I On-Demand Photon Pairs: Entanglement or Necessity?

[Swamp Thing]

When I search for "on demand photon pairs" I find that most publications are about entangled pairs.

I found no papers and articles about on-demand pairs where the photons are identical in all respects except their direction, and whose polarization & frequency are the same across pairs and within pairs. It always seems to involve one other parameter (e.g. polarization) whose value is random from pair to pair, but is correlated across each pair.

Just wondering whether this is because
(A) entangled on-demand pairs are more useful and interesting than non-entangled ones

OR

(B) because there is some theoretical constraint that makes it necessary that they should have an additional degree of freedom (apart from direction) and that they should have correlated values in that degree of freedom.

 

[.Scott]

If I understand what you are looking for, the answer is "B".
A particle with a known attribute (such as spin orientation) and its exact duplicate cannot exist.

 

[zonde]

Just wondering whether this is because
(A) entangled on-demand pairs are more useful and interesting than non-entangled ones

OR

(B) because there is some theoretical constraint that makes it necessary that they should have an additional degree of freedom (apart from direction) and that they should have correlated values in that degree of freedom.

It's option A. Look at this paper Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory
They are using two down-conversion crystals to get overlapping HH and VV pairs. If you would use just one crystal you would get HH or VV pairs only.

 

[Swamp Thing]

It's option A. Look at this paper Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory
They are using two down-conversion crystals to get overlapping HH and VV pairs. If you would use just one crystal you would get HH or VV pairs only.

Thanks.
So, for the Type-1 process, we get all photon pairs with their polarizations matching the pump beam. No polarization entanglement. And when we pre-select pairs traveling out in specific directions, we get rid of the spatial (mode) entanglement as well. What about frequency? From the article, it seems highly likely that all photons have to be the same frequency, but the authors don't make this clear.

 

[Swamp Thing]

Oh, OK, they have red filters before both detectors. So I guess the frequencies are all the same.

 

[DrChinese]

Thanks.
So, for the Type-1 process, we get all photon pairs with their polarizations matching the pump beam. No polarization entanglement. And when we pre-select pairs traveling out in specific directions, we get rid of the spatial (mode) entanglement as well. What about frequency? From the article, it seems highly likely that all photons have to be the same frequency, but the authors don't make this clear.

A couple of points about Type I. These pairs are normally frequency/wavelength entangled. The formula is f1 + f2 = k where k is the frequency of the source laser (which is essentially constant).

They may or may not be polarization entangled, it depends on the specific setup. If there are 2 crystals positioned orthogonal to each other, they will be polarization entangled. Sometimes that is not necessary for an experiment, and then only 1 crystal is needed.

 

[Misha]

About the original post. Such states mostly considered within the context of Fock states. For instance, the first paper that Google returns after 'generation fock states' search

Generation of Fock states in a superconducting quantum circuit

Fock states are interesting on their own, regardless entanglement.