# What tells us photons were ever in a superposition in EPR

Hi,

I know that entanglement is real and that it tells us something profound about the nature of quantum objects like electrons and photons. I can't explain to a family member how we know that two twin photons in the EPR experiment started off in a superposition. In other words, how do we know that the photons acquired their spin up/down properties when measured? Am I wrong about it?

Thanks

Andrew

## Answers and Replies

phyzguy
When we do the EPR experiment, we don't just start with two random photons. We start with two very carefully prepared photons that result from the decay of a known state. So we know that their spins are correlated.

DrChinese
Gold Member
I know that entanglement is real and that it tells us something profound about the nature of quantum objects like electrons and photons. I can't explain to a family member how we know that two twin photons in the EPR experiment started off in a superposition. In other words, how do we know that the photons acquired their spin up/down properties when measured?

One's initial reaction is that the 2 particles acquired their spin properties at T=0 (when initially entangled). However, that simple assumption leads to a contradiction with experiment. That contradiction was first discovered by John Bell, around 30 years after the EPR paper (and about 10 years after Einstein's death).

Look up Bell's Theorem, or alternately go to a web page I created which will give you a minimally taxing explanation:

http://drchinese.com/David/Bell_Theorem_Easy_Math.htm

Sadly, this is not the kind of thing you can explain over a family dinner.

Thanks for your time and patience :)

jfizzix
Gold Member
Hi,

I know that entanglement is real and that it tells us something profound about the nature of quantum objects like electrons and photons. I can't explain to a family member how we know that two twin photons in the EPR experiment started off in a superposition. In other words, how do we know that the photons acquired their spin up/down properties when measured? Am I wrong about it?

Thanks

Andrew

In the EPR scenario, you can prove that they started off in a superposition because if they were in a classically correlated mixture (e.g., sending out randomly spin-up pairs or spin-down pairs), the correlations could only be so strong. This comes from assuming the uncertainty principle is correct. For spin-1/2 observables, the uncertainty principle would say that the particle cannot have both a perfectly defined horizontal component of spin and a vertical component of spin. This uncertainty principle also holds true when conditioning on a third variable (whatever variable determines whether up-up is sent or down-down), and because of this, the conditional uncertainties (i.e., the uncertainty of measuring the spin of B conditioned on the outcome of A) would also have to obey the same limit.

However, there's no real limit to the correlations between these observables, so if you measure near-perfect correlations between the two spins no matter how you orient your measurement axis (e.g., horizontal/vertical), then you rule out the possibility that there's a classical explanation of the correlations, and they must indeed be in an entangled superposition.

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