Where is the way where light dwelleth? Job 38:19
"Up to today, most experiments have tested entanglement over spatial gaps.
The assumption is that the 'nonlocal' part of quantum nonlocality refers to the entanglement of properties across space.
-----But what if entanglement also occurs across time?
Is there such a thing as temporal nonlocality?
The answer, as it turns out, is yes.
Just when you thought quantum mechanics couldn't get any weirder, a team of physicists reported that they had successfully entangled photons that never coexisted.
Previous experiments involving a technique called 'entanglement swapping' had already showed quantum correlations across time, by delaying the measurement of one of the coexisting entangled particles; but Eli Megidish and his collaborators were the first to show entanglement between photons whose lifespans did not overlap at all.
Here's how they did it.
First, they created an entangled pair of photons, '1-2' (step I in the diagram below). Soon after, they measured the polarisation of photon 1 (a property describing the direction of light's oscillation) – thus 'killing' it (step II).
Photon 2 was sent on a wild goose chase while a new entangled pair, '3-4', was created (step III). Photon 3 was then measured along with the itinerant photon 2 in such a way that the entanglement relation was 'swapped' from the old pairs ('1-2' and '3-4') onto the new '2-3' combo (step IV).
Some time later (step V), the polarisation of the lone survivor, photon 4, is measured, and the results are compared with those of the long-dead photon 1 (back at step II).
The data revealed the existence of quantum correlations between 'temporally nonlocal' photons 1 and 4.
That is,
What on Earth can this mean? Prima facie, it seems as troubling as saying that the polarity of starlight in the far-distant past – say, greater than twice Earth's lifetime – nevertheless influenced the polarity of starlight falling through your amateur telescope this winter.
Perhaps the measurement of photon 1's polarisation at step II somehow steers the future polarisation of 4, or the measurement of photon 4's polarisation at step V somehow rewrites the past polarisation state of photon 1.
In both forward and backward directions, quantum correlations span the causal void between the death of one photon and the birth of the other."
ScienceAlert
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The assumption is that the 'nonlocal' part of quantum nonlocality refers to the entanglement of properties across space.
-----But what if entanglement also occurs across time?
Is there such a thing as temporal nonlocality?
The answer, as it turns out, is yes.
Just when you thought quantum mechanics couldn't get any weirder, a team of physicists reported that they had successfully entangled photons that never coexisted.
Previous experiments involving a technique called 'entanglement swapping' had already showed quantum correlations across time, by delaying the measurement of one of the coexisting entangled particles; but Eli Megidish and his collaborators were the first to show entanglement between photons whose lifespans did not overlap at all.
Here's how they did it.
First, they created an entangled pair of photons, '1-2' (step I in the diagram below). Soon after, they measured the polarisation of photon 1 (a property describing the direction of light's oscillation) – thus 'killing' it (step II).
Photon 2 was sent on a wild goose chase while a new entangled pair, '3-4', was created (step III). Photon 3 was then measured along with the itinerant photon 2 in such a way that the entanglement relation was 'swapped' from the old pairs ('1-2' and '3-4') onto the new '2-3' combo (step IV).
Some time later (step V), the polarisation of the lone survivor, photon 4, is measured, and the results are compared with those of the long-dead photon 1 (back at step II).
The data revealed the existence of quantum correlations between 'temporally nonlocal' photons 1 and 4.
That is,
entanglement can occur across two quantum systems that never coexisted.
Perhaps the measurement of photon 1's polarisation at step II somehow steers the future polarisation of 4, or the measurement of photon 4's polarisation at step V somehow rewrites the past polarisation state of photon 1.
In both forward and backward directions, quantum correlations span the causal void between the death of one photon and the birth of the other."
ScienceAlert