Richard Holt, of Western’s Physics & Astronomy Department, has received an international accolade recognizing his contributions to physics. He is now one of the newest Fellows of the prestigious American Physical Society (APS). “In this case they cited two contributions I made” says Holt, “one was a paper in 1969 which I did when I was a graduate student at Harvard and the other was some work that I did in the 1980s in our laser spectroscopy lab at Western and they’re both related to quantum mechanics.”
Born in Brooklyn, Holt spent his childhood in the Bronx and Yonkers. He then went off to Harvard for his undergraduate Bachelor of Arts in Physics. He remained to work on his PhD there as well and it was during this period where Holt became part of the history of physics.
In the early 1960s a number of theoretical physicists were not satisfied with the theoretical side of quantum mechanics. This unease was a remnant of the debate between Albert Einstein and Niels Bohr years earlier about the peculiar nature of the quantum world of molecules, atoms, electrons and photons. What Einstein found so unsettling was abandoning the classical idea of cause and effect of the measurements made in this microscopic world in exchange for the indefinite probabilities held by Bohr.
In 1935, Einstein and two colleagues Boris Podolsky and Nathan Rosen (EPR) published a paper in the journal Physical Review to counter Bohr’s interpretation. What EPR realized is that there are quantum mechanical states in which you can take a system that can produce two pieces that can be separated by any distance where one of them can have a value of some property and the other one will have a correlated value. Even though both are in an indefinite state, the instant that a measurement is determined on the state of one part, the other one follows suit in a corresponding way. Einstein talked about this entanglement as “Spooky actions at a distance” and thought that “no reasonable definition of reality could be expected to permit this.” As Holt comments, “This essentially showed the world that quantum mechanics was bizarre; it didn’t show that it was wrong.”
According to EPR, quantum mechanics is fine for generating workable statistics but as it stood the theory was incomplete. Quantum mechanics must have a deeper theory that gives a step by step explanation of what everything is doing at every moment and he reasoned that quantum mechanics had left out variables from the equation and these were called hidden variables. For a time, the two positions seemed to be deadlocked within the physics community without a resolution until John von Neumann produced a proof that showed that no such hidden variable theory could equal the statistical predictions of quantum mechanics and most physicists accepted this “impossibility proof.” However, around 1966 John Bell found a flaw in von Neumann’s proof where the measurements required needed completely incompatible measuring devices. Bell favored the EPR situation and thought that quantum mechanics allowed stronger correlations between these distant measurements than any theory in which, in principle, the result of the measurement was determined before it was made where everything was as Einstein wished. As abstract as it is, this inequality formulated by Bell presented a particular example pointing to a way of testing whether Einstein or Bohr were right.
Holt first heard of Bell’s Inequality through a graduate level quantum mechanics course at Harvard, “the professor told us, ‘this is a very interesting paper, you should look it up.’ So I looked it up and I said, Hmm. I didn’t think much about it...I missed my early run at that.“ But other people had taken a look at it, most notably John Clauser who was a graduate student at Columbia at the time. He is one of these people like Einstein who doesn’t mind being in a minority of one who absolutely disliked quantum mechanics, even though he’s very good at it. He was determined to prove that it was wrong and he realised the example that Bell had used wasn’t all that suitable.” Clauser decided to use a new technique involving atoms emitting one photon and then almost immediately another one that sometimes goes in an opposite direction. He thought that measuring the polarization of these photon pairs would be a good experimental test. At the same time, Abner Shimony at Boston University had the same realization and they found each other when Clauser submitted an abstract for a meeting of the American Physical Society and very graciously agreed to work together and publish. Holt relates, “Abner was just across the river from Harvard and he knew that my thesis supervisor (Frank Pipkin) and I were doing experiments on atoms emitting photon pairs so he and his graduate student Michael Horne came across the river and we had a lovely meeting. I said ‘this is great’ to my thesis supervisor ‘let’s just do it in six months then I’ll get back to what I’ve been working on.’ So once again I wasn’t quite prescient. The famous 1969 paper is the one in which we showed, first of all, how you could modify Bell’s Inequality to make it a practical testable experiment, which does require some extra assumptions because in a real practical experiment there’s always some inefficiencies. You have to do what’s called a fair sampling assumption where if you lose some of the photons, you didn’t lose them in a biased way to change the results. So they were working on that, but neither of them really knew how to do the proper atomic physics calculation to show what quantum mechanics predicts in a particular atomic system. You had to realize that the two photons don’t always come out exactly back to back and when you do the math you find that they come off in many, many directions and it’s a much more complicated calculation. What’s absolutely necessary is that you show a feasible experiment using practical conditions to demonstrate a result that was different from the complete range allowed by what is called the hidden variables in the inequality. People refer to it as the CHSH (Clauser, Horne, Shimony, Holt) Inequality and what I did was show how to do the atomic physics calculation. I’m the last author but I think it was an important contribution.”
Since then many measurements were made using this method and the results showed that Einstein was wrong. “The correlation was too strong to explain without quantum mechanics so you really essentially had to believe in the weirdness,” says Holt. Ten years later Alain Aspect’s series of experiments further refined the results. Ironically, since researchers were already using quantum mechanics to perform calculations, the results were treated more as a curiosity. “After we published our paper, not that many people noticed it,” says Holt. “But then the citations started going up an up and now it is almost 2,000, there’s about 150 every year, and what happened was that a whole new field was born called quantum information science. We can make a perfect cryptography scheme which, if we use single photons, and send them in this indeterminate state, if anybody eavesdrops they have to determine their state and they have to remove the randomness. You can’t put the randomness back in, especially if it has to be correlated with another photon they don’t have since it is with the person who sent the message so you can always tell by comparing your photons at the sent and received ends whether somebody had eavesdropped.” Quantum mechanics also allows investigations into a novel way of calculating called quantum computing which would use qubits instead of the binary bits used in computers today. As Holt says “if you wanted to do serious calculations in physics then an ordinary computer will never be good enough because the problems are just too difficult, but if you had a quantum computer you could do them.”
After Harvard, Holt went on to complete a post doctoral degree at Brown when just by chance he met up with an old friend (Prof. S. David Rosner) who had been a graduate student the same time he was at Harvard. His friend was working for the same supervisor and then he came here to Western to be a faculty member “so I came up thinking that I would spend a couple of years and before too much time had passed they offered me a faculty position and I found things interesting so I stuck around.”
It was at Western where Holt garnered his second citation for nomination as a fellow of the APS. “The other thing that I did in the 1980’s was to initiate a very precision measurement of effects you get from combining quantum mechanics and relativity as applied to the prediction of properties of atoms that have two electrons.” Prior to this, the available techniques only allowed for hydrogen as the only atom that could really be studied. Holt’s system of choice was the ion of lithium. He was able to capitalize on the calculating tools developed by Gordon Drake in Windsor, who had come up with some startling advances. Holt says that Drake had “made advances where you could calculate helium which made it actually as accurate as hydrogen for the part without relativity and then you could start testing the relativity part, that’s what we did in the 1980s. There we are just concentrating on measuring all the decimal places and seeing if the theory gives the right answer.” Holt admitted that this work was “very different from checking to see if the theory is fundamentally weird and counterintuitive.”