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Physical Review A

Dynamics of collective-dephasing-induced multiatom entanglement

Y. Li, Yefeng Mei, Huy Minh Nguyen, P. R. Berman, A. Kuzmich

Published Nov 17, 2022• 9 citations
Publisher

Abstract

The authors study the dynamics of dephasing induced by Rydberg-Rydberg interactions and show that, as a result of these interactions, an initially unentangled Rydberg spin wave evolves into an entangled Dicke state. The results of this study have relevance to broad classes of applications for collective atomic systems, including driving of collective atomic qubits, on-demand generation of single photons, and preparation of entangled states involving atoms or light.

DOI
10.1103/physreva.106.l051701

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Physical Review A•Jan 16, 2024•4 citations
The authors study the dynamics of dephasing induced by Rydberg-Rydberg interactions and show that, as a result of these interactions, an initially unentangled Rydberg spin wave evolves into an entangled Dicke state. The results of this study have relevance to broad classes of applications for collective atomic systems, including driving of collective atomic qubits, on-demand generation of single photons, and preparation of entangled states involving atoms or light.
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An approach to fast entanglement generation based on Rydberg dephasing of collective excitations (spin waves) in large, optically thick atomic ensembles is proposed. Long-range 1/r(3) atomic interactions are induced by microwave mixing of opposite-parity Rydberg states. The required long coherence times are achieved via four-photon excitation and readout of long wavelength spin waves. The dephasing mechanism is shown to have favorable, approximately exponential, scaling for entanglement generation.
This thesis explores the effects of decoherence and dephasing on single- and many- body dynamics of spin-systems. A particular realisation of the spin systems that the work focuses on are highly excited, Rydberg states of atoms. A software library ARC (Alkali Rydberg Calculator) for calculating properties of Rydberg states in alkali metals is presented, with particular attention to the multi- atom and multi-level effects that influence many-body dynamics in realistic systems, and properties related to terahertz imaging with alkali atom Rydberg states. Dressed-state electromagnetically induced transparency (EIT) is proposed as a way of preparing uniform-phase spin-waves in ladder excitation schemes, making the collective excitation storage insensitive to motional dephasing. Proof of concept dressed state EIT experiments are presented. Strong resonant dressing is also theoretically analysed as a way of preparing velocity superposition of spin-waves. The developed theoretical model is in a good agreement with existing experimental data on single-photon many-atom quantum beats in diamond excitation schemes. By modelling the strongly driven Rydberg ensembles, many-body dynamics of driven-dissipative spin systems is analysed. Working in the limit of strong dephasing, the effects of fluctuations, the shape of interaction potential, spatial correlations and motion on non-equilibrium phase diagrams and the occurrence of bistability are examined. An ensemble averaged mean field model is introduced as an exact solution for completely uncorrelated ensembles. It is shown that the van der Waals interaction does not allow the occurrence of bistability, for which a finite dipolar core is required. The short-range interaction potential shape is found to have a profound influence on non-equilibrium phase diagrams, controlling the size of fluctuations in the dynamics. For a frozen system, several methods for identifying and quantifying bistable phases are introduced, and phase diagrams are reconstructed. It is shown that the temperature of external degrees of freedom, i.e. spin motion, can drive a non-equilibrium transition into the bistable phase.
Based on the dipole blockade effect and with the aid of the superatom (SA) model, we propose a scheme to investigate the correlated evolution of two Rydberg sub-superatoms (SSAs), formed by two spatially separated atomic Rydberg sub-ensembles but in the same blockade region. Starting from the pure separable states, we investigate the in-phase or anti-phase correlated dynamics and explore how two Rydberg SSAs entangle with each other mediated by a single Rydberg excitation. Starting from the entangled states, we discuss the robustness of the system against decoherence induced by the dephasing rate. Our results show that both the correlated evolution of two Rydberg SSAs and their collective-state entanglement are usually sensitive to the number of each Rydberg SSA. This allows us to coherently manipulate the Rydberg ensemble over long distances from the single-quantum level to the mesoscopic level by changing the number of atoms. Furthermore, the method for dividing an SA into two SSAs and obtaining their spin operators without any approximation can be readily generalized to the case of many SSAs. It may have potential promising applications in quantum information processing and provide an attractive platform to study the quantum-classical correspondence, many-body physics and so on.
Quantum information is a rapidly developing field that merges the principles of quantum mechanics with information theory to explore new ways of processing and transmitting information. Qubits, which utilize superposition and entanglement, offer the potential for exponentially faster computation and secure communication methods that are fundamentally different from classical systems. This thesis contributes to the future of quantum information by studying two key platforms using neutral atoms: atomic arrays and atomic ensembles. It summarizes essential theoretical and experimental methods in the field and discusses key findings. To achieve entanglement among atomic qubits, Rydberg interactions are utilized, with two main effects discussed in this thesis. The Rydberg blockade effect is introduced where atoms trapped in an ensemble can form a two-level superatom. A state-insensitive optical lattice is used to achieve long coherence time for both the ground and Rydberg states. Long-lived Rabi oscillations and Ramsey interference are reported to demonstrate the collective enhancement of √N in a superatom. Furthermore, Rydberg dephasing is separated from Rydberg blockade and is investigated as a method to generate single photons from multiply excited states. This thesis presents both theoretical models and temporal evolution measurements of Rydberg dephasing from an initially unentangled Rydberg spinwave into an entangled single photon state. Additionally, this thesis examines the collective atomic dipole moment of a superatom, ⟨E⟩. Theoretical and experimental work are conducted to study the dipole moment when the phase-matched emission is combined with a coherent field in a homodyne detection method. This work advances our understanding of Rydberg interactions and collective dipole moments, contributing to the rapidly developing field of quantum information. The goal is to pave the way for the next era of quantum information through collaborative efforts and innovative research.
Interactions between Rydberg atoms can result in a dipole blockade for which the probability Pn{P}_{n}Pn​ to have nnn atomic excitations is reduced significantly when $n\ensuremath{\ge}2$. Ideally, this allows one to create a single collective Rydberg excitation in an atomic ensemble with P1=1{P}_{1}=1P1​=1. A single-photon source is realized by mapping this atomic excitation into a propagating light field. Even if ${P}_{n}\ensuremath{\ne}0$ for $n\ensuremath{\ge}2$, a single-photon source can be approximated if interaction-induced dephasing damps the contributions from multiply excited collective states into the preferred spatial field mode. Two quantities that can be used as figures of merit for these single-photon sources are the second-order correlation function g(2){g}^{(2)}g(2) associated with the phase-matched field emitted by the sample and the second-order correlation function g(A){g}^{(A)}g(A) associated with the atoms. Here we demonstrate that interaction-induced dephasing can lead to significant differences between g(2){g}^{(2)}g(2) and g(A){g}^{(A)}g(A) even if ${P}_{2}\ensuremath{\ll}{P}_{1}\ensuremath{\approx}1$. Theoretical expressions are derived for these quantities and it is shown that ${g}^{(2)}\ensuremath{\le}{g}^{(A)}$. It is also shown that there is a distinct advantage for minimizing g(2){g}^{(2)}\phantom{\rule{4pt}{0ex}}g(2) and g(A){g}^{(A)}g(A) by using adiabatic pulsed fields rather than constant amplitude fields to excite the ensemble. These results maybe useful for optimizing Rydberg single-photon sources.