Science by Marilyn: Quantum Temporal Entanglements

[Kathryn Burke]

Quantum Temporal Entanglement


Most of us have heard about quantum entanglement. It is that odd property in which two quantum systems meet and entangle then even across the distance of thousands of light years it becomes impossible to measure the features of one system (such as its position, momentum and polarity) without instantly steering the other into a corresponding state. What is now called quantum non-locality, the eerie link that appears to exist between entangled particles.

The assumption is that the 'nonlocal' part of quantum nonlocality refers to the entanglement of properties across space. The idea that information is transferred at some superluminal speed. And many experiments have shown that. A new ‘trick’ has been quantum entanglement ‘swapping’.
In this experiment two independent pairs of entangled photons, A1-A2 and B1-B2, are emitted by autonomous sources. By taking a joint measurement on one photon in each pair (A1 and B1), these photons fall into an entangled state (later verified using detectors). The two remaining photons (A2 and B2) are projected on an entangled state despite being unaware of the other's presence and never having previously interacted. Hence the entanglement of the initial pairs has been “swapped’ onto A2 and B2.
And you thought that quantum mechanics was as Einstein stated, “Spooky action at a distance.”
However, what if entanglement also occurs across time? Is there such a thing as temporal nonlocality? As
Just when you thought quantum mechanics couldn't get any weirder it turns out the answer to temporal nonlocality is a resounding yes.
As in the previous mentioned experiment 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 life spans did not overlap at all.
Here's how they did it.
First, they created an entangled pair of photons, '1-2'. Soon after, they measured the polarization of photon 1 (a property describing the direction of light's oscillation) – thus 'killing' it.
Photon 2 was sent on a wild goose chase while a new entangled pair, '3-4', was created. 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.

Some time later, the polarization of the lone survivor, photon 4, is measured, and the results are compared with those of the long-dead photon 1 at the beginning of the experiment.

The upshot? 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.

What does this mean? On the face of the experiment 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.
Even more bizarrely it implies that the measurements carried out by your eye upon starlight falling through your telescope somehow dictated the polarity of photons more than 9 billion years old.
Lest this scenario strike you as too outlandish, Megidish and his colleagues can't resist speculating on possible and rather spooky interpretations of their results. It means that perhaps the measurement of photon 1's polarization somehow steers the future polarization of 4, or the measurement of photon 4's polarization somehow rewrites the past polarization 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.
So does that mean that somehow everything is already pre-written or somehow what happens now rewrites the past? In either case this has some troubling implications. However, all is not completely lost.
In developing his theory of special relativity, Einstein deposed the concept of simultaneity from its Newtonian pedestal. As a consequence, simultaneity went from being an absolute property to being a relative one. There is no single timekeeper for the Universe; precisely when something is occurring depends on your precise location relative to what you are observing, known as your frame of reference. This has been observed in so called time dilation, due to speed or gravity time for one observer seems different than the other observer. Hence what you experience is dependent on your frame of reference.

So the key to avoiding strange causal behaviour (steering the future or rewriting the past) in instances of temporal separation is to accept that calling events 'simultaneous' carries little metaphysical weight, which on its own is weird enough.
What it tells me is the universe and the quantum physics underlying it are a whole lot mind blowing and yet fascinating than one ever thought!
But then again maybe it is all relative.