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Entanglement Swapping 
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#1
Feb2312, 04:31 PM

PF Gold
P: 802

Hi there,
If we setup an experiment for entanglement swapping, the two entangled pairs of photons are: Group 1: A, B Group 2: X, Y Take B and Y and perform a Bellstate measurement on them. Obviously A and X are now entangled. But for this to occur, must B and Y have no definite polarisation? 


#2
Feb2312, 04:59 PM

Sci Advisor
PF Gold
P: 5,374




#3
Feb2312, 05:06 PM

P: 1,583




#4
Feb2312, 07:45 PM

PF Gold
P: 802

Entanglement Swapping
Okay:
Because if we measure particles A and X, the entanglement with B and Y is lost. So when you go to entangle B and Y, because no entanglement exists between A and B, and X and Y, A and X won't become entangled? When we talk of teleportation, is it when we measure A, Y takes on the same polarisation? 


#5
Feb2412, 09:02 AM

Sci Advisor
PF Gold
P: 5,374




#6
Feb2412, 09:38 PM

PF Gold
P: 802

Even if the entanglement is broken between A and B, because you've measured A?



#7
Feb2512, 07:16 PM

PF Gold
P: 802

Okay, I've read this paper: http://www.univie.ac.at/qfp/publicat...es/200106.pdf
And I am lost!! So what I understand: photons 1 and 4 show entanglement correlations; i.e. both are HH or VV, never seeing HV or VH. "This is the socalled entanglement swapping , which can also be seen as teleportation either of the state of photon 2 over to photon 4 or of the state of photon 3 over to photon 1." This is where I am confused, especially if photons 1 and 4 already have polarisations. Teleporting the state of photon 3 to 1, etc. in this case makes my brain fry. 


#8
Feb2612, 11:48 AM

Sci Advisor
PF Gold
P: 5,374




#9
Feb2712, 11:00 AM

PF Gold
P: 1,376

I wouldn't say that teleportation is very good word for entanglement swapping. 


#10
Feb2712, 07:57 PM

PF Gold
P: 802

If we implement the same experimental setup, and measure photons 1 and 4 before measurement occurs on photons 2 and 3  I assume when the polarising beam splitter combines particles 2 and 3 this is when the entanglement switch occurs. So if we detect photons 1 and 4 in both H (or V polarisations), prior to reaching the filters and detectors photons 2 and 3 take on V (or H polarisations)  the opposite of what the others take?



#11
Feb2712, 11:07 PM

PF Gold
P: 1,376

So how they disappear when we perform Bell state measurement? We throw them out because their pair photons reach the same detector (either both reach detector 2 or detector 3) and so we don't have 4fold coincidence in 1,2,3 and 4. 


#12
Feb2912, 11:54 PM

PF Gold
P: 802

If we analyse photon 3 in the 45 basis, according to QM it gets entangled with the filter so it both passes and fails at the same time. If we then measure photons 1 and 4, so they show VV or HH, will photon 4 take the opposite polarisation of photons 1 and 4, or will it written in the 45 basis so take on V or H, without dependence on what photon 1 takes? 


#13
Mar112, 11:23 PM

PF Gold
P: 1,376

Anyways you get entanglement for photons 1 and 4 only (in 4fold coincidences) when you measure photons 2 and 3 in +/ (+45/45) basis after PBS. 


#14
Mar812, 09:16 PM

PF Gold
P: 802

For entanglement swapping: is entanglement created between photons 2 and 3 when they hit the beam splitter (in this experiment scheme: http://www.univie.ac.at/qfp/publicat.../200106.pdf)? Is entanglement swapping the same thing as creating fourphoton entanglement? 


#15
Mar1012, 12:22 AM

PF Gold
P: 1,376

When we detect VH in 1 and 4 we have two photons in 2 and no photons in 3. And what do you mean by fourphoton entanglement? So entanglement swapping happens when you register 4fold coincidence in coincidence counter i.e. when filtering is performed. 


#16
Mar1312, 12:47 AM

PF Gold
P: 802

If I measured photon 2 in 45 basis and 3 in 135 basis, wouldn't that indicate whether the photons were described by the GHZ state? If one passes, the other fails. From the GHZ state, you can't have photon 2 in 45 and photon 3 in 135. 


#17
Mar1312, 11:20 AM

PF Gold
P: 1,376

If you measure photon 2 in 45 basis and 3 in 135 basis then photons 1 and 4 will have positive correlation for +45 and +135 measurement and negative correlation (minimal rate of fourfold coincidences) for +45 and +45 measurement. 


#18
Mar1312, 11:35 AM

Sci Advisor
PF Gold
P: 5,374

With entanglement swapping, you have 2 entangled pairs (which is N=4) but the 4 are never entangled all at once. 


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