Two State System: QM Explained by Sakurai

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Discussion Overview

The discussion revolves around the concept of a two-state system in quantum mechanics, particularly in the context of the Stern-Gerlach experiment as presented in Sakurai's modern quantum mechanics text. Participants explore the definition and implications of two-state systems, including examples and their relevance in quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant seeks clarification on what is meant by a two-state system as referenced in Sakurai's text.
  • Another participant explains that the two-state system refers to the two observable states of silver atoms in the Stern-Gerlach experiment, indicating two positions of the electron.
  • A further contribution defines a two-state system as one that can exist in one of two states separated by an energy barrier, using the example of an electron with spin +1/2 or -1/2.
  • It is noted that many systems can be modeled as two-state systems, including atoms with two involved energy levels and quantum bits (qubits).
  • A participant suggests that Feynman's Lectures provide a valuable perspective on the Stern-Gerlach experiment.
  • One participant expresses understanding after confirming the existence of an energy barrier between the two states of a free electron.

Areas of Agreement / Disagreement

Participants generally agree on the definition and examples of two-state systems, but there is no explicit consensus on the implications or broader applications of these systems beyond the examples provided.

Contextual Notes

Some assumptions about the nature of energy barriers and the specific conditions under which two-state systems are applicable remain unaddressed, and the discussion does not resolve the complexities of modeling various systems as two-state systems.

neelakash
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I have started to read from Sakurai's modern QM.At very first,he refers to two state systems in context of Stern Gerlach experiment.Indeed he says that this system is a two state system.Can anyone please tell me what is meant by a two state system?
 
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In QM a two state system (sometimes called a two-level system) is a system which can be in one of just wo states, and these states are separated by an energy barrier.
The most archetypical two-state system is a free electron which have either spin +1/2 or spin -1/2. This is the reason why you see two beams in a S-G experiment.

However, many systems can be MODELLED as two-state systems, e.g. an atom participating in a process where only two energy levels are involved.
Another example would be a quantum bit (qubit), which, by definition is a two-state system. Thi is why a qubit can be descibed using formalism that was originally developed to describe spin; even it the qubit is actually implemented using e.g. an electronic circuit..
 
The Feynman Lectures Vol III (Quantum) does an excellent job with the Stern-Gerlach approach. Very worthwhile read if your interested in such.
 
and these states are separated by an energy barrier

So,there is an energy barrier between the two possible states of the free electron?

Now,I can see more clearly.Thank you...
 

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