Every OU Sooners football game begins with a coin toss. No one knows ahead of time whether the coin will turn heads or tails. In the quantum world, it is possible to have coins – which, as far as we can tell based on some of the most precise measurements ever made – are in a state of both heads and tails. This is called a superposition state. Only when we make a measurement, do we find either a heads or tails. It's random!
Processes of this type are now used for random number generation. Even more astonishing, there are pairs of "entangled" coins. When we toss them, the results are also random. But when one comes up heads, the other will come up heads, even if it is tossed thousands of miles away. Experimental confirmation of this "spooky" result is one of the things recognized by the 2022 Nobel Prize in Physics.
These entangled states can have properties far beyond only the two end states of heads and tails, and by manipulating these entangled states, we can build new forms of computers, sensors, and possibly secure communication systems.
Experimentally we don’t look at large objects like coins but sub-nanoscopic systems such as photons, electrons, atoms and molecules and things like that. They all have measurable internal properties such as polarization, spin, and energy states. These can exist in a combination of states or entangled states. To give an example, one of the properties of photon is its polarization as it can have a horizontal or a vertical polarization – think of it just like the heads or tails for the coin. If you take 2 entangled photons and send them in opposite directions the quantum postulate states that their internal property (which is this entangled state) will remain fixed for both these photons. At any point when the photons are received by two receivers who could be separated by say thousands of kilometers, for example one could be in United States another person could be in Australia, when one of them measures the polarization, they will either measure a horizontal or vertical polarization. The unrealistic result here is that whatever polarization is measured in Australia will be deterministic of what is measured in the U.S.! In other words, these correlations can in principle persist over great distances. These experiments were done chronologically from the 1960s to the late 1990s by the three scientists who won the Nobel Prize award this year.
The history behind the award is equally fascinating and throws some light on how creative research gets done. In the 1930s, Einstein, after his extraordinary contribution to the origin of quantum mechanics became highly frustrated with the kind of nonintuitive consequences that one saw using quantum mechanics. Along with two authors, Podolsky and Rosen, he published a very famous paper (EPR paper) pointing out the flaws in quantum mechanics which needed to be fixed to create a worldview consistent with what we perceive to be real.
For several decades there was this raging debate between Einstein and Niels Bohr, who was another great scientist of the last century, and they took opposing positions on quantum mechanics with Einstein questioning quantum mechanics on account of phenomena such as entangled states, which really bothered him, versus Niels Bohr being comfortable with the idea that quantum mechanics can create situation which are counter intuitive.
For many years after their deaths the question relating to these issues were in a kind of holding pattern, until a brilliant young scientist named John Stewart Bell. While a physics student at Queen’s University of Belfast in the 1940s, Bell was unhappy with the way he was taught quantum mechanics and was constantly thinking about the famous Einstein paper and the debates between Einstein and Bohr. He eventually became an accelerator physicist and at CERN as a particle physicist in the ‘60s but in his part time he was thinking a great deal about this controversy between Einstein and Niels Bohr.
Eventually, he published a paper which is now referred to as the Bell’s Inequality in an obscure journal as he was concerned about the reaction of the science establishment. The journal does not exist today, and his work remained obfuscated with limited citations. Bell took the abstract ideas of Einstein and others and boiled them down into a simple statement about quantum mechanics – Bell’s inequality. It elegantly demonstrates that quantum mechanics makes predictions about simple physical systems, such as properties of pairs of atoms or photons, that cannot be reconciled with Einstein’s position that a proper description of reality should be sensible and intuitive, and not feature strange, entangled quantum particles. In the simplest terms, Bell demonstrated that Einstein was wrong: a tossed quantum coin really does land on both heads and tails at the same time!
John Clauser, then a graduate student, was intrigued by this paper, and he took it to his thesis advisor at Columbia University and proposed to do some experiments to verify Bell's hypothesis. His professor advised him to avoid this controversy and what he termed as more philosophy rather than science and asked him to focus on other projects. He complied, but he kept thinking about this paper, and at a later stage in his life, he performed the first beautiful experiments on quantum entanglement showing Bell’s inequality to be real. His work was not well cited, and he probably suffered somewhat professionally.
A decade and a half later Alaine Aspect took over the same problem and did it at an even higher level of accuracy and rigor and published some very beautiful papers. The response of the scientific community was lukewarm, and he eventually decided to quit this field and go do other brilliant experiments in different fields. Last, but not least Zeilinger came around the towards the end of the millennium and performed some outstanding experiments literally putting the nail on the coffin so to say and even verified some ideas in quantum teleportation where the entanglement from one system could be transferred to another. Clauser pioneered the question of how to test Bell’s inequality, while Aspect and Zeilinger thought about how to close every potential loophole. These experiments were technologically heroic for their times. Today we are recognizing the importance of this primarily because of the enhanced interest in quantum science and engineering.
What is the value of all this is that the entangled state is a very fragile state and so any external disturbance like an electric field magnetic field, or in specific cases, even physical vibrations of an optical fiber carrying entangled photons could have a significant effect on the entanglement and therefore it becomes an extraordinarily sensitive sensor for external perturbations. To give you an idea, if you create an optical communication system where the fibers carry entangled photons then when an interloper taps into these photons (I mean interfere in the line by listening to it) that alone would immediately change the entanglement in the photon and will tip off the people that the system is now unsafe, and they can take remedial measures. Also, this sort of entanglement using atoms and other systems could also be of great value. In fact, sensors that are based on these entangled atoms can essentially be fantastic gravitational detectors as they can measure gravitational anomalies, for example a heavy object buried underground. When several of these quantum elements (called Qubits) are in an entangled state, they enable a novel form of quantum computing enabling processes not easily feasible in conventional computers.
This entire story has some morals that we can take away. There is outstanding research work out there that we don’t pay attention to simply because they are not highly cited because they are too original. In the case of all these scientists they persisted in following their instincts in pursuing this problem with a passion despite significant pushbacks from the scientific establishment. Yet again, many university researchers follow the hot fields creating a bandwagon effect – we saw a lot of this over the last few decades: high Tc superconductors, highly correlated manganite systems, diluted magnetic oxides, 2D materials, Inorganic perovskites and so on. It really requires a level of self-confidence and a true gut feeling for what could be important to make breakthrough research of this kind at a time when it is not so popular. The peer review process which very often gets affected by factors such as citation index, hotness of a field, etc. tend to make research such as these ever more difficult.
