In the quest for signatures of coherent transport we consider exciton trapping in the continuous-time quantum walk framework. The survival probability displays different decay domains, related to distinct regions of the spectrum of the Hamiltonian. For linear systems and at intermediate times the decay obeys a power-law, in contrast to the corresponding exponential decay found in incoherent continuous-time random walk situations. To differentiate between the coherent and incoherent mechanisms, we present an experimental protocol based on a frozen Rydberg gas structured by optical dipole traps.PACS numbers: 05.60. Gg, 32.80.Rm, 34.20.Cf Recent years have seen an upsurge of interest in coherent energy transfer, given the experimental advances in manipulating and controlling quantum mechanical systems. From the theoretical side, such investigations are of long standing; see, e.g., [1]. Here, tight-binding models, which model coherent exciton transfer, are closely related to the quantum walks (QW An appropriate means to monitor transport is to follow the decay of the excitation due to trapping. The long time decay of chains with traps is a well studied problem for classical systems [8,9]: for an ensemble of chains of different length with traps at both ends the averaged exciton survival probability has a stretched exponential form exp(−bt λ ), with λ = 1/3 (see, e.g., [9]). In contrast, quantum mechanical tight-binding models lead to λ = 1/4 [10, 11]. However, up to now only little is known about the decay of the quantum mechanical survival probability at experimentally relevant intermediate times.Here we evaluate and compare the intermediate-time decays due to trapping for both RW and QW situations by employing the similarity of the CTRW and the CTQW formalisms. Without traps, the coherent dynamics of excitons on a graph of connected nodes is modeled by the CTQW, which is obtained by identifying the Hamiltonian H 0 of the system with the CTRW transfer matrix T 0 , i.e., H 0 = −T 0 ; see e.g. [3, 12] (we will set ≡ 1 in the following). For undirected graphs, T 0 is related to the connectivity matrix A 0 of the graph by T 0 = −A 0 , where (for simplicity) all transmission rates are taken to be equal. Thus, in the following we take H 0 = A 0 . The matrix A 0 has as non-diagonal elements A the Rydberg gases considered in the following, the coupling strength is roughly H (0) k,j / 1 MHz, i.e., the time unit for transfer between two nodes is of the order of a few hundred nanoseconds.The states |j associated with excitons localized at the nodes j (j = 1, . . . , N ) form a complete, orthonormal basis set (COBS) of the whole accessible Hilbert space, i.e., k|j = δ kj and k |k k| = 1. In general, the time evolution of a state |j starting at time t 0 = 0 is given by |j; t = exp(−iH 0 t)|j ; hence the transition amplitudes and the probabilities read α kj (t) ≡ k| exp(−iH 0 t)|j and π kj (t) ≡ |α kj (t)| 2 , respectively. In the corresponding classical CTRW case the transition probabilities follow from a master equation as ...

We present the experimental observation of the antiblockade in an ultracold Rydberg gas recently proposed by Ates et al. [Phys. Rev. Lett. 98, 023002 (2007)]. Our approach allows the control of the pair distribution in the gas and is based on a strong coupling of one transition in an atomic three-level system, while introducing specific detunings of the other transition. When the coupling energy matches the interaction energy of the Rydberg long-range interactions, the otherwise blocked excitation of close pairs becomes possible. A time-resolved spectroscopic measurement of the Penning ionization signal is used to identify slight variations in the Rydberg pair distribution of a random arrangement of atoms. A model based on a pair interaction Hamiltonian is presented which well reproduces our experimental observations and allows one to deduce the distribution of nearest-neighbor distances.

