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Physical Review A

Witnessing entanglement from N -qubit radiation emission

P. Milman

Published Oct 17, 2006• 3 citations
Publisher

Abstract

We show how radiation emitted from a collection of noninteracting two-level particles, which can be treated as qubits, can be used to detect a wide class of entangled states of this system. This result is an example of a macroscopical manifestation of entanglement that can be inferred from direct measurements. Some examples of physical effects directly related to the collective emission of radiation, such as photon echoes and superradiance, are discussed, suggesting some ways of how to experimentally witness entanglement with simple and standard techniques.

DOI
10.1103/physreva.74.042317

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We show how radiation emitted from a collection of noninteracting two-level particles, which can be treated as qubits, can be used to detect a wide class of entangled states of this system. This result is an example of a macroscopical manifestation of entanglement that can be inferred from direct measurements. Some examples of physical effects directly related to the collective emission of radiation, such as photon echoes and superradiance, are discussed, suggesting some ways of how to experimentally witness entanglement with simple and standard techniques.
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Localized radiation sources are analyzed with respect to the relation of nonclassicality and quantum entanglement of the emitted light. The source field parts of the radiation emitted in different directions are closely related to each other. As a consequence, nonclassicality of the light fields in one direction directly implies entanglement of the field modes in different directions. This implication can be extended to multipartite entanglement and multitime quantum correlations. Given that a nonclassical effect is observed, our approach explicitly yields the multipartite entanglement witnesses. Two examples are considered, the fluorescence radiation of a system of two-level atoms and of excitons in a semiconductor quantum well.
Quantum theory predicts that entanglement can also persist in macroscopic physical systems, albeit difficulties to demonstrate it experimentally remain. Recently, significant progress has been achieved and genuine entanglement between up to 2900 atoms was reported. Here, we demonstrate 16 million genuinely entangled atoms in a solid-state quantum memory prepared by the heralded absorption of a single photon. We develop an entanglement witness for quantifying the number of genuinely entangled particles based on the collective effect of directed emission combined with the non-classical nature of the emitted light. The method is applicable to a wide range of physical systems and is effective even in situations with significant losses. Our results clarify the role of multipartite entanglement in ensemble-based quantum memories and demonstrate the accessibility to certain classes of multipartite entanglement with limited experimental control.The presence of entanglement in macroscopic systems is notoriously difficult to observe. Here, the authors develop a witness which allow them to demonstrate entanglement between millions of atoms in a solid-state quantum memory prepared by the heralded absorption of a single photon.
The certification of entanglement is a vital task in quantum technology. To theoretically quantify and experimentally detect entanglement of many qubits, we suggest to identify quantum entanglement by measuring the statistical response of a quantum system to an arbitrary nonlocal parametric evolution. Our suggestion does not basing on the tomographic reconstruction of the quantum state, or the realization of witness operators. The protocol requires two collective settings for any number of parties and is robust against noise and decoherence occurring after the implementation of the parametric transformation. The bounds for witnessing quantum entanglement with strength of nonlocal interaction are shown with the help of Ising nonlocal Hamiltonian. Furthermore to illustrate its user friendliness we demonstrate multipartite entanglement in different experiments with ions and photons by analyzing published data on fidelity visibilities and variances of collective observables.
We describe an experimentally straightforward method for preparing an entangled W state of up to 100 qubits. Our repeat-until-success protocol relies on detection of single photons from collective spontaneous emission in free space. Our method allows entanglement preparation in a wide range of qubit implementations that lack entangling qubit-qubit interactions. We give detailed numerical examples for entanglement of neutral atoms in optical lattices and of nitrogen-vacancy centres in diamond. The simplicity of our method should enable preparation of mesoscopic entangled states in a number of physical systems in the near future.