Yesterday, the 2022 Nobel Prize in Physics was announced. It was awarded to John F. Clauser, Alain Aspect, and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” This topic is dear to my heart: the exciting weirdness of quantum physics was one of the most critical factors in choosing to become a physicist. After several people asked me about it, I wondered — why not write a brief post about it? I only hope my simplifications will not result in my physicist friends coming to look for me with torches and pitchforks. So, here we go.
Classical physics posits two principles or axioms: locality and realism. Both of these will seem obvious.
- Locality means that nothing can travel faster than the speed of light (this specific limit is set by relativity; locality only says that there is a limit); thus, things can only be influenced by what’s near them. You cannot be affected by a black hole exploding millions of light years away *today* because it will take millions of years for that information and any consequences to reach you.
- Realism means that things exist irrespective of whether we observe them. All physical systems have well-defined properties at all times (e.g., speed, position). Any uncertainty must be due to our lack of knowledge about the system.
Quantum physics seems to violate this. Experimental results (and theoretical predictions) show probabilistic outcomes for identical initial system states, violating realism. Furthermore, it seems like a change in some part of the system affects the rest instantaneously, no matter how far the parts are from each other – also violating locality. When a quantum system with several parts is prepared correctly, the parts can be related to each other in very special ways that cannot be reproduced outside the quantum realm. We call this “entanglement.”
Einstein is famous for having complained about all this (“God does not play dice”), and together with two other scientists (Rosen, Podolsky), a paradox was posited in 1935. First, suppose we prepare a particular system with two parts in a quantum entangled state. Then, assume we separate the two parts, taking them very far away from each other without disturbing them. Lastly, suppose we interact with one of the parts, thereby changing its state. If the consequences of that interaction immediately reached the other part of the system, changing its state (as quantum physics seemed to say), one would be violating locality (and relativity): information would have been transmitted faster than the speed of light.
In fact, the paradox only violates locality: for this result to violate relativity, you would need to be able to do something useful immediately with the other part. Unfortunately, since the results are probabilistic, for you to leverage what you know about the other part of the system, you also need to know what you did to the first part. And that information will still be limited by the speed of light constraint (e.g., a phone call).
These scientists resolved the paradox by positing a set of unknown or “hidden” variables. These variables would predetermine the outcome of any interactions with the system, so no information would need to be transmitted. If only you had known the correct variables and told the other person what kind of disturbance you’d effect on the first part, you could have predicted the outcome for both the first and the second part of the system. It simply looks probabilistic because you are missing information. And poof, both realism and localism are back on the table.
Of course, as you may expect, this is not the full story. In 1964, Bell, another scientist, challenged this conclusion mathematically. He derived a set of equations (well, inequalities) that would allow one to test whether this was the case. If there were, in fact, hidden variables that predetermined the outcomes, there are mathematical consequences as to what you can possibly measure when investigating the two-part system. Bell showed that quantum physics’ predictions violated these.
A few years later, in 1972, Clauser experimentally tested Bell’s predictions. It seemed like entanglement was a real thing! Unfortunately, the experiment wasn’t perfect: there were still ways in which these hidden variables could have been real (“loopholes”). Most of the work that came after this served two purposes: 1) to develop the potential of quantum entanglement and information as resources that can be exploited, both theoretically and experimentally (see the 2012 Nobel Prize to Haroche and Wineland), and 2) to close these loopholes and put the quantum weirdness on unshakable ground.
This year’s prize was awarded to crucial scientists in this journey: Clauser, Aspect, and Zeillinger. Their joint achievements rejected the loopholes and showed that entanglement was genuine and that quantum information science had a bright future. Aspect closed the most significant loophole in Clauser’s experiment in 1982, but there were still more. Zeillinger would be the leader in closing other gaps and showing quantum teleportation (which is very exciting, but not what you think) experimentally.
Now you know: quantum physics is both weird and real. But that weirdness is precisely what makes quantum computing and related fields so exciting and powerful.