What happened?
Researchers at the University of Regensburg and the Max Planck Institute for the Structure and Dynamics of Matter have combined atomic-scale scanning tunnelling microscopy with attosecond laser pulses. Their instrument followed individual electron wave packets while electrons tunnelled between an atomically sharp metal tip and a silver surface.
Two carefully delayed near-infrared light pulses changed the energy barrier between the tip and surface. By adjusting the delay and recording the resulting electric current, the team reconstructed tunnelling events shorter than one femtosecond while retaining enough spatial resolution to map a single copper atom on the silver.
The measurements and quantum simulations showed that the electron response lagged behind the driving light field by about 500 attoseconds. They also showed that squeezing the tunnelling event into a shorter time changes the spatial spread of the electron wave packet. This experimentally reveals a coupled space-time limit for this particular ultrafast process.
The simple version
A scanning tunnelling microscope brings an extremely sharp conducting tip close to a surface. The tip does not touch. When the gap is tiny, quantum tunnelling gives electrons a probability of crossing the barrier, producing a measurable current.
Ordinary scanning tunnelling microscopy gives a remarkably sharp picture of where atoms are, but it is not normally fast enough to show how an electron changes during an attosecond event. The new experiment adds a pair of ultrashort light pulses that act like a start signal and an adjustable stopwatch.
An electron in quantum mechanics is not a hard dot following one exact path. It is described by a wave packet: a spread of possible positions and momenta. The experiment showed that changing when tunnelling occurs also changes how far that wave packet spreads across space.
Worked equations
How short is an attosecond?
An attosecond is one billionth of a billionth of a second.
- Prefix: atto = 10^-18
- One femtosecond: 1 fs = 1000 as
The simulated response delay
Even a delay this small can be resolved when the laser waveform is controlled precisely.
- Convert: 500 x 10^-18 s = 5.00 x 10^-16 s
Position-momentum uncertainty
Localising an electron more tightly in position requires a wider spread of possible momentum values.
- This is the standard Heisenberg position-momentum uncertainty relation.
- The reported space-time coupling is a dynamical effect, not a new time-position version of this equation.
Why it matters
Many processes that control electronics, light-sensitive materials and chemical bonds begin with electrons moving over atomic distances. A microscope that can resolve both the location and the timing of that motion gives physicists evidence that was previously hidden inside an averaged current.
The instrument can test quantum calculations of tunnelling directly. In this experiment, the quantum simulation reproduced the measured response and identified the roughly 500-attosecond delay. Agreement between a time-resolved measurement and a full simulation makes the model more useful for future materials.
The work could eventually help researchers trigger and observe charge transfer in a chosen molecule or bond. That is a long-term possibility, not a finished technology, but the experiment supplies the combination of spatial and temporal control such work would require.
Physics you already know
This links directly to electric current. The microscope does not photograph an electron with visible light; it measures the small current produced when electrons cross the gap between tip and sample.
It also links to wave-particle duality. Calling the electron a wave packet explains why its position is spread out and why changing the energy and timing of the driving field can alter that spatial spread.
Quantum tunnelling means the wavefunction extends into and beyond an energy barrier. The electron can therefore be detected on the other side even when a classical particle with the same energy could not cross.
The two infrared light pulses provide a pump-probe style measurement. Changing their relative delay builds up a sequence of measurements, rather like reconstructing a very fast film from many carefully timed snapshots.
Science ideas to understand
What was directly measured?
The experiment measured tunnelling current as the delay between two ultrashort light pulses was changed. The electron wave-packet behaviour was reconstructed from these repeated measurements and compared with quantum simulations.
Why the sharp tip matters
A scanning tunnelling microscope concentrates the measurement into an atom-sized region. That spatial selectivity is what allows an attosecond timing technique to become an atomic-scale microscope.
Common misconception
The result does not show an electron travelling along a visible miniature path. Quantum mechanics predicts a changing wavefunction and probabilities, which the current measurements test.
A Level stretch
The uncertainty principle applies to position and momentum; there is no ordinary Heisenberg uncertainty relation between position and time because time is a parameter rather than a position-like quantum observable. The space-time limit here arises from the dynamics: shortening and strengthening the light-driven tunnelling event changes the energy and momentum components that form the wave packet.
The paper reports isolated tunnelling transients shorter than one femtosecond and localisation on the angstrom scale. One angstrom is 10 to the power minus 10 metres, a useful scale for distances between atoms.
The light cannot be described adequately as only a classical wave or only a stream of independent photons in this extreme regime. The field waveform drives the barrier, while multiphoton absorption also contributes to the electron dynamics.
A key limitation is repetition. The microscope reconstructs probabilities from many controlled events; it is not a camera following one named electron continuously along a classical trajectory.
Key words
Quick pupil questions
What did the attosecond scanning tunnelling microscope measure?
It measured light-driven electron tunnelling between a sharp tip and a silver surface with atomic spatial resolution and attosecond timing.
How long is 500 attoseconds?
Five hundred attoseconds is half of one femtosecond, or five ten-quadrillionths of a second.
Does the experiment create a new Heisenberg uncertainty principle?
No. Position and momentum still obey the standard uncertainty relation. The reported space-time limit is a dynamical coupling between the duration and spatial spread of a driven electron wave packet.
How does this link to A Level Physics?
It applies electric current, infrared radiation, wave-particle duality, quantum tunnelling, uncertainty and standard-form time conversions.