- #1
Derek P
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The light source in the DCQE experiment of Kim et al is a laser which illuminates two slits which are immediately followed by an SPDC. The latter is therefore excited coherently in two narrow stripes referred to as "slits". The output has a small amplitude of down-converted entangled pairs. The authors state "A pair of entangled photons, photon 1 and photon 2, is then emitted from either atom A or atom B by atomic cascade decay."
Consider just the "erased" cases. Photon 2 is entangled with photon 1. So when, for example, an |A>+|B> detection occurs, photon 1 is assigned to the |A>+|B> "interference pattern". That's not the problem here.
But according to what I just quoted, the pair is emitted by a single atom, a single slit. To simplify things a bit, It is either |A>|A> or |B>|B>. However, later in the paper, in (2) it has become a superposition.
I would put this down to careless language by Kim et al, but it seems to me this does not get rid of the problem. |A>|A> + |B>|B> at the detectors can only have evolved from |A>|A> + |B>|B> at the SPDC. Which can only have evolved from |α>+|β> where |α> and |β> are the excited states of atom A and atom B respectively. The photon pair is therefore emitted by two, widely separated atoms acting together.
Personally I have no difficulty with this picture, because I would expect the possible different emission times to result in a superposition of |α> → |A>|A> and |β> → |B>|B>. However apparently this is not a standard description. I would dearly love to know what is the standard description given that the alleged emission from a single atom in a single slit gives rise to a state that refers to both slits.
Consider just the "erased" cases. Photon 2 is entangled with photon 1. So when, for example, an |A>+|B> detection occurs, photon 1 is assigned to the |A>+|B> "interference pattern". That's not the problem here.
But according to what I just quoted, the pair is emitted by a single atom, a single slit. To simplify things a bit, It is either |A>|A> or |B>|B>. However, later in the paper, in (2) it has become a superposition.
I would put this down to careless language by Kim et al, but it seems to me this does not get rid of the problem. |A>|A> + |B>|B> at the detectors can only have evolved from |A>|A> + |B>|B> at the SPDC. Which can only have evolved from |α>+|β> where |α> and |β> are the excited states of atom A and atom B respectively. The photon pair is therefore emitted by two, widely separated atoms acting together.
Personally I have no difficulty with this picture, because I would expect the possible different emission times to result in a superposition of |α> → |A>|A> and |β> → |B>|B>. However apparently this is not a standard description. I would dearly love to know what is the standard description given that the alleged emission from a single atom in a single slit gives rise to a state that refers to both slits.
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