope that entanglement can be made reversible even under weaker assumptions than we have made in our work — notably, without having to rely on probabilistic transformations. The issue is that answering these questions appears significantly more difficult, requiring the solution of mathematical and information-theoretic problems that have evaded all attempts at solving them thus far. Understanding the precise requirements for reversibility to hold thus remains a fascinating open problem,” Regula added.
This new work is published in the journal Nature Communications.
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
