Myth 4: Quantum Computers Communicate Instantaneously
“I cannot seriously believe in it because the theory cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky action at a distance.” — Albert Einstein
Entanglement is surely the most misunderstood concept in quantum physics, often depicted as a kind of mystical connection enabling instant communication across vast distances. This misunderstanding has led to widespread speculation about quantum computers exploiting this phenomenon to achieve instantaneous data transfer. But like the myths before it, this concept of “spooky action at a distance” as a computational resource strays far from the realities of quantum mechanics and the operational principles of quantum computing.
While the quantum data within quantum computers is indeed entangled, we can reframe our understanding such that this should seem inevitable rather than miraculous.
Where did entanglement come from?
In 1935, Albert Einstein and two colleagues, Boris Podolsky and Nathan Rosen, wrote a paper elucidating the conceptual problem quantum physics posed for our classical notions of space and time. Either quantum physics disobeyed Einstein’s theory of relativity, or quantum physics was not a complete theory. Einstein dismissed the former — the infamous “spooky action at a distance” — suggesting that there must be some deeper reality behind the equations of quantum physics. Since the theory didn’t specify what these might be, they came to be called “hidden variables.”
After a New York Times headline read “Einstein Attacks Quantum Theory,” attention was drawn to the feature Einstein, Podolsky, and Rosen identified. Erwin Schrödinger was the first to name it and its own English translation, calling it entanglement. Of it, he said entanglement is “the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” Then, he left physics to become an Irish biologist. Besides some wordy and pedantic public debates between Einstein and Niels Bohr, not much more was said about entanglement for decades. War and the shift from science to engineering in quantum physics produced the “shut up and calculate” generation, which frowned upon what they saw as fruitless philosophical matters. Of course, there are always a brave few.
One of the brave souls was John S. Bell. In 1964, he proposed an experiment that could rule out exactly the kind of hidden variables that the now-late Einstein hoped for. His proposed experiment — and many refinements that came later — are known as Bell experiments. The quantities these experiments measure are compared to Bell inequalities. Entanglement is necessary to violate the inequalities, which in turn rule out local hidden variables. John Clauser and his student, Stuart Freedman, performed the first real Bell experiment eight years later. This could have ended the debate, but the hope for hidden variables was strong. People began looking for so-called “loopholes” that might leave room for Einstein’s desires. Alain Aspect and Anton Zeilinger followed with their own experiments to close the loopholes a decade later. In doing so, they paved the way for extremely precise control of entangled quantum states, which ushered in a new era of quantum technology.
Explainer-level nonsense
The gist of any quantum entanglement story is that it arises when particles interact and create a “link,” which we call entanglement. Importantly, entanglement remains no matter how far apart they might be. The state of each individual particle is not well defined, but their joint (entangled) state is. Thus, the two particles must be considered as a single entity spread across a potentially vast distance. If you believe that story, I agree it’s mystical, just as the internet told you.
The mechanics of what is going on when actually creating entangled states of quantum things seem to corroborate this story, though. We’ve been creating entangled states of photons, for example, this way for decades. A single high-energy photon enters a special transparent material that converts it into two lower-energy photons that fly off in different directions. Because of an ambiguity in which photon has which property, quantum physics says they are entangled. Numerous experiments have verified the predictions of measuring the two entangled photons, with the latest separations being tens of kilometers. In fact, each new experiment has accompanying press releases staking their claim on the current quantum entanglement distance record.
The fact that physical properties, like polarization in the case of photons, are correlated in these experiments makes it feel like a real link has been created. But that’s not the only way to create entanglement.
Cutting the link
Quantum mechanics makes accurate predictions in the context of entangled systems. It doesn’t actually contain or suggest the model of entanglement as a physical connection between two distant objects, though. That model is wrong. To see why, consider that entanglement can be created between two particles without ever having them interact. They can be so far away from each other that not even light signals can reach one another, meaning nothing physical could have mediated the “link.”
First, imagine two atoms separated by a large distance and both in an excited state. When either atom decays, it releases a photon that is detected at a central station midway between the two distant atoms. Since the detector cannot distinguish which of the two atoms decayed, the state of the pair becomes correlated. Quantum mechanics dictates they must be entangled, but it says nothing about a physical link.
