How Scientists Have Learned To Work With the Quantum World – Walter Bradley Center for Natural and Artificial Intelligence

Posted: November 30, 2020 at 3:57 pm


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In last weeks 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: Whats superposition? Whats going on there?

Enrique Blair: Quantum superposition is really a mathematical description. We use wave functions to describe these particles. Theres 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. Thats 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. Its not one or the other, its something of both.

Robert J. Marks: Its kind of like Invisible Boy (pictured) in Mystery Men. When you dont look, zero and one are both there.

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: Thats right. Oddly enough, there is no mathematical definition that rigorously describes measurement. Its one we havent 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 functionwhen 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 thats all you get.

You get probabilities for outcomes. You cant predict with certainty which outcome is going to result when you make a measurement. Thats the subject of one of the papers we wrote recently, just using quantum mechanics to make something thats a truly random number generator.

You know well that computers cant generate random numbers because theyre deterministic.

Robert J. Marks: Which is really surprising because you see random numbers used a lot in gaming machines, like in casinos.

And theyre not random numbers, theyre 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. Thats amazing.

Note: Heres a random number generator (RNG) for sale at Amazon. Why cant we just think up and write down random numbers? That doesnt 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, thats true. Like I said, the Schrdinger equation describes the time evolution of the system if you dont measure it or dont look at it or dont interact with it. But then once you measure it, you get one of these probabilities and you radically change the wave function and its in the state that corresponds to the result that you got. Previous to that, its 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 Truth about Schrdingers cat.

Previous: Heres why the quantum world is just so strange. It underlies our universe but it follows its own rules, which dont 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.

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How Scientists Have Learned To Work With the Quantum World - Walter Bradley Center for Natural and Artificial Intelligence

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November 30th, 2020 at 3:57 pm

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