We humans always reach for analogies, and with every new wave of technology we apply it to things we don’t know. The human brain has variously been described as a mechanical Babbage machine, an electrical circuit board, a silicon chip, and now… (pace Roger Penrose) a quantum computer.

Unfortunately, none of these analogies is enlightening and all are profoundly limiting. For example, the “electrical circuit” idea seems to have led neuroscientists to ignore some important physical aspects of neurons which, belatedly, are just beginning to receive the attention they deserve. So before we rush off to apply the nascent field of quantum computing to the brain it may be worthwhile to think about why the brain is not, and cannot be, any sort of quantum device.

Quantum effects occur only at the smallest scales, and require exquisitely fine-tuned conditions in which to occur. Hence the complexity and expense of creating even the simplest qbit. As the “observer effect” means any external interaction (photon, electron, atom, etc.) there’s no quantum effects at larger scales. The brain is 1.4kg of larger-scale system operating at body temperature. These two simple facts alone tell us there can be no quantum effects going on.

Furthermore, there’s no need to posit quantum effects. It’s pretty clear that the brain, like so much of nature, is analog. Analog permits a far greater range of outcome options than binary computing (which is another reason making analogies with current tech is so very misleading and so very unhelpful). When you have analog you don’t really need quantum effects at all.

Neuroscience is a fascinating area of exploration, but perhaps it’s best not to introduce unnecessary wild speculations that on an a priori basis are entirely implausible. Far better to search for how the brain really works, than to impose yet another entirely misleading analogy?