Making Waves on Reentry


A new approach to measuring electrons’ quantum states looks at the tail end of their escape

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One of the most basic of quantum mechanical phenomena occurs when light strikes matter, removing an electron as it hits. To physicists, this process – photoemission – can be used as a window into the quantum world: It projects the quantum state of matter onto free electrons, providing an opportunity for measuring this state. Photoemission is also one of the fastest phenomena in nature – mere tens or hundreds of attoseconds. That is, it occurs within a few quintillionths of a second, so that attempting to measure it directly is highly challenging. Prof. Nirit Dudovich and her group in the Weizmann Institute of Science recently demonstrated an “upside-down” scheme for investigating the quantum properties of escaped electrons.   

“Most physicists examine the quantum state of matter by pulling the electron out using an energetic photon. But this approach obscures important information,” says Dudovich, who is in the Physics of Complex Systems Department. “We completely turned this basic order around using a process known as photo-recombination. While others measure an electron released by a photon, we measure a photon emitted by the electron colliding with the atom as the two are reunited. These are two sides of the same coin, but it turns out that the ability to measure the emitted photon – basically, an ultra-rapid pulse of light – provides access to hitherto inaccessible information.”  The quantum state of matter is imprinted in the wave properties of the emitted ultrashort pulses, and it is these that Dudovich and her group sought.

Dudovich’s lab is uniquely suited for this sort of research, as she develops advanced measurement techniques that enable her and her group members to study ultrafast phenomena with attosecond resolution (an attosecond is a billionth of a billionth of a second). Dudovich and her team had been looking for a way to resolve the wave properties of attosecond light pulses. Wave properties are defined by wavelength, amplitude and phase.

Phase measurements are most commonly conducted with an interferometer; optical interferometers can be found in almost every optical physics lab. An interferometer, as its name suggests, looks at waves that interfere with one another – for example, light waves or those of charged particles – and uses the interference patterns to reveal the wave nature of the quantum phenomena that produce them. Dudovich explains that, like the ripples in a pool of water – which can appear organized or random – the interference patterns from photons or electrons can be recorded and analyzed to reveal their individual phases and how they are organized. The ability to resolve the phases of light or other waves from various sources can then reveal new aspects of the structure of quantum matter.

One day around two years ago, Doron Azoury, one of her students, called Nirit in excitement. An unexpected signal that appeared during a routine calibration test revealed a faint sign of interference between two attosecond pulses. This accidental discovery motivated the group to introduce substantial upgrades to the experimental setup, until they were able to demonstrate attosecond accuracy in their interferometer.

It was a really exciting experiment, to know that we have the ability to directly measure such fundamental effects in physics

Continuing to investigate, the group came to understand that the interference they were observing was the signature of electrons reuniting with their atoms through photo-recombination. They realized the method was so precise, it could be used not just to observe, but also to measure this reunion. Dudovich, Azoury and team members Michael Krüger and Omer Kneller designed new experiments to explore the wave properties of the photoemission process. The results of one set of experiments, recently published in Nature Photonics, show that phase measurements were used in their lab to fully resolve the photoemission process.

“These phases have been calculated – you can find them in textbooks. But this is the first time they have been directly measured in this really accurate way. We measured them with attosecond precision, and our measurements agreed perfectly with the calculations. It was a really exciting experiment, to know that we have the ability to directly measure such fundamental effects in physics. Hopefully our results will be used as a platform to reveal new ultrafast phenomena in more complex systems,” says Dudovich.

Prof. Nirit Dudovich's research is supported by the Helen and Martin Kimmel Award for Innovative Investigation; the Crown Photonics Center; the Jay Smith and Laura Rapp Laboratory for Research in the Physics of Complex Systems; the Rosa and Emilio Segre Research Award; the Wolfson Family Charitable Trust; the Jacques and Charlotte Wolf Research Fund; and the estate of Raymond Lapon. Prof. Dudovich is the incumbent of the Robin Chemers Neustein Professorial Chair.