[ziofill]

Quantum physicist here. My PhD back in the day was about the entanglement between downconverted photons. I've thought about this more than I like to admit.

While I appreciate the blog post, it seems a bit disingenuous. I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.

The MAGIC about delayed choice measurements is that they work even when the temporal order is UNDETERMINED. By this I mean that the two measurements of A and B can be set up to occur so close in time to each other that there is no time for a signal travelling at the speed of light to travel between the two events. Under this condition, you can witness both orderings (A measured before B and B measured before A) just by changing your reference frame. Under these conditions, the delayed choice experiment STILL WORKS!

In this case, there cannot be any argument like "but the idler was measured first", because "first" does not make any sense.

[sdwr]

I dug up this photo that says you can create a bell inequality with a single source of light

https://physics.stackexchange.com/questions/318967/can-bells...

Does that mean quantum calculations are just a fancy way of describing correlated probabilities, and have nothing to do with spooky action?

[DesiLurker]

I was fascinated by this experiment when I first learned about it. At one point I thought of a modified version where you let the 'B' go across a event horizon of a Black-hole. in this case there would be a clear before and after right? will DCQE still work across singularity boundary?

[anotherpaulg]

Good point!

This is easy to picture if you imagine widely spreading out the equipment used for the eraser experiment. If the signal hitting the screen and idler hitting one of the detectors are space-like separated events... the OP's explanation no longer seems to apply.

[Strilanc]

The delayed choice experiment doesn't contain a bell inequality, so spacelike seperation doesn't really mean much here. You can reproduce the results with local classical models.

[Filligree]

> I hope everyone understand that if you take two entangled photons A and B and detect A before B, then the outcome of the measurement of B must depend on the outcome of the earlier measurement of A, because measuring A causes the collapse of the joint state and determines the wavefunction of B undergoing the later measurement.

This bakes in an assumption that collapse happens, which I don't believe everyone agrees with...

[ziofill]

Sure sure, you can ignore that wording. The point is that the first measurement determines the state of the particle undergoing the second measurement.

[nyeah]

The author talks about "two downconverted photons" each at half the energy, in that simple linear experiment. Is that mainstream physics? If so ... big ask, but do you happen to have a butt-simple reference at the undergrad QM level? It feels like I need more equipment than a fixed pair of slits to downconvert frequencies.

[jfengel]

Yes, that's the main way we produce entangled photons. You put in a photon; you get out two photons, each with half the energy. To follow conservation laws, they must have complementary values of things like polarization: if one is up-down polarized, the other must be left-right polarized.

Those values are linked: if you measure one, the other will have the opposite. You can tell that it's not just pre-set values by measuring at a 45 degree angle, so you get some up-down and some left-right in each measurement. Take a bunch of those, and you'll see that the expected correlation between your measurements follows what quantum mechanics predicts, and not classical mechanics.

It's usually done with beta barium borate. You can buy it at optical suppliers:

https://eksmaoptics.com/nonlinear-and-laser-crystals/nonline...

[nyeah]

Thanks. Somehow I thought the author was trying to explain a simple linear two-slit experiment using two photons, each with half the energy. I think it was a reading problem on my end.