Metabolic quantum limit to the information capacity of magnetoencephalography
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Abstract
Magnetoencephalography, the noninvasive measurement of magnetic fields produced by brain activity, utilizes quantum sensors such as superconducting quantum interference devices and atomic magnetometers. Combining the energy resolution limit of magnetic sensing with the brain's metabolic power, we derive a technology-independent bound on the information capacity of such measurements. Depending only on geometry, neural metabolism, and Planck's constant, this bound yields a maximum information rate of 2.2~Mbit/s for the human brain. We also show that the measurable magnetic field has a finite angular bandwidth. Higher multipole components are geometrically suppressed and fall below the quantum-limited noise floor, limiting the spatial complexity of neural current patterns encoded in the external field. Because the energy resolution limit implies noise variance grows linearly with bandwidth, temporal and spatial bandwidths compete, establishing a fundamental spatio-temporal trade-off. These results unravel the fundamental limits of noninvasive brain imaging, and may inspire the synthesis of neuroscience with modern quantum technology.