Building bridges between strong-field physics and quantum optics

Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)

A working group at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has made an important contribution to investigating the fundamental connections between strong field physics and quantum optics: The researchers have investigated for the first time how electrons emitted by metallic needle tips after light is shone on them behave in strong quantum light. Their study has now been published in the journal Nature Physics.

For a long time, the two areas of strong-field physics and quantum optics were considered independent areas of physics research without any significant overlaps. Whilst strong-field physics focuses on the behavior of material, such as atomic gases, in intense light fields, quantum optics focuses on researching special quantum properties of light that cannot be described within the framework of classical physics. Strong-field physics requires intense laser rays, in other words rays with innumerable photons, whilst quantum optics focuses on the physics of fewer photons. Over recent years, the two areas have come closer together, leading to the emergence of the area of strong-field quantum optics. A group of physicists from FAU and the Max Planck Institute for the Science of Light, led by Prof. Dr. Peter Hommelhoff and Prof. Dr. Maria Chekhova have now made an important experimental contribution to the advancement of this field of research.

The special properties of quantum light

Light can appear in special forms that are so exotic that they represent pure quantum light. One example is the “bright squeezed vacuum”. This state of light, that can be created in the laboratory in optical crystals, has various properties that differ from normal light. In particular, the central electrical field is equivalent to zero at all times. Nevertheless, the light exhibits significant fluctuations in the electric field.

Quantum light states such as squeezed light are typically very weak, often consisting of only a few protons. For a long time, it was impossible to create stronger let alone extremely intense quantum light in experiments. The group led by Maria Chekhova, Professor of Optics at FAU and head of a research group at the Max Planck Institute for the Science of Light, has been conducting research into the creation of such intense quantum light states for a long time and has in recent years developed methods for producing an intense squeezed vacuum.

Strong-field physics with intense quantum light

The group led by Peter Hommelhoff, Chair of Laser Physics at FAU, has been investigating the interaction between intense light fields and material for a long time now. At Hommelhoff’s Chair, electrons are released from extremely sharp needle points using ultrashort pulses of light. These electrons are then accelerated through the optical electrical field at the surface of the metal tips. If the electrons are directed back to the tip, they can be elastically repelled and receive additional kinetic energy from the light field. Finally, the researchers measure the distribution of kinetic energy in the electrons; typically, a plateau is observed in the electron spectrum. This is a region in the spectrum in which the count rate for the electrons remains the same over a certain spectral area, where otherwise the count rate falls significantly as the kinetic energy increases. The reason for this is the backscattering of the electrons described above. A plateau in the spectrum is therefore the decisive feature of strong-field physics.

In the past, strong-field experiments were conducted practically exclusively with intense laser light, partly due to the lack of other forms of intense light. The collaboration between the two research groups led by Professors Hommelhoff and Chekhova has now made it possible to explore new avenues. The researchers had several questions: Can strong-field processes at metallic needle tips also be initiated using quantum light, in particular bright squeezed vacuum with a mean electric field of zero? What differences are there to the electron spectra when laser light is used?

High-energetic electrons and a hidden plateau

The researchers have now answered these questions in an article recently published in the journal “Nature Physics”. In experiments, they observed that significantly higher electron energies can be reached in comparison to classical laser light if they use intense vacuum light to drive the strong-field processes. However, at first they did not notice any plateau in the energy spectrum of the electrons. It was only when they compared the number of photons in the respective light pulses that they were able to reconstruct the familiar plateau that is a feature of strong-field physics. It is hidden in the quantum light-driven electron spectrum.

What is striking about this experiment is that it is the first time that the above-mentioned strong-field phenomena have been observed in a light state with a mean electrical field of zero. This would be unthinkable under classical conditions, and demonstrates the particular properties of quantum light.