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Alternative fuels play a critical role in strategies aiming at the decarbonization of civil aviation. In this context, Power-to-Liquid fuels produced from renewable electricity, water and carbon dioxide represent a scalable and sustainable option [1]. An in-depth understanding of the entire process chain is key to identify the levers in future scale-up scenarios for PtL capacity deployment. This is not only true for the conversion and upgrading steps, but also extends to the green hydrogen and carbon dioxide feedstocks required. Direct capture from air (DAC) is a potential solution for sustainable CO2 provision at scale. It effectively closes the carbon cycle and enables decoupling the carbon source from fossil infrastructures and biomass. On the other hand, data availability is comparatively low, as DAC is a novel technology and only a small number of operational plants with a total annual output of around 10 t have been realized [2] orders of magnitude lower than what would be required in a net-zero emission scenario. Consequently, DAC, while representing a corner stone in the Power-to-Liquids process chain, is associated with considerable uncertainties when it comes to its potential for future widespread application. Several technology options based on solid or liquid sorbents exist, employing different temperature ranges and typically demanding both thermal and electrical energy. To deepen the understanding and illustrate the associated trade-offs, we employ future technology analysis and consolidate the results in a comparative multi-criteria assessment, taking into account energy demand, scalability, integrability, land requirements and future cost estimations, among other metrics. Further, we apply the learnings as an input to a thorough sensitivity study of the Power-to-Liquids production and supply chain, shedding light onto possible levers and bottlenecks. Finally, we draw a brief comparison to other fuel choices. References [1] Batteiger, V.; Ebner, K.; Habersetzer, A.; Moser, L.; Schmidt, P.; Weindorf, W.; Rakscha, T., Power-to-Liquids - A scalable and sustainable fuel supply perspective for aviation., German Environment Agency. (2022), available at: https://www.umweltbundesamt.de/en/publications [2] IEA, Direct Air Capture (2021), available at: https://www.iea.org/reports/direct-air-capture

The EU Horizon 2020 project HyFlexFuel successfully demonstrated hydrothermal liquefaction (HTL) fuel production chains from different feedstock types to upgraded kerosene products. Now the question arises which commercial scale HTL system design is associated with the lowest environmental impact and production costs. The contribution addresses this research question by establishing a comprehensive process model for different feedstock types (sewage sludge, straw, miscanthus and microalgae) based on experimental biocrude production and upgrading campaigns in pilot and laboratory scale. This model enables evaluating different process configurations and serves as basis for subsequent system analyses by applying techno-economic and life cycle analyses (TEA and LCA). Upgraded biocrude using sewage sludge, representing a waste stream in wastewater management, can be produced at near-competitive price levels. Compared to conventional jet fuel production, greenhouse gases are reduced significantly. However, sewage sludge is a limited resource and only limited amounts of jet fuel could be substituted. Lignocellulosic feedstock such as straw or miscanthus are available in larger quantities and provide the opportunity to produce large amounts of sustainable aviation fuels at moderate costs.

We report on observations of thermal motion of a single, Doppler-cooled ion along the axis of a linear radio-frequency quadrupole trap. We show that for a harmonic potential the thermal occupation of energy levels leads to Gaussian distribution of the ion's axial position. The dependence of the spatial thermal spread on the trap potential is used for precise calibration of our imaging system's point spread function and sub-milliKelvin thermometry. We employ this technique to investigate the laser detuning dependence of the Doppler temperature., Comment: 5 pages, 4 figures

We report on injection locking of optically excited mechanical oscillations of a single, trapped ion. The injection locking dynamics are studied by analyzing the oscillator spectrum with a spatially selective Fourier transform technique and the oscillator phase with stroboscopic imaging. In both cases we find excellent agreement with theory inside and outside the locking range. We attain injection locking with forces as low as 5(1)×10^(-24) N so this system appears promising for the detection of ultraweak oscillating forces.

Red-detuned laser pumping of an atomic resonance will cool the motion of an ion or atom. The complementary regime of blue-detuned pumping is investigated in this work using a single, trapped Mg+ ion interacting with two laser beams, tuned above and below resonance. Widely thought of as a regime of heating, theory and experiment instead show that stimulated emission of centre-of-mass phonons occurs, providing saturable amplification of the motion. A threshold for transition from thermal to coherent oscillating motion has been observed, thus establishing this system as a mechanical analogue to an optical laser -— a phonon laser. Such a system has been sought in many different physical contexts.

