May 30th, 2013 by Michael Ricciardi
Entanglement of Photon Pairs From Two Different Times
In the bizarro sub-atomic realm of quantum physics, a particle can occupy two different states at the same time (a state known as superposition), and, two particles (like two particles of light, or photons) can become entangled — a curious, coupled state in which an action (like a measurement) upon one particle instantly causes a correlated change in the other.
For example, a photon can be spin-polarized vertically or horizontally. If one were to measure the spin of one of the entangle pair of photons, and found it to have a vertical spin, then its entangled twin will always have a horizontal spin, and vice versa. This holds true even if the two entangled particles have been separated by a great distance (like opposite ends of the universe, in theory; see Bell’s Theorem).
This might seem to violate Special Relativity (and the speed of light constant) but, quantum physicists assure us, it does not (but Einstein called it “spooky action at a distance” nonetheless).
One other thing to know: before these measurements take place, the actual spin states of either of the two particles are uncertain/unknowable.
This quantum entanglement effect has been demonstrated many times in laboratory experiments, and, in a few ‘real world’ demonstrations of entanglement with quantum cryptography experiments (in which the entangled state was maintained for a couple of hundred miles distance, before decaying).
But all these experiments have used entangled particles that were entangled at the same time — in the same time. This would seem to be obvious; how could one possibly entangle two particles from different times?
Well, as with all things quantum, bizarre is the norm. Recently, particle physicists in Israel have demonstrated the entanglement of two photos that “don’t exist at the same time”. Time-separated entanglement is predicted by standard quantum theory, but has until now never been successfully demonstrated.
Recall that superposition allows a photon to be in two states at once, but once measured, the uncertainty of this superposed waveform (of the particle) “collapses” into one or the other state (vertically or horizontally polarized). The uncertainty also applies to two-particle systems; each photon in a two photon system can be forced into an uncertain, both-at-once state, but also remain correlated (entangled) despite the uncertain state of either. And, again, once you try to determine the spin state of one photon, its superposed state collapses, and, the other particle’s state collapses.
We might assume this to be true for even for two entangled pairs of photons (separated by any arbitrary distance)…but, what about two pairs of photons separated not just in space, but in time?
That is exactly what Eli Megidish, Hagai Eisenberg, and colleagues at the Hebrew University of Jerusalem, set out to prove — and they succeeded, apparently, using a technique called entanglement swapping.
To effect this, the team applied a pulsed laser light to a special crystal to create two entangled pairs of photons, which were designated pair 1 and 2, and pair 3 and 4 (note: the pairs were created separately in time). Now, at the start of the experiment, particles 1 and 4 are not entangled (as they come from different pairs), but, by cleverly manipulating particles 2 and 3 (also from different pairs), they were able to create the “swapped” entangled state.
The trick to doing this is to project a particle into a definite state by ‘projective measurement’ (recall that the act of measurement causes the superposed state to collapse). So, while photons 2 and 3 were initially unentangled, the physicists were able to conduct a projective measurement to determine whether the two pairs were in one of two different entangled states or the other. In doing so, the measurement entangles the photons even as it cause the superposed states to collapse. As an example, if the physicists selected (via projective measurement) only the events in which photons 2 and 3 ended up in the first entangled state, then that measurement consequently entangles photons 1 and 4 (photons from two different pairs created at two different times; see diagram, below).
The effect can be elucidated by visualizing a linkage of four gears (i.e., joining two pairs of gears); by enmeshing the inner most pair of gears, one establishes a link between the outer two.
You can read more about this crazy-sounding experiment in the Wired post ‘Physicists Create Quantum Link Between Photons That Don’t Exist at the Same Time’ by Adrian Cho (ScienceNOW)
[above] In standard entanglement swapping (top), entanglement (blue shading) is transferred to photons 1 and 4 by making a measurement on photons 2 and 3. The new experiment (bottom) shows that the scheme still works even if photon 1 is destroyed before photon 4 is created. (Image: AAAS/Science via Wired.com)
First Ever ‘Direct Observation’ of Electron Orbitals
Mysterious Structure in Collider Data Could be New Particle…But Which?
It could be a second form of the Higgs Boson, or an exotic “superpartner” particle predicted by Super Symmetry Theory (SUSY) like a gluino, wino (that’s ‘wee – no’), photino, or a neutralino…possibly; it also could be a form of recently discovered Majorana fermion (which is its own antimatter particle), or maybe a graviton (the posited but never validated gravity particle)…or, more tantalizingly, an unparticle (which mediates a fifth force of Nature)…or possibly even a chameleon particle (that could hold the key to understanding dark energy and dark matter)…
But whatever it turns out to be, right now, there is no consensus amongst physicists. Instead, physicists are cryptically calling it Y (4140)*. The never-before-seen “structure” was officially acknowledged by a panel of physicists at the April 2013 American Physical Society meeting in Denver, Colorado.
“We don’t know what it is. It could be some sort of exotic new combination of quarks, though not the so-called ‘charmonium’ that involves charm quarks, or something else entirely,” said Kai Yi, physicist at the University of Iowa and one of the panelists [source: see link below].
Readers may recall that a mysterious particle (or signal) was detected by the Tevatron collider at Fermilab shortly before it was officially (and permanently) shuttered in later 2009. Quite unexpected, it was speculated that the mysterious signal (with an energy range of 120 – 160 GeV) could be an “unknown form of matter” whose confirmation might “alter Quantum Theory”.
Since then, two detectors attached to the Large Hadron Collider (LHC) have also caught glimpses of some mysterious structure, or particle…could the two machines have detected the same thing? Stay tuned.
Much of this hunting for exotic particles at higher energies and shorter distances is motivated by what is known as the Hierarchy Problem which describes the large discrepancy between characteristics of the weak nuclear force (that mediates atomic decay) and the gravitational force. For example, it remains a deep puzzle as to why this weak force is 1032 times stronger than the force of gravity.
It should be noted that the LHC — the most powerful ‘atom smasher’ in the world — has to date only run at half it’s power. the massive machine is currently down for maintenance and repairs and won’t be operational again until 2015. So, hopefully, there are more surprises (like maybe confirmation of SUSY, and thus, one form of String Theory) to come from the world’s most powerful “atom smasher.”
But, before we get our new particle hopes up too high, one last note of caution: a recent experiment — called the The Belle Experiment — conducted by an international consortium of more than 400 physicists and engineers, found no evidence confirming the existence of this mysterious structure, or particle.
For more on this mysterious structure, check out the Yahoo News story ‘Atom Smashers Find Something Not So Charm-ing’ by Jesse Emspak
* The Particle Data Group has renamed Y(4140), per naming convention, to X(4140)
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