Quantum Physics


How Big Can Entanglement Get?

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.

Image: Flickr user Domiriel, adapted under a Creative Commons license.

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.

Entanglement 101

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.

Go Deeper
Author’s picks for further reading

About.com: EPR Paradox
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.

Wired: Have We Been Interpreting Quantum Mechanics Wrong This Whole Time?
Natalie Wolchover on new experiments that may reveal the true meaning of quantum mechanics.

Tell us what you think on Twitter, Facebook, or email.

Andrew Zimmerman Jones

    Andrew Zimmerman Jones is a known member of organizations such as American Mensa and the National Association of Science Writers. Having earned a degree in physics from Wabash College and a master's degree in Mathematics Education from Purdue University, he has gone on to such disreputable activities as becoming the Physics Expert at About.com Physics and co-authoring String Theory For Dummies, and occasionally publishing works of philosophy, reviews of board games, and other leisurely activities. You can follow him on Twitter, Facebook, or Google+.

    • Rich

      Beam me up, Scotty. Just a few trillion more entanglements and the teleporter is here. Also instant communication over Any distance.

    • Richard

      Here are some things that may help enhance understanding for your scientists.

      • dave geiger


        Will you please send a copy of all six views that you posted to admin@iwyfy.com?

        I would also ask for a corrosponding email so that i might discuss your papers with you.


    • Richard

      Now for the good stuff!

    • Richard

      When this material has been understood I will say more.

    • Richard

      In this information, look beyond the obvious.

    • Hal Fisher

      they are going to come from other dimensions to tell us to knock it off.

    • kart_125cc

      I disagree with the analogy of entanglement. A better one would that conveys the conundrum and what the outcome is would be more like this:

      Suppose you give a hat to Peter and a hat to Macy. Into each hat, you put both a green piece of paper and a red piece of paper. You send them in opposite directions. If Peter reaches into the hat and randomly pulls out one of the pieces of paper, suppose it is red, you would then know instantly that when Macy reaches in to randomly pulls out a piece of paper from her hat, it will also be red and not green.

      The error in the analogy in the article is that by putting only one piece of paper in the envelope, the state is already predetermined. The reality is that both states exist and it is not until observed (pulling a paper out of the hat) that the state is determined. The state is determined at the time the paper is pulled out of the hat -by the act of pulling the paper out of the hat, not at the time the paper is put into the envelope as the analogy in the article would imply.

      The analogy in the article fails to convey the “freaky” nature of entanglement. When one paper is put in one envelope and the other in the other envelope, there is nothing “freaky” about knowing what color is in the second envelope. But my analogy conveys the “freaky” aspect that you don’t (normally) know which paper will be puled out of either hat because either paper could be pulled out – yet how is it possible that randomly pulling one color in the first hat determines which color will be pulled out of the second hat? When the natural expectation would be that since both colors are also in the second hat, presumably either could equally be choose in that hat also.

      I think that part of the problem with the analogy in the article is that the writer is sort of conflating it with the Schrodinger’s cat concept (incorrectly, I might add).

      • Prakash Acharya

        Thank you for the clarification.

      • OWilson

        It’s a clumsy analogy, I’ll admit.
        The “information” that was supposedly transmitted instantaneously, was nothing of the kind. As you say it was predetermined, and certainly not instantaneous.
        Remember the protagonists Peter and Macy took a certain amount of time to manually carry take that “information” in opposite directions.
        As an aside, the quantum state of a random piece of paper does not have to “collapse” into red or green. It could just as easily collapse into purple.
        An example of proven mathematical theory, versus reality.
        Sort of like “The Big Bang”.

    • Mike Guy

      Well written – this is the true mechanics of how the Universe works – Each and every one of us has a Quantum Computer – it’s called our brain – and it has the power to impose our consciousness or reality onto the world – it can entangle and decohere our reality and evaluate the symmetry of all the consciousness of all life – where science has a habit of excluding the conscious experience of reality from the equation, it will always fail to fine tune its accuracy…

    • dave geiger

      To andrew,

      Your paper is well written, but our brains are already both a quantum computer and room temperature superconductor. Would not a better use of resources be to raise our own consciousness to allow ourselves as a species to deal with our own entanglements in our own species first?

    • I’m just an engineer, but I know BS when I see it
      Nature is not complicated
      our problem is that we are 100% on the wrong path
      trying to use the Big Bang to explain the universe
      when magic is needed to explain the Big Bang

      Same with the other side – the smallest of small
      we are 100% on the wrong path there too

      It’s got to be so simple that a 5th grader can understand how the universe works
      if not, it’s wrong…..

    • Kenneth Epps

      In my profession, disaster recovery, entangled storage would present a great leap forward. If the platters of spinning disk could be entangled, I know that’s a huge if, offsite replication would no longer need a fat pipe that costs thousands of dollars a month. It’s fascinating stuff, and impressive the scale that has been achieved.

    • David Eddy

      It is essential to untangle the mysteries of what are the non-physical concepts of mind and the actual processes of atom level of physical reality. Inductive and deductive mind functions are essential to our understanding of what happens at any level of physical reality. Knowing what the atoms are doing at the atomic level is
      essential to the understanding of what is happening at the human level of
      reality. What happens at the atomic level of reality, for example when there is an atomic explosion that can destroys things at the human level of physical reality, needs to be known.
      At the human level of reality we are also experiencing the entanglement of each other as we live together on earth. We are all affected by the reality of interdependence.

      It is essential that we use our God given intelligence to avoid the destruction of earth as a place for people to exist.

    • Anonymous

      Entanglement is a ‘relationship’ it seems, but how is that relationship created?

      When I was a boy I placed the head of a screwdriver on a relatively strong magnet for a week or two and the head became magnetized. This memory is what comes to mind when, as a layperson, I hear entanglement described.

      How does one create entangled particles? Could someone describe the relationship between them? Is it only binary…meaning, of all the characteristics a particle can have, are characteristics with only an ‘either/or’ (1 or 0) value ‘entagled’? What about those characteristics with three or more potential states? Or is it just one characteristic, and which is it? Are ‘entangled’ relationships somehow ascribed by the observer?

    • McCann

      Superposition simply implies that conscious beings always have choices, and that entanglement (multiple states) is/are an illusion. For alive and conscious beings, complete decoherence never occurs, although regarding finite and defined events (observable reality), everything that has ever been observed or can be observed has already decohered. The moment is seemingly faster than the speed of light, because it is already here.