Our intuition has evolved to deal with the macroscopic world: the world of things you can hold in your hand and see with your naked eyes. But many of the discoveries of the last century, particularly those in quantum physics, have called into question virtually all of those physical intuitions. Even Albert Einstein, whose intuitions were often spot-on, couldn’t bridge the gap between his intuition and the predictions of quantum theory, particularly when it came to the notion of quantum entanglement. Yet we’ve been able to make some peace with quantum mechanics because, for most intents and purposes, its strangest effects are only felt on the micro scale. For everyday interactions with ordinary objects, our intuition still works just fine.
Now, though, physicists are entangling bigger and bigger objects—not just single particles but collections of thousands of atoms. This seemingly-esoteric research could have real technological implications, potentially doubling the accuracy of atomic clocks used in applications such as GPS. But it also challenges the artificial barrier we’ve set up between the microscopic scale, where quantum mechanics rules, and the macroscopic world, where we can count on our intuition. Quantum weirdness is going big.
What is entanglement, anyway, and why did it get Einstein tied in knots? For a mundane analogy, image you put a red piece of paper in one opaque envelope and a green piece of paper in an identical envelope. Now, randomly hand an envelope to each of two kids, Peter and Macy, and have them walk in opposite directions. There is no way to know which kid has which color, but you can say with certainty that one of the following two “states” describes the situation.
State 1: Peter has the red paper and Macy has the green paper
State 2: Peter has the green paper and Macy has the red paper
Since the state of each piece of paper is absolutely tied into the state of the other one, the red paper and the green paper are entangled with each other. If Peter looks into his envelope and finds the red paper, then you instantly know that Macy has the green paper, because you must be in State 1. The papers represent an entangled system, because they can’t be fully described independently of each other. If you describe Peter’s paper as “either red or green” and Macy’s paper as “either green or red,” but don’t connect their two situations together, then you have an incomplete description of the system.
At this point, you’re likely thinking: “So what?” And rightly so. As with most things in the universe, this entanglement gets a lot stranger when you stick the word “quantum” in front of it. In the mundane example, the entanglement came about purely because of our ignorance. We didn’t know for sure which envelope each paper was in, but we were certain that they were really in those envelopes.
Quantum mechanics, however, does not seem to work if you try to hold this level of certainty. So let’s try the same scenario, but instead of regular paper, imagine that we are instead using some “quantum paper” that (though not real) obeys the traditional rules of quantum mechanics. In such a quantum system, Peter’s unseen quantum paper exists in a bizarre state where it is both red and green at the same time. Macy’s quantum paper is similarly in such an undetermined state. This isn’t to say that the paper is a color that is a mix of red and green, but rather that each piece of paper exists in a superposition of states where it is both “a red paper” and “a green paper,” even though it is not in a state that makes it “a red and green paper.”
That is, of course, until someone actually looks in the envelope to determine the state (called “collapsing the wavefunction” in quantum terminology). If Peter looks in his new quantum envelope and sees a green paper, quantum physics would say that his paper has collapsed into the “green” state. But remember that his paper is entangled with Macy’s paper, so when his collapses into the “green” state the whole entangled system collapses into State 2.
If you’re thinking that something sounds fishy here, you’re in good company, since that’s exactly what Albert Einstein thought when he and colleagues came up with this challenge to quantum mechanics. (Their version of this Einstein-Podolsky-Rosen paradox, or EPR paradox, involved decaying particles rather than hypothetical quantum paper.) The idea that, by looking at his quantum paper, Peter could have any effect on Macy’s quantum paper struck Einstein as bizarre, and he ultimately dubbed it “spooky action at a distance” because it seemed to violate the rule that nothing could communicate faster than the speed of light.
Spooky or not, a century of physics research has shown that this does appear to be what happens. At the moment Peter observes the color of his quantum paper, Macy’s quantum paper ceases to be both red and green and instantly becomes definitely one or the other. Because the two pieces of quantum paper are entangled, this would be true no matter where in the universe Macy went with her paper.
What Can Entangle?
Of the many deep and profound questions related to quantum entanglement, the size of the entangled system is one that has always been of interest to physicists. The original EPR paradox only described pairs of individual particles, not pieces of paper or even molecules. So, what happens when you try to scale entanglement up to bigger objects? Maintaining a superposition of states is a very delicate operation. Most particle interactions cause the superposition of states to collapse into a single state, a process called “decoherence.” Even a stray light particle, a photon, could knock the whole entangled system out of its superposition state and into a single definitive state. This is why we don’t experience this quantum behavior in our everyday life, because pretty much everything we experience has already undergone decoherence.
Or has it? One worldview, called the “many worlds interpretation” of quantum physics, takes the superposition of states as seriously as possible. It suggests that decoherence never actually happens, that the array of possible states never collapses into one single state. Each possible state is “real,” though they don’t all manifest themselves in the reality that we are experience. We experience merely one limb on a branching tree of possibilities. If Macy looks in her envelope first and finds a green paper (State 1), there exists another branch where she finds a red one (State 2), and because her paper is entangled with his, Peter will always find his paper in the corresponding state.
While many physicists find the many-worlds interpretation an intriguing prospect, it doesn’t actually solve the question of how big we can make a system that exhibits this bizarre superposition behavior in a way that is perceptible to us. Some things seem to be in a superposition and some things don’t, even if the many worlds interpretation applies. How far can we push that boundary in our experiments? Is it possible for non-microscopic objects to demonstrate quantum behaviors?
Creating entangled systems has always been tricky. Though the EPR paradox was proposed in 1935, it wasn’t until the early 1980s that scientists were able to actually test it with a real physical system. Entangling more than a handful of particles was incredibly difficult, but technology gradually improved. In 2005, when researchers created an entanglement among six atoms, it was considered a major breakthrough.
Because of the delicate nature of quantum systems, it is key to keep the entanglement safe from random motion of the particles, which can cause a collapse. This has traditionally involved cooling the atoms to limit motion, but in recent years scientists have even been able to entangle objects at room temperature. In a 2011 paper, physicists described an experiment where two tiny diamonds released vibrational energy in an entangled system. The fact that these larger systems can display properties of entanglement has highlighted the challenge in drawing clear lines between the “quantum” and “non-quantum” worlds.
Only a decade after six-atom entangled systems were considered cutting-edge, the number to beat was 100 atoms entangled together. That record seems to have now been blown out of the water, as a March 2015 paper in the journal Nature indicated a record of 3,000 cooled atoms entangled together, with the researchers stating with confidence that they thought they could scale their process up to millions of atoms.
More significantly, being able to create complex, stable entangled systems is an essential component in the development of quantum computers. First proposed by Nobel laureate Richard Feynman in the 1980s, quantum computers would exploit the bizarre behavior of quantum superposition to perform calculations exponentially more quickly than classical computers. It would represent an astounding revolution in information technology, if the technical hurdles can be overcome to make it a reality.
When all is said and done, one thing seems clear: there is more to quantum reality than was dreamt of in even Einstein’s philosophy.
Author’s picks for further reading
A detailed looked at this famous paradox and its implications.
The Nature of Reality:
Can Quantum Computing Reveal the True Meaning of Quantum Mechanics?
Scott Aaronson on quantum computing and its potential to answer deep unsolved questions about quantum theory.
Have We Been Interpreting Quantum Mechanics Wrong This Whole Time?
Natalie Wolchover on new experiments that may reveal the true meaning of quantum mechanics.