Chapter 3.13_ Quantum Decoherence

Quantum decoherence is a phenomenon related to quantum noise.
It describes the loss of quantum coherence, which is essential for quantum behaviors like superposition, interference, and entanglement.
Once decoherence occurs, quantum systems begin to behave according to classical mechanics rules.

Coherence is crucial for interference patterns, seen in:
- Laser: Emits coherent light of a single wavelength and phase.
- Torch: Emits incoherent light with varying wavelengths and phases, thus failing to create clear interference patterns.
Quantum Coherence: Refers specifically to the ability of probability waves from different quantum states to interfere, which relies on definite phase relations.
Non-coherent waves cannot generate clear interference, highlighting the essential nature of coherence in quantum phenomena.

Waves in superposition can experience:
- Constructive Interference: When waves add up to greater amplitudes.
- Destructive Interference: When waves cancel each other out.
The nature of superposition is foundational to quantum behavior that leads to observable interference patterns.

A perfectly isolated quantum system would theoretically maintain coherence indefinitely.
Interaction with the environment leads to decoherence, similar to a wrapped gift losing its pristine state when opened.
- Example: If a measurement is made, such as observing electrons in a double-slit experiment, the interference pattern vanishes.
Quantum cryptography is impacted similarly when measurement attempts interrupt quantum states, exposing potential eavesdropping.
Various types of quantum noise foster these interactions, which disrupt quantum states and lead to decoherence.

Quantum Decoherence: Describes transition from quantum to classical behavior through environmental interaction.
- Causes loss of coherence and explains the absence of quantum superpositions in larger, macroscopic objects.
Wavefunction Collapse: The act of measurement causes the reduction of superposition to a single outcome.
- Illustrated through the analogy of a spinning coin:
  - Initially coherent when spinning in a vacuum;
  - Environmental interaction (e.g., air) causes wobbling and loss of coherence.
  - Measurement (stopping the coin) results in heads or tails, exemplifying wavefunction collapse.
Key distinctions:
- Decoherence: Environmental interaction leads to loss of coherence but does not select an outcome.
- Wavefunction Collapse: Measurement determines a definitive state.
- Environment may trigger decoherence; however, the act of measurement remains critical in collapse.