Fock State: A Picture from Physical Point of View

[KFC]

I know there were some discussions in the forum about fock state. In text, fock state described as a state with fixed number of particle but unpredictable phase. Instead of talking about the math, could anyone show me a clear picture from physical point of view what is fock state? In the phase space, will fock state always be a circle with one direction squeezed (like ellipse)?

 

[soarce]

I think that the Fock state would be represented like a line, i.e. P(n, \theta) \propto \delta(n)
Check for instance this article on the number-phase Wigner function: http://journals.aps.org/pra/abstract/10.1103/PhysRevA.52.3474 , I will try to get a copy.

Also you can try:
- the webpage of Lvovsky's group contains some of their papers on experiments releated to Fock states: http://people.ucalgary.ca/~lvov/Alex-pub.html
- If you have access to some library look for the book Quantum optics in phase space by W. Schleich.

 

[soarce]

I managed to retrieve a copy of the PhyRevA article <<Moderator note: Deleted content>>.

PS: Can I upload the copy here on forum as attachment?

<<Moderator note: Parts of this message has been deleted.>>

 

[Orodruin]

I managed to retrieve a copy of the PhyRevA article.

PS: Can I upload the copy here on forum as attachement?

No, you are not allowed to do this. APS charges ca $160 for the right to host an article in this fashion. However, many people here will be able to access the article through university subscriptions so it can still be relevant to give the reference: http://dx.doi.org/10.1103/PhysRevA.52.3474

Thread re-opened, but please abide by the copyright laws.

 

[MisterX]

I know there were some discussions in the forum about fock state. In text, fock state described as a state with fixed number of particle but unpredictable phase. Instead of talking about the math, could anyone show me a clear picture from physical point of view what is fock state? In the phase space, will fock state always be a circle with one direction squeezed (like ellipse)?

I don't know where you are getting this. A Fock state, also known as a number state, is just a state where the number of excitations is definite. For example a Fock state might something like

"3 photons of momentum $k_1$ and seven of momentum $k_2$".

Something that is not a Fock state would be superposition of number states, such as

"1 photon with momentum $k$, prob. amplitude $a_1$" + "2 photons with momentum $k$, prob. amplitude $a_2$". $a_1\neq 0 ,\,a_2 \neq 0$

This would not be a Fock state (although it still is in Fock space).

As for the phase being unpredictable, this can be seen by finding the expectation of the field operators in a Fock state. You will find that it is zero - it doesn't represent a wave with a particular phase, so when you find the average value of the field, it comes out to be zero. For a state more closely represents a classical wave, see a coherent state (which is not a Fock state - it contains all possible number states of one particular momentum in a superposition).