The Beautiful Physics of Spin Qubits

Frank A. Wilczek

When I was asked to talk about quantum beauty I was a little startled, because the beauty of quantum theory is something that practising physicists, in the course of their work, rarely think about or mention. But when I gave the idea a chance, it really caught my imagination. And that’s why I’m here. Quantum beauty really is a wonderful, true thing to talk about. I’m going to sneak up on quantum beauty by putting it in historical context. The right context, I think, is a broader question: Does the world embody beautiful ideas? That is a question that people have thought about for a long time. Its intellectual history deserves volumes and syllabi. Here, though, I want to keep things brief and entertaining, so I shall spin a simple tale of heroes. Pythagoras and Plato intuited that the world should embody beautiful ideas; Newton and Maxwell demonstrated how the world could embody beautiful ideas, in specific impressive cases. Finally, in the twentieth century in modern physics, and especially in quantum physics, we find a definitive answer: Yes! – the world does embody beautiful ideas.

Sangchul Oh, Yun-Pil Shim, Mark G. Friesen +1 more

Indubala I. Satija

Abstract This chapter tells the magic of the most beautiful equation in physics—namely the Dirac equation that predicted spin and antiparticles and guided physicists to the ultimate theory of quantum physics—the quantum field theory, all emerging by wedding relativity and quantum mechanics. The theory predicted the electron g-factor of 2—agreeing with the experimentally known value at that time and establishing the importance of the fine-structure constant in quantum physics. Here is a discussion of spacetime algebra where Hestenes addresses the question of what exactly the quantum spin is and how it generates magnetic moment that is proportional to the spin. Answers may be lurking in de Broglie’s proposal of an electron possessing an internal clock and the concept of Zitterbewegung that provides a window to the particle’s substructure in quantum mechanics.

Vanita Srinivasa, Jacob M. Taylor, Charles Tahan

P. J. Lewis

I have argued (Lewis 2007) that anyone who adopts the Everettian approach to the foundations of quantum mechanics must also accept the (unpopular) ‘halfer’ solution to the Sleeping Beauty puzzle. Papineau and Durà-Vilà (2009) have responded with an argument that it is perfectly cogent both to be an Everettian and to accept the (popular) ‘thirder’ solution to Sleeping Beauty. Here I attempt to rebut their argument, and to clarify my original position. My argument depends on the claim that there are strong structural parallels between the Everettian treatment of a simple spin measurement and a simplified version of the Sleeping Beauty puzzle. The simple spin measurement is a measurement of the x-spin of a spin-1/2 particle whose state is an eigenstate of z-spin. The simplified Sleeping Beauty scenario is one in which Sleeping Beauty is told that she will (definitely) be woken on Monday and on Tuesday, with memory erasure in between. In each case, the agent's subjective experience has a branching structure, and she becomes uncertain about her location in this structure. On the basis of this analogy, I argue that the same subjective probability assignment is appropriate in each case to quantify the agent's uncertainty. But the accepted assignment in the Sleeping Beauty case does not allow for pre-measurement probabilities other than 1 when applied to the Everettian case, and the required assignment in the Everettian case entails the problematic halfer assignment when applied to the Sleeping Beauty case.