## Matter in Strong Laser Fields – at the Frontiers of Feasibility

The investigation of the interaction of matter with laser pulses by now has reached a level at which fundamental aspects such as the quantum nature of both light and matter, relativity and couplings among the involved particles have become key issues and substantial challenges alike. Theory helps to explore the effects of extremely strong fields, even such that will be reached experimentally only in the near future. This requires the search for solutions of the many-body time-dependent Schrödinger and Dirac equations. Furthermore, quantum electrodynamics, nuclear effects and pair creation are considered.

Another topic is the role of time in quantum processes such as tunnel ionization in ultrastrong fields. In particular, the question for the time span an electron needs for tunnelling is controversial to date: Does this take time or is it instantaneous? Theoretical considerations based on a concept published by Nobel laureate Eugene Wigner in 1955 predict a finite tunnelling time. A recent joint theoretical and experimental study at MPIK using a refined model succeeded in translating the Wigner time into an observable quantity. An accurate analysis of electrons emerging from noble gases in ultrashort circularly polarized laser pulses gave evidence of a finite tunnelling time up to 180 attoseconds (1 as = 10^{–18} s).

Availability of frequency combs in the X-ray region will allow stringent tests of physical theories and exact measurements of fundamental constants. A recently proposed method to produce such frequency combs is based on a gas, that is pumped by laser pulses into a metastable state, from which an optical frequency comb populates the radiative state. This results in the emission of an X-ray frequency comb.

The action of intense laser fields on atoms, i. e., mainly on the electrons that can easily be accelerated, has been studied intensively during the last decades. In contrast, atomic nuclei which are bound by the strong interaction, can hardly be influenced even by extremely strong optical light fields. Instead, direct excitation of nuclei by gamma radiation of megaelectron Volts (MeV) photon energy may be opened in the not too far future by gamma lasers such as ELI. First theoretical approaches to this new kind of laser-matter interaction have been developed at MPIK. Gamma photon absorption leads to the excitation of single nucleons, creating so-called particle-hole pairs. The nuclear response however tends to drive the system towards statistical equilibrium. New nuclear physics insights arise when the absorption of several photons is fast enough to compete with the equilibration leading to excitation of many nucleons (compound nucleus). The theoretical studies have shown that multiple photon absorption may produce compound nuclei in the so-far unexplored regime of several hundred MeV excitation energies.

Theory of Collective and Relativistic Quantum Dynamics in Strong Laser Fields (pdf)

## Strong-Field Quantum Electrodynamics – Modifying the Vacuum

In the language of quantum electrodynamics (QED), electromagnetism is described as the exchange of so-called virtual photons between charged particles. Another consequence of this theory is the fact that there is no empty space, i. e., the vacuum is filled with virtual particles. Though their existence is only allowed for a very short time – given by quantum uncertainty – the presence of an average number of virtual particles can be detected by high-precision experiments. At the same time, QED is the to date best tested theory in physics at all.

Of particular interest is the QED in extremely strong fields. Besides high-precision calculations of the inner structure of matter (e. g. highly charged ions) theory deals with the fundamental question of radiation reaction: when a charged particle is accelerated in an electromagnetic field, it emits electromagnetic radiation which in turn acts back on the particle’s motion. Intense laser fields can help to test experimentally the underlying equations. Quantum aspects of radiation reaction in electron dynamics should show up in studies using already available laser systems. This is also of importance for many-particle ensembles like a laser-generated relativistic plasma.

Very strong electric fields will influence the charged virtual particles in the quantum vacuum such that the vacuum becomes polarized changing its optical properties: The conditions were calculated under which light is scattered by a matterless double slit (nearby foci of two ultra-intense laser beams). Very strong fields also prevail in the vicinity of the nuclei of heavy elements.

The interplay of theory and experiment significantly contributes to the determination of fundamental constants such as the g-factor (ratio of angular momentum and magnetic momentum) of the electron. On the one hand, comparison with precision experiments permits validation of QED predictions, while on the other hand theory helps to determine natural constants like the electron mass: its current value is by a factor of 13 more accurate.

**Division Keitel Division Blaum**

Laser-Modified Quantum Electrodynamics, Nuclear and High-Energy Processes (pdf)

Penning Traps: Precision Measurements on Single Ions (pdf)

## Laser Astrophysics – Cosmic Accelerators in the Laboratory Scale

Already to date, highly intense laser fields enable the acceleration of particles to energies up to the order of gigaelectron Volts (GeV). This opens the possibility to reproduce physical conditions in the laboratory, as they prevail in extreme astrophysical processes. In close collaboration with external experimental groups, MPIK researchers developed models for the production of ultrarelativistic lepton beams consisting of electrons and positrons in equal amounts as well as gamma rays. Thereby, the conversion of bremsstrahlung to electron-positron pairs could be identified as a substantial mechanism. The investigation of such highly energetic processes on laboratory scale is of great importance for astrophysics: Cosmic gamma-ray bursts for example, to our present knowledge emerge from the extremely collimated ultrarelativistic leptonic jets which are emitted along the rotation axis of certain types of collapsing stars.

Laser-Modified Quantum Electrodynamics, Nuclear and High-Energy Processes (pdf)

## Quantum Control – Steering Atoms and Nuclei with Lasers

Spectroscopy – the measurement of the absorption and emission of light as it interacts with matter – is one of the most important tools of physics. Line spectra are observed in the case of resonant interaction. Under certain conditions, they interfere with a continuous background and asymmetric line shapes (“Fano profiles”) emerge. This can be illustrated as the superposition of coupled oscillations. Using ultrashort infrared laser pulses, it is possible to control the temporal evolution and thus the quantum interference – for example to achieve the inversion of absorption into emission.

Quantum optics with X-radiation emerged in the last years as a new field. It exploits the wave nature of light which provides additional parameters for quantum control. An important step here is the preparation of robust superpositions of quantum states. Theoretical predictions for this could already be confirmed in an experiment with the atomic nuclei of an iron sample which – embedded in a resonator for X-ray light – could be brought into the desired superposition of states. The generation of X-rays with a small energy spread is crucial for future applications but still a big challenge for experiments. In a recent joint theoretical and experimental study, it was shown that the macroscopic motion of a sample can shuffle light intensity within the spectrum such that it is enhanced at the desired energy without losses. The principle used here is based on the effect discovered by Rudolf Mößbauer at the precursor institute of MPIK in 1958.