  The exceptionally large polarisability of highly excited Rydberg atoms (six
orders of magnitude higher than ground-state atoms) makes them of great
interest in fields such as quantum optics, quantum computing, quantum
simulation and metrology. If however they are to be used routinely in
applications, a major requirement is their integration into technically
feasible, miniaturised devices. Here we show that a Rydberg medium based on
room temperature caesium vapour can be confined in broadband-guiding
kagome-style hollow-core photonic crystal fibres. Three-photon spectroscopy
performed on a caesium-filled fibre detects Rydberg states up to a principal
quantum number of n = 40. Besides small energy level shifts we observe narrow
lines confirming the coherence of the Rydberg excitation. Using different
Rydberg states and core diameters we study the influence of confinement within
the fibre core after different exposure times. Understanding these effects is
essential for the successful future development of novel applications based on
integrated room temperature Rydberg systems.
