Intense quantum light generates ‘weird’ electron behavior

Photon-number distributions of various light sources have been studied extensively, but little is known about the statistical distribution of electrons emitted under the effect of intense quantum light.

However, researchers at the Max Planck Institute for the Science of Light (MPL) and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have made a discovery that highlights this phenomenon.

Studying electron emission from quantum light

Published in the journal Nature Physics, the study led by Prof. Maria Chekhova at MPL and Prof. Peter Hommelhoff at FAU reveals extreme and highly unusual statistical events in electron-number distributions obtained when nanometer-sized metal needle tips are illuminated with ultrashort pulses of bright quantum light.

These findings prove that the number of electrons is influenced by the light statistics and contribute to a deeper understanding of the process of electron emission, which could help further improve electron microscopes.

Understanding electron emission

Electron emission is the process by which electrons are released from a material, typically a metal or semiconductor, into a vacuum or another medium. There are several types of electron emission, each with its own unique characteristics and underlying mechanisms.

Thermionic emission

• Occurs when a material is heated to high temperatures, causing electrons to gain enough energy to overcome the work function and escape the surface.
• Commonly used in vacuum tubes, cathode ray tubes (CRTs), and electron microscopes.

Photoelectric emission

• Happens when a material absorbs photons (light particles) with sufficient energy to overcome the work function, causing electrons to be ejected from the surface.
• The basis for photoelectric sensors, solar cells, and photomultiplier tubes.

Field emission

• Involves applying a strong external electric field to a material, which narrows the potential barrier and allows electrons to tunnel through the barrier and escape the surface.
• Used in field emission displays, electron microscopes, and vacuum microelectronics.

Secondary electron emission

• Occurs when a material is bombarded with high-energy particles (e.g., electrons or ions), causing the emission of low-energy secondary electrons from the surface.
• Important in electron multipliers, photomultiplier tubes, and scanning electron microscopes.

Schottky emission

• A combination of thermionic and field emission, where an external electric field lowers the work function, making it easier for electrons to escape the surface at lower temperatures.
• Used in high-brightness electron sources and vacuum microelectronics.

The study of electron emission is crucial for understanding the behavior of materials under various conditions and for developing advanced technologies such as electron microscopes, vacuum electronics, and energy conversion devices.

By manipulating the factors that influence electron emission, researchers can design materials and devices with specific electronic properties tailored to various applications.

Needle tips, electrons, and quantum light

In this collaborative project, the researchers illuminate nanometer-sized metal needle tips with pulses of classical light and quantum light. They then detect the electrons released from the metal and study their statistical properties.

When classical light is used, the electrons follow a Poissonian distribution, meaning that each electron is emitted independently of the others. This results in only slight variations in the number of electrons emitted from pulse to pulse.

‘Bright squeezed vacuum’ quantum light

By switching to a quantum light source called bright squeezed vacuum, which exhibits strong photon-number fluctuations, the researchers demonstrated that the statistics of photons can be transferred to electrons.

Using this technique, they measured extreme statistical events with up to 65 electrons from a single light pulse, with an average value of 0.27 electrons per pulse.

To put this into perspective, if the electrons followed a Poissonian distribution, the probability of such an event — an outlier exceeding the mean by a factor of 240 — would be as low as 10-128.

By changing the number of modes of the squeezed vacuum, the scientists could tailor the electron-number distribution on demand.

“Our results show that photon statistics are imprinted from the driving light onto the emitted electrons, opening the door to new sensor devices and strong-field optics with quantum light and electrons,” says Maria Chekhova, research group leader at MPL.

Raisin muffins and electron distributions

Jonas Heimerl, an FAU Ph.D. student, explains the dimensions of this discovery using an everyday example.

“If you spread raisins on muffins, the probability of finding a certain number of raisins in the muffin follows a Poisson distribution. Let us assume that there is an average (mean) of two raisins per muffin,” Heimerl pontificated.

“It may therefore happen that there are no raisins or five raisins in the muffin, but in most cases, there will be two. However, the probability of getting more than 50 raisins is impossible with a Poisson distribution,” he explained.

In contrast, the multi-electron events observed in these experiments were like finding 480 raisins in a single muffin — a discovery that would undoubtedly delight any raisin lover.

Quantum light opens doors to tech innovation

In summary, this research from MPL and FAU has unveiled the surprising influence of quantum light on electron emission.

By illuminating nanometer-sized metal needle tips with bright squeezed vacuum, they have demonstrated that the statistical properties of photons can be imprinted onto electrons, leading to extreme and highly unusual electron-number distributions.

This discovery deepens our understanding of the fundamental processes governing electron emission and opens doors to exciting possibilities for the development of new sensor devices, advancements in electron microscopy, and the exploration of strong-field optics with quantum light and electrons.

As we continue to unravel the mysteries of the quantum world, these findings serve as a testament to the power of innovative research and the potential for quantum phenomena to revolutionize our understanding of the universe.

The full study was published in the journal Nature Physics.

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