The Heisenberg uncertainty principle forbids knowing simultaneously the position and the momentum of an electron. German researchers have just uncovered a comparable limit between position and temporal evolution — and have measured it directly for the first time thanks to laser pulses on the attosecond timescale. The study appears in Nature Photonics.
What you will learn
- What an attosecond is — and why it is the time scale on which electrons move
- Why attempting to pin down the exact moment of an electron in time necessarily broadens its spatial distribution
- What future applications this discovery enables, from ultrafast electronics to chemically triggered reactions
An uncertainty principle Heisenberg had not formulated
The famous Heisenberg uncertainty principle states that one cannot determine with arbitrary precision the position and the momentum of a particle at the same time — not because of a lack of instruments, but because nature fundamentally forbids it. Between position and time, however, this principle had never been formalized.
Researchers from the Ultrafast Nanoscopy Center in Regensburg, in collaboration with the Max Planck Institute in Hamburg, have just observed and quantified for the first time that it is also impossible to measure the position of an electron and its temporal evolution with arbitrary precision at the same time. This fundamental spatiotemporal limit of quantum mechanics, suspected but never measured directly, is described in Nature Photonics.
The attosecond: the time scale of electrons
To grasp the significance, one must appreciate the orders of magnitude at play. An atom is about ten million times smaller than a millimeter. An attosecond is a billionth of a billionth of a second — the same relation as a second to the age of the universe. It is at this scale that electrons move, a thousand times faster than atoms and molecules.
To watch these motions, the team developed a new laser system that generates ultrashort pulses. These pulses steer the motion of electrons from a atomically thin metallic tip toward a silver surface, over a distance of merely a few atomic diameters. By adjusting the delay between two successive laser pulses, the researchers can directly observe the electrons’ response — and measure the minute delay of 500 attoseconds with which an electron follows the light field.
More precise timing, less spatial precision
The core of the discovery lies in the trade-off between temporal resolution and spatial distribution. Raffael Spachtholz, co-author of the study, states it plainly: “The more one wishes to determine the electron’s position in time with precision, the more energy must be supplied. Consequently, the electron’s wave packet expands further in space.”
This is the temporal mirror of Heisenberg’s uncertainty principle — a fundamental constraint on the simultaneous knowledge of an electron’s spatial location and its temporal evolution, not a technological limit.
Fortunately, despite a strong temporal excitation, the wave packets retain enough spatial resolution to enable atom-scale microscopy — opening the way to an ultrafast camera for quantum behaviors.
Applications that touch the human imagination
These results open up substantial prospects. Specifically triggering chemical reactions by transferring electrons at the atomic scale in both time and space, and watching how chemical bonds break or change in real time — these are the immediate aims of the team.
In the longer term, electronics operating at the intrinsic speed of electron motion would be hundreds of thousands of times faster than today’s CMOS technology. “The potential applications are currently limited more by human imagination than by nature,” conclude the project leaders.
