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 in ⁴⁵Sc between the ground state and the 12.4-keV isomeric state with a long lifetime of 0.47 s (ref. ¹⁰). The scientific potential of ⁴⁵Sc was realized long ago, but applications require ⁴⁵Sc resonant excitation, which in turn requires accelerator-driven, high-brightness X-ray sources¹¹ that have become available only recently. Here we report on resonant X-ray excitation of the ⁴⁵Sc 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 $${\mathrm{12,389.59}}_{+0.12\left({\rm{syst}}\right)}^{\pm 0.15\left({\rm{stat}}\right)}\,{\rm{eV}}$$12,389.59 + 0.12 syst ± 0.15 stat 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.

We propose a technique for $\gamma$ photon quantum memory through a Doppler frequency comb, produced by a set of resonantly absorbing nuclear targets that move with different velocities. It provides a reliable storage, an on-demand generation, and a time sequencing of a single $\gamma$ photon. This scheme presents the first $\gamma$-photon-nuclear-ensemble interface opening a new direction of research in quantum information science.

We prove that the amplification condition for a multifrequency field in an atomic system with multiple splitting of the operating levels implies a population inversion between the eigenstates of the density matrix. This explains the possibility of amplification without population inversion when a coherent superposition state of sublevels is prepared.

We consider a new way to trap light pulses inside a gaseous medium by using additional electromagnetic fields. The drive field controls the transparency and the dispersion of the medium. Applied auxiliary fields provide additional control on the propagation of the probe pulse via the dragging effect resulting from the population redistribution between hyperfine levels of different velocity groups. The density matrix calculation is performed to prove that these effects are experimentally feasible.

In a recent work (Antonov et al., Physical Review Letters 123, 243903 (2019)), it was shown that it is possible to amplify a train of attosecond pulses, which are produced from the radiation of high harmonics of the infrared field of the fundamental frequency, in the active medium of a plasma-based X-ray laser modulated by a replica of the infrared field of the same frequency. In this paper, we show that much higher amplification can be achieved using the second harmonic of the fundamental frequency for modulating of a hydrogen-like active medium. The physical reason for such enhanced amplification is the possibility to use all (even and odd) sidebands induced in the gain spectrum in the case of the modulating field of the doubled fundamental frequency, while only one set of sidebands (either even or odd) could participate in amplification in the case of the modulating field of the fundamental frequency due to the fact that the spectral components of the high-harmonic field are separated by twice the fundamental frequency. Using the plasma of hydrogen-like C⁵⁺ ions with an inverted transition wavelength of 3.38 nm in the water window as an example, it is shown that the use of a modulating field at a doubled fundamental frequency makes it possible to increase the intensity of amplified attosecond pulses by an order of magnitude in comparison with the previously studied case of a fundamental frequency modulating field.