Nobel ‘22: Explained
The Nobel prize awarded this year to Alain Aspect, John F. Clauser, and Anton Zeilinger finds its roots in the former half of the 20th Century.
The 1930s saw quantum mechanics take the center stage in the physics world. Albert Einstein, Boris Podolsky, and Nathan Rosen were on one side of the field while Niels Bohr and Erwin Schrödinger were on the other. At the heart of this fiery clash lay quantum entanglement.
Einstein, Podolsky, and Rosen believed all aspects of reality should have a concrete and fully knowable existence. All objects — from the Andromeda galaxy to a photon of light — should have precisely defined properties that can be discovered through measurement.
Bohr, Schrödinger, and other proponents of the nascent quantum mechanics, however, were adamant in their belief that reality appeared to be fundamentally uncertain; a particle does not possess certain properties until the moment of measurement.
Entanglement emerged as a decisive way to distinguish between these two possible versions of reality. But it is not entanglement that is of particular interest here — it is the assumptions on which we understand entanglement.
Up until the advent of quantum mechanics, and more specifically, the introduction of entanglement, physics was infected with the idea of local realism. This was an unintended consequence of classical field theories — sort of just fell out of the maths. Local realism was a quick way of saying two things: the principle of locality and the principle of realism. Locality is the assumption that the cause of any physical change must be local. That is, an object is influenced directly only by its local surroundings. The principle of realism says that the properties of objects are real and exist in our physical universe independent of our minds. These may sound awfully metaphysical but their importance prevails.
One of the implications of local realism was the idea that two particles separated by some arbitrary distance in space cannot instantaneously communicate with each other. From a classical perspective, this made complete sense; of course, nothing travels faster than the speed of light. We are all victims of the cosmic speed limit. Physicists were comfortable with locality and realism for the most part. But here’s where things got interesting. Enter Einstein.
Einstein was a poster boy for local realism. He attempted to reformulate physics in a way that obeyed the principles of locality and realism. And he did succeed: general relativity was a reformulation of gravity in a way that obeyed locality. A product of these principles was the idea that complete knowledge of a system at a particular instant in time will yield complete knowledge of the system at any other moment in time. In Laplace’s words, we may regard the present state of the universe as the effect of its past and the cause of its future. Then, an intellect who knew all forces that set nature in motion and all positions of all particles could yield a single formula for the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.
But the advent of quantum theory called for an exception to the principles of locality and realism.
Laplace’s, Einstein’s, Podolsky’s, and Rosen’s arguments seem fairly logical. But it’s not quite that simple.
What Is Entanglement?
Entanglement is an incredibly profound feature of quantum theory. It is the physical phenomenon that occurs when a group of particles is generated, interacts, or shares spatial proximity in a way such that the quantum state of each particle cannot be described independently of the state of the others, including when the particles are separated by a large distance.
To understand this kind of quantum connection, I’ll take the conventional road of considering two electrons. Electrons have a property called spin, which, when measured, can take one of two values, “up” or “down.” Measuring spin has an inherent amount of randomness to it. Similar to how tossing a coin yields either heads or tails, measuring spin yields up or down.
The proponents of local realism rested on a thought experiment involving two such electrons in an entangled state: A and B. Although it is impossible to measure both the momentum and the position of particle B exactly (Heisenberg’s uncertainty principle), it is possible to measure the exact position of particle A and, by extension, deduce the exact position of particle B. The same with the exact momentum of particle A; the exact momentum of particle B can be found out. Similarly, measuring electron A’s spin will yield immediate knowledge about electron B’s spin. Since they are entangled, it is possible for measurement here to affect an electron infinitely away. This is instantaneous. And since information cannot travel faster that the speed of light, the possibility of both particles communicating to inform each other and react to preserve the correlation, would violate Einstein’s relativity theory.
Drawing an equally classical comparison, it is almost as if flipping one coin could send out a signal that instantaneously ensured the proper outcome of the other coin at the precise moment of measurement. Of course, this isn’t our reality at the larger scale but at the very fundamental level, this is where we stand. It only gets better from here.
Einstein suspected that entanglement would prove the death knell of quantum mechanics because it seemed to fly in the face of a central tenet of relativity — that no information could travel faster than the speed of light. No measurement of one electron should be able to instantly influence measurement in some distant place.
