In last week’s podcast, “Enrique Blair on quantum computing,” Walter Bradley Center director Robert J. Marks talks with fellow computer engineer Enrique Blair about why quantum mechanics is so strange. But scientists have learned to work with QM, despite many questions, like how to work with particles that can be in two different places (quantum superposition):
[Starts at approximately 13:16.] The Show Notes and transcript follow.
Excerpts from the transcript:
Robert J. Marks: What’s superposition? What’s going on there?
Enrique Blair: Quantum superposition is really a mathematical description. We use wave functions to describe these particles. There’s a wave function for the photon going through Slit One and a wave function for the photon going through Slit Two. To describe it going through both slits, we have a linear combination of those two wave functions and so you have a more general wave function. That’s the heart of quantum computing because in classical computing, we have bits like zero or one. And in quantum computing, we like to use these superpositions of zero and one. It’s not one or the other, it’s something of both.
Note: “ Invisible Boy is a resident of Champion City who spent most of his adolescent life ignored even by his own father. Eventually he discovered that after years of being overlooked, he had developed the power of invisibility, but it only works as long as no one (including himself) is looking at him.” – Mystery Men Fan Wiki
Enrique Blair: That’s right. … Oddly enough, there is no mathematical definition that rigorously describes measurement. It’s one we haven’t quite figured out yet.
Robert J. Marks: Tell us what a wave function is.
Enrique Blair: A wave function describes the state of a quantum system and it contains everything we can know about that quantum system. But we manipulate these things or we extract meaning from them using quantum mechanical operators. These operators describe things like time evolution or the total energy of the system, or some observable quantity like position or momentum.
The wave function itself is not the probability density. You have to take the magnitude squared. And then you get probabilities.
Note: It amounts to doing mathematics with probabilities rather than exact figures. “In the experiments about atomic events we have to do with things and facts, with phenomena that are just as real as any phenomena in daily life. But the atoms or the elementary particles themselves are not as real; they form a world of potentialities or possibilities rather than one of things or facts. (– Werner Heisenberg, a quantum mechanics pioneer, Physics and Philosophy, p. 186)”
Enrique Blair (pictured): Okay. The wave function—when you take its magnitude squared— you get the probabilities of various outcomes for measurement when you also use an operator. But really, the stunning thing is that’s all you get.
You get probabilities for outcomes. You can’t predict with certainty which outcome is going to result when you make a measurement. That’s the subject of one of the papers we wrote recently, just using quantum mechanics to make something that’s a truly random number generator.
You know well that computers can’t generate random numbers because they’re deterministic.
Robert J. Marks: Which is really surprising because you see random numbers used a lot in gaming machines, like in casinos.
And they’re not random numbers, they’re pseudo-random numbers. They actually use an algorithm.
Physics and engineering professor Craig Lent has talked about randomness and the ability of quantum mechanics to generate true randomness. In fact, this is the only pure source of randomness there is. He said you can go to amazon.com and buy yourself a random number generator based on quantum mechanics that really spits out 100% random numbers. That’s amazing.
Note: Here’s a random number generator (RNG) for sale at Amazon. Why can’t we just think up and write down “random” numbers? That doesn’t really work because humans always think in patterns, whether we notice them or not. And if we try to write an algorithm to produce random numbers, that is a pattern too. Quantum mechanics can, however, generate random numbers because there is no specific prior position.
Robert J. Marks: In the quantum world, when you measure something, you kind of mess around with the wave equation when you measure it. And then it collapses in accordance to its probability. Is that kind of the way it is?
Enrique Blair: Yeah, that’s true. Like I said, the Schrödinger equation describes the time evolution of the system if you don’t measure it or don’t look at it or don’t interact with it. But then once you measure it, you get one of these probabilities and you radically change the wave function… and it’s in the state that corresponds to the result that you got. Previous to that, it’s a quantum superposition of many different states.
Note: Is quantum mechanics practical? “Quantum computers, as their name implies, operate on the bizarre principles of quantum mechanics to manipulate information, and are poised to revolutionize our computing capabilities. With companies like IBM and Google already building the first prototypes, they are expected to propel technology forward with greater speed, accuracy, and security by completing tasks that would be otherwise impossible for ordinary computers to handle.” – Advanced Science News More on how that works later.
Next: The final ambiguous truth about Schrödinger’s cat. Schrödinger came up with the cat illustration to explain quantum mechanics to interested people who were not physicists. We don’t see quantum paradoxes outside the lab because everything we see consists of far too many quantum subsystems for any one particle to stand out.
Here are the earlier discussions:
How scientists have learned to work with the quantum world.
It’s still pretty weird, though. Wave function mathematics can work with particles that may be in different places (quantum superposition). QM can also generate truly random numbers we can use.
Here’s why the quantum world is just so strange. It underlies our universe but it follows its own “rules,” which don’t make sense to the rest of us. Computer engineer Enrique Blair explains to Robert J. Marks the simple experiment that shows why so many scientists find the quantum world “mind-blowing.”
- 00:54 | Introducing Dr. Enrique Blair, a professor of electrical and computer engineering at Baylor University
- 03:08 | The history of quantum mechanics
- 13:16 | Quantum superposition
- 21:50 | Schrödinger’s cat
- 27:45 | Why didn’t Einstein like quantum mechanics?
- 28:51 | Quantum entanglement
- 32:58 | Applications of quantum mechanics
- 34:53 | Quantum dots
- 37:31 | Quantum computing
- 43:48 | The use of quantum computers
- 47:55 | Quantum supremacy
- 55:32 | Quantum communication
- 58:47 | The future of quantum computing
- Enrique Blair’s website
- Copenhagen Interpretation of Quantum Mechanics at Standford Encyclopedia of Philosophy
- Young’s double-slit experiment at Encyclopædia Britannica
- Planck’s explanation of black-body radiation at Encyclopædia Britannica
- Quantum superposition at Wikipedia
- Nobel Prize in Physics 1932 — Werner Heisenberg
- Nobel Prize in Physics 1933 — Erwin Schrödinger
- Many-Worlds Interpretation of Quantum Mechanics at Stanford Encyclopedia of Philosophy
- Schrödinger’s cat at Wikipedia
- Quantum entanglement at Wikipedia
- Quantum bit (qubit) at Wikipedia
- Quantum dots at Wikipedia
- Shor’s algorithm at Wikipedia
- RSA at Wikipedia
- Grover’s algorithm at Wikipedia
- Quantum supremacy at Wikipedia
- IBM on Google’s claim of quantum supremacy
- Quantum communication at MIT Technology Review
- Adiabatic quantum computation at Wikipedia