We study Dicke superradiance as collective and coherent absorption and (time-delayed) emission of photons from an ensemble of ultracold atoms in an optical lattice. Since this process depends on the coherence properties of the atoms (e.g., superfluid
ity), it can be used as a probe for their quantum state. In analogy to pump-probe spectroscopy in solid-state physics, this detection method facilitates the investigation of nonequilibrium phenomena and is less invasive than time-of-flight experiments or direct (projective) measurements of the atom number (or parity) per lattice site, which both destroy properties of the quantum state such as phase coherence.
The quantum Zeno effect describes the inhibition of quantum evolution by frequent measurements. Here, we propose a scheme for entangling two given photons based on this effect. We consider a linear-optics set-up with an absorber medium whose two-phot
on absorption rate $xi_{2gamma}$ exceeds the one-photon loss rate $xi_{1gamma}$. In order to reach an error probability $P_{rm error}$, we need $xi_{1gamma}/xi_{2gamma}<2P_{rm error}^2/pi^2$, which is a factor of 64 better than previous approaches (e.g., by Franson et al). Since typical media have $xi_{2gamma}<xi_{1gamma}$, we discuss three mechanisms for enhancing two-photon absorption as compared to one-photon loss. The first mechanism again employs the quantum Zeno effect via self-interference effects when sending two photons repeatedly through the same absorber. The second mechanism is based on coherent excitations of many atoms and exploits the fact that $xi_{2gamma}$ scales with the number of excitations but $xi_{1gamma}$ does not. The third mechanism envisages three-level systems where the middle level is meta-stable ($Lambda$-system). In this case, $xi_{1gamma}$ is more strongly reduced than $xi_{2gamma}$ and thus it should be possible to achieve $xi_{2gamma}/xi_{1gamma}gg1$. In conclusion, although our scheme poses challenges regarding the density of active atoms/molecules in the absorber medium, their coupling constants and the detuning, etc., we find that a two-photon gate with an error probability $P_{rm error}$ below 25% might be feasible using present-day technology.
Nuclear excitons known from Mossbauer spectroscopy describe coherent excitations of a large number of nuclei -- analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser bas
ed on these coherent excitations. In contrast to the free electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology -- but our design should be feasible in the UV regime, for example.
