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Moscow University Physics Bulletin

On the Effect of Interatomic Interactions on the Efficiency of the Rydberg Blockade

Yu. V. Dumin

Published Nov 1, 2021• 0 citations
Publisher

Abstract

Abstract The process of the Rydberg blockade, which is currently regarded as one of the most promising approaches to quantum computation, can become significantly less effective due to interatomic interactions that shift the excitation frequency of initially blocked particles to the region of resonance with external laser radiation. In this article, we develop an analytical theory of this effect based on a self-consistent model of two Rydberg atoms acting on each other with their dipole electric fields and experiencing a Stark splitting of energy levels under the action of these fields. It follows from our calculations that the typical interatomic distances at which the unblocking effect due to ‘‘parasitic’’ resonances begins to occur are in rough agreement with earlier qualitative estimates; however, the number of such resonances can be much larger than in recent numerical simulations of some particular systems. Therefore, the possibility of breaking the Rydberg blockade at small interatomic distances should be carefully taken into account in any future applications to quantum computing.

DOI
10.3103/s0027134921060035

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Abstract The process of the Rydberg blockade, which is currently regarded as one of the most promising approaches to quantum computation, can become significantly less effective due to interatomic interactions that shift the excitation frequency of initially blocked particles to the region of resonance with external laser radiation. In this article, we develop an analytical theory of this effect based on a self-consistent model of two Rydberg atoms acting on each other with their dipole electric fields and experiencing a Stark splitting of energy levels under the action of these fields. It follows from our calculations that the typical interatomic distances at which the unblocking effect due to ‘‘parasitic’’ resonances begins to occur are in rough agreement with earlier qualitative estimates; however, the number of such resonances can be much larger than in recent numerical simulations of some particular systems. Therefore, the possibility of breaking the Rydberg blockade at small interatomic distances should be carefully taken into account in any future applications to quantum computing.
Systems of cold, interacting Rydberg atoms are of interest because they exhibit a wide range of collective quantum phenomena. This thesis describes experimental and theoretical work related to these systems with an emphasis on two phenomena: Rydberg excitation blockades and quantum-state-changing collisions. I begin by presenting a detailed calculation of the interaction energies of two Rydberg atoms for a variety of orientations, quantum states, and applied electric fields. The calculated energies are used to interpret the experimental results that follow. I then describe measurements showing that the widths of the distributions of Rydberg excitation number are related to the effectiveness of a Rydberg excitation blockade. These distributions are investigated as a function of principal quantum number, n, and applied electric field. In the presence of a Forster resonant field, sub-Poissonian distributions are measured. I next present a spectroscopic measurement of the energy level shift of a system of atoms which collectively shares two Rydberg excitations. The qualitative nature of the shift depends sensitively on the type of interactions between the atoms (van der Waals or dipole-dipole) and may be changed by changing the quantum state or applied electric field. This measurement provides the first spectroscopic proof that Rydberg excitation blockades are operative. Finally, I describe measurements of the probability for quantum state-changing and Penning-ionizing collisions to provide evidence for intrinsic, electric field-free-interaction resonances and complex many-body interactions in systems of cold Rydberg atoms.
