Two State System: QM Explained by Sakurai

[neelakash]

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?

 

[premagg]

He is referring to stern-gerlach exp where they found that there were two lines of silver atom.Now two state means just two positions of electron.
http://en.wikipedia.org/wiki/Two-level_system

 

[f95toli]

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..

 

[Billygoat]

The Feynman Lectures Vol III (Quantum) does an excellent job with the Stern-Gerlach approach. Very worthwhile read if your interested in such.

 

[neelakash]

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...