Author
Royal Swedish Academy of Sciences
3 reading cards from 3 books · 1973–2025.
3 cards
Press release: The 1973 Nobel Prize in Physics · 1973
A quantum particle neither jumps the wall nor breaks it: its wave passes through weakened, and what is left on the far side is a chance.
Picture a ball you roll towards a hill. If it is not fast enough, it climbs a little, stops and rolls back. Never, not even once in a million tries, does it turn up on the other side. That is how the big world works, the world of balls, cars and people. In the very small world of electrons and protons the rule changes. There, a particle that does not have enough energy to get over an obstacle still has a small chance of being found beyond it. Physicists call this tunnelling, although no tunnel is dug and the wall stays whole. Why does it happen? In the lessons on the double-slit experiment we saw that a particle also behaves like a wave, and the wave tells you where you are likely to find it. When the wave hits a wall it does not stop dead at the surface. It goes a little way into the wall and fades gradually, much as sound comes through your neighbour's wall, weaker. If the wall is thin enough, something of the wave is left on the far side. That remainder means a real chance that the particle is detected beyond. The sound analogy has a limit: the sound arrives quieter, whereas the particle that gets through arrives whole, with all its energy. Not a ‘piece’ of an electron passes, but the whole electron, only more rarely. The most important part is how fast the chance falls. It does not halve when you double the wall's thickness; it collapses. For an electron, each extra layer as thick as a small atom divides the chance by roughly ten. The height of the wall matters too and, very much, the mass. A proton, almost two thousand times heavier than an electron, gets through the same wall vastly more rarely. That is why tunnelling shows up with electrons, with atomic nuclei and in the cores of stars, but never with a ball or with a person who bumps into a wall.
“These waves can penetrate a barrier that would be a forbidden area if the particle was considered in the classical way. The term tunneling phenomenon refers to this property – the particle “tunnels” through the forbidden area.”
Press release: The 1986 Nobel Prize in Physics · 1986
The microscope sees atoms and flash memory keeps data because tunnelling depends enormously on distance and voltage.
The first invention came in 1957. Leo Esaki, a young researcher at the Japanese company that would become Sony, made a diode (a part that lets current pass only one way) with an extremely thin separating layer. The current crossed the layer by tunnelling, and the diode behaved strangely: at a certain voltage, if you pushed harder, the current went down. That made it very fast. The Esaki diode earned him the 1973 Nobel Prize in Physics, together with Ivar Giaever and Brian Josephson. The second: the scanning tunnelling microscope, built in 1981 by Gerd Binnig and Heinrich Rohrer at IBM's laboratory in Zurich. A needle sharpened down to a single atom at its tip moves over a surface without touching it, at a distance of about two atoms. Electrons tunnel between the needle and the surface. Because the chance of getting through falls very fast with distance, the current changes about tenfold if the needle rises or falls by a tenth of a nanometre. By holding the current steady and following how the needle moves up and down, you can draw the surface atom by atom. The Nobel came in 1986. In 1989, at IBM, Don Eigler and Erhard Schweizer used such a needle to move 35 xenon atoms and spell the letters I, B and M. The third is in your pocket: the flash memory in your phone and on USB sticks. Each bit is a tiny island of conductor buried in an insulator. To write, you apply a high voltage and electrons tunnel through the insulator onto the island. Without the voltage, the insulator is thick enough for them to stay there for years, with the phone switched off. The same tunnelling is also a problem: when the insulator in transistors shrank to a few atoms thick, electrons began leaking through it, and the industry had to switch to other materials.
“An electrical potential between the tip of the stylus and the surface causes an electric current to flow between them despite the fact that they are not in contact. […] Recording the vertical movement of the stylus makes it possible to study the structure of the surface atom by atom.”
Press release: The Nobel Prize in Physics 2025 · 2025
A circuit you can hold in your hand can tunnel like a single particle, if it is cold and quiet enough.
Until now, tunnelling has been the business of tiny particles: an electron, a proton, an alpha particle. When you put many particles together, quantum effects usually wash out, which is why a ball does not pass through a wall. In the early 1980s the physicist Anthony Leggett asked a precise question: can a large, human-made object, made of very many particles, behave quantum-mechanically as a whole? If so, we might see it tunnel. At the University of California, Berkeley, professor John Clarke, postdoctoral researcher Michel Devoret and doctoral student John Martinis built a circuit out of superconductors, materials through which current flows with no loss at all when they are very cold. Between two pieces of superconductor was a thin insulating layer, a so-called Josephson junction. Cooled close to absolute zero, the circuit behaves strangely: billions of electron pairs move together, like a single giant ‘particle’ that fills the whole circuit. At first the current flows with no voltage at all, like a marble caught in a dip. Classically, the marble should stay there. Yet it escaped by tunnelling, and the escape showed up simply, on an instrument: a voltage suddenly appeared. They also showed the second quantum feature: the circuit took in energy only in portions of precise sizes, not in any amount, like an atom. That is why people speak of ‘artificial atoms’. Later Martinis used exactly these energy steps to build a qubit, the unit of information in a quantum computer: the lowest step means zero, the first step up means one. Superconducting circuits are today one of the main routes to quantum computers. The 2025 Nobel Prize in Physics went to the three ‘for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit’.
“It is wonderful to be able to celebrate the way that century-old quantum mechanics continually offers new surprises. It is also enormously useful, as quantum mechanics is the foundation of all digital technology.”