The atoms never interacted or exchanged any information. In fact, they couldn’t have — the only signal that something had happened made it halfway between them when the entanglement was created. Are we to believe a physical link of double that size was immediately generated? I hope not. So, what’s going on then?
Classical entanglement
Imagine if, instead of atoms, there were two distant boxes, each with a ball in it. The ball might be removed and sent to you from either box. At some median location, you receive a ball in the post. Immediately, the boxes become correlated — one is empty, and the other is not — because you don’t know which box the ball came from. While the boxes, still separated by a great distance, instantly formed this connection, it is simply your ignorance and future expectations about what might be revealed that define the correlation. There is certainly no mystical “link” that physically manifested between the boxes the moment the post arrived, and the same is true for atoms. The point here is that, like correlated bits, entanglement is simply correlated qubits.
Now, of course, there must be some difference between the very classical “balls in boxes” situation and the quantum atoms. In the classical world, correlated events cause one another or can be traced back to a common cause that could have determined the outcome. An infamous example is the fact that cities with more police have more crime. Neither causes the other, though. The confounding factor is city size — bigger cities have both more police and more criminals simply because they have more people. There is always something that explains correlations in classical information. In the case of the balls, in principle, someone could know the whole situation — which box was empty and where each ball was. That’s just not possible with atoms and photons.
When we attempt to “explain” quantum correlations, we naively and unavoidably constrain ourselves to stories phrased in classical information. These are essentially hidden variables, which we ought to know won’t do. You can explain quantum entanglement, but it must be phrased in quantum information. Demanding a classical explanation of entanglement is like demanding the behavior of rabbits be explained in terms of apples.
Technobabble
The point of mathematics is simplifying things that would require otherwise long-winded and complicated sentences. So, we replace the things we are talking about with symbols and numbers. If classical bits are unknown, we write them as a list of probabilities (p1, p2, p3, …). In the case of the two boxes, our ignorance of the contents of each box is a probabilistic bit, as introduced in the previous chapter. The first box is associated with a pair of probabilities (p1, p2). Again, this is just a way more succinct way than writing (the probability that this box has a ball, the probability that this box has no ball). Between the two boxes, there are four possible situations, which would have a list of numbers like (q1, q2, q3, q4).
Now, here’s the important point: if the list of four probabilities for the pair of them can’t be equally described as two separate lists of two numbers for each of them, then the information they share must be correlated. Mathematically, you can take this as the definition of correlation. For example, (0, 0.5, 0.5, 0) represents the situation when one ball is received at the central location. There is zero chance both are empty and zero chance both have a ball. We are certain one is empty, and one has a ball — we just don’t know which is which. Since we don’t know what box it came from, either each box is empty or contains a ball with 0.5 probability. Each box alone has the same probability pair (0.5, 0.5), but these individual lists don’t capture the complete situation — a bigger list is always needed to capture the correlations.
You now know what regular old classical correlation is. Luckily, entanglement is not much different. Let’s recall the definition of a qubits. Instead of two positive numbers that add up to one, a qubit is represented by two numbers (which could be negative) that add up to one after you square them. For example, (0.6, −0.8) represents a qubit. Clearly, these are neither positive nor do they add to one. But if you square each of them, you get (0.36, 0.64), which adds to one. When you square the numbers in the qubit list, it tells you the probability of each possible answer to the question the qubit represents. If one atom is described by a qubit (0.6, −0.8), then we would find it excited with a probability of 0.36 and decayed with a probability of 0.64.
For the pair of atoms, the two-qubit state has four numbers. For example, (0, 0.6, −0.8, 0) tells us that only one atom will be found in the excited state, but with unequal probabilities. If the list representing the pair of atoms can’t be equally represented by two smaller lists for each atom individually, they are correlated. But since they are qubits instead of bits, we give such a list a new name: entanglement. That’s it. In quantum information, entanglement is correlated qubits.
