The question "What is the second law of quantum?" is based on a misunderstanding. There is not a well-defined "second law of quantum mechanics" in the same way there are laws of thermodynamics. However, the spirit of the second law of thermodynamics, which deals with entropy and the tendency towards disorder, has connections to quantum mechanics and how quantum systems evolve. It's more accurate to say concepts related to the second law are explored within quantum mechanics.
Understanding the Relationship: Thermodynamics and Quantum Mechanics
While a direct "second law of quantum" doesn't exist, here's how the ideas of entropy and disorder relate to quantum systems:
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Entropy and Information: In classical thermodynamics, entropy is often described as a measure of disorder. In quantum mechanics, entropy can also be related to the amount of information we have about a quantum system's state.
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Quantum Decoherence: Quantum systems, when isolated, can exist in superpositions of states. However, when they interact with their environment, they undergo decoherence, which causes them to lose their quantum properties and behave more classically. This can be seen as a movement towards a more "disordered" or less well-defined state. Decoherence is, in a loose sense, analogous to the increase in entropy in thermodynamics.
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Von Neumann Entropy: This is a measure of entropy for quantum states, which is a quantum analog to the Gibbs entropy in classical statistical mechanics. It quantifies the "mixedness" of a quantum state. A pure quantum state has zero Von Neumann entropy, while a mixed state has a positive entropy. Evolution often leads toward mixed states through interactions, increasing the entropy.
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Irreversibility: Just like in classical thermodynamics, quantum processes often exhibit irreversibility. While the fundamental laws of quantum mechanics (e.g., the Schrödinger equation) are time-reversible, the interactions with the environment and the loss of quantum coherence lead to apparent irreversibility in observed phenomena.
Examples:
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Quantum Measurement: The act of measuring a quantum system forces it to collapse from a superposition of states into a single, definite state. This collapse involves information loss and can be seen as a type of irreversible process, conceptually linked to entropy increase.
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Thermalization of Quantum Systems: When interacting quantum systems are brought together, they tend to thermalize – reaching a state of thermal equilibrium. This process involves the redistribution of energy and a corresponding increase in entropy, mirroring the second law of thermodynamics.
Conclusion:
While there isn't a formally defined "second law of quantum mechanics," the core concepts of entropy, disorder, and irreversibility, which are central to the second law of thermodynamics, have analogs and implications in the behavior of quantum systems. The interaction with the environment and the processes of decoherence and measurement often lead to behavior that parallels the increase in entropy observed in classical thermodynamics. The question's premise is flawed as stated.
