# Verify the equality

1. Apr 6, 2014

### StevieTNZ

Hi there

I'm reading through Valerio Scarani's "Six Quantum Pieces" and have hit an exercise which requires verification of an equality.

where the letters in the brackets indicate the photons.

How does one verify the above equality. The answer given in the book is
That is where I am stuck. I know the different bell-states, but entangle swapping photons that are entangled differently I can't figure out what the outcomes will be.

Any help much appreciated
Stevie

2. Apr 7, 2014

### atyy

I think Psi+(AB) means that you have to put labels on the kets: Psi+(AB) = 1/2 |HA>|VB> + |VA>|HB>.

So the LHS should be |HD>|VA>|HB>|HC> + |VD>|HA>|HB>|HC> + |HD>|VA>|VB>|VC> + |VD>|HA>|VB>|VC>. Then do the same on the RHS, and use brute force.

3. Apr 7, 2014

### StevieTNZ

I guess my confusion lies not with expanding the right hand side, but how the author obtains those bell-states in the first place.

Any clarity on that would be great!

4. Apr 9, 2014

### StevieTNZ

Apparently the right hand side is an expansion of |Psi+>{AB} |Phi+>{CD}, not of |Psi+>{DA} |Phi+>{BC}. Now I'm extra confused at how the author obtains
= Phi+(AB)Psi+(CD) + Phi-(AB)Psi-(CD) + Psi+(AB)Phi+(CD) + Psi-(AB)Phi-(CD)
from
|Psi+>{AB} |Phi+>{CD}, not of |Psi+>{DA} |Phi+>{BC}

5. Apr 10, 2014

### StevieTNZ

Okay, so I expand the right hand side:
=(|H>|H>+|V>|V>(AB) |H>|V>+|V>|H>(CD)) + (|H>|H>-|V>|V>(AB) |H>|V>-|V>|H>(CD)) + (|H>|V>+|V>|H>(AB) |H>|H>+|V>|V>(CD)) + (|H>|V>-|V>|H>(AB) |H>|H>-|V>|V>(CD))

That done, I don't see how that is equivalent to |H>|V>+|V>|H>(DA) and |H>|H>+|V>|V>(BC).

If instead photons D&A are entangled as |H>|V> - |V>|H>, what impact does that have on the equality presented?

If the photons D&A and B&C are both in the bell-state |H>|H> + |V>|V> or |H>|H> - |V>|V>, would detecting photons A & B in the bell-state Phi+ mean photons C & D are in that bell-state also? Or if D&A are entangled as |H>|H> + |V>|V> and B&C are entangled as |H>|H> - |V>|V>, would we find photons C&D in the opposite bell-state that photons A & B are found in (i.e C&D in Psi-, A&B in Psi+)?

6. Apr 11, 2014

### atyy

Maybe try expanding it further? The same way the left hand side is expanded in post #2?

7. Apr 11, 2014

### naima

Do not use labels. Always write things in the same order A B C D i.e HVHH for A = H, B = V C = H and D = H.
It is easy for le right side (they are in the good order. For the left side you ave DA and BC terms, take Psi+(DA)Phi+(BC)
you have a HH term for BC, you write it and a HV for DA. You write V at the left and H at the right of the HH and you get VHHH for ABCD and so on for the four terms of the left side.

Here we begin with a pair of entangled photons BC and DA they are in Phi+ and Psi+ Next we make a bell state measurement on AB. there are four possibles results. According to the result you get, the equality tells you in which Bell State is the CD pair.

Does anybody know how one can say BC are Phi+ and AD in Psi+? I suppose that it can not be known before AB Bell measurement. But how are they post selected?

8. Apr 11, 2014

### atyy

I agree with naima. The idea of using the labels is to first write the expansion in the "wrong*" order, since that's how it's presented in the book. Then rearrange every term so that the labels are in the order ABCD.

*It's not wrong, since labels instead of ordering are used to indicate ABCD. One can notate either way, which is why I put "wrong" in quotes.

9. Apr 11, 2014

### StevieTNZ

I'm not entirely sure how I would start going about that. Any guidance must appreciated.

10. Apr 12, 2014

### naima

First write the 16 terms of the right side (it is easy) and then the four of the left side the way i gave you.

We have here Psi+Phi+ swapping. In another thread we had Psi-Psi- swapping.
How many other swapping possibilities?

11. Apr 12, 2014

### atyy

As naima says, there are 16 terms on the right. For example the first term is expanded into 4 terms:

(|H>|H>+|V>|V>(AB))( |H>|V>+|V>|H>(CD))
= |HA>|HB>|HC>|VD>+|HA>|HB>|VC>|HD>+|VA>|VB>|HC>|VD>+|VA>|VB>|VC>|HD>

12. Apr 12, 2014

### StevieTNZ

I expanded further and after cancelling I get the following:
|H>(A)|H>(B)|H>(C)|V>(D) + |V>(A)|V>(B)|V>(C)|H>(D) + |H>(A)|V>(B)|H>(C)|H>(D) +
|V>(A)|H>(B)|V>(C)|V>(D)

13. Apr 13, 2014

### atyy

Taking the LHS:
|HD>|VA>|HB>|HC> + |VD>|HA>|HB>|HC> + |HD>|VA>|VB>|VC> + |VD>|HA>|VB>|VC>

Arranging each term to be in ABCD order:
|VA>|HB>|HC>|HD> + |HA>|HB>|HC>|VD> + |VA>|VB>|VC>|HD> + |HA>|VB>|VC>|VD>

which doesn't seem to be the same :(

Can you double check your RHS, or is my LHS wrong?

14. Apr 13, 2014

### StevieTNZ

Find attached my right hand side calculation. As far as I can tell, it is correct.

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15. Apr 14, 2014

### atyy

Is there a typo in the book, should it be this?

Psi+(DA)Phi+(BC) = Phi+(AB)Psi+(CD) + Phi-(AB)Psi-(CD) + Psi+(AB)Phi+(CD) - Psi-(AB)Phi-(CD)

16. Apr 16, 2014

### StevieTNZ

Should it be that, if the way to verify the equality is to expand as I have done and cancel like terms, and re-arrange the photons as they're originally entangled?

17. Apr 16, 2014

### atyy

I think so, but my algebra is terrible. I was going to check that the method is correct by trying to verify Eq 2.12 of http://arxiv.org/abs/0910.4222.

18. Apr 20, 2014

### StevieTNZ

Did you manage to check if your method is correct by verifying that equality?

19. Apr 21, 2014

### naima

There is a typo in the answer given by the author:
Phi+ * Psi+ is (HH+VV)(HV+ VH)
The result cannot contain HVHV

20. Apr 21, 2014

### atyy

Yes, seems to work.