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In quantum mechanics, particles such as electrons act like waves and can even interfere with themselves—a striking and counterintuitive feature that defies our classical view of reality. We know this kind of interference happens in space, where different paths can overlap and combine, but what if we could take it further? What if we could control quantum interference in time, where electrons created at different moments interfere?

In a new study in Â鶹ÒùÔºical Review Letters, a team of researchers developed a novel technique—chirped laser-assisted dynamic interference—to manipulate temporal quantum interference during photoionization.

By using extreme-ultraviolet pulses with time-varying central frequency, in combination with intense infrared laser fields, they guided electron motion with unprecedented precision.

Careful tuning of the pulse timing and intensity enabled the researchers to control the photoemission process such that electrons emitted at different times reached the same final energy, allowing their quantum trajectories to interfere coherently. This interference gave rise to well-defined fringe patterns in the photoelectron spectra, revealing key information about the underlying ultrafast physics.

These findings, supported by advanced simulations and experiments at ELI ALPS, a leading European facility dedicated to ultrafast science, represent the first clear experimental observation of this elusive phenomenon, long theorized but previously obscured by competing multiphoton interference effects. ELI ALPS offers unique access to ultrashort light pulses across a broad frequency range—from the terahertz (10¹² Hz) to the X-ray (10¹⁸–10¹⁹ Hz) region and is open to end-users and developers worldwide.

The study is the result of a close collaboration between researchers at Politecnico di Milano, Lund University, IFN-CNR, ETH Zurich and ELI ALPS.

Offering deeper insight into how matter responds to intense laser fields at the quantum level, this breakthrough gives researchers a powerful new tool to manipulate the behavior of electrons on attosecond timescales (a billionth of a billionth of a second), unlocking new possibilities for quantum technologies and ultrafast electronics.