Quantum Optical Phenomena in Nuclear Resonant Scattering

With the advent of high-brilliance, accelerator-driven light sources such as modern synchrotron radiation sources or x-ray lasers, it has become possible to extend quantum optical concepts into the x-ray regime. Owing to the availability of single photon x-ray detectors with quantum efficiencies close to unity and photon-number resolving capabilities, fundamental phenomena of quantum optics can now also be studied at Angstrom wavelengths. A key role in the emerging field of x-ray quantum optics is taken by the nuclear resonances of Mossbauer isotopes. Their narrow resonance bandwidth facilitates high-precision studies of fundamental aspects of the light-matter interaction. A very accurate tuning of this interaction is possible via a controlled placement of Mossbauer nuclei in planar thin-film waveguides that act as cavities for x-rays. A decisive aspect in contrast to conventional forward scattering is that the cavity geometry facilitates the excitation of cooperative radiative eigenstates of the embedded nuclei. The multiple interaction of real and virtual photons with a nuclear ensemble in a cavity leads to a strong superradiant enhancement of the resonant emission and a strong radiative level shift, known as collective Lamb shift. Meanwhile, thin-film x-ray cavities and multilayers have evolved into an enabling technology for nuclear quantum optics. The radiative coupling of such ensembles in the cavity field can be employed to generate atomic coherences between different nuclear levels, resulting in phenomena including electromagnetically induced transparency, spontaneously generated coherences, Fano resonances and others. Enhancing the interaction strength between nuclei in photonic structures like superlattices and coupled cavities facilitates to reach the regime of collective strong coupling of light and matter where phenomena like normal mode splitting and Rabi oscillations appear. These developments establish Mossbauer nuclei as a promising platform to study quantum optical effects at x-ray energies. In turn, these effects bear potential to advance the instrumentation and applications of Mossbauer science as a whole.