What Is Quantum Entanglement?
In quantum physics, the entanglement of particles describes a relationship between their fundamental properties that can't have happened by chance. This could refer to states such as their momentum, position, or polarisation.
Knowing something about one of these characteristics for one particle tells you something about the same characteristic for the other.
Think of a pair of gloves. If you found a right glove alone in your drawer, you can be certain the missing glove would fit your left hand. The two gloves could be described as entangled, as knowing something about one would tell you something important about the other that isn't a random feature.
In fashion, this concept isn't all that strange. But the concept poses a problem for quantum mechanics.
Does quantum entanglement work with 'reality'?
The physicists Niels Bohr and Werner Heisenberg argued an object's state only truly existed once it became associated with a measurement, which meant somebody needed to observe it experimentally. Until then, its nature was merely a possibility.
To other physicists, such as the famous Albert Einstein and Erwin Schrödinger, this was as preposterous as saying a cat inside a box is neither alive nor dead until you look.
Finally two physicists Boris Podolsky and Nathan Rosen collaborated with Einstein to come up with a thought experiment, where two objects interact in some way.
By measuring one of them, we might be able to work out some of its partner details without needing to measure it directly, thanks to its 'entangled' history.
"Spooky action at a distance"
In response to this dilemma (now called the EPR or Einstein-Podolsky-Rosen paradox) Bohr suggested that the state of both objects simply became 'real' at the same time, as if they instantly swapped details on this experimental intrusion across a distance.
Einstein dismissed this idea as a 'spooky action', claiming on multiple occasions that "God does not play dice".
Decades later, Bohr's ideas still stand strong, and the strange nature of quantum entanglement is a solid part of modern physics. Physics really is fundamentally 'spooky' after all.
Action at a distance
Most of the time, the world seems — if not precisely orderly — then at least governed by fixed rules. At the macroscale, cause-and-effect rules the behavior of the universe, time always marches forward and objects in the universe have objective, measurable properties.
But zoom in enough, and those common-sense notions seem to evaporate. At the subatomic scale, particles can become entangled, meaning their fates are bizarrely linked. For instance, if two photons are sent from a laser through a crystal, after they fly off in separate directions, their spin will be linked the moment one of the particles is measured. Several studies have now confirmed that, no matter how far apart entangled particles are, how fast one particle is measured, or how many times particles are measured, their states become inextricably linked once they are measured.
For nearly a century, physicists have tried to understand what this means about the universe. The dominant interpretation was that entangled particles have no fixed position or orientation until they are measured. Instead, both particles travel as the sum of the probability of all their potential positions, and both only "choose" one state at the moment of measurement. This behavior seems to defy notions of Einstein's theory of special relativity, which argues that no information can be transmitted faster than the speed of light. It was so frustrating to Einstein that he famously called it "spooky action at a distance."
To get around this notion, in 1935, Einstein and colleagues Boris Podolsky and Nathan Rosen laid out a paradox that could test the alternate hypothesis that some hidden variable affected the fate of both objects as they traveled. If the hidden variable model were true, that would mean "there's some description of reality which is objective," Ringbauer told Live Science.
Then in 1964, Irish physicist John Stewart Bell came up with a mathematical expression, now known as Bell's Inequality, that could experimentally prove Einstein wrong by proving the act of measuring a particle affects its state.
In hundreds of tests since, Einstein's basic explanation for entanglement has failed: Hidden variables can't seem to explain the correlations between entangled particles.
But there was still some wiggle room: Bell's Inequality didn't address the situation in which two entangled photons travel faster than light.
A little wiggle left
In the new study, however, Ringbauer and his colleagues took a little bit more of that wiggle room away. In a combination of experiments and theoretical calculations, they show that even if a hidden variable were to travel from entangled photon "A" to entangled photon "B" instantaneously, that would not explain the correlations found between the two particles.
The findings may bolster the traditional interpretation of quantum mechanics, but that leaves physicists with other headaches, Ringbauer said. For one, it lays waste to our conventional notions of cause and effect, he said.
For another, it means that measurements and observations are subjective, Ognyan Oreshkov, a theoretical physicist at the Free University of Brussels in Belgium, told Live Science.
If the state of a particle depends on being measured or observed, then who or what is the observer when, for instance, subatomic particles in a distant supernova interact? What is the measurement? Who is "inside" the entangled system and who is on the outside observing it? Depending on how the system is defined, for instance, to include more and more objects and things, the "state" of any given particle may then be different, Ringbauer said.
"You can always draw a bigger box," Ringbauer said.
Still, realists should take heart. The new findings are not a complete death knell for faster-than-light interpretations of entanglement, said Oreshkov, who was not involved in the current study.
