158
R. Röhlsberger and J. Evers
To summarize, we have succeeded in observing Rabi oscillations in the X-ray
regime by using a double cavity set-up, and confirmed the collective strong coupling
regime via a quantum optical model. The temporal evolution of the system is marked
by the coherent exchange of a photon between two different collective excited nuclear
states. The result also points the way to coherent control of X-rays over matter.
Already minor changes in the layer structure, such as the positioning of the layers,
can create new effective multilevel systems with completely different dynamics,
and facilitate artificial quantum optical systems in the X-ray range, with tunable
dispersions and interactions.
3.9 Nuclear Quantum Optics with Advanced Sources
of X-Rays
A further boost for the field of nuclear quantum optics will be enabled not only by an
increase of the spectral flux provided by the x-ray source, but also by qualitatively new
properties of the radiation source. One of those is the so-called photon degeneracy
η, which is a parameter that gives the number of resonant photons per mode of the
radiation field. In the case of a pulsed radiation source where the pulse length is
much shorter than the nuclear lifetime, η is simply given by the number of photons
per pulse within the nuclear resonance bandwidth. If η is significantly larger than
1, one obviously enters the regime of multiphoton excitation of nuclear ensembles
which could open new avenues for nuclear quantum optics, ranging from stimulated
emission to nonlinear optics. At most present-day sources, however, values for η are
typically much lower than 1, implying that in the majority of the pulses that interact
with the sample there is no resonant photon. This means that there is typically only
one nuclear resonant photon at a time in the setup. Under favorable conditions,
e.g., when large bunch charges like in few-bunch filling mode of the storage ring
are realized, there can be a sufficiently large fraction of bunches containing two
or more resonant photons. The situation changes drastically with the advent of xray laser sources operating in the regime of hard x-rays like the LCLS in Stanford
(USA) [136], SACLA in Himeji (Japan) [137], and the European XFEL in Hamburg
(Germany) [138–141]. At these sources, values for η larger than 10
2 at 14.4 keV
can be expected. In fact, a first experiment devoted to NRS has been recently been
performed at SACLA, demonstrating superradiant emission from nuclear excited
states with close to 10
2 photons per pulse [142]. In this section we are going to
discuss two new types of future sources of x-rays that have great potential to further
stimulate the field of nuclear quantum optics. These are the ultimate realizations
of synchrotron radiation sources, i.e., diffraction limited storage rings (DLSR) and
x-ray free electron laser oscillators (XFELO).
R. Röhlsberger and J. Evers
To summarize, we have succeeded in observing Rabi oscillations in the X-ray
regime by using a double cavity set-up, and confirmed the collective strong coupling
regime via a quantum optical model. The temporal evolution of the system is marked
by the coherent exchange of a photon between two different collective excited nuclear
states. The result also points the way to coherent control of X-rays over matter.
Already minor changes in the layer structure, such as the positioning of the layers,
can create new effective multilevel systems with completely different dynamics,
and facilitate artificial quantum optical systems in the X-ray range, with tunable
dispersions and interactions.
3.9 Nuclear Quantum Optics with Advanced Sources
of X-Rays
A further boost for the field of nuclear quantum optics will be enabled not only by an
increase of the spectral flux provided by the x-ray source, but also by qualitatively new
properties of the radiation source. One of those is the so-called photon degeneracy
η, which is a parameter that gives the number of resonant photons per mode of the
radiation field. In the case of a pulsed radiation source where the pulse length is
much shorter than the nuclear lifetime, η is simply given by the number of photons
per pulse within the nuclear resonance bandwidth. If η is significantly larger than
1, one obviously enters the regime of multiphoton excitation of nuclear ensembles
which could open new avenues for nuclear quantum optics, ranging from stimulated
emission to nonlinear optics. At most present-day sources, however, values for η are
typically much lower than 1, implying that in the majority of the pulses that interact
with the sample there is no resonant photon. This means that there is typically only
one nuclear resonant photon at a time in the setup. Under favorable conditions,
e.g., when large bunch charges like in few-bunch filling mode of the storage ring
are realized, there can be a sufficiently large fraction of bunches containing two
or more resonant photons. The situation changes drastically with the advent of xray laser sources operating in the regime of hard x-rays like the LCLS in Stanford
(USA) [136], SACLA in Himeji (Japan) [137], and the European XFEL in Hamburg
(Germany) [138–141]. At these sources, values for η larger than 10
2 at 14.4 keV
can be expected. In fact, a first experiment devoted to NRS has been recently been
performed at SACLA, demonstrating superradiant emission from nuclear excited
states with close to 10
2 photons per pulse [142]. In this section we are going to
discuss two new types of future sources of x-rays that have great potential to further
stimulate the field of nuclear quantum optics. These are the ultimate realizations
of synchrotron radiation sources, i.e., diffraction limited storage rings (DLSR) and
x-ray free electron laser oscillators (XFELO).