Thread: Delayed Choice Entanglement Swapping

Before you read any further, I wanted to set some parameters for the thread.

I'm going to be posting up a study from mid-April. I haven't had the chance to go through the methodology myself, yet. Ideally, others in here who are well-versed in Quantum physics will contribute. What that means is, if you don't understand the topic, and can't understand the physics being discussed here, please just read, or ask for clarification. Please do not post opinions based on what you've heard, or your personal beliefs.

Please, please, please, no pseudoscientists in here.

http://www.nature.com/nphys/journal/...nphys2294.html

Motivated by the question of which kind of physical interactions and processes are needed for the production of quantum entanglement, Peres has put forward the radical idea of delayed-choice entanglement swapping. There, entanglement can be ‘produced a posteriori, after the entangled particles have been measured and may no longer exist’. Here, we report the realization of Peres’s gedanken experiment. Using four photons, we can actively delay the choice of measurement—implemented through a high-speed tunable bipartite-state analyser and a quantum random-number generator—on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one of two mutually exclusive quantum states in which the photons are either entangled (quantum correlations) or separable (classical correlations). This can also be viewed as ‘quantum steering into the past’.

As I study this experiment, I'll be posting up my breakdown of the methodology, any potentially omitted conclusions from the data, and basically... a critique of the study. It will probably take a week or two, based on my schedule.

For the physicists in here, enjoy! For those who aren't physicists, feel free to ask questions of the physicists. We'll try to make it clear what was going on in the study.

Extraordinary claims, require extraordinary evidence. I'm going into this analysis looking for their flaw(s). I encourage the others to, as well.

Well, I have a question already. The first sentence in the quote says, "Motivated by the question of which kind of physical interactions and processes are needed for the production of quantum entanglement,...". It was my understanding that they already knew how to produce entanglement, and had been doing it for some time. Is the difference here that they are trying to retroactively entangle? And how does this differ from other delayed-choice experiments (not the methodology, but the desired result)?

Originally Posted by fifthofnovember

Well, I have a question already. The first sentence in the quote says, "Motivated by the question of which kind of physical interactions and processes are needed for the production of quantum entanglement,...". It was my understanding that they already knew how to produce entanglement, and had been doing it for some time. Is the difference here that they are trying to retroactively entangle? And how does this differ from other delayed-choice experiments (not the methodology, but the desired result)?

Good questions.

This is investigating a specific method of inducing quantum entanglement. There have been numerous theoretical models worked out, but only a few have been tested. This very much builds on earlier work by Strekalov (et al) in 1996. However, Strekalov specifically didn't account for variations in timing. He focused solely on whether a single photon pair was in the system at any given point. This experiment was an attempt to account for the post-selection aspect of the thought experiment.

Jaques (et al) in 2007 did investigate the post-selection entanglement, but his results helped demonstrate instantaneous entanglement. The retroactive entanglement that has been indicated in this paper, appears to be a bit unique. Moreover, Jaques results appeared to be more limited in scope, compared to the results from Xiao-song's seemingly more expansive conclusions.

Based purely on what I've read from the experimenters, it seemed that even they were a bit taken aback by the results. That, of course, is merely supposition.