An international research collaboration, including teams from the Regensburg Center for Ultrafast Nanoscopy (RUN) and the Max Planck Institute in Hamburg, has experimentally demonstrated a fundamental quantum limit affecting the precise measurement of electrons. This newly observed "space-time limit" indicates that an electron's location and its behavior over time cannot both be determined with arbitrary precision. The finding introduces a fundamental trade-off: increased precision in determining an electron's temporal movement leads to a less confined quantum wave packet in space.
This limitation differs from Werner Heisenberg's well-known uncertainty principle, which places restrictions on simultaneously measuring a particle's position and momentum with unlimited precision. Heisenberg's principle does not impose the same constraints on position and time. The newly observed space-time limit, however, reveals an inherent restriction built into the laws of nature, rather than a limitation of scientific instruments.
The research involved developing a new laser system to observe electron behavior at extremely high speeds, providing finer time resolution than previous experiments. This system allowed the researchers to control and study the quantum motion of individual electrons directly. In their experiments, precisely timed laser pulses propelled electrons from a sharp metal tip towards a silver surface, a distance of only a few atomic diameters. The resulting electrical current was then measured to detect this movement.
The electrons traversed the gap through quantum tunneling, a process where particles can pass through an energy barrier that would be impassable under classical physics. Quantum tunneling is already integral to various modern technologies, including scanning tunneling microscopes and semiconductor devices. In this specific experiment, the team achieved attosecond precision in determining when tunneling occurred. An attosecond is one billionth of a billionth of a second.
This ability to resolve the intrinsic space-time limit of electronic wave packets was achieved through atomic-scale lightwave-driven scanning tunneling microscopy with attosecond time resolution. The researchers employed a novel pump-probe-like experiment, using two time-delayed near-infrared pulses to form phase-controlled single-cycle waveforms. This method allowed them to identify isolated electron tunneling transients shorter than one femtosecond. The measured spatial extension was found to depend on the interplay of multi-photon and field-driven dynamics, a finding confirmed by full quantum simulations.
The ability to localize the attosecond-confined tunneling wave packet on the angstrom scale allowed the researchers to map a single copper adatom on a silver surface. This fusion of attosecond science with atomic-scale scanning tunneling microscopy creates new possibilities for studying wavefunction dynamics within atoms, molecules, and solids.
The discovery holds significant implications for the future of quantum technology and ultrafast electronics. Many future technologies, such as faster computer chips, quantum information systems, advanced energy materials, and precisely directed chemical reactions, rely on controlling electrons at scales beyond the reach of conventional electronics. By understanding and potentially manipulating this space-time limit, scientists may pave the way for electronics hundreds of thousands of times faster than current technology.