Irreversible entanglement manipulation; entropy, energy, Carnot cycle; second law of thermodynamics; quantum systems; entanglement entropy.
The second law of thermodynamics states that the amount of entropy of any closed system can never decrease. This law has been applied to macroscopic systems, but quantum systems are mostly studied in the microscopic regime. Scientists have uncovered a notion of ‘entanglement entropy’ that mimics the role of thermodynamical entropy, but it was unknown whether entanglement can always be reversibly manipulated.
Researchers Ludovico Lami and Bartosz Regula have demonstrated that manipulation of entanglement is fundamentally irreversible, putting to rest any hopes of establishing a second law of entanglement. This new result relies on the construction of a particular quantum state which is very ‘expensive’ to create using pure entanglement. As a result, it is inherently impossible to transform this state into another and back again.
The approach used does not presuppose what exact transformation protocols are used, ruling out the reversibility of entanglement in all possible settings. It applies to all protocols, assuming they don’t generate new entanglement themselves. This means that describing quantum entanglement is not as simple as scientists had hoped.
Rather than being a drawback, the vastly greater complexity of the theory of entanglement compared to the classical laws of thermodynamics may allow us to use entanglement to achieve feats that would otherwise be completely inconceivable. This paper is published in the journal Nature Physics.
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