The fascinating world of quantum tunneling
Quantum mechanics and this year’s Nobel Prize
Without quantum mechanics, there would be no modern electronics, from mobile phones to computers, and no photonic technologies such as lasers or LEDs. In this essay, Fredrik Kihlborg elaborates on this year’s Nobel Prize in physics: tunneling.
During the first half of the twentieth century, the theory of quantum mechanics emerged from the minds of giants — Planck, Einstein, Bohr, Heisenberg, Schrödinger, Born, de Broglie, Dirac, and Pauli — who together reshaped our understanding of reality at its most fundamental level. It stands as one of the most successful theories in the history of mankind, confirmed time and again by experiment. Its predictions match reality with almost absurd precision and it underpins nearly every aspect of modern science and technology.
Without quantum mechanics, we wouldn’t grasp the nature of chemical bonds and reactions, and we couldn’t design most of the pharmaceutical drugs we take for granted today, and there would be no modern electronics, from mobile phones to computers, and no photonic technologies such as lasers or LEDs.
The compelling behaviour of atoms and particles
Quantum mechanics describes the behaviour of the world at the very smallest scales — those of atoms and particles. This is where many of the juicy intricacies that govern materials occur, and where things behave in ways that defy our everyday, macroscopic intuition. Here, particles exist only at discrete energy levels (so-called quanta) and can exist in multiple places at once (superposition). They behave both as particles and waves (the wave–particle duality) and it is fundamentally impossible to know both the precise position and the momentum of a particle at the same time (limited by the Heisenberg uncertainty principle). Under certain conditions, two particles can become linked so that their states remain perfectly correlated, no matter how far apart they are (entanglement). And they can even pass through energy barriers they shouldn’t be able to, according to classical physics (tunneling).
Does tunneling happen only in the microscopic world?
We don’t see our colleagues pass through walls on their way to the coffee room — unless they’re moving fast enough to qualify as a workplace incident rather than a quantum effect. And if you ever feel you’re attending two meetings simultaneously, don’t worry — it’s probably not quantum superposition. Just close a Teams window. So why is it that we typically don’t observe quantum phenomena in the macroscopic world?
That’s because every macroscopic object consists of an enormous number of particles, and all of them would have to act in perfect unison to produce a measurable quantum effect. Our bodies, for example, contain around 10²⁸ particles (give or take), and every single one would need to tunnel at exactly the same time and in exactly the same direction for you to pass through a wall. Since walls are usually a bit thicker than a few nanometres, this would have to happen repeatedly — many, many times. That’s rather improbable, even by quantum-mechanical standards. In theory it isn’t impossible, just so fantastically unlikely that it would never occur once in the entire lifetime of the universe.
This year’s Nobel Prize in physics
The Nobel Prize in Physics 2025 went to John Clarke, Michel H. Devoret and John M. Martinis who achieved the tremendous feat of designing a macroscopic system in which particles acted in perfect unison, allowing quantum effects to be observed directly. They used a so-called Josephson junction, consisting of two superconducting materials separated by an ultra-thin oxide layer only a few nanometres thick. The layer was thin enough for electrons (or electron pairs to be correct) to tunnel across, and the measured energy levels were discrete — in perfect agreement with quantum mechanics. It was an astonishing accomplishment, both experimentally and theoretically.
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- The groundbreaking technologies that tunneling could unlock
- Quantum mechanics at AFRY
Let’s go back to our unfortunate colleague who doesn’t tunnel through the wall on his way to the coffee room. If he doesn’t break the wall, he doesn’t get through. End of story (though possibly followed by a loud noise and a request for medical assistance).
However, unlike our coffee-seeking colleague, light does pass through frosted glass and sound does leak through walls. That’s because both light and sound are waves — electromagnetic and vibrational waves, respectively. The same is true for particles – they behave both as particles and as waves at the same time, a phenomenon known as the wave–particle duality. This is not just a beautiful way of describing it or a convenient mathematical trick. It’s a well-established experimental fact.
Take an electron as an example. When it’s confined by a potential barrier, its wave (or wavefunction to be correct) doesn’t stop abruptly at the boundary — it extends slightly into the forbidden region, meaning there is a small probability of finding the electron on the other side. If we apply an electric field, the wave is pushed closer to the barrier, and the chance of tunnelling increases. Raising the temperature gives the electron more energy, again making tunnelling more likely.
