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[Ed. note: This article was recently published (July 6, 2019) on the Science Alert web site. As a footnote on the Science Alert story notes: "This article was originally published at Aeon and has been republished under Creative Commons." Viewing the source HTML at Aeon, I discovered it was originally published 02-Feb-2018. Though the material is somewhat dated, it was the first I'd heard of this and thought it sufficiently interesting to share with the SoylentNews community. --martyb]
Entanglement of particles, i.e. quantum nonlocality, is routinely demonstrated in particles separated by space.
But space and time are related, leading to a team of physicists demonstrating that quantum entanglement can occur across time with particles that shared no concurrent existence.
Just when you thought quantum mechanics couldn't get any weirder, a team of physicists at the Hebrew University of Jerusalem reported in 2013 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.
One might be curious how a measurement done on one particle might be instantly reflected on another that doesn't exist yet, so here is how this was accomplished:
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 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.
The physicist's speculation on what this means is somewhat reminiscent of a cat in a box:
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.
For this to begin to make sense, recall that simultaneity is not the absolute Newtonian property you perceive, but per Einstein
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.
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.
It is only a frame-specific property, a choice among many alternative but equally viable ones – a matter of convention, or record-keeping.
The lesson carries over directly to both spatial and temporal quantum nonlocality.
Hopefully the temporal entanglement of entire objects is next. Imagine checking out the final episode of a show on your entangled TV, realizing it is terrible, and avoiding the entire series which the studios don't even make because nobody watched it...
Journal Reference
E. Megidish, et al. Entanglement Swapping between Photons that have Never Coexisted Phys. Rev. Lett. 110, 210403 DOI:10.1103/PhysRevLett.110.210403
(Score: 1, Informative) by Anonymous Coward on Sunday July 07 2019, @02:16AM (2 children)
Classical analogies like this suffer from missing a key component that really distinguishes quantum entanglement from conventional correlation: the role of the measurement. In the "quantum" case, your gloves might also have a "colour", say, red or blue. The scenario would be that you can make only one measurement, handedness or colour. This is where entanglement plays a fundamentally stronger role: your gloves can be created in such a way (entangled) that they will always have opposite results, regardless of which measurement you choose, and yet taken individually the results will be distributed uniformly randomly.
Even this extension to the analogy struggles, though, because in reality a glove can have both a colour and a handedness at the same time. The quantum properties we're talking about don't really exist simultaneously. A photon's polarization, for example, can be described as components along horizontal and vertical axes, say, or alternatively along diagonal axes. But these are just different ways to describe the same property of a photon (i.e., which way it's electric field oscillates). And yet, measuring in either the horizontal/vertical basis OR in the diagonal basis is precisely the sort of thing we'd do to illustrate entanglement. Each polarized photon of an entangled pair will be measured to have an apparently random direction, yet their directions will always be opposite each other if they are both measured in the same basis, regardless of what basis that is, and even if that basis is chosen after the photons were actually created.
(Score: 2) by Rupert Pupnick on Tuesday July 09 2019, @04:16PM (1 child)
So the fundamental limitation of Quantum Teleportation [tm], is that you don’t know what you’re sending, only that it’s the opposite, in some sense, of the piece that remains at the transmitter once either side has collapsed the wave function (e.g. measured at the receive side)?
Is this useful?
(Score: 2) by maxwell demon on Saturday July 13 2019, @06:19AM
No, quantum teleportation is a specific protocol that allows to send arbitrary given quantum states by sending classical information (which before was not thought to be possible because you cannot completely measure the quantum state). It uses quantum entanglement, but consists of more than just measuring the parts of the known shared entangled state (in particular, it obviously involves the original quantum state to be sent, and most importantly, it involves actually sending information).
The Tao of math: The numbers you can count are not the real numbers.