1973 · Tunneling: Esaki, Giaever & Josephson
Quantum mechanics says a particle can do the impossible: walk through a wall. Not over it, not around it — through it. This is quantum tunneling: a particle's wave leaks into regions classical physics forbids, so there is always a small chance of finding it on the other side. Three physicists turned this ghostly effect into hard reality — and one of their discoveries now sits inside every superconducting quantum computer.
Leo Esaki, working at Sony in Japan, built the first device to exploit tunneling: the tunnel diode (1958), where electrons tunnel through a semiconductor junction. Ivar Giaever — a Norwegian mechanical engineer turned physicist at General Electric — crafted sandwiches of metal films separated by oxide layers just atoms thick, and watched electrons tunnel through them in superconductors, confirming deep predictions about the superconducting energy gap.
Then came the shock. Brian Josephson, a 22-year-old PhD student at Cambridge, predicted in 1962 that pairs of electrons could tunnel through a barrier with zero resistance — a supercurrent flowing through an insulator. The great John Bardeen, himself a double Nobel laureate, said it was impossible. Within a year, experiments proved the student right. The 1973 Nobel Prize in Physics was split: half to Esaki and Giaever "for their experimental discoveries regarding tunneling phenomena in semiconductors and superconductors, respectively," half to Josephson "for his theoretical predictions of the properties of a supercurrent through a tunnel barrier."
गहरे उतरें — गणित और सटीकता
Tunneling comes straight from Schrödinger's equation: inside a barrier, the wavefunction doesn't vanish — it decays exponentially, so a thin enough barrier lets a finite wave leak through. The transmission probability falls off roughly as \(e^{-2\kappa L}\), where \(L\) is the barrier thickness. Josephson's predictions were sharper — two exact relations for the supercurrent \(I\) through the junction as a function of the quantum phase difference \(\varphi\) and the voltage \(V\):
\[I = I_c \sin\varphi, \qquad V = \frac{\hbar}{2e}\,\frac{d\varphi}{dt}\]
The first (DC effect) says a supercurrent flows with no voltage at all; the second (AC effect) says a steady voltage makes the current oscillate at a frequency set only by fundamental constants — an effect now used to define the volt.
Why this prize matters for quantum computing: the Josephson junction is the heart of the superconducting qubit. A Josephson junction acts as a nonlinear inductor — the one circuit element with no classical analogue — which turns a superconducting circuit into an artificial atom with unequally spaced energy levels. Take the lowest two levels, and you have a transmon qubit: the workhorse of IBM's and Google's quantum processors. Every superconducting quantum computer ever built runs on Josephson's 1962 prediction.
मुख्य बातें
- Quantum tunneling lets particles pass through classically forbidden barriers — the wavefunction leaks through.
- Esaki (tunnel diode, semiconductors) and Giaever (tunneling in superconductors) proved it experimentally in solids.
- Josephson, aged 22, predicted supercurrent tunneling through a barrier; Bardeen said impossible; experiment proved him right.
- The 1973 prize was split: half to Esaki and Giaever, half to Josephson.
- The Josephson junction is the core element of superconducting (transmon) qubits used by IBM and Google.
अपनी समझ परखें
Q1.What is quantum tunneling?
Schrödinger's equation lets the wavefunction decay (not vanish) inside a barrier, so thin barriers have measurable transmission probability.
Q2.Why did Josephson's 1962 prediction cause such a stir?
A graduate student's theory overruled a double Nobel laureate's objection within a year — one of physics' great upsets.
Q3.How does the 1973 prize connect to today's quantum computers?
The Josephson junction is the only non-classical circuit element; it creates the artificial atom whose two lowest levels form the qubit.
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