Breaking the light speed barrier with quantum tunneling – the debate gets heated

At the scale of atoms and electrons, walls are not always walls. A particle that lacks the energy to climb a barrier can still appear on the far side, a curiosity known as “quantum tunneling.”

In the heart of the Sun, this quirk lets hydrogen nuclei fuse quickly enough to keep daylight streaming toward Earth.

Physicists have puzzled over how long the hop through the barrier actually takes. Some experiments hinted that the leap happens so swiftly that it appears faster than light, brushing against the limits set by Einstein’s special relativity.

Others clocked a small but finite delay. The debate has rolled on because time, slippery even in classical physics, becomes downright elusive in the quantum world.

Quantum tunneling defies classical limits

Classical rules say a baseball that can’t clear a fence bounces back, end of story. In the quantum picture, particles behave as waves that seep into and even through obstacles.

A sliver of the wave survives inside the forbidden zone, giving the particle a genuine chance to show up on the other side.

Engineers tap that chance in flash memory, Josephson junctions, and scanning tunneling microscopes that map surfaces atom by atom.

Because the wave spreads out, deciding where to mark the particle’s position is tricky.

Earlier timing experiments fixed on the crest of the outgoing wave and reported that the particle covered more ground than a free-flying counterpart in the same span, implying superluminal travel.

Those results stirred excitement but also skepticism, as the crest is a moving pattern, not a point-like traveler.

Trying to define “time”

Physicists Patrik Schach and Enno Giese of TU Darmstadt think the stopwatch itself was misread. “But the particle does not follow a path in the classical sense,” objects Giese.

If you can’t pin down where the traveler is, measuring when it arrives gets murky. Instead of treating the atom as a marble darting through a tunnel, the pair propose using the atom as its own clock.

Their design, outlined in Science Advances, borrows a page from atomic timekeeping. They prepare each atom in a blend of two internal energy states and send it toward an optical barrier.

With a technique akin to a Ramsey sequence, the two states tick against each other while the atom either tunnels or reflects. A reference atom that faces no barrier runs alongside, giving a direct comparison.

Pocket watches and quantum tunnels

“Time is what you read off a clock.” Einstein’s quip fits snugly here. The oscillation between the two energy levels works like a wristwatch small enough to ride the atom.

If tunneling slows the tick, the phase between the levels shifts. Detecting that shift after a second laser pulse reveals how much time elapsed during the barrier encounter.

Calculations show the tunneled clock lags by about 10^-26 seconds – roughly a hundred septillionths of a second. That delay, though vanishingly small, is measurable with clouds of ultracold atoms whose collective signal boosts sensitivity.

The approach dodges the need to guess where the particle is at each instant and stays faithful to quantum principles.

Building a fair stopwatch

A companion atom that never meets the barrier keeps regular time, letting researchers spot any lag or lead. “The tunneled clock is slightly older than the other,” says Schach.

The outcome undercuts earlier claims of speeds beyond light and reframes the question: tunneling might feel instantaneous, yet its inner clock shows the journey costs a pinch of time.

Capturing such a slight slippage is no picnic. Lasers must maintain phase stability with dizzying precision, and stray magnetic fields must be kept in check.

Using atom clouds spreads the work over many identically prepared clocks, and raising the transition frequency – much like switching from a wristwatch to an optical clock – amplifies the effect.

Why does any of this matter?

Sorting out tunneling time is more than settling an academic quarrel. Quantum engineers push devices toward ever thinner barriers in transistors and sensors, where picosecond wiggles can make or break performance.

A crisp measure of tunneling time feeds directly into models that predict leakage currents, coherence losses, and heat budgets in next-generation chips and quantum processors.

On the fundamental side, timing the hop tests how quantum mechanics meshes with relativity.

If the Darmstadt scheme proves that tunneling never outpaces light once the clock rides with the particle, one more apparent paradox evaporates, tightening the weave between the two towering theories of modern physics.

“We are currently discussing this idea with experimental colleagues and are in contact with our project partners,” adds Giese.

Laboratories already pushing atom interferometers to probe gravity or hunt dark matter may pivot to give the tunneling clock a spin.

Success would not just timestamp a famous quantum trick; it would hand scientists a new metrological tool, ready to keep pace with particles that treat walls like suggestions rather than commands.

The full study was published in the journal Science Advances.

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