We investigate Coherent Population Trapping in a strongly interacting ultracold Rydberg gas. Despite the strong van der Waals interactions and interparticle correlations, we observe the persistence of a resonance with subnatural linewidth at the single-particle resonance frequency as we tune the interaction strength. This narrow resonance cannot be understood within a meanfield description of the strong Rydberg-Rydberg interactions. Instead, a many-body density matrix approach, accounting for the dynamics of interparticle correlations, is shown to reproduce the observed spectral features.PACS numbers: 42.50. Gy,42.50.Ct,32.80.Ee Coherent population trapping (CPT), i.e. the population of a quantum state decoupled from a resonant light field, serves as a paradigm for a quantum interference effect [1]. First observed in 1976 [2], CPT with its related phenomena electromagnetically induced transparency (EIT) [3,4] and stimulated Raman adiabatic passage (STIRAP) [5] has provided the basis for a large variety of effects and applications in many areas of physics, such as high-resolution spectroscopy, coherent control, metrology, quantum information and quantum gases. While CPT, EIT and STIRAP are generally described in terms of isolated single-atom interactions with coherent light fields, the situation becomes more involved when interactions between the particles need to be considered.To gain initial insights into the effects of interactions on the quantum interference in CPT, consider two atoms with a three-level ladder structure with states |1 , |2 and |3 as shown in Fig. 1(a). The atoms are exposed to two resonant coherent light fields and interact only if both of them are in the highly excited atomic state |3 . In the case of non-interacting atoms the population accumulates in the two-body product state of the single-particle dark state |d which is a coherent superposition of |1 and |3 [1]. This state is defined as the eigenstate of the total Hamiltonian with vanishing coupling to the coherent light field. When turning on the interparticle interaction this state is no longer a dark state as it is no longer an eigenstate of the total Hamiltonian. As pointed out in [6], the two interacting atoms, nevertheless, possess two dark states |d ± . These states are dissipative due to the admixture of the intermediate, decaying state |2 , but are significantly populated by optical pumping. While these states have dissipative character, they do not contain the state |33 and are, thus, immune to interactions.In a first approach to a many-particle system one could apply a meanfield model by replacing many-body opera- PSfrag replacementsFIG. 1: (a) Excitation scheme ( 87 Rb). Ω1 and Ω2 are the Rabi frequencies at 780 and 480 nm, respectively, δ is the detuning of the upper transition; (b) calculated Rydberg state population, produced by the two-step sequence described in the text. The upper panel shows the result of a meanfield calculation, which predicts a strong shift and broadening of the resonance line. On the contrary, the...

The dynamics of vibrational wave packets in triplet states of rubidium dimers (Rb2) formed on helium nanodroplets are studied using femtosecond pump-probe photoionization spectroscopy. Due to fast desorption of the excited Rb2 molecules off the droplets and due to their low internal temperature, wave packet oscillations can be followed up to very long pump-probe delay times 1.5 ns. In the first excited triplet state (1) 3 Σ + g , full and fractional revivals are observed with high contrast. Fourier analysis provides high-resolution vibrational spectra which are in excellent agreement with ab initio calculations.

In a recent work [Phys. Rev. Lett. 98, 023004 (2007)] we have investigated the influence of attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive longrange interatomic forces. Redistribution to nearby states induced by black body radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue-detuned from the atomic resonance

Abstract. We investigate a possible mechanism for the autoionization of ultracold Rydberg gases, based on the resonant coupling of Rydberg pair states to the ionization continuum. Unlike an atomic collision where the wave functions begin to overlap, the mechanism considered here involves only the long-range dipole interaction and is in principle possible in a static system. It is related to the process of intermolecular Coulombic decay (ICD). In addition, we include the interaction-induced motion of the atoms and the effect of multi-particle systems in this work. We find that the probability for this ionization mechanism can be increased in many-particle systems featuring attractive or repulsive van der Waals interactions. However, the rates for ionization through resonant dipole coupling are very low. It is thus unlikely that this process contributes to the autoionization of Rydberg gases in the form presented here, but it may still act as a trigger for secondary ionization processes. As our picture involves only binary interactions, it remains to be investigated if collective effects of an ensemble of atoms can significantly influence the ionization probability. Nevertheless our calculations may serve as a starting point for the investigation of more complex systems, such as the coupling of many pair states proposed in [1].

Vibrationally resolved photoionization spectra of RbHe exciplexes forming on He nanodroplets are recorded using femtosecond pump-probe spectroscopy with amplitude-shaped probe pulses. The time-evolution of the spectra reveals an exciplex formation time ~10 ps followed by vibrational relaxation extending up to ≳ 1 ns. This points to an indirect, time-delayed desorption process of RbHe off the He surface.

Homo-and heteronuclear alkali quartet trimers of the type K 3−n Rb n (n = 0, 1, 2, 3) formed on helium nanodroplets are probed by one-color femtosecond photoionization spectroscopy. The obtained frequencies are assigned to vibrations in different electronic states by comparison to high level ab initio calculations of the involved potentials including pronounced Jahn-Teller and spin-orbit couplings. Despite the fact that the resulting complex vibronic structure of the heavy alkali molecules complicates the comparison of experiment and theory we find good agreement for many of the observed lines for all species.