Oct 10, 2022
The 2022 Nobel Prize in Physics is a fitting end to “spooky” entanglement
“Science progresses one funeral at a time.” — Max Planck
Alain Aspect, John Clauser, and Anton Zeilinger recently won the 2022 Nobel Prize in Physics for “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” This is a beautiful bookend to the “spooky” era of quantum physics. I’m sure it is seen as the ultimate vindication for those that pioneered it during a time when the reaction to it was “shut up and calculate.” But the “shut up and calculate” bar has moved well beyond these foundations. Indeed, we now routinely ask high-school students to demonstrate and calculate quantities related to quantum entanglement. Today’s brave souls — those researchers who are now told they are “wasting their time” on foundational problems — are not studying entanglement. We have moved way beyond that.
What was so spooky anyway?
In 1935, Albert Einstein and two colleagues (Boris Podolsky and Nathan Rosen) wrote a paper elucidating the conceptual problem quantum entanglement posed for our classical notions of space and time. Either quantum physics disobeyed Einstein’s theory of relativity (in some sense), or quantum physics was not a complete theory. Einstein dismissed the former, referring to it as “spooky action at a distance.” The paper is the source for concepts such as the EPR paradox, EPR pairs, and the like. In fact, entanglement was coined by Erwin Schrodinger in response to the paper, claiming it was the single essential feature of quantum physics. The conclusion of EPR, on the other hand, was that there must be some deeper reality behind the equations of quantum physics that could be known. Since the theory didn’t specify what these might be, they were referred to as “hidden variables.”
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 them was John Bell. In 1964, he proposed an experiment that could rule out exactly the kind of hidden variables that the now-late Einstein was hoping for. His experiment, and the many refinements, are known as Bell experiments, and the quantity measured, which rules out Einstein’s variables, is called a Bell inequality. Eight years after Bell, John Clauser and his student Stuart Freedman performed the experiment. This might have been the end of 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. Aspect and Zeilinger followed with experiments of their own to close the loopholes. In doing so, they paved the way for extremely precise control of entangled quantum states, which ushered in a new era of quantum technology.
The death of spooky entanglement
As the Prize citation points out, the detailed study and understanding of entanglement gave rise to quantum information science, a now mature and multidisciplinary field that underpins the latest achievements in cryptography, sensing, communication, and computation. It also has produced novel insights into problems older than quantum physics itself, including the nature of gravity, causality, complexity, reality, and even free will. In short, entanglement is no longer as “spooky” as even the likes of Einstein once thought.
Clauser’s first experiment was performed over 50 years ago. It unsettled a generation of physicists, but much of that generation is gone. The loss of life is sad, but, as the late “father of quantum physics” himself pointed out, there is a poetic cycle in human intellectual pursuits wherein a new generation easily accepts the known facts and goes on to produce new surprises. In science, what was once unsettling but true becomes the comfortably mundane foundation for the next set of temporarily exciting discoveries.
The above quote is a popular paraphrasing. What Max Planck said in his autobiography was less punchy but equally profound: “A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” There is now an entire generation of quantum information scientists that have “grown up” with entanglement. The foundational investigations of entanglement gave birth to quantum information science, but from the perspective of quantum information, entanglement is natural, necessary, and inevitable. How can such a thing be spooky?
What’s spooky now?
A Nobel Prize in Physics for quantum information is well-deserved and, dare I say, long overdue. But let’s now focus our attention not on the past but on the present quandaries in quantum physics. Indeed, Aspect himself is a co-founder of a quantum computing company and works at the forefront of experimental many-body physics, trying to understand emergent phenomena in quantum gases. Simple particles following rules can get together to create the complexity of the world we see around us, and no one has a satisfactory answer for how that happens and why — that is an example of today’s quantum “spookiness.”
But there are many more examples of counterintuitive features of quantum physics worth mulling over. When I was a graduate student over ten years ago, I worked on “negative” probability and was told by my thesis committee to work on something practical instead. Christina Giarmatzi, a research fellow in my group at the University of Technology Sydney, is a pioneer of the new field of quantum causality, which upends our notion of cause and effect. Indeed, even entanglement can be seen as a special case of contextuality, which I have written an introduction to elsewhere.
Einstein’s “spooky” entanglement is to the foundations of quantum physics what nostalgic Christmas music is to radio play time in December. Most physicists have moved beyond it. It’s time everyone else does as well.