In essence, tunnelling is a matter of probability. For a localized particle — say, an electron bound in a molecular orbital — there is a region where it’s most likely to be found, but also a tiny chance that it exists farther away. If this faint tail of probability overlaps with a neighbouring orbital, the electron can effectively “hop” across. Classically, the electron would be trapped by an energy barrier too high to climb, but in quantum mechanics it can slip through that barrier as a wave.
Flash memory (the kind used in USB sticks) is a great example of tunneling in action. These devices consist of semiconducting silicon with an ultra-thin oxide layer. When an electric field is applied across the oxide, electrons can tunnel through it into a floating gate, where they become trapped. The presence or absence of these trapped electrons changes the conductivity of the silicon, defining whether the bit represents a 0 or a 1.
But tunneling isn’t always a welcome effect. In modern silicon chips, the smallest feature size of transistors is just 10–20 nanometres — about 50 atoms across and roughly a hundred times smaller than a red blood cell. At these scales, quantum tunneling becomes a major design challenge. Electrons can slip through barriers that were once perfect insulators, causing leakage currents that waste power and generate heat.
No tunneling – no stars
Tunneling isn’t just for technology, it’s also what keeps the universe running. Fusion inside stars depends on tunneling – protons fuse into helium only because they can quantum-tunnel through their mutual repulsion. So, without tunnelling there would be no stars. And tunnelling is also what fuels life on our planet – electron transfer in photosynthesis would be far too slow for plants, and life, to function.
In the case of the Josephson junction of this year’s Nobel Prize, pairs of electrons reside in quantum states on either side of a thin oxide layer. Because of their wave-like nature, the states on both sides overlap slightly, giving the pairs a finite probability to tunnel through the barrier. This collective tunneling of electron pairs is what makes the Josephson junction such a striking demonstration of macroscopic quantum behaviour.
The theory of quantum mechanics has been around for over a century, and ways to harness quantum effects have been thoroughly explored at the theoretical level for decades. But the realization of quantum technologies has long been limited by our ability to measure with sufficient sensitivity and to manipulate systems with sufficient precision. Today, manufacturing and sensing technologies are reaching a level of maturity that allows quantum technologies to begin emerging in practical applications.
Quantum computers are already here in small scale. They utilize the fact that particles (qubits) can be in more than one state at the same time (so-called superpositions) among other quantum mechanical phenomena. As the size of these computers increase (in number of cubits) we will be able to make some types of calculations orders of magnitudes faster than today. We’ll use quantum computers not to write emails faster, but to understand nature, design new materials, and uncover patterns that classical logic simply can’t reach. A couple of examples are modelling of complex biomolecules to make new pharmaceuticals and in optimization of logistics and finance by exploring many possible combinations simultaneously.
Remember quantum entanglement, which we mentioned briefly in the introduction, where particles are connected in such a way that the state of one instantly reflects the state of the other, no matter the distance between them. This makes it possible to share an encryption key and be certain that no one else has intercepted it — guaranteed by the laws of nature themselves. The technology already exists today, but it’s still limited by how sensitive the entangled states are. As these systems become more robust, we’ll start to see them used more widely to secure communication between banks, government agencies, and others handling sensitive information. Unlike traditional cryptography, which relies on mathematical complexity, quantum cryptography is protected by the fundamental principles of physics, making it immune even to attacks from future quantum computers.
Quantom entanglement makes it possible to share an encryption key and be certain that no one else has intercepted it — guaranteed by the laws of nature themselves.
Quantum sensors will give us the possibility to measure with ridiculous precision. In the quantum realm, even the smallest disturbance — a shift in distance, energy, or magnetic field — can have a dramatic effect, which can be used for sensing. For example, tunneling or spin polarization (the tiny “twist” of particles) could act as the transduction mechanisms. Scanning tunneling microscopes (STM) and magnetic resonance imaging (MRI) use this already today, but in very large machines. The quantum sensors of the future will be small and portable.
At AFRY, as a project management and engineering company, we tend to focus more on designing systems than on developing the underlying theories behind them. But to design the best systems, we need to be experts at using new technologies — and that means understanding what they can bring.
Quantum mechanics opens an entirely new world of possibilities. It’s the science that explains everything from the birth of space and time at the Big Bang, to the fusion furnaces of stars, to the quantum “hairs” on the edges of black holes, and even the restless froth of empty space itself. It’s also the foundation of many of the technologies that have transformed our lives, and the source of the next generation still to come. What could be more inspiring than that?
