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Zusammenfassung Die resonante Anregung der⁴⁵Scandium‐Kernresonanz und die präzise Messung ihrer Energie eröffnen neue Möglichkeiten für Anwendungen in der Ultrapräzisions‐Röntgenspektroskopie bis hin zur Entwicklung einer Kernuhr. Damit kann zum Beispiel die Gravitationsrotverschiebung erstmals auf sehr kurzen Längenskalen überprüft werden. Darüber hinaus demonstriert dieses Experiment das große Potenzial von Self‐Seeding‐Röntgenlasern mit hoher Pulsrate als vielversprechende Plattform für die Spektroskopie von extrem schmalbandigen Kernresonanzen. Die nächsten Schritte in Richtung einer Kernuhr auf der Basis von⁴⁵Scandium erfordern eine weitere Erhöhung des spektralen Photonenflusses unter Verwendung verbesserter Röntgenlaserquellen bei 12,4 keV und die Entwicklung von Frequenzkämmen, die bis zu dieser Energie reichen. 

Optical quantum memories are key elements in modern quantum technologies to reliably store and retrieve quantum information. At present, they are conceptually limited to the optical wavelength regime. Recent advancements in x-ray quantum optics render an extension of optical quantum memory protocols to ultrashort wavelengths possible, thereby establishing quantum photonics at x-ray energies. Here, we introduce an x-ray quantum memory protocol that utilizes mechanically driven nuclear resonant⁵⁷Fe absorbers to form a comb structure in the nuclear absorption spectrum by using the Doppler effect. This room-temperature nuclear frequency comb enables us to control the waveform of x-ray photon wave packets to a high level of accuracy and fidelity using solely mechanical motions. This tunable, robust, and highly flexible system offers a versatile platform for a compact solid-state quantum memory at room temperature for hard x-rays. 

Abstract Resonant oscillators with stable frequencies and large quality factors help us to keep track of time with high precision. Examples range from quartz crystal oscillators in wristwatches to atomic oscillators in atomic clocks, which are, at present, our most precise time measurement devices¹. The search for more stable and convenient reference oscillators is continuing^2–6. Nuclear oscillators are better than atomic oscillators because of their naturally higher quality factors and higher resilience against external perturbations^7–9. One of the most promising cases is an ultra-narrow nuclear resonance transition in⁴⁵Sc between the ground state and the 12.4-keV isomeric state with a long lifetime of 0.47 s (ref. ¹⁰). The scientific potential of⁴⁵Sc was realized long ago, but applications require⁴⁵Sc resonant excitation, which in turn requires accelerator-driven, high-brightness X-ray sources¹¹that have become available only recently. Here we report on resonant X-ray excitation of the⁴⁵Sc isomeric state by irradiation of Sc-metal foil with 12.4-keV photon pulses from a state-of-the-art X-ray free-electron laser and subsequent detection of nuclear decay products. Simultaneously, the transition energy was determined as$${\mathrm{12,389.59}}_{+0.12\left({\rm{syst}}\right)}^{\pm 0.15\left({\rm{stat}}\right)}\,{\rm{eV}}$$ ${\mathrm{12,389.59}}_{+0.12\left(\mathrm{syst}\right)}^{\pm 0.15\left(\mathrm{stat}\right)}\mathrm{eV}$with an uncertainty that is two orders of magnitude smaller than the previously known values. These advancements enable the application of this isomer in extreme metrology, nuclear clock technology, ultra-high-precision spectroscopy and similar applications.