Spontaneous emission

Definition

Spontaneous emission is a process by which an excited atom or molecule transitions to a lower energy state without external influence, emitting a photon in the process. This phenomenon plays a crucial role in understanding how quantum systems interact with their environment and is essential for the derivation of Fermi's golden rule, which provides a framework for calculating transition probabilities between states due to perturbations.

5 Must Know Facts For Your Next Test

1. Spontaneous emission is responsible for the natural light emitted by atoms and molecules when they return to their ground state.
2. This process can be characterized by a specific lifetime, which indicates how long an excited state typically exists before decaying.
3. In quantum mechanics, the rate of spontaneous emission can be derived from perturbation theory, specifically leading to Fermi's golden rule.
4. The emitted photon during spontaneous emission is random in direction and phase, leading to the phenomenon of incoherent light.
5. Spontaneous emission is essential for various technologies, including lasers, where it initiates the chain reaction of stimulated emissions.

Review Questions

• How does spontaneous emission differ from stimulated emission in terms of photon production and coherence?
  • Spontaneous emission occurs without any external influence, resulting in the random emission of a photon from an excited atom or molecule. In contrast, stimulated emission involves an incoming photon triggering the release of another photon from an excited state, producing two photons that are coherent in phase and direction. This distinction highlights how spontaneous emission contributes to the initial generation of photons, while stimulated emission amplifies them in devices like lasers.
• Describe the relationship between spontaneous emission and Fermi's golden rule in predicting transition probabilities.
  • Fermi's golden rule provides a method for calculating the probability of transitions between quantum states due to perturbations. Spontaneous emission is an example of such a transition, where an excited state decays into a lower energy state naturally. By applying time-dependent perturbation theory, we can derive rates for spontaneous emission that help quantify how likely it is for an atom or molecule to emit a photon when transitioning states.
• Evaluate the implications of spontaneous emission on quantum technologies and their applications in modern physics.
  • Spontaneous emission has significant implications for quantum technologies, particularly in the development of lasers and quantum information systems. Understanding this process is crucial for controlling light-matter interactions, enhancing coherence times, and improving efficiency in quantum communication and computing. As technologies advance, manipulating spontaneous emission could lead to breakthroughs in creating more effective quantum devices that leverage both spontaneous and stimulated emissions for improved performance.

Related terms

stimulated emission: A process where an incoming photon stimulates an excited atom or molecule to drop to a lower energy state, resulting in the emission of an additional photon that is coherent with the incoming one.

transition probability: The likelihood of a quantum system transitioning from one energy state to another, often calculated using time-dependent perturbation theory.

Lifetimes: The average time an excited state lasts before undergoing spontaneous emission, which is related to the probabilities of transition between states.