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arXiv:1907.05957 · Physical Review A

Time-resolved buildup of two-slit-type interference from a single atom

Jonas Wätzel, Andrew James Murray, Jamal Berakdar

Published Jul 12, 2019• 2 citations
arXivHTMLPDF

Abstract

A photoelectron forced to pass through two atomic energy levels before receding from the residual ion shows interference fringes in its angular distribution as a manifestation of a two-slit-type interference experiment in wave-vector space. This scenario was experimentally realized by irradiating a rubidium atom by two low-intensity continuous-wave lasers [Pursehouse et al., Phys. Rev. Lett. 122, 053204 (2019)]. In a one-photon process, the first laser excites the $5p$ level whereas the second uncorrelated photon elevates the excited population to the continuum. This same continuum state can also be reached when the second laser excites the $6p$ state, and the first photon then triggers the ionization. As the two lasers are weak and their relative phases uncorrelated, the coherence needed for generating the interference stems from the atom itself. Increasing the intensity or shortening the laser pulses enhances the probability that two photons from both lasers act at the same time, and hence, the coherence properties of the applied lasers are expected to affect the interference fringes. Here, this aspect is investigated in detail, and it is shown how tuning the temporal shapes of the laser pulses allows for tracing the time dependence of the interference fringes. We also study the influence of applying a third laser field with a random amplitude, resulting in a random fluctuation of one of the ionization amplitudes and discuss how the interference fringes are affected.

DOI
10.1103/physreva.100.013407

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A photoelectron forced to pass through two atomic energy levels before receding from the residual ion shows interference fringes in its angular distribution as a manifestation of a two-slit-type interference experiment in wave-vector space. This scenario was experimentally realized by irradiating a rubidium atom by two low-intensity continuous-wave lasers [Pursehouse et al., Phys. Rev. Lett. 122, 053204 (2019)]. In a one-photon process, the first laser excites the 5p5p5p level whereas the second uncorrelated photon elevates the excited population to the continuum. This same continuum state can also be reached when the second laser excites the 6p6p6p state, and the first photon then triggers the ionization. As the two lasers are weak and their relative phases uncorrelated, the coherence needed for generating the interference stems from the atom itself. Increasing the intensity or shortening the laser pulses enhances the probability that two photons from both lasers act at the same time, and hence, the coherence properties of the applied lasers are expected to affect the interference fringes. Here, this aspect is investigated in detail, and it is shown how tuning the temporal shapes of the laser pulses allows for tracing the time dependence of the interference fringes. We also study the influence of applying a third laser field with a random amplitude, resulting in a random fluctuation of one of the ionization amplitudes and discuss how the interference fringes are affected.
A single-atom "double-slit" experiment is realized by photoionizing rubidium atoms using two independent low power lasers. The photoelectron wave of well-defined energy recedes to the continuum either from the 5P or 6P states in the same atom, resulting in two-path interference imaged in the far field using a photoelectron detector. Even though the lasers are independent and not phase locked, the transitions within the atom impart the phase relationship necessary for interference. The experiment is designed so that either 5P or 6P states are excited by one laser, before ionization by the second beam. The measurement cannot determine which excitation path is taken, resulting in interference in wave-vector space analogous to Young's double-slit studies. As the lasers are tunable in both frequency and intensity, the individual excitation-ionization pathways can be varied, allowing dynamic control of the interference term. Since the electron wave recedes in the Coulomb potential of the residual ion, a quantum model is used to capture the dynamics. Excellent agreement is found between theory and experiment.
We report an experiment of observation of classical double-slit interference fringes of two-photon interference. In the experiment, a commercial continuous-wave multi-mode F-P laser diode without either mode-locked or frequency-locked is used as the light source, the density of photons is far more than the single-photon level, and the path difference of the long and short paths is far more than the longitudinal coherence length of the laser diode. The temporal stable and clearly visible spatial-distributed pattern, i.e. first-order interference fringes, was observed .Contradict to the prediction of single-photon interference mechanism, the interference happened far beyond the coherence length, the occurrence possibility and time duration of the interference fringes decrease with the reduction of mode number of the LD, and the time difference between the fringes disappearance and the light vanish is equal to the time difference of the two paths. After discussion, we came to the conclusion that the observed phenomena can be understood in time-resolved two-photon interference mechanism. We reveal a new method to perform two-photon first-order interference, and this help to understand the nature of two-photon interference and also can be useful for quantum information science.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
In this thesis we report an experimental study of the interference between one- and two-photon ionization processes from the ground state of rubidium atom. This interference is observed by measuring the photoelectron angular distribution under the influence of fundamental and second harmonic laser fields. Externally controlling the relative phase and amplitude between these two laser fields, we obtained interference in the angular distribution patterns. This study represents the first observation of the phase interference in photoionization angular distributions. The photoelectron angular distributions resulting from the single-photon ionization of cesium and rubidium from its ground state to the continuum were measured. This is the first set of measurements of the single photon photoelectron angular distributions at a variety of energies in vicinity of the Cooper minimum from a single initial state of alkali metals. The results show a strong variation of asymmetry parameter β\betaβ from its nonrelativistic value of 2. Spin-orbit coupling displaces the minimum in the cross section for excitation to the $\rm \varepsilon P\sb{1/2}$ and $\rm \varepsilon P\sb{3/2}$ and results in a rapid variation of the angular distribution with photon energy in this region. Through the measurement of two-photon ionization angular distributions the quantum defects and spin-orbit coupling were investigated. These studies help us to understand the phase interference interaction, and the data in those studies would be used to analyze the interference processes.
Abstract We demonstrate how strong-field multiphoton transitions between dynamically shifted atomic levels can be traced in the energy spectra of emitted photoelectrons. Applying an ultrafast and intense laser pulse, two-photon Rabi oscillations are induced between two bound states of an atom. A third photon from the same pulse directly ionizes the atom, thus the emitted photoelectrons coherently probe the underlying dynamics. As the instantaneous energy of photoelectrons follows the pulse intensity envelope, modulated by the ac Stark shifts, electrons emitted with the same energy but at different times—at the rising and falling edge of the pulse—will interfere leading to pronounced dynamic interference pattern in the spectra. We investigate this phenomenon both numerically and analytically by developing a minimal three-state model that incorporates two-photon coupling and dynamically shifted atomic levels. On the example of atomic lithium (2 s → → 4 s → continuum) we show how the individual ac Stark shifts and the two-photon Rabi frequency are reflected through the asymmetry, shifting and splitting of the interference structure of the computed photopeaks.
Multiphoton interference is an important phenomenon in modern quantum mechanics and experimental quantum optics, and it is fundamental for the development of quantum information science and technologies. Over the last three decades, several theoretical and experimental studies have been performed to understand the essential principles underlying such interference and to explore potential applications. Recently, the two-photon interference (TPI) of phase-randomized weak coherent states has played a key role in the realization of long-distance quantum communication based on the use of classical light sources. In this context, we investigated TPI experiments with weak coherent pulses at the single-photon level and quantitatively analyzed the results in terms of the single- and coincidence-counting rates and one- and two-photon interference-fringe shapes. We experimentally examined the Hong-Ou-Mandel-type TPI of phase-randomized weak coherent pulses to compare the TPI effect with that of correlated photons. Further experiments were also performed with two temporally- and spatially separated weak coherent pulses. Although the observed interference results, including the results of visibility and fringe shape, can be suitably explained by classical intensity correlation, the physics underlying the TPI effect needs to be interpreted as the interference between the two-photon states at the single-photon level within the utilized interferometer. The results of this study can provide a more comprehensive understanding of the TPI of coherent light at the single-photon level.