Owing to the unique physical characteristics of Rydberg atoms, which play an important role in quantum information and quantum computation, the theoretical and applied research of Rydberg atoms have become the hot spots of scientific research in recent years. With the large polarizability of Rydberg atoms, even a small electric field could cause a considerable electric dipole moment, resulting in a strong dipole-dipole interaction between Rydberg atoms. The multiple excitations of the Rydberg states are strongly inhibited because of the strong dipole interaction between atoms within a mesoscopic interaction (blockade) region. We call this phenomenon the dipole blockade effect. The dipole blockade effect makes it possible to build single-photon quantum devices, implement quantum gates, generate quantum entanglement, and simulate many-body quantum problems, etc. A Rydberg atomic ensemble in the same blockade region can be regarded as a superatom. In the same way, if these atoms trapped in two optical dipole traps, each sub-ensemble can be considered as a sub-superatom which is closely related to the superatom. According to the fact that two Rydberg sub-superatoms can be strongly correlated due to sharing no more than one excited Rydberg atom, we study correlated collective excitation and quantum entanglement between two Rydberg sub-superatoms in a steady state. With the superatom model, the problem of exponentially increasing system size with the number of atoms can be circumvented to a certain extent in studying many-body physics. By solving the two-body Lindblad’s master equation accurately, we obtain the analytical expressions for the collective excitation probabilities of the two sub-superatoms, and the concurrence measuring the bipartite entanglement between them. Our results show that they are all sensitive to the number of atoms in each Rydberg superatom: the bigger (including more atoms) the Rydberg superatom, the higher the collective Rydberg excitation probability is. And that the maximally entangled state can only be obtained with two equal-sized Rydberg superatoms. When this condition is fulfilled, the mesoscopic entanglement can be generated by adding the number of atoms in each Rydberg superatom. This may provide an attractive platform for studying the quantum-classical correspondence and have potential promising applications in quantum information processing.
High resolution laser Stark excitation of np (60<n<85) Rydberg states of ultracold cesium atoms shows an efficient blockade of the excitation attributed to long-range dipole-dipole interaction. The dipole blockade effect is observed as a quenching of the Rydberg excitation depending on the value of the dipole moment induced by the external electric field. Effects of ions which could match the dipole blockade effect are discussed in detail but are ruled out for our experimental conditions. Analytic and Monte Carlo simulations of the excitation of an ensemble of interacting Rydberg atoms agree with the experiments and indicate a major role of the nearest neighboring Rydberg atom.
The spatial evolution of the Rydberg blockade is studied taking into account Stark-split energy levels across several manifolds. We find that the unexpected restoration of a blockaded Rydberg excitation at small interatomic distances, e.g., experimentally observed by P.Schauss, et al. [Nature 491, 87 (2012)], can be explained by the perturbed energy levels from neighboring manifolds that enter the energy window of excitation defined by the bandwidth of the exciting laser. The same mechanism can also explain why the pair correlation function of Rydberg atoms remains nonzero in the entire region of Rydberg blockade.
Abstract The dipole blockade effect at laser excitation of mesoscopic ensembles of Rydberg atoms lies in the fact that the excitation of one atom to a Rydberg state blocks the excitation of other atoms due to the shift in the collective energy levels of interacting Rydberg atoms. It is used to obtain the entangled qubit states based on single neutral atoms in optical traps. In this paper, we present our experimental results on the observation of the dipole blockade for mesoscopic ensembles of 1–5 atoms when they are detected by the selective field ionization method. We have investigated the spectra of the three-photon laser excitation 5S1/2 → 5P3/2 → 6S1/2 → nP3/2 of cold Rydberg Rb atoms in a magneto-optical trap. We have found that for mesoscopic ensembles this method allows only a partial dipole blockage to be observed. This is most likely related to the presence of parasitic electric fields reducing the interaction energy of Rydberg atoms, the decrease in the probability of detecting high states, and the strong angular dependence of the interaction energy of Rydberg atoms in a single interaction volume.
Abstract Splitting the energy levels of a hydrogen-like atom by the electric field nonuniform at the atomic scale is studied. This situation is important for the multi-level treatment of the phenomenon of Rydberg blockade (Dumin Y V 2014 J. Phys. B: At. Mol. Opt. Phys. 47 175502 ). An explicit formula for the energy levels is derived. A typical value of the energy shift by the electric field gradient turns out to be proportional to the fourth power of the principal quantum number (i.e. the square of atomic size), as would be expected from a qualitative consideration. Finally, the fine spatial structure of the Rydberg blockade is analyzed when the electric-field-gradient term plays the dominant role, and the results are compared with the experimental data.