Figure 2
Illustration: Alan Stonebraker

Figure 2: The entanglement of the qubit electron spins is controlled by exciting a chosen control donor into a spatially more extended state. Dopant wave functions fix the scale of the separations over which entanglement is effective to a mere 10–20 nm. But light can only be focused down to 1000–2000 nm. However, different controls (hence different gates) can be selected by exploiting the variations in excitation energies from one place to another, from whatever cause, even from surface steps. So here the red laser excites one control, entangling two qubits. Next, the green laser excites another control, in the case shown entangling a different pair of qubits without affecting those controlled by the red laser. Spectral selectivity is combined with spatial selectivity. In this way, a sequence of optical pulses of chosen wavelengths and durations can control quantum operations in a “patch” of about one optical wavelength across. Each patch might contain perhaps 20 gates.