Core Principles
Quantum Tunneling
The ability of a quantum particle to pass through an energy barrier it would not have enough energy to climb classically.
Definition
Quantum tunneling is the effect by which a particle can appear on the far side of a barrier even though a classical particle with the same energy would be stopped. Because a quantum particle is described by a spreading wave, part of the wave leaks through the barrier. Tunneling powers the Sun, makes flash memory work, and is the key mechanism in superconducting circuits and quantum annealers.
In depth
Tunneling is common in technology. Scanning tunneling microscopes image surfaces atom by atom by measuring the tunneling current between a sharp tip and the surface, which depends extremely strongly on distance. Flash memory stores bits by tunneling electrons through thin insulating layers. Tunnel diodes and some transistors rely on it, and as transistors shrink, unwanted tunneling leakage is a design limit.
In quantum computing, tunneling appears in two places. In superconducting circuits, the Josephson junction is a thin insulating barrier between two superconductors through which Cooper pairs tunnel. The junction behaves as a nonlinear inductor, and that nonlinearity is what makes a circuit into a qubit with distinct energy levels. Without tunneling, superconducting qubits would not exist.
In quantum annealing, tunneling is the proposed mechanism for finding low-energy states of optimization problems. A classical algorithm such as simulated annealing climbs over energy barriers using thermal energy, whereas a quantum annealer can tunnel through tall, thin barriers. Whether this gives a real advantage on practical problems is debated, with some benchmarks showing speedups for specially constructed problems and classical algorithms matching in others.
Technical Definition
For a particle of energy E meeting a barrier of height V greater than E and width a, the transmission probability falls off roughly as e to the minus 2 a times the square root of 2m(V minus E) over h-bar. The wave function decays exponentially inside the barrier and a nonzero amplitude emerges on the other side.
Visual Explanation: An Analogy
Imagine rolling a ball toward a hill it cannot climb. Classically it rolls back. A quantum ball is more like a fuzzy cloud, and a little of the cloud seeps through the hill so there is a small chance of finding the ball on the other side.
Key facts
| Item | Detail |
|---|---|
| Transmission | Falls exponentially with barrier width and the square root of barrier height |
| Josephson junction | The tunnel barrier that makes superconducting qubits possible |
| Scanning tunneling microscope | Invented in 1981, Nobel Prize in 1986 |
| Nobel Prize 2025 | Awarded to Clarke, Devoret, and Martinis for macroscopic quantum tunneling in circuits |
| Stars | Fusion in the Sun depends on protons tunneling through their mutual repulsion |
Where the idea comes from
Friedrich Hund identified tunneling in molecular spectra in 1927, and George Gamow used it the next year to explain alpha decay. In 1985 Clarke, Devoret, and Martinis showed tunneling of a macroscopic circuit variable, the work cited in the 2025 Nobel Prize in Physics, which laid the foundation for superconducting qubits.
Real-World Use Cases
- Operating Josephson junctions in superconducting qubits.
- Escaping local minima in quantum annealing.
- Imaging surfaces with scanning tunneling microscopes.
- Understanding leakage and memory behavior in nanoscale electronics.
How it differs from related ideas
- Thermal activation crosses a barrier by climbing over it, while tunneling passes through it.
- Tunneling is a wave effect, related to the same mathematics as interference.
- Tunneling is probabilistic, and its rate depends strongly on barrier shape.
Common Misconceptions
- Tunneling does not mean particles travel faster than light or disappear and reappear. The effect is a feature of wave propagation.
- It is not limited to the microscopic world. Circuits with billions of electrons have shown it.
- It does not guarantee that quantum annealers beat classical algorithms.
Frequently asked questions
What is quantum tunneling?
The passage of a particle through an energy barrier that it could not cross classically, because its wave extends into and through the barrier.
How does tunneling relate to quantum computers?
It underlies the Josephson junction in superconducting qubits and the mechanism proposed for quantum annealing.
Who won the Nobel Prize for tunneling in circuits?
John Clarke, Michel Devoret, and John Martinis shared the 2025 Nobel Prize in Physics for work on macroscopic quantum tunneling in superconducting circuits.
Related Terms
Keep exploring
- Algorithm: Quantum Annealing
- Algorithm: Adiabatic Quantum Computation
- Hardware: D-Wave Advantage2
- Hardware: Google Willow