Both sides made their case. The localists and the non-localists. The localists, along with David Bohm, came up with what’s now called local hidden variable theories. Essentially, the idea is this:
Local-hidden variable theories envisage a world of particles that all have definite momenta and positions, albeit the values of which are generally inaccessible. The particles are deterministically “steered” or “guided” by a universal field which is described by the quantum wave function. It is sometimes said that this view is a return to a classical picture of the world, embracing atomistic particularity and determinism. But it is also sometimes accepted that Bohm’s view offers a more complete description of reality.
Eventually, this long-standing debate saw a potential end when John Stewart Bell came along. He figured there must be some experimental method to determine the validity of non-locality. In other words, there must be an event that yields different observations in different realities.
Well, in 1964, Bell proposed an experiment that could settle the debate. The details are rather involved, but the general idea was for two physicists to measure the spins of entangled particles along different axes: not just up and down but sometimes, randomly, left and right or in other directions. If Einstein was right, and the particles secretly had predetermined spins all along, then the act of switching the axis of measurement should have no effect on the outcome. Bell calculated that if the universe was truly quantum mechanical, and entanglement was as spooky as it seemed, the axis-switching would lead to correlated spin measurements more often than would be possible in classical theories like relativity. The idea Bell proposed is now called Bell’s theorem.
The essence of it is this: If locality holds and a measurement of one particle cannot instantly affect the outcome of another measurement far away, then the results in a certain experimental setup can be no more than 67% correlated. If, on the other hand, the fates of entangled particles are inextricably linked even across vast distances, as in quantum mechanics, the results of certain measurements will exhibit stronger correlations.
Bell’s genius formulated an experiment and it took Aspect, Clauser, and Zeilinger to realize it in various experimental forms. The work proved Schrödinger right and Einstein wrong. It demonstrated the follies of our understanding and that quantum mechanics was the operating system of the universe.
John Clauser, of Lawrence Berkeley National Laboratory and the University of California, Berkeley, and Stuart Freedman, a graduate student, were the first to take Bell’s experiment from the page into the lab. Clauser realized that the experiment would be more feasible if it involved not spinning electrons but polarized photons — particles of light. Like the spin direction of an electron, the polarization of a photon can take on one of two values relative to the orientation of a filter. Polarized sunglasses, for example, block photons that are polarized one way and let in photons polarized in the other manner.
Initially, physicists including Richard Feynman discouraged Clauser from pursuing the experiment, arguing that quantum mechanics needed no further experimental proof. But Bell personally encouraged Clauser to see the research through, and in 1972 Clauser and Freedman succeeded in realizing Bell’s experiment. They generated pairs of entangled photons and used lenses to measure their polarization directions. Unsure what he would find, Clauser had placed a $2 bet that his experiment would prove Einstein right. To his surprise, his results vindicated Bell’s prediction over Einstein’s. The photons’ states appeared correlated in a way that precluded any hidden-variable theory. Clauser’s lost bet was a huge win for quantum mechanics.
Clauser’s and Freedman’s work, however, still wasn’t entirely convincing. They used fixed orientations of the lenses, allowing for a subtlety: If a hidden variable that coordinates the photons’ polarizations somehow depends on the experimental positioning of the lenses, Einstein could yet be right. The idea of hidden variable theories provided relief to a large part of the physics world. As always, however, there were a group of people that disagreed.
Enter: Alain Aspect. Aspect performed a series of incredibly stringent bell tests in Paris, culminating in a devilishly sophisticated experiment in 1982. In that test, the orientation of the lenses would randomly change during the billionths of a second that the photons spent flying from the emitter to the lens. In this way, the initial lens configuration was erased and could have no influence on any secret process setting the polarization at the moment of their emission. Once more, the experiment was found in favor of Bell and quantum mechanics.
Only the slimmest of loopholes remained. Could a secret and nonrandom process that was somehow set in motion at the beginning of the experiment determine how the lenses would update? Anton Zeilinger’s research at the University of Vienna further narrowed this remaining sliver of doubt. In a 2017 experiment, he led a team that used the colors of photons emitted from distant stars hundreds of years ago to determine the settings of the experiment. If some cosmic conspiracy was creating the illusion of entanglement, it would have had to begin centuries before the births of the experimenters. Once again, entanglement prevailed and quantum mechanics reigned.
It was this that Aspect, Zeilinger, and Clauser were awarded their Nobel prize. Their work in proving that the Universe is not locally real. Their experiments have essentially vindicated quantum mechanics and demonstrated the fall of any hidden variable theory. The Universe is, after all, random.
