A quick refresher on the impossible
Roll a ball at a hill. If it does not have enough energy to reach the top, it rolls partway up and comes back — always. That is classical common sense. A quantum particle facing a barrier it cannot energetically surmount does something that has no everyday equivalent: now and then, it simply *appears on the other side*, having passed through the forbidden region without ever being on top of it. This is quantum tunnelling, and it is not a metaphor or an approximation — it genuinely happens, constantly.
The reason is the wavefunction. A particle is described by a wave of probability, and when that wave meets a barrier it does not stop dead at the wall. Instead it leaks a little way *into* the forbidden region, fading exponentially — an evanescent wave. If the barrier is thin enough that some of the wave is still nonzero on the far side, then there is a real, calculable chance the particle is found over there. The thinner and lower the barrier, the larger that chance — quantitatively, the tunnelling probability.
Seeing single atoms: the scanning tunnelling microscope
The most beautiful use of tunnelling is the scanning tunnelling microscope (STM), invented in 1981, which can image a surface atom by atom. The trick: bring an extremely sharp metal tip — ideally ending in a single atom — to within a hair's-breadth of a conducting surface, close enough that the gap between them is a barrier thin enough for electrons to tunnel across. Apply a small voltage, and a tiny tunnelling current flows through that empty gap. No touching required; the electrons cross the void by tunnelling.
Now the exponential earns its keep. Because the tunnelling current changes so violently with the gap width, even an atom-sized bump under the tip — a fraction of a nanometre — sends the current swinging measurably. Sweep the tip back and forth across the surface while a feedback loop nudges its height to keep the current constant, and the record of those height adjustments traces out the surface's bumps and dips: a map at the scale of individual atoms. The STM does not use lenses or light at all; it feels the surface with a quantum current.
The payoff was historic: for the first time, humans could *see* individual atoms laid out on a surface, and eventually even nudge them one at a time into chosen patterns. An entire field — nanotechnology — leans on this ability to look and poke at the atomic scale, all built on a particle's willingness to cross a gap it classically cannot.
Tunnel diodes and the flash in your phone
Tunnelling also shows up inside electronics. The tunnel diode, devised by Leo Esaki in the late 1950s, is a semiconductor component built so that electrons tunnel across an ultra-thin junction. Because tunnelling responds almost instantly to voltage — there is no slow drift of charges, just a wave leaking through — these devices can switch extraordinarily fast and were prized for high-frequency circuits. Esaki shared a Nobel Prize for showing tunnelling at work in solids.
The most widespread example is in your pocket. Flash memory — the storage in phones, cameras, and solid-state drives — works by parking electrons on a tiny isolated island of conductor, walled off by an insulating barrier. To write or erase a bit, a voltage forces electrons to *tunnel* through that barrier onto or off the island; once there, with no voltage applied, the barrier is too thick for them to tunnel back out on any human timescale, so the bit stays put for years. Every photo and song you have saved is held in place by the careful tuning of a tunnelling barrier — thin enough to write through, thick enough to remember.
It is worth noting that tunnelling is not always a friend. As chip transistors shrink toward a few nanometres, their insulating layers grow so thin that electrons tunnel across where they should not, leaking current and wasting power. The same effect that we harness on purpose in flash memory becomes a limit on how small ordinary transistors can be — a reminder that tunnelling is a fundamental, always-on feature of the quantum world, not an optional gadget.
The same effect, all over nature
Tunnelling is not just a laboratory trick; nature has been using it forever. The radioactive alpha decay of heavy nuclei is tunnelling — an alpha particle escapes a nucleus it has nowhere near enough energy to climb out of, by tunnelling through the barrier holding it in. The Sun shines because protons in its core tunnel through their mutual repulsion to fuse; without tunnelling, the core would be too cool for fusion and the stars would not burn. The same loophole that lets you store a photo keeps the Sun alight.
So the lesson of this guide is double. Tunnelling is genuinely strange — a particle reaching where its energy forbids — yet it is also utterly ordinary, woven into your phone's memory, into a microscope that maps atoms, into the radioactivity of rocks, and into starlight. The quantum loophole is not a curiosity at the edge of physics; it is a working part of the machinery of the universe, and of the machinery on your desk.