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Laser Physics

W-state preparation and entanglement dynamics in Rydberg atomic system based on the collective excitation enhancement

Kui Hou, Daqiang Bao, Chengjie Zhu, Yaping Yang

Published Nov 9, 2018• 5 citations
Publisher

Abstract

Abstract We present a theoretical scheme to realize the highly entangled W state in the neutral atomic system by using the Rydberg blockade effect. Considering three neutral atoms trapped in an optical lattice with an equilateral triangle geometric configuration, we show that only one atom can be excited to the Rydberg excite state due to the Rydberg blockade effect. By choosing a non-zero detuning from the exact two-photon resonance in order to compensate for the blockade error caused by the finite interaction energy, we show that the three atoms W state can be generated. Moreover, we show that three atoms are entangled when W state is achieved with high fidelity.

DOI
10.1088/1555-6611/aaea7b

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Abstract We present a theoretical scheme to realize the highly entangled W state in the neutral atomic system by using the Rydberg blockade effect. Considering three neutral atoms trapped in an optical lattice with an equilateral triangle geometric configuration, we show that only one atom can be excited to the Rydberg excite state due to the Rydberg blockade effect. By choosing a non-zero detuning from the exact two-photon resonance in order to compensate for the blockade error caused by the finite interaction energy, we show that the three atoms W state can be generated. Moreover, we show that three atoms are entangled when W state is achieved with high fidelity.
Interaction between Rydberg atoms can significantly modify Rydberg excitation dynamics. Under a resonant driving field the Rydberg-Rydberg interaction in high-lying states can induce shifts in the atomic resonance such that a secondary Rydberg excitation becomes unlikely leading to the Rydberg blockade effect. In a related effect, off-resonant coupling of light to Rydberg states of atoms contributes to the Rydberg anti-blockade effect where the Rydberg interaction creates a resonant condition that promotes a secondary excitation in a Rydberg atomic gas. Here, we study the light-matter interaction and dynamics of off-resonant two-photon excitations and include two- and three-atom Rydberg interactions and their effect on excited state dynamics in an ensemble of cold atoms. In an experimentally-motivated regime, we find the optimal physical parameters such as Rabi frequencies, two-photon detuning, and pump duration to achieve significant enhancement in the probability of generating doubly-excited collective atomic states. This results in large auto-correlation values due to the Rydberg anti-blockade effect and makes this system a potential candidate for a high-purity two-photon Fock state source.
We investigate the collective excitation effect in a scheme where three identical Rydberg atoms are arranged in an equilateral triangular lattice. By using a static electric field polarizing the atomic dipoles, the dipole–dipole interactions between two Rydberg atoms are essentially anisotropic and can even disappear in several special resonance cases. For that fact, we observe the collectively enhanced excitation probability of a single Rydberg atom in resonant areas in the case of strong blockade, and that of double or triple Rydberg atoms in the case of partial blockade. To give more evidence for this collective excitation enhancement, we study the two-body quantum correlation between three Rydberg atoms, as well as the dependence of the blockade radius on the length of triangle sides, which present a good agreement with the excitation properties.
Since the establishment of quantum mechanics, quantum entanglement has become one of the most important realms in quantum physics. On the one hand, it reflects some of the most fascinating features, such as quantum coherence, probability and non-locality and so on. On the other hand, it proves to be an indispensable resource of quantum information processing and quantum computation, which is considered to greatly promote the development of human science and technology. In the past decades, inspired by advances in quantum information theory and quantum physics, people have been searching for suitable systems with great enthusiasm to prepare the robust and manipulable quantum entanglement. Recently, Rydberg atoms have been considered to be a good candidate for many quantum information and quantum computation tasks. Compared with general neutral atoms, Rydberg atoms with large principal quantum number have several advantages in the quantum information and computation service. Firstly, they have finite lifetimes much larger than general neutral atoms, which indicates that the long-time entanglement between Rydberg atoms can be achieved. Secondly, due to the high-excitation level, Rydberg-excitation atoms have long-ranged dipole-dipole interaction much stronger than ground state atoms. This strong atomic interaction leads to the so-called blockade effect: when one atom is excited to Rydberg level, the excitation of the neighboring atoms will be strictly suppressed due to the energy shift induced by the strong atomic interaction. On the contrast, if the energy shift is compensated for by the detuning between the energy levels and the driven laser field, these atoms can be excited with higher probability simultaneously. These effects imply that Rydberg atoms provide an excellent platform for investigating the quantum information and quantum computation process, and many important achievements based on them have been achieved. Encouraged by these researches on entanglement and Rydberg atoms, in this paper, we study the steady-state and transient dynamical properties of two-body entanglement and the Rydberg-excitation properties in a dilute gas of Rydberg atoms, which can be represented by a tetrahedrally arranged interacting four-atom model. By solving numerically the master equation of four atoms involving Rydberg level, we investigate the higher-order Rydberg excitations and bipartite entanglement, which is estimated by concurrence. Our results show that the bipartite entanglement can only achieve its maximal value in the strongest dipole blockade regime rather than anti-blockade one (the high-order Rydberg excitations). Furthermore, the physical essence of quantum entanglement is analyzed theoretically in relevant regimes. Our work can naturally extend to more complicated atomic space structures, and might be treated as a good platform for fulfilling many quantum information tasks by employing the quantum entanglement.
Abstract We propose a scheme for generating the three-dimensional (3D) entanglement between two Rydberg atoms. Two laser fields are imposed to drive two transitions of each atom. The Rydberg antiblockade is induced through the suitable choice of the relation among parameters. We propose to offset Stark shifts by introducing detuning compensation so as to induce an effective resonant three-level system concluding two collective ground states and the double-excitation state, instead of introducing the complicated auxiliary levels and laser fields. Four 3D entangled states are calculated out, and numerical simulations show that the high-fidelity 3D entanglement can be generated. The effect of the spontaneous radiations of the Rydberg states on the generation of the 3D entanglement is analyzed. In addition, we show the numerical search algorithm so as to get optimized parameters for shortening operation time and thus decreasing the decoherence accumulation.
We report the generation of entanglement between two individual 87Rb atoms in hyperfine ground states |F=1,M=1> and |F=2,M=2> which are held in two optical tweezers separated by 4 microm. Our scheme relies on the Rydberg blockade effect which prevents the simultaneous excitation of the two atoms to a Rydberg state. The entangled state is generated in about 200 ns using pulsed two-photon excitation. We quantify the entanglement by applying global Raman rotations on both atoms. We measure that 61% of the initial pairs of atoms are still present at the end of the entangling sequence. These pairs are in the target entangled state with a fidelity of 0.75.
We study the Rydberg blockade in a system of three atoms arranged in different two-dimensional geometries (linear and triangular configurations). In the strong blockade regime, we observe high-contrast, coherent collective oscillations of the single excitation probability and an almost perfect van der Waals blockade. Our data are consistent with a total population in doubly and triply excited states below 2%. In the partial blockade regime, we directly observe the anisotropy of the van der Waals interactions between |nD> Rydberg states in the triangular configuration. A simple model that only uses independently measured two-body van der Waals interactions fully reproduces the dynamics of the system without any adjustable parameter. These results are extremely promising for scalable quantum information processing and quantum simulation with neutral atoms.