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arXiv:1809.01857 · Physical Review A

Collective effects in Th229 -doped crystals

Brenden Scott Nickerson, Wen-Te Liao, Adriana Pálffy

Published Dec 26, 2018• 12 citations
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Abstract

Vacuum-ultraviolet-transparent crystals have been proposed as host lattice for the coherent driving of the unusually low-lying isomer excitation in $^{229}\mathrm{Th}$ for metrology and quantum optics applications. Here the possible collective effects occurring for the coherent pulse propagation in the crystal system are investigated theoretically. We consider the effect of possible doping sites, quantization axis orientation, and pulse configurations on the scattered light intensity and signatures of nuclear excitation. Our results show that for narrow-pulse driving, the rather complicated quadrupole splitting of the level scheme is significantly simplified. Furthermore, we investigate complex driving schemes with a combination of pulsed fields and investigate the occurring interference process. Our theoretical results support experimental attempts for first direct driving of the nuclear transition with coherent light.

DOI
10.1103/physreva.98.062520

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Vacuum-ultraviolet-transparent crystals have been proposed as host lattice for the coherent driving of the unusually low-lying isomer excitation in 229Th^{229}\mathrm{Th}229Th for metrology and quantum optics applications. Here the possible collective effects occurring for the coherent pulse propagation in the crystal system are investigated theoretically. We consider the effect of possible doping sites, quantization axis orientation, and pulse configurations on the scattered light intensity and signatures of nuclear excitation. Our results show that for narrow-pulse driving, the rather complicated quadrupole splitting of the level scheme is significantly simplified. Furthermore, we investigate complex driving schemes with a combination of pulsed fields and investigate the occurring interference process. Our theoretical results support experimental attempts for first direct driving of the nuclear transition with coherent light.
As a unique feature, the 229{}^{229}229Th nucleus has an isomeric transition in the vacuum ultraviolet that can be accessed by optical lasers. The interference effects occurring in the interaction between coherent optical light and an ensemble of 229{}^{229}229Th nuclei are investigated theoretically. We consider the scenario of nuclei doped in vacuum ultraviolet transparent crystals and take into account the effect of different doping sites and therefore different lattice fields that broaden the nuclear transition width. This effect is shown to come into interplay with interference effects due to the hyperfine splitting of the ground and isomeric nuclear states. We investigate possible experimentally available situations involving two-, three- and four-level schemes of quadrupole sublevels of the ground and isomeric nuclear states coupling to one or two coherent fields. Specific configurations which offer clear signatures of the isomer excitation advantageous for the more precise experimental determination of the transition energy are identified. Furthermore, it is shown that population trapping into the isomeric state can be achieved. This paves the way for further nuclear quantum optics applications with 229{}^{229}229Th such as nuclear coherent control.
The impact of coherent light propagation on the excitation and fluorescence of thorium nuclei in a crystal lattice environment is investigated theoretically. We find that in the forward direction, the fluorescence signal exhibits characteristic intensity modulations dominated by a sped-up initial decay signal that is orders of magnitude faster. This feature can be exploited for the optical determination of the isomeric transition energy. In order to obtain a unmistakable signature of the isomeric nuclear fluorescence, we put forward a novel scheme for the direct measurement of the transition energy via electromagnetically modified nuclear forward scattering involving two fields that couple to three nuclear states.
Nuclear coherent population transfer to the 229m^{229m}229mTh isomer using x-ray pulses
Population of the 8 eV 229m^{229m}229mTh isomer via the second nuclear excited state at 29.19 keV by means of coherent x-ray pulses is investigated theoretically. We focus on two nuclear coherent population transfer schemes using partially overlapping x-ray pulses known from quantum optics: stimulated Raman adiabatic passage (STIRAP), and successive πππ pulses. Numerical results are presented for three possible experimental setups. Our results identify the Gamma Factory as the most promising scenario, where two ultraviolet pulses combined with relativistically accelerated ions deliver the required intensities for efficient isomer population. Our simulations require knowledge of the in-band and cross-band nuclear transition probabilities. We give theoretically predicted values for the latter and discuss them in the context of recent experiments.
The interaction between super-intense coherent x-ray light and nuclei is studied theoretically. One of the main difficulties in driving nuclear transitions arises from the very narrow nuclear excited state widths, which limit the coupling between lasers and nuclei. In the context of direct laser–nucleus interaction, we consider the nuclear width broadening that occurs when in solid targets, when the excitation caused by a single photon is shared by a large number of nuclei, forming a collective excited state. Our results show that cooperative effects mostly contribute with a modest increase to the nuclear excited state population except in the case of 57 Fe, where the enhancement can reach almost two orders of magnitude. Additionally, an update is given of previous estimates of the nuclear excited state population and signal photons for x-ray lasers interacting with solid-state and ion beam nuclear targets taking into account the experimental advances of x-ray coherent light sources. The presented values are an improvement by orders of magnitude and are encouraging as to the future prospects of nuclear quantum optics.
The direct interaction between nuclei and strong electromagnetic fields generated by novel coherent x-ray light sources such as the x-ray free electron laser is studied. We investigate the case of various stable or long-lived isotopes with energetically low-lying excited states, where the laser drives the nuclear transition from the ground to the first excited state. We therefore consider the nucleus as a two-level system including the corresponding hyperfine levels in our theoretical description. The performance of present and upcoming x-ray free electron laser facilities in coherently controlling the nuclear excited state population is analyzed for setups involving both solid state as well as accelerated nuclear targets to bridge the gap between photon and transition frequency. Current and predicted laser parameters are applied to calculate excited state populations and signal photon rates of nuclei after a radiation pulse, finding significant improvements compared to former calculations. Furthermore, we determine the parameters necessary to transfer substantial amounts of the nuclear ground state population to the excited state, and therefore to bring nuclear Rabi-oscillation within reach. Additionally, we take into account for the first time collective effects that take place in solid state targets and show that they may provide a significant effect in broadening natural linewidths of nuclear levels depending on the experimental setup. Our findings support the case of nuclear quantum optics for XFEL applications. Therefore, one hopes that eventually coherent control of nuclear transitions can be successfully applied in a number of areas in nuclear physics including preparation, control and detection methods.
In this thesis, aspects of the use of cryogenically cooled rare-earth-ion-doped crystals in the fields of quantum optics and quantum information processing are addressed. The experiments and theoretical considerations referred to in this thesis are based on the coherent transient interaction between light and the ions in the crystals. A photon echo experiment has been performed where faint optical pulses were accumulated in the crystal. In this experiment, the possibility of single photons acting as two of the optical fields in the photon echo process was investigated. Different aspects on the experiment, which can be viewed as delayed self-interference of a single photon, are discussed in this thesis. A scheme for the possible implementation of quantum information processing in rare-earth-ion-doped crystals is presented. A key feature in this scheme is that small groups of ions, which can be optically manipulated and which interact in a controlled way, can be extracted from a collection of randomly positioned ions in the crystal host. Initial experimental investigations of the scheme, indicating the possibility of implementing quantum gates, are also presented in this thesis. In another experiment, the application of magnetic fields of up to 5 T was seen to increase the lifetime of information stored as frequency-dependent modulation of the population of thulium ions doped into a YAG crystal by several orders of magnitude. This mechanism might be useful in data storage and information processing applications. An experiment in which the same type of crystal was used as the active medium in a photon-echo-based pulse compression experiment, where microsecond long pulses were compressed to nanosecond duration using frequency-chirped light pulses, is also reported in this thesis.