Interference
Lists of probabilities change by multiplying and adding up the individual numbers to create new ones. As pointed out in the last chapter, multiplying or adding positive numbers can only produce more positive numbers. Whereas, with qubits, the list can change in drastically different ways because adding negative numbers to positive numbers can lead to cancellation. Borrowing terminology from wave mechanics, this is often referred to as interference, where two waves cancel when the crest of one meets the trough of another.
Quantum computers perform calculations in far fewer steps than classical computers by using interference — choreographing the cancellation of unwanted numbers in qubits of information. While the computer doesn’t use entanglement as some physical fuel, we can show that without it, the computations it performs can be easily simulated with classical digital computers. That is, a quantum computer, made of atoms and photons, for example, that never realizes entanglement is no less “quantum” than anything else but also no more powerful than a digital computer. In some sense, though, this is not surprising. After all, a digital computer that never produces correlated bits would be extremely useless — no more powerful than flipping a bunch of fair coins.
Beam me up
Correlated qubits are necessary for quantum computation and can be seen as a resource for primitive information-processing tasks. The whimsical names of these tasks don’t help our myth-busting endeavor, however.
Take, for example, quantum teleportation. Entanglement is necessary to “teleport” quantum information between locations using only classical information. While this doesn’t mean teleportation in the sci-fi sense of instantly transporting matter, it does involve the transfer of quantum information in a way that’s not possible without quantum entanglement. When thinking about entanglement as a physical connection, things like teleportation do indeed sound like science fiction. However, quantum teleportation is just shifting the location where information is stored in an efficient way. If I were narrating the protocol, I might say the following.
“Two qubits are correlated. I take one and compare it to a third. Now, two of the three qubits have been read. Having learned something about that comparison, I manipulate the unread qubit so that it’s described in the same state as the third one before reading it.”
It’s not that you are meant to follow the logic there — the protocol itself is not trivial. However, compare this to an explanation from Popular Mechanics (A. Thompson, March 16, 2017).
“If we take two particles, entangle them, and send one to the moon, then we can use that property of entanglement to teleport something between them. If we have an object we want to teleport, all we have to do is include that object in the entanglement… After that, it’s just a matter of making an observation of the object you want to teleport, which sends that information to the other entangled particle on the moon. Just like that, your object is teleported, assuming you have enough raw material on the other side.”
Thinking about quantum information as physically corresponding to classical objects quickly descends into magical thinking. Not only does it not help explain the concepts, but it further mystifies quantum physics and gives it the illusion that supernatural forces are at play.
Just correlations
Most of what you hear and read about quantum entanglement is the shooting down of attempts to force it into a classical worldview, but framed with headlines like “quantum physicists just proved nature is spooky.” Technically, we call the results no-go theorems because they rule out theories that would restore classical objectivity to quantum physics. Classical objectivity is comforting because it provides a reliable and persistent model of the world. It allows us to predict and control our environment with remarkable ease as we cobble together rigid objects to act as simple machines that extend our natural abilities and more complicated ones that have enabled a mostly cooperative global technological society.
We found and exploited regular patterns in the world. But the comfort of objectivity is an illusion — an illusion that has made “physics” synonymous with “objective reality.” This is not a problem for classical information since there is a perfect correspondence between bits and easily recognizable binary alternatives in the objective world we have created. But that’s a very narrow view influenced by the success of classical physics and engineering. Quantum physics, with things like entanglement, throws a wrench into this neat classical picture of the world.
Yes, entanglement challenges our deepest desires for a universe of rigid cause-and-effect relationships. However, as noted when debunking Myth 1, unless you “speak” quantum information natively, you won’t “understand” entanglement in the mechanical way you understand most other things. But, from a higher vantage point, you can appreciate it. Classical correlations are relationships between bits of information. The easiest high-level description of entanglement is the correlation between quantum bits (qubits) of information. In the same way that correlated bits are ubiquitous and inevitable, so is entanglement. Quantum bits belong to a theory of information where correlation is the norm, not the exception. Sure, to encode entangled qubits faithfully into the world might be an engineering challenge, but the concept is ultimately substrate-independent, living in the abstract realm of information and algorithms — no mysterious links, instantaneous communication, or spooky actions.
This was part of the following book, and it is free here on Medium!
Physical copies are available on Amazon: What You Shouldn’t Know About Quantum Computers.