The new study "rules out only one specific model where the influence goes from the outcome of one measurement to the outcome of the other measurement," Oreshkov said. In other words, that photon A is talking to photon B at faster-than-light speeds.
Another possibility, however, is that the influence starts earlier, with the correlation in states somehow going from the point at which the photons became entangled (or at some point earlier in the experiment) to the measured photons at the end of the experiment, Oreshkov added. That, however, wasn't tested in the current research, he said.
Most physicists who were holding out for a nonlocal interpretation, meaning one not constrained by the speed of light, believe this latter scenario is more likely, said Jacques Pienaar, a physicist who was recently at the University of Vienna in Austria.
"There won't be anybody reading this paper saying, 'Oh, my God, I've been wrong my whole life,'" Pienaar, who was not involved in the current study, told Live Science. "Everybody is going to find it maybe surprising but not challenging, they'll very easily incorporate it into their theories."
Beyond Bell's Inequality
The new study suggests it may be time to retire Bell's Inequality, Pienaar said.
"I think that people are too focused on, too obsessed with Bell Inequalities," Pienaar said. "I think it's an idea which was really amazing and changed the whole field, but it's run its course."
Instead, a tangential idea laid out in the paper may be more intriguing – the development of a definition of causality on the quantum scale, he said.
If people focus on cracking quantum entanglement from these new perspectives, "I think lots of cool discoveries could be made," Pienaar said.
How do scientists explain quantum entanglement?
In the video below, Caltech faculty members take a stab at explaining entanglement. Featured: Rana Adhikari, professor of physics; Xie Chen, professor of theoretical physics; Manuel Endres, professor of physics and Rosenberg Scholar; and John Preskill, Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter.
Unbreakable Correlation
When researchers study entanglement, they often use a special kind of crystal to generate two entangled particles from one. The entangled particles are then sent off to different locations. For this example, let's say the researchers want to measure the direction the particles are spinning, which can be either up or down along a given axis. Before the particles are measured, each will be in a state of superposition, or both "spin up" and "spin down" at the same time.
If the researcher measures the direction of one particle's spin and then repeats the measurement on its distant, entangled partner, that researcher will always find that the pair are correlated: if one particle's spin is up, the other's will be down (the spins may instead both be up or both be down, depending on how the experiment is designed, but there will always be a correlation). Returning to our dancer metaphor, this would be like observing one dancer and finding them in a pirouette, and then automatically knowing the other dancer must also be performing a pirouette. The beauty of entanglement is that just knowing the state of one particle automatically tells you something about its companion, even when they are far apart.
Are particles really connected across space?
But are the particles really somehow tethered to each other across space, or is something else going on? Some scientists, including Albert Einstein in the 1930s, pointed out that the entangled particles might have always been spin up or spin down, but that this information was hidden from us until the measurements were made. Such "local hidden variable theories" argued against the mind-boggling aspect of entanglement, instead proposing that something more mundane, yet unseen, is going on.
Thanks to theoretical work by John Stewart Bell in the 1960s, and experimental work done by Caltech alumnus John Clauser (BS '64) and others beginning in the 1970s, scientists have ruled out these local hidden-variable theories. A key to the researchers' success involved observing entangled particles from different angles. In the experiment mentioned above, this means that a researcher would measure their first particle as spin up, but then use a different viewing angle (or a different spin axis direction) to measure the second particle. Rather than the two particles matching up as before, the second particle would have gone back into a state of superposition and, once observed, could be either spin up or down. The choice of the viewing angle changed the outcome of the experiment, which means that there cannot be any hidden information buried inside a particle that determines its spin before it is observed. The dance of entanglement materializes not from any one particle but from the connections between them.
Relativity Remains Intact
A common misconception about entanglement is that the particles are communicating with each other faster than the speed of light, which would go against Einstein's special theory of relativity. Experiments have shown that this is not true, nor can quantum physics be used to send faster-than-light communications. Though scientists still debate how the seemingly bizarre phenomenon of entanglement arises, they know it is a real principle that passes test after test. In fact, while Einstein famously described entanglement as "spooky action at a distance," today's quantum scientists say there is nothing spooky about it.
"It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case," says Thomas Vidick, a professor of computing and mathematical sciences at Caltech. "There can be correlation without communication," and the particles "can be thought of as one object."
Networks of Entanglement
Entanglement can also occur among hundreds, millions, and even more particles. The phenomenon is thought to take place throughout nature, among the atoms and molecules in living species and within metals and other materials. When hundreds of particles become entangled, they still act as one unified object. Like a flock of birds, the particles become a whole entity unto itself without being in direct contact with one another. Caltech scientists focus on the study of these so-called many-body entangled systems, both to understand the fundamental physics and to create and develop new quantum technologies. As John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter, says, "We are making investments in and betting on entanglement being one of the most important themes of 21st-century science."
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