PARTICLES on opposite ends of the universe can link quantum mechanical hands. The phenomenon hints at an entirely new aspect of the quantum reality underlying all matter.
The effect is a sort of inversion of one of the most famous and profound quantum properties, called entanglement. Two entangled particles share a single quantum state: they behave as one and cannot be described individually. Measuring one instantaneously affects the other, no matter how far apart they become, an oddity that prompted Einstein to describe entanglement as 鈥渟pooky action at a distance鈥.
However, for this to happen the particles must have interacted in some way when they came into existence, which may mean only a small fraction of the particles in the universe are entangled at any given time. Cosmic connections make no such demands. 鈥淭hey have no interaction, they have no idea that the other particle even existed,鈥 says Jeff Tollaksen of Chapman University in Orange, California.
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鈥淓ntangled particles have to interact. These cosmic connections make no such demands鈥
The effect is based on work by Yakir Aharonov, also at Chapman University, in the 1960s. He and his colleagues showed that, mathematically speaking, a system鈥檚 properties can be influenced by measurements made in the future. Aharonov has been studying the strange consequences of this 鈥減ost-selection鈥 process ever since.
Now Aharonov, Tollaksen, Sandu Popescu and their colleagues show mathematically that post-selection should link any two particles every time their quantum properties are measured, no matter where they are in the universe. In other words, all particles everywhere could be linked, provided they have been post-selected in some way. 鈥淚s that mind-blowing or is that mind-blowing?鈥 Tollaksen says.
Knowing that such an extraordinary claim would need to be backed up, the researchers devised a thought experiment to demonstrate the simplest case in which a cosmic correlation between two particles would be obvious and testable. The thought experiment, which the team calls the 鈥渜uantum pigeonhole effect鈥, reveals how quantum particles subvert the rules of regular mathematics.
Imagine you have to house three pigeons in two pigeonholes. In the classical world, it鈥檚 obvious that at least two pigeons will have to share.
Swap your pigeons
But for quantum particles, this is not necessarily true. Swap your pigeons for electrons, and send them three at a time into an interferometer. This device splits each electron into two, sends them along both paths simultaneously, then brings them together again.
Then 鈥減ost-select鈥 some of the electrons to influence their past state. You do this by measuring the electrons鈥 states when they exit the interferometer, selecting electrons in a particular state that is significantly different to the one they had when they entered. That should create quantum links between those electrons, Tollaksen says (see diagram).FIG-mg29802301.jpg
To show the links are really there, you need to know what the electrons are doing inside the device. Another quantum effect, called superposition, means that each electron takes both paths at once. If any two of the three electrons share one arm, which must happen classically, their identical electrical charges will repel one another, deflecting their trajectories ever so slightly. That deflection should be detectable when the electrons exit the interferometer.
Because superposition is a delicate state, if you try to measure the path any electron took, it will pick a side and you will find it in one or the other. But if you check the paths of any pair of the three original electrons, the detector will show no deflection has happened. In other words, you can have three pigeons and two boxes, and yet no two pigeons are ever found in the same box. A linkage must exist, because it鈥檚 as if the particles know the other is there, and avoid each other.
鈥淵ou can have three pigeons and two boxes, and yet no two pigeons are ever found in the same box鈥
Furthermore, this works no matter how many pigeons you have. 鈥淵ou can put an infinite number of pigeons in two boxes, and no two pigeons will be in the same box,鈥 says Tollaksen ().
The quantum pigeonhole effect is the simplest way to test the idea that unrelated particles can be correlated simply by being post-selected. You would only need to bring the particles together in the interferometer to test that they are in fact correlated. In theory, these electrons could be spread across the universe, never come anywhere near each other, and mere post-selection would link them up.
Given that this is happening to real particles all the time, either due to human measurements or natural interactions, does this mean that particles everywhere in the universe are correlated? 鈥淚鈥檇 say that the answer to that is yes,鈥 says Tollaksen.
This is a surprising breakthrough, says Paul Davies at Arizona State University in Tempe. 鈥淚t鈥檚 remarkable that it鈥檚 still possible to discover something fundamentally new about quantum mechanics, which has been around for nearly 100 years,鈥 he says. 鈥淗ere we see a richer, more complex set of long-range correlations that nobody knew existed before.鈥
What鈥檚 more, Davies points out that the effect emerges from standard quantum mechanics. 鈥淚t鈥檚 really important to emphasise that this is not somebody鈥檚 alternative to quantum mechanics. Anybody since the 1920s could have discovered what we are talking about now.鈥
While Davies thinks it鈥檚 too early to predict practical implications, Tollaksen thinks they could be far-ranging.
Entanglement eventually spawned the entire field of quantum cryptography and quantum computing. But while entanglement is extremely tricky to work with, since creating entangled particles is very difficult, that鈥檚 not the case with cosmic correlations. 鈥淗ere we have this non-classical resource basically everywhere,鈥 says Tollaksen. 鈥淭hat simple fact could turn out to have unique technological utility.鈥
Scott Aaronson of the Massachusetts Institute of Technology is more reserved. He thinks the idea is interesting and fun, but points out that the effect only works in certain circumstances, which may dampen its universality.
鈥淚t is at least as much a statement about the weirdness of post-selection, as it is about the weirdness of quantum mechanics itself,鈥 he says. 鈥淚f you just have ordinary quantum mechanics without post-selection, then you don鈥檛 get the effect they鈥檙e talking about.鈥 You also have to be careful how you ask about whether any two pigeons share a hole, he says.
Ultimately, someone will have to perform the experiment for real. 鈥淚t would actually be much more shocking and revolutionary if people did the experiments we suggest and they turned out to be false,鈥 says Tollaksen. 鈥淭hen we would have the very first ever experimental proof that quantum mechanics is wrong. But I鈥檓 sure that won鈥檛 happen.鈥
This article appeared in print under the headline 鈥淭he cosmic entanglement鈥