Phase-sensitive nuclear target spectroscopy

Abstract

Mössbauer nuclei feature exceptionally narrow resonances at hard x-ray energies, which render them ideal probes for structure and dynamics in condensed-matter systems, and a promising platform for x-ray quantum optics and fundamental tests. However, a direct spectroscopy at modern x-ray sources such as synchrotrons or xray free-electron lasers is challenging, because of the broad spectral bandwidth of the delivered x-ray pulses, and because of a limited spectral resolution offered by x-ray optics and detectors. To overcome these challenges, here, we propose a spectroscopy technique based on a spectrally narrow reference absorber that is rapidly oscillating along the propagation direction of the x-ray light. The motion induces sidebands to the response of the absorber, which we scan across the spectrum of the unknown target to gain spectral information. The oscillation further introduces a dependence of the detected light on the motional phase at the time of x-ray excitation as an additional controllable degree of freedom. We show how a Fourier analysis with respect to this phase enables one to selectively extract parts of the recorded intensity after the actual experiment, throughout the data analysis. This allows one to improve the spectral recovery by removing unwanted signal contributions. Our method is capable of gaining spectral information from the entire measured intensity, and not only from the intensity at late times after the excitation, such that a significantly higher part of the signal photons can be used. Furthermore, it not only enables one to measure the amplitude of the spectral response but also its phase.

Ab initio quantum models for thin-film x-ray cavity QED

“…Nevertheless, a rich variety of quantum optical coherence effects has already been observed, both in nuclear forward scattering [13][14][15][16][17][18][19][20] and thin-film cavity setups [21][22][23][24][25][26][27]. More possibilities have been theoretically suggested [28][29][30][31][32][33][34][35][36][37][38]. With the first successful experiment at an x-ray free electron laser [39], including photon-numberresolved detection [40][41][42], this platform provides an exciting candidate for nonlinear or correlated quantum effects with extreme transitions.…”

Nuclear quantum memory for hard x-ray photon wave packets

“…Mössbauer nuclei with their energetically sharp resonances are a potential material system. The prominent 14.4 keV nuclear resonance of 57 Fe was already used to establish many quantum optical phenomena in the x-ray regime [see (13) and references therein]. The photon-nuclei interaction benefits from high nuclear densities in solids (~ 10 28 m −3 ) and from a resonant cross section being about 400 times higher than the electronic scattering cross section at this energy, resulting in large optical thicknesses even in physically thin absorbers.…”

“…The photon-nuclei interaction benefits from high nuclear densities in solids (~ 10 28 m −3 ) and from a resonant cross section being about 400 times higher than the electronic scattering cross section at this energy, resulting in large optical thicknesses even in physically thin absorbers. Even at room temperature, nuclear resonances in solids exhibit ultrahigh quality factors and long coherence lifetimes, e.g., for 57 Fe, Q ~10 13 and τ 0 = 141 ns, respectively. These are important prerequisites for quantum memories where phase and amplitude of the stored qubit must be preserved.…”

“…It is the latter approach that enables the formation of a quantum memory for hard x-ray photon wave packets using a set of moving resonant nuclear 57 Fe absorbers (12). With the small transition linewidths and the typical high transition energies E 0 , a sizeable Doppler shift ∆E = E 0 v/c, c being the speed of light, is achieved at comparatively small velocities v (a velocity of 0.1 mm/s readily shifts the 57 Fe transition by its natural linewidth). The proposed quantum memory concept is based on moving multiple resonant absorbers with different but equidistantly spaced velocities to form a Doppler nuclear frequency comb (NFC) in the resonant absorption structure.…”

“…Motivated by extending the quantum memory concept to the hard x-ray regime, we experimentally implement a robust NFC with up to seven teeth. After excitation with synchrotron x-ray pulses, the prepared NFC dramatically reshapes the temporal response of the coherent collective nuclear excitation of 57 Fe nuclei in the forward scattering direction. This results in the formation of x-ray wave packets on the single-photon level with durations inverse to the frequency comb's bandwidth.…”