B New retrocausality experiment ?

1. Aug 8, 2016

Nicky665

It seems that this was published last year on IJQF

"In the EPR experiment, each measurement
“What spin value
has this particle
along this orientation?

We propose
a new setting where the question is reversed: “What is the orientation along which
this particle has this spin value?” It turns out that the orientation is similarly subject
to nonlocal effects. To enable the reversal, each particle's interaction with a beam-
splitter at t1 leaves its spin orientation superposed. Then at t2, the experimenter selects
an “up” or “down” spin value for this yet-undefined orientation. Only after the two
particles undergo this procedure, the two measurements are completed, each particle
having its spin value along a definite orientation. By Bell's theorem, it is now the
“choice” of orientation that must be nonlocally transmitted between the particles
upon completing the measurement. This choice, however, has preceded the
experimenter’s selection. This seems to lend support for the time-symmetric
interpretations of QM, where retrocausality plays a significant role
"

2. Aug 8, 2016

Strilanc

I think they're just describing a system where the choice of orientation is determined by other quantum stuff.

I can make a quantum logic circuit in Quirk that does the same thing. Since Quirk doesn't do anything retrocausal, I don't see how this experiment favors retrocausality except in the sense of the authors liking those kinds of interpretations more:

The circuit works like this:

- The top two wires are entangled into an EPR pair.
- The next two wires are the "first qubit choosers". We have three orientations to choose between, so we place them into a uniform superposition $\frac{1}{\sqrt 3} |00\rangle + \frac{1}{\sqrt 3} |01\rangle + \frac{1}{\sqrt 3} |10\rangle$.
- The last two wires are the "second qubit choosers". Same state as the previous state.
- Use the choosers to conditionally rotate the top qubits.
- Measure the top qubits.

You now have measurement results but because you haven't measured the choosers yet you don't know the orientations the measurement results correspond to. Whether the measurements agree or not does give you some information, though. And because the orientations are in superposition there will now be some entanglement present between the chooser qubits (for basically the same reason that performing a parity measurement can entangle two initially separated qubits into an EPR pair).