3 Quantum Optical Phenomena in Nuclear Resonant Scattering
109
ensembles of identical emitters was by far not trivial for a long time. This situation
has changed significantly in recent years, e.g., due to the development of storing
and manipulating of atoms in electromagnetic traps, but also by the possibility to
prepare resonant atoms in solid state environments in a very controlled fashion. It
turns out that such a controlled preparation of identical atoms is naturally realized for
certain experimental settings involving Mössbauer isotopes, and that the engineering
of cooperative effects in turn is indispensable for implementing advanced quantum
optical schemes with nuclei.
Collective and virtual effects in the interaction of identical atoms with single
photons are the source of intriguing phenomena in atomic physics and quantum optics
[9–22] extending into the regime of hard x-rays [23–29]. The cooperative character
of the interaction modifies the decay rate [30] (see Fig. 3.3) and shifts the resonance
energy of the atomic ensemble as compared to a single atom [31], also known as the
collective Lamb shift. Nowadays, these effects are becoming increasingly attractive
to create entangled atomic ensembles [32] for applications ranging from quantum
memories [33], quantum information processing [14] to radiative transport of energy
in light-harvesting systems [34]. In particular, as discussed in this review, they also
allow for the design of cooperative nuclear level schemes [28, 29, 35–40].
The collective decay rate of an ensemble of identical resonant atoms was introduced by Dicke in his pioneering work on superradiance [30]. In contrast to the
atomic Lamb shift, the collective Lamb shift emerges when a virtual photon emitted
from one atom is not absorbed by the same atom but by another atom within the
ensemble [31, 41]. The investigation of the collective Lamb shift induced by virtual
processes has received stimulated theoretical interest [15, 18, 19, 31, 41–44] that
has been accompanied by recent experimental studies [27, 45–48]. Virtual transitions not only lead to a shift of the transition energy, but have an interesting effect
on the collective decay rate as well [19–21]: They partially transfer population from
the initially superradiant state into slowly decaying states, resulting in a trapping of
the atomic excitation. On the other hand, virtual transitions open additional decay
channels for otherwise trapped states. It lies at the heart of superradiance that the
presence of many identical atoms opens a large number of potential decay channels
for collective excitations. From that perspective such systems are appealing examples
for open and marginally stable quantum many-body systems [49].
3.1.4 X-Ray Cavities as Enabling Tool for Nuclear Quantum
Optics
Today it is possible to experimentally access collections of identical resonators in a
controlled fashion, ranging from atomic Bose-Einstein condensates to quantum dots
in solid state systems. Moreover, laser technology has reached a level of advancement
that allows to control the light-matter interaction down to timescales of attoseconds.
Currently this field of research progresses to shorter and shorter wavelengths into
109
ensembles of identical emitters was by far not trivial for a long time. This situation
has changed significantly in recent years, e.g., due to the development of storing
and manipulating of atoms in electromagnetic traps, but also by the possibility to
prepare resonant atoms in solid state environments in a very controlled fashion. It
turns out that such a controlled preparation of identical atoms is naturally realized for
certain experimental settings involving Mössbauer isotopes, and that the engineering
of cooperative effects in turn is indispensable for implementing advanced quantum
optical schemes with nuclei.
Collective and virtual effects in the interaction of identical atoms with single
photons are the source of intriguing phenomena in atomic physics and quantum optics
[9–22] extending into the regime of hard x-rays [23–29]. The cooperative character
of the interaction modifies the decay rate [30] (see Fig. 3.3) and shifts the resonance
energy of the atomic ensemble as compared to a single atom [31], also known as the
collective Lamb shift. Nowadays, these effects are becoming increasingly attractive
to create entangled atomic ensembles [32] for applications ranging from quantum
memories [33], quantum information processing [14] to radiative transport of energy
in light-harvesting systems [34]. In particular, as discussed in this review, they also
allow for the design of cooperative nuclear level schemes [28, 29, 35–40].
The collective decay rate of an ensemble of identical resonant atoms was introduced by Dicke in his pioneering work on superradiance [30]. In contrast to the
atomic Lamb shift, the collective Lamb shift emerges when a virtual photon emitted
from one atom is not absorbed by the same atom but by another atom within the
ensemble [31, 41]. The investigation of the collective Lamb shift induced by virtual
processes has received stimulated theoretical interest [15, 18, 19, 31, 41–44] that
has been accompanied by recent experimental studies [27, 45–48]. Virtual transitions not only lead to a shift of the transition energy, but have an interesting effect
on the collective decay rate as well [19–21]: They partially transfer population from
the initially superradiant state into slowly decaying states, resulting in a trapping of
the atomic excitation. On the other hand, virtual transitions open additional decay
channels for otherwise trapped states. It lies at the heart of superradiance that the
presence of many identical atoms opens a large number of potential decay channels
for collective excitations. From that perspective such systems are appealing examples
for open and marginally stable quantum many-body systems [49].
3.1.4 X-Ray Cavities as Enabling Tool for Nuclear Quantum
Optics
Today it is possible to experimentally access collections of identical resonators in a
controlled fashion, ranging from atomic Bose-Einstein condensates to quantum dots
in solid state systems. Moreover, laser technology has reached a level of advancement
that allows to control the light-matter interaction down to timescales of attoseconds.
Currently this field of research progresses to shorter and shorter wavelengths into