Witnessing entanglement without entanglement witness operators.
Quantum mechanics predicts the existence of correlations between composite systems that, although puzzling to our physical intuition, enable technologies not accessible in a classical world. Notwithstanding, there is still no efficient general method to theoretically quantify and experimentally detect entanglement of many qubits. Here we propose to detect entanglement by measuring the statistical response of a quantum system to an arbitrary nonlocal parametric evolution. We witness entanglement without relying 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. 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.
Quantum Science and Technology in Arcetri, 50125 Florence, Italy; National Institute of Optics, National Research Council, 50125 Florence, Italy; European Laboratory for Non-Linear Spectroscopy, 50125 Florence, Italy.
Luca
L
Pezzè
Institute of Theoretical Physics and Department of Physics, Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China.
Yan
Y
Li
Institute of Theoretical Physics and Department of Physics, Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China wdli@sxu.edu.cn.
Weidong
W
Li
Quantum Science and Technology in Arcetri, 50125 Florence, Italy; National Institute of Optics, National Research Council, 50125 Florence, Italy; European Laboratory for Non-Linear Spectroscopy, 50125 Florence, Italy.
Augusto
A
Smerzi
Fisher information; entanglement detection; quantum entanglement; quantum technology; trapped ions
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