I Photon Entanglement: Identifying & Using an Entangled Pair

[RobbyQ]

In this video how are the entangled photons later used and actually identified as an entangled pair amongst billions of others.

Also does he really mean the photon is split or is the quantised energy split with half frequencies?

 

[.Scott]

He is using a "Parametric Down Converter" (PDC). So if you send violet photons (400nm) into this PDC
and all goes well you get two deep red photons coming out. The violet photon "splits" into two red photons, each with half the energy.

The photon pairs that are entangled are most readily identified as the red ones coming from the device. So if you put a filter on the output that blocks all violet photons, all of the photons emitted from the device will be the entangled ones.

 

[RobbyQ]

He is using a "Parametric Down Converter" (PDC). So if you send violet photons (400nm) into this PDC
and all goes well you get two deep red photons coming out. The violet photon "splits" into two red photons, each with half the energy.

The photon pairs that are entangled are most readily identified as the red ones coming from the device. So if you put a filter on the output that blocks all violet photons, all of the photons emitted from the device will be the entangled ones.

Thanks. But how do you associate 2 entangled photons out of the whole bucket load of entangled photons. And how do they measure the shared quantum state of 2 entangled photons and put it to practical use once there is a state collapse from observation?

 

[.Scott]

Thanks. But how do you associate 2 entangled photons out of the whole bucket load of entangled photons.

The pair of photons will be emitted at the same time. If you need to "associate" them, one way is to dim the light intensity down to slow the average rate that the photons are emitted. If the time between photon pairs is long enough (say about a nanosecond), then you are able to detect and count individual photons.

And how do they measure the shared quantum state of 2 entangled photons and put it to practical use once there is a state collapse from observation?

If demonstrating the Bell inequality is considered a "practical use", then direct each photon from the pair to a separate polarization detector. The entire set up is described here: Bell Inequality Test

 

[DrChinese]

Thanks. But how do you associate 2 entangled photons out of the whole bucket load of entangled photons. And how do they measure the shared quantum state of 2 entangled photons and put it to practical use once there is a state collapse from observation?

Just to add to @.Scott ‘s correct answer: the pair is entangled if they are detected within a specific coincidence time window, let’s say 10 nanoseconds. Note that the timing is adjusted for the relative length each one travels. Commonly in normal situations, only one pair is seen in any time window regardless of laser intensity because only 1 in perhaps 10 million down converts.

In many PDC setups, the split photons veer off at a slight angle while the rest go straight ahead. That makes it possible to distinguish them as well.

The fidelity of the detected pairs is checked by performing a Bell test. A common version is called a CHSH inequality which yields an S value which must be above 2, higher means better quality. Typical values are 2.4 and have experimental accuracy to 4 or more standard deviations.

Once you have a good source, you can perform other experiments.