Why can't transistors keep shrinking forever?
The smallest parts of today's transistors measure from a few nanometres to a few tens of nanometres. A nanometre is a millionth of a millimetre, and a silicon atom is about 0.2 nanometres wide. So those parts are only a few dozen atoms across.
About how many silicon atoms fit side by side across a 10 nanometre feature? 10 nm ÷ 0.2 nm per atom = 50 atoms.
At this scale, electrons stop behaving like little balls running along wires. They behave like waves, and a wave can leak through a thin wall that a ball could never cross. Physicists call this quantum tunnelling. In a transistor it means current leaks through a switch that should be off: wasted energy, heat and errors.
Chip makers fight it with new materials and new transistor shapes, and they keep winning, but each step is harder and more expensive. Quantum physics is a problem for ordinary chips.
It is also an opportunity. Instead of fighting quantum behaviour, what if we built a computer that uses it? That is the idea of a quantum computer. It is not a smaller version of today's chips: it computes by different rules, which the rest of this course explains.
1 nanometre = 10^−9 metres. Atoms across a feature = its width ÷ the size of an atom: 10 nm ÷ 0.2 nm = 50.
The chance that an electron tunnels through a barrier falls exponentially with the barrier's thickness d, roughly as e^−2κd. Make the barrier a little thinner and the leakage grows many times.
When switches are a few dozen atoms wide, quantum effects take over: a problem for ordinary chips, and the starting point for quantum computers.