Quantum entanglement — what Einstein called “spooky action at a distance” — involves a peculiarly strong correlation between two particles (typically, polarized photons) such that measuring a property of one particle simultaneously determines the property of the second particle — even when those two particles are separated by great distances (theoretically, even when the particles are at opposite ends of the universe). In quite recent years, quantum entanglement has even been observed for multi-particle systems.
However, despite these laboratory successes, Einstein’s critique of this entanglement phenomenon (which was never demonstrated during his lifetime) still manages to assert itself; such a correlation, or “communication”, asserted Einstein, could only occur at physically local distances (technically: local relativistic causality, where determining variables are in relative proximity to the measured particles). This causal necessity is called locality, or “local realism”, and is a consequence of Einstein’s “principle of local action”.
Analogously, a pool ball cannot influence another pool ball (analogous to the correlation between two photons) unless it at some point physically (locally) touches the second pool ball — even if through an intermediate source, such as another pool ball (where the kinetic energy is transferred through neighboring pool balls). This makes intuitive, “common” sense. The “spooky action at a distance” described in Quantum Mechanics — in which the behavior of one entangled particle seems to “know” or influence what the other entangled particles is doing — appears to violate this basic, common sense notion. This is one of the main reasons why Einstein initially rejected components of Quantum Theory.*
However, because previous entanglement experiments did not control for variables that might be enabling local causality, “true” quantum entanglement (as proposed in the famous Bell’s theorem) could not be definitively demonstrated, or proven. The previous lack of experimental control creates a loophole, of sorts, in which Einstein’s principle of local action — that two distant objects can not have direct influence on each other — might still be valid (sometimes referred to as the ‘hidden local variables theory’).
Closing the Locality Loophole – Photonic Physics Fun With Alice, Bob, Charlie…and Randy
But quite recently, quantum physicists at the Institute for Quantum Computation (IQC) in Waterloo, Canada, were able to demonstrate 3-particle entanglement with non-locality for the first time. The successful experiment may make possible multi-party communication and other advanced quantum information technologies.
For this newest real-world experiment, the IQC team conceived of a clever plan to exclude the possibility of some hidden variable(s) controlling the entangled state, thus closing the locality loophole. The team had to first generate photon triplets which exist in a quantum state known as a Greenberger-Horne-Zeilinger (GHZ) state (a state that involves three or more particles).
The choice to study photons in the GHZ state has a dual purpose. Paper co-author Professor Kevin Resch, Canada Research Chair in Optical Quantum Technologies, explains:
“Correlations measured from quantum systems can tell us a lot about nature at the most fundamental level. Three-particle entanglement is more complex than that of pairs. We can exploit the complex behaviour to rule out certain descriptions of nature or as a resource for new quantum technologies.” [source: see link, below]
Now, in the classic quantum thought experiment, the locations/recipients of the two entangled particles are traditionally given the proper names “Alice” and “Bob”, and any third party is named “Charlie”. The team uses these classic designations to demark the key players in the entanglement experiment.
Using detectors set up, ironically, in several local spots — several trailers positioned several hundred meters from the IQC lab — the researchers at the lab (Alice) then beamed the entangled photons to these locations but with a time-delay in the transmission of the first photon. The other two photons were beamed (through two telescopes) to two other trailers (Bob, Charlie) separated from each other, and Alice, by approximately 700 meters. And, just to maintain a “space-like” separation between all three particles, they introduced a fourth party (“Randy”), located in a third trailer, that would act as a randomizing agent; Randy would randomly choose a correlation measurement for Alice (the lab team) to conduct on her photon (before it left the lab), without any local influence from either Bob or Charlie. (above right) Photons generated in the lab were beamed to separate trailers in a field on the University of Waterloo campus.
Each detector at each trailer was integrated with a time-tagging device and a random number generator. The random number generators determined how the photon arriving at each detector would be measured, independently of any other measurements. The time-tagging devices made certain that each measurement transpired within an extremely short time frame (just three nanoseconds). This minute time window is deemed to be too quick for any possible communication or information transfer between particles to occur. This impossibility of local communication is crucial for proving quantum nonlocality (and, by way of Bell’s Theorem, nonrealism). Finally, by measuring the strength of the correlations between the three entangled photons (in the GHZ state), the team was able to verify entanglement and thus prove that quantum nonlocality is a real feature of the quantum domain. (right) Trailers parked more than 600m from the researchers lab on the University of Waterloo campus were used to separate three photons, proving the theory of quantum nonlocality.
Regarding the all-around success of the experiment, co-author and Professor Thomas Jennewein observes:
“The idea of entangling three photons has been around for a long time. It took the right people with the right knowledge to come together to make the experiment happen in the short time it did. IQC had the right mix at the right time.”
From High Tech Experiment to Practical Technology – What it All Means for Future Communications
By demonstrating a nonlocal correlation between three distributed and entangled particles, the team provided ‘proof of principle’ that communication beyond conventional “pairwise” communication is possible, and hopes that their results will lead to new “multipartite quantum communication protocols, including Quantum Key Distribution (QKD), third man cryptography and quantum secret sharing.”
Lead author Chris Erven from the University of Bristol elaborates on these possibilities:
“The interesting result is that we now have the ability to do more than paired quantum communication. QKD, so far, has been a pairwise system – meaning that it works best and with less assumptions when you’re only talking with one other person. This is the first experiment where you can now imagine a network of people connected in different ways using the correlations between three or more photons.”
The research team was comprised of students and faculty from the Institute for Quantum Computing and the Department of Physics and Astronomy in the Faculty of Science at the University of Waterloo. The experimental findings were published in Nature Photonics under the title ‘Experimental Three-Particle Quantum Nonlocality under Strict Locality Conditions’.
Some source material (including extended quotes and experiment photos) for this post came from the March 23, 2014 University of Waterloo News release: Experiment opens the door to multi-party quantum communication
* In an essay entitled “Quanten-Mechanik Und Wirklichkeit” (‘Quantum Mechanics and Reality’, 1948, Dialectica), Einstein wrote: “(…) The following idea characterises the relative independence of objects far apart in space, A and B: external influence on A has no direct influence on B; this is known as the Principle of Local Action, which is used consistently only in field theory. If this axiom were to be completely abolished, the idea of the existence of quasienclosed systems, and thereby the postulation of laws which can be checked empirically in the accepted sense, would become impossible. (…)”
And, long before Einstein, Sir Isaac Newton (1642–1727), upon contemplating the idea of action-at-a-distance, proclaimed it “so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it”
Diagram (second image from the top): Artistic rendering of the generation of an entangled pair of photons by spontaneous parametric down-conversion as a laser beam passes through a nonlinear crystal. Inspired by an image in Dance of the Photons by Anton Zeilinger. However, this depiction is from a different angle, to better show the “figure 8” pattern typical of this process, clearly shows that the pump beam continues across the entire image, and better represents that the photons are entangled. Credit: J-Wiki at en.Wikipedia