From: mk_thisisit
Decoherence is a phenomenon that describes the loss of quantum coherence in a system due to its interaction with the environment. It is a key concept for understanding the emergence of the classical world from quantum mechanics [05:33:00]. Professor Wojciech Żurek is noted as a leading physicist and the creator of the theory of quantum Darwinism [02:06:00] [02:11:00], which builds upon decoherence.
The Problem of Superposition
In quantum mechanics, an object can exist in a superposition of multiple states simultaneously [03:12:00]. For example, a glass could be “both here and there” [00:00:00] [03:27:00], or a coin could be “at the same time the obverse and reverse” [03:36:00]. This is known as quantum superposition [04:43:00].
Albert Einstein, in correspondence with Max Born, raised concerns that quantum mechanics’s predictions for such objects contradict what we observe in the classical world [02:40:00] [04:06:00] [15:29:00]. The core of the issue is why we don’t experience such superpositions in everyday life [17:44:00].
How Decoherence Works
Decoherence is the process that eliminates these quantum superpositions, causing objects to appear in a single, defined state [04:53:00] [06:38:00].
The environment plays a crucial role in decoherence [06:00:00]:
- Information Transfer: Any correlation that transfers information from a quantum object to its environment acts as a “measurement” [06:03:00]. This doesn’t have to be a human observer or a device; it can be interactions with photons, air particles, or other environmental factors [06:07:00] [06:18:00].
- Loss of Coherence: When environmental elements like photons or air particles “know” where a glass is (e.g., by reflecting off it into our eyes) [06:21:00] [06:31:00], this transfer of information disrupts the coherence, forcing the object to “decide” on a single position [06:46:00].
Decoherence and Quantum Darwinism
Decoherence serves as the foundation for the theory of quantum Darwinism [08:48:00].
- Selection of Pointer States: The environment selectively interacts with a quantum system, disturbing most states but allowing certain “Pointer States” to survive [08:33:00] [09:03:00] [09:34:00] [10:08:00]. These are the states resistant to environmental disturbance [14:16:00] [14:35:00].
- Replication of Information: Quantum Darwinism further posits that information about these surviving states is not just preserved, but replicated in many copies throughout the environment [10:59:00] [12:36:00] [14:40:00]. This allows observers to gain information about the object by interacting with only a tiny fraction of these copies [11:49:00] [12:03:00].
- Analogy to Darwinian Evolution: The theory is named “Darwinism” because there is a “selective information” process, similar to natural selection, where certain quantum states “survive” and “multiply” (i.e., their information is replicated) [12:52:00] [12:58:00].
Prohibition of Cloning
Professor Żurek is also the creator of the theory on the “ban on cloning” [02:15:00], specifically the No-Cloning Theorem in quantum theory [13:08:00]. This theorem states that an unknown quantum state cannot be perfectly cloned [13:13:00]. In the context of decoherence, it’s not that a state is literally multiplied, but rather information about the state is replicated [13:30:00] [13:32:00].
Decoherence and the Classical World
Decoherence is crucial for explaining how our classical world emerges from quantum physics [05:33:00]. It explains why we observe objects in definite states (e.g., a glass being either “here” or “there”) [03:45:00], rather than the superpositions allowed by quantum mechanics [03:19:00]. Professor Żurek’s research, along with his colleagues, has helped to find the “point of contact” between classical and quantum physics [18:17:00].
Implications for Quantum Computing
Decoherence is the main problem and a significant challenge in building quantum computers [24:52:00] [25:28:00].
- Vulnerability to Environment: For a quantum computer to function, its quantum bits (qubits) must interact strongly with each other to perform calculations [25:47:00]. However, even a weak interaction with the external environment can derail these delicate quantum processes through decoherence [25:59:00] [26:06:00].
- Experimental Challenges: This presents a fundamental contradiction: strong internal interaction for computation versus extreme isolation from the environment to maintain quantum coherence [25:37:00]. This makes building large-scale quantum computers very ambitious [24:49:00].
- Quantum Simulation: While universal quantum computers are challenging, simulating or emulating various strange quantum systems using specialized quantum devices shows more promising results [26:36:00]. Experiments can be conducted to observe how many “topological defects” are created during simulated phase transitions in these quantum systems [0:45:53] [0:46:03] [0:46:06].
Conclusion
Understanding decoherence allows physicists to explain why macro-scale objects appear classical, even though they are fundamentally quantum [17:55:00]. It is a critical bridge between the quantum and classical realms [18:01:00] and continues to be a central topic in quantum physics research.