Abbas Syed Omar

The equations of motion for a spin magnetic dipole in nuclear magnetic resonance imaging (MRI) and a qubit’s Bloch-sphere state vector in quantum computing (QC) share the same mathematical form. Semi-classical solutions in both fields show strong behavioral similarities: in MRI, the elevation and azimuthal angles describe the physical orientation of the spin, while in QC, they are purely mathematical parameters—yet both follow the same equations. This paper introduces qubit behavior, excitation, and readout from an electrical engineering perspective, requiring little or no prior knowledge of quantum theory. It also highlights how decades of MRI expertise in spin control can inform qubit manipulation in quantum gates, especially when gate times approach coherence limits. Under such conditions, additional control pulses may be needed to refocus dephasing while maintaining the desired unitary operation—a challenge directly analogous to the inhomogeneous-dephasing refocusing routinely performed in MRI.

This paper presents a fully classical, nonlinear, closed-form solution for the dynamics of the spin magnetic dipole moment in magnetic resonance imaging (MRI) under continuous-wave (CW) magnetic excitation. Unlike the commonly used linearized small-signal approaches or the quantum operator-based Rabi solution, the analysis here is developed entirely within a classical framework using rotating-frame vector calculus. The derived equations capture the complete angular evolution of the spin vector, provide insight into the reactive energy exchange with the environment, and show that no net energy transfer occurs over a full excitation cycle. This behavior is analogous to that of reactive circuit elements and has fundamental similarities with the stimulated emission radiation in optical Lasers. To the best of the author’s knowledge, this represents the first such nonlinear large-signal analysis based solely on classical physics.

There have been lots of debates about the concept of entanglement since Einstein’s famous statement “spooky action at a distance.” This contribution first reviews the main stages of the acceptance history of the concept of quantum entanglement. It then presents a comparison between the quantum and the physical coupling of two entangled quantum resonators, where the term physical is used in its classically intuitive sense. It is shown that both conform to Einstein’s theory of relativity and do not allow for either instantaneous or superluminal transmission of information. The physical model relies on connecting the two quantum resonators via a waveguide operating below the cutoff frequency of its dominant mode. Apparent superluminality might take place within the isolated lossless system. Coupling the system to its surroundings for conducting a measurement or observation breaks such apparent superluminality and makes the system conform to relativity constraints. A comprehensive transient analysis of the physical model might be needed to understand its role in the entanglement and how it interacts with the quantum one. It is, however, worth mentioning that although the analysis presented in this article is valid for resonant circuits, it cannot be applied to entangled moving particles, like electrons and photons, because of the lack of physical coupling in these cases.

Most publications on quantum computing and its main building block—the qubit—rely on quantum theory and use its terminology. It is therefore not an easy task for electrical engineers to fully understand the concept of qubits and how they are designed, controlled, read out, and used for computation.

Presents corrections to the paper, Health aspects of millimeter-wave exposures in 5G and beyond.