We apply a recently demonstrated method for precision spectroscopy on strong transitions in trapped ions to measure both fine-structure components of the $3s\text{\ensuremath{-}}3p$ transition in ${^{24}\text{M}\text{g}}^{+}$ and ${^{26}\text{M}\text{g}}^{+}$. We deduce absolute frequency reference data for transition frequencies, isotope shifts, and fine-structure splittings that in particular are useful for comparison with quasar absorption spectra, which test possible space-time variations of the fine-structure constant. The measurement accuracy improves previous literature values, when existing, by more than two orders of magnitude.

Absolute frequency measurements on single trapped ions reached tremendous accuracies by probing narrow transitions in tightly confined ions (resolved sideband regime) [1,2]. A variety of interesting transitions may hardly be studied in this regime, because their transition linewidths well exceed common (secular) trap frequencies. Accurate spectroscopy in the so-called weak binding limit is challenging, since an interrogating laser induces detuning dependent heating and cooling which distorts the line profile. We present both theory and experimental demonstration of a spectroscopic method (Fig. 1) which essentially removes these limitations and allows us to observe a well understood line shape with high signal-to-noise ratio (Fig. 2) [3].

Cooling of atoms and ions [1,2] using a red-detuned laser has had a profound impact on science and technology [3–5]. In this work simultaneous laser cooling and blue-detuned laser pumping of a Mg+ ion in a Paul trap is studied. Blue-detuned pumping is conventionally referred to as the heating regime, and in early work, remarkably complex behaviors (bistability and limit cycles) have been associated with this regime [6,7]. These behaviors have so far not been fully explained. Here, it is shown that blue-detuned pumping, as opposed to heating, causes stimulated emission of center-of-mass phonons, leading to coherent oscillatory motion of the ion in analogy with a laser. Mechanical amplification is calculated as well as the threshold pumping condition for oscillation. A single ion in a linear radio-frequency trap is studied to verify these predictions (figure 1 left panel). Blue-detuned pumping of the magnesium D2 transition at 279.6 nm provides amplification along the long axis of the ion trap so as to excite only axial oscillations. A slightly off-axis, red-detuned beam cools the center-of-mass motion to approximately 1 mK. The experimental arrangement is identical to that detailed in ref. [8].

The $1S\text{\ensuremath{-}}2S$ two-photon transition in singly ionized helium is a highly interesting candidate for precision tests of bound-state quantum electrodynamics (QED). With the recent advent of extreme ultraviolet frequency combs, highly coherent quasi-continuous-wave light sources at 61 nm have become available, and precision spectroscopy of this transition now comes into reach for the first time. We discuss quantitatively the feasibility of such an experiment by analyzing excitation and ionization rates, propose an experimental scheme, and explore the potential for QED tests.

We demonstrate a method for precision spectroscopy on trapped ions in the limit of unresolved motional sidebands. By sympathetic cooling of a chain of crystallized ions, we suppress adverse temperature variations induced by the spectroscopy laser that usually lead to a distorted line profile and obtain a Voigt profile with negligible distortions. We applied the method to measure the absolute frequency of the astrophysically relevant $D2$ transition in single $^{24}\mathrm{Mg}^{+}$ ions and find $1\text{ }072\text{ }082\text{ }934.33(16)\text{ }\text{ }\mathrm{MHz}$, a nearly 400-fold improvement over previous results. Further, we find the excited state lifetime to be 3.84(10) ns.

A trapped Mg+ion is laser cooled within a Paul trap, and is simultaneously excited using a continuous blue-detuned pump wave. The ion subsequently exhibits coherent oscillatory motion with a well-defined threshold.

A cooled, trapped Mg^+ ion is excited using a continuous, blue-detuned pump. Coherent oscillatory motion is observed. It is shown to result from stimulated emission of center-of-mass phonons, constituting the mechanical analog of a laser.

Mechanical amplification by stimulated phonon emission is reviewed in two optomechanical systems: an optical resonator system and a trapped-ion system. The physics of these devices and observations of threshold and line narrowing are presented.