We observe and study the phenomenon of Anderson localization in a system of true quantum kicked rotors. Nitrogen molecules in a supersonic molecular jet are cooled down to 27 K and are rotationally excited by a periodic train of 24 high-intensity femtosecond pulses. Exponential distribution of the molecular angular momentum -the most unambiguous signature of Anderson localization -is measured directly by means of coherent Raman scattering. We demonstrate the suppressed growth of the molecular rotational energy with the number of laser kicks and study the dependence of the localization length on the kick strength. Both timing and amplitude noise in the pulse train is shown to destroy the localization and revive the diffusive growth of angular momentum.The periodically kicked rotor is one of the simplest systems whose classical motion is chaotic, as manifested by the unbounded diffusive growth of its energy with the number of kicks. In contrast, the energy growth of a quantum kicked rotor (QKR) is suppressed due to the interference of quantum interaction pathways [1,2]. The effect has been linked to Anderson localization [3] of the electronic wave function in disordered solids [4]. Similarly to the latter, the wave function of the quantum rotor does not grow wider in the angular momentum space with every consecutive kick, but instead localizes near the initial rotational state, with the probability amplitude falling exponentially away from it.The exponential distribution around the localization center is considered a necessary component and a distinct signature of Anderson localization. Although it has been demonstrated [5] in a cold-atom analogue of the QKR [6], exponentially localized states have not yet been observed in a system of true quantum rotors. A natural choice for such a system -a diatomic molecule subject to short kicks from a pulsed external field (microwave, optical or THz), has been discussed in multiple theoretical proposals [7][8][9][10]. In a series of recent works [9,[11][12][13], Averbukh and coworkers suggested a strategy to observe and study a number of QKR effects in an ensemble of molecules exposed to a periodic sequence of ultra-short laser pulses. The effects of a quantum resonance [14,15] and Bloch oscillations [16] have been verified experimentally. An onset of Anderson localization in laser-induced molecular alignment has been reported [17], but the direct evidence of the exponentially localized states and the suppressed growth of the rotational energy has not been shown.The difficulty of demonstrating Anderson localization with molecular rotors stems from a number of experimental challenges. First, the need to assess the shape of the rotational distribution calls for a sensitive detection method capable of resolving individual rotational states. According to the theoretical studies [11], the population of a few tens of rotational states must be measured with high sensitivity over the range of at least two orders of magnitude. Second, for the localized state not to be smeared out due ...
High-repetition-rate PIV measurements were performed in the trisonic wind tunnel facility at the Bundeswehr University Munich in order to investigate the boundary layer parameters on a generic rocket model and the recirculation area in the wake of the model at Mach numbers up to Mach = 2.6. The data are required for the validation of unsteady flow simulations. Because of the limited run time of the blow-down wind tunnel, a high-repetition-rate PIV system was applied to obtain the flow statistics with high accuracy. The results demonstrate this method's potential to resolve small-scale flow phenomena over a wide field of view in a large Mach number range but also show its limitations for the investigations of wall-bounded flows.
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