Blockade threshold logic. Coulomb-blockade single-electron transistors and quantum-dot majority gates are fabricated, measured devices. The threshold-from-physics is real and old.
A single-electron perceptron net. Capacitively-summed dot neurons have been demonstrated, but they're niche and stochastic (tunnelling is probabilistic — a noisy neuron, sometimes usefully so).
An entangled quantum perceptron. A dot can host a qubit; a true superposition-valued perceptron ran on IBM Q (Tacchino 2019). Whether entanglement buys an advantage is open.
Source and drain bracket a tiny island. Two input gates and a bias gate couple to it through capacitances you set (the weights). Toggle the inputs; their gate voltages add into the island as induced charge q = b + C₁·x₁ + C₂·x₂. The island conducts — a single electron tunnels through — only when q is pushed past the blockade edge. That on/off is the neuron firing.
The blockade isn't a metaphor. Sweep the island's charge and its conductance rises and falls in Coulomb oscillations — sharp peaks at every half-integer electron (a charge state is degenerate, current flows) and dead valleys between (blockaded). The peaks need the charging energy to beat the thermal blur: E_C = e²/2C_Σ > ~3.5 k_BT. Raise the temperature and watch the threshold dissolve.
The honest twist: the dot's native nonlinearity is periodic, not a single sigmoid — it fires at every half-integer charge. You get a clean perceptron by operating at one edge (Module I). Used across many periods it's really a periodic-activation neuron — closer to a Fourier feature than a textbook threshold. The dot gives you more than a perceptron; the perceptron is one slice of it.
Because it's a linear sum through a single threshold, one dot draws exactly one straight cut across its inputs. That's enough for AND and OR — and, just like its silicon cousin, never enough for XOR. The escape is the same: couple a second dot. Two capacitively-linked dots are a quantum-dot cellular automaton majority gate — a hidden layer made of charge. Coupled dots are depth.