Entanglement correlations vs. timelike measurement events?

[LarryS]

The experiments that have been done to demonstrate that entangled particles are correlated (or anti-correlated) have been designed so that Alice and Bob's measurement events are intentionally space-like separated. Thus demonstrating that the "ghostly action at a distance" is supraluminal or instantaneous.

Have there been any such experiments that demonstrate, either intentionally or accidentally, that the correlations still hold up when the two measurement events are time-like separated? Just wondering.

Thanks in advance.

 

[DrChinese]

Have there been any such experiments that demonstrate, either intentionally or accidentally, that the correlations still hold up when the two measurement events are time-like separated? Just wondering.

Thanks in advance.

Great question, and the answer is yes. Specifically, photons can be entangled that have never co-existed. I'd call that time-like separation.

While it might seem impossible to entangle particles at different times, it can be and has been done using entanglement swapping. (Side note: normal Bell experiments do not actually attempt to measure spin or polarization at the exact same time anyway.)

https://arxiv.org/abs/0911.1314
Quantum systems that have never interacted can become nonlocally correlated through a process called entanglement swapping. To characterize nonlocality in this context, we introduce local models where quantum systems that are initially uncorrelated are described by uncorrelated local variables. While a pair of maximally entangled qubits prepared in the usual way (i.e., emitted from a common source) requires a visibility close to 70% to violate a Bell inequality, we show that an entangled pair generated through entanglement swapping will already violate a Bell inequality for visibilities as low as 50% under our assumption.

https://arxiv.org/abs/1209.4191
The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.

 

[vanhees71]

It should, however be stressed, that within "no-nonsense interpretations" of quantum theory, i.e., such as local relativistic quantum field theory, upon which the most successful theory so far is based, the Standard Model of elementary particle physics, there is no "ghostly action at a distance" but only local measurements at two far-distant places on parts of an entangled system, which is entangled, because it was prepared as such in the very beginning long before any of the two "measurement events" (usually "clicks" of photon detectors at each of the experimentalists' places), which can be taylored to be space like (i.e., simultaneous in one inertial system of reference) or time-like (i.e., objectively one later than the other). No matter what, since there is no influence of the one measurement on the outcome of the other measurement, it doesn't make any difference in the outcome. The correlations due to entangledment, that are stronger than in any local deterministic model, are due to the preparation and not to the mutual influence of the measurements at far-distant places. There are very long debates on this already in these forums, leading to basically nothing from the point of view of physics!

 

[LarryS]

It should, however be stressed, that within "no-nonsense interpretations" of quantum theory, i.e., such as local relativistic quantum field theory, upon which the most successful theory so far is based, the Standard Model of elementary particle physics, there is no "ghostly action at a distance" but only local measurements at two far-distant places on parts of an entangled system, which is entangled, because it was prepared as such in the very beginning long before any of the two "measurement events" (usually "clicks" of photon detectors at each of the experimentalists' places), which can be taylored to be space like (i.e., simultaneous in one inertial system of reference) or time-like (i.e., objectively one later than the other). No matter what, since there is no influence of the one measurement on the outcome of the other measurement, it doesn't make any difference in the outcome. The correlations due to entangledment, that are stronger than in any local deterministic model, are due to the preparation and not to the mutual influence of the measurements at far-distant places. There are very long debates on this already in these forums, leading to basically nothing from the point of view of physics!

Can you recommend any recent papers dealing with entanglement in QFT? My two QFT books (Ryder and Zee) don't even reference it. Thanks.

 

[vanhees71]

Read Weinberg, Quantum Theory of Fields, vol. I. There is a careful discussion about the "linked-cluster principle", "causality", and all that.

 

[DrSteveBigTuna]

Has anyone from the paper "Entanglement Swapping between Photons that have Never Coexisted"
put the optical circuit into the "usual" quantum circuit format visa-vi
Nielsen/Chuang text or
the Marinescu text
( a non-implementation format i.e. )

 

[platosuniverse]

The experiments that have been done to demonstrate that entangled particles are correlated (or anti-correlated) have been designed so that Alice and Bob's measurement events are intentionally space-like separated. Thus demonstrating that the "ghostly action at a distance" is supraluminal or instantaneous.

Have there been any such experiments that demonstrate, either intentionally or accidentally, that the correlations still hold up when the two measurement events are time-like separated? Just wondering.

Thanks in advance.

Good question and it is "ghostly or spooky action at a distance" as Einstein said. Information can't go from point A to point B faster than the speed of light yet when particles are entangled information that's not useful is shared amongst two separate points in space and time. Basically 1 bit like a string of a's or a random string of HT's.

The first problem is semantics. When people hear particles they think of particles of sand or particles of salt. I think they should be called subatomic states. QFT says particles are excitations of underlying quantum fields. Excitations is very different than particles. It's just like imaginary time. Gauss said it should be called lateral instead of imaginary. Imaginary gives the impression of something that's mysterious.

There have been experiments that show correlations in time.

Entanglement Between Photons that have Never Coexisted

In this work we demonstrate how the time at which quantum measurements are taken and their order, has no effect on the outcome of a quantum mechanical experiment, by entangling two photons that exist at separate times. This is achieved by first creating one photon pair (1-2) and right away measuring photon 1 (see Fig. 1). Photon 2 is delayed until a second pair (3-4) is created and photons 2 and 3 are projected onto the Bell basis. When photon 1 is measured in a certain basis, it does not ’know’ that photon 4 is going to be created, and in which basis it will be measured. Nevertheless, photons 1 and 4 exhibit quantum correlations despite the fact that they never coexisted. The possibility of two photons that do not overlap in time, but still exhibit entanglement, was discussed theoretically in a system of atoms and photons [20]. Recently, entanglement swapping was demonstrated with a delayed choice, where all four photons were created simultaneously, but photons 1 and 4 were measured before a choice had been made whether to entangle them or not.

https://arxiv.org/pdf/1209.4191.pdf

 

[jfizzix]

The experiments that have been done to demonstrate that entangled particles are correlated (or anti-correlated) have been designed so that Alice and Bob's measurement events are intentionally space-like separated. Thus demonstrating that the "ghostly action at a distance" is supraluminal or instantaneous.

Have there been any such experiments that demonstrate, either intentionally or accidentally, that the correlations still hold up when the two measurement events are time-like separated? Just wondering.

Thanks in advance.

It's also worth looking up Leggett-Garg inequalities:

https://en.wikipedia.org/wiki/Leggett–Garg_inequality

These inequalities are like Bell inequalities in time in that their violation reals quantum behavior in time that defies classical intuition.