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arXiv:1212.0972 · New Journal of Physics

Proving the generation of genuine multipartite entanglement in a single-neutron interferometer experiment

Daniel Erdösi, Marcus Huber, Beatrix C. Hiesmayr, Yuji Hasegawa

Published Feb 22, 2013• 20 citations
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Abstract

We present experimental evidence of the generation of distinct types of genuine multipartite entanglement between three degrees of freedom (spin, energy and path) within single-neutron quantum systems. This is achieved via the development of new spin manipulation apparatuses for neutron interferometry and the entanglement is detected via appropriately designed and optimized nonlinear witnesses. Via the applied criteria we reveal even finer properties of the type of the genuine multipartite entanglement that is produced in the experiment. In a subsequent analysis we show the extraordinarily high fidelity of the generated entangled states, proving the outstanding level of controllability of these systems.

DOI
10.1088/1367-2630/15/2/023033

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We present experimental evidence of the generation of distinct types of genuine multipartite entanglement between three degrees of freedom (spin, energy and path) within single-neutron quantum systems. This is achieved via the development of new spin manipulation apparatuses for neutron interferometry and the entanglement is detected via appropriately designed and optimized nonlinear witnesses. Via the applied criteria we reveal even finer properties of the type of the genuine multipartite entanglement that is produced in the experiment. In a subsequent analysis we show the extraordinarily high fidelity of the generated entangled states, proving the outstanding level of controllability of these systems.
We prove experimentally the predicted existence of a three-qubit quantum state with genuine multipartite entanglement which can be certified solely from its separable two-qubit reduced density matrices. The qubits are encoded into different degrees of freedom of a pair of correlated photons and the state is prepared by letting the photons to propagate through a linear optical circuit. The presence of genuine multipartite entanglement is verified by finding numerically a fully decomposable entanglement witness acting nontrivially only on the reductions of the global state. Our result confirms viability of detection of emerging global properties of composite quantum systems from their parts which lack the properties.
Entanglement is a remarkable peculiarity in quantum mechanics. It occurs in quantum systems that consist of space-like separated parts, or in systems whose observables belong to disjoint Hilbert spaces. The latter is the case in single-neutron systems. Entangled states are renowned for exhibiting non-classical correlations between observables of individual sub-systems. In a perfect Si-crystal interferometer experiment entanglement between three degrees of freedom in a single-neutron system is created. The prepared entanglement of spin, path and energy is induced by interaction with an oscillating magnetic field. The generated Greenberger-Horne-Zeilinger (GHZ) state is analyzed with an inequality derived by Mermin, yielding a value M = 2.558(4) 2, which exhibits a clear violation of the classical assumption. In addition observation of a GHZ entanglement, consisting of spin, momentum and total energy, in a neutron polarimetric experiment is presented. Here the advantages of neutron polarimetry, such as high contrast or insensitivity to ambient disturbances, are utilized resulting in final value of M = 3.936(2) 2.
Tomographic measurements of Bell states of a single-neutron system in an interferometer were carried out. The spin-path joint measurements exhibit clear evidence of entanglement. Reconstructions of the density matrices of, practically complete, Bell states were accomplished. The fidelities as well as the concurrences of the generated states are determined by 0.79 and 0.75 as well as 0.59 and 0.52, respectively, which are again clear evidence of the spin-path entanglement in a single-neutron system.
The development of direct probes of entanglement is integral to the rapidly expanding field of complex quantum materials.Here we test the robustness of entangled neutrons as a quantum probe by measuring the Clauser-Horne-Shimony-Holt contextuality witness while varying the beam properties.Specifically, we show that the mode entanglement of the spin and path subsystems of individual neutrons prepared in two different experiments using two different apparatuses persists even after varying the entanglement length, coherence length, and neutron energy difference of the paths.The two independent apparatuses acting as entangler-disentangler pairs are static-field magnetic Wollaston prisms and resonance-field radio-frequency flippers.Our results show that the spatial and energy properties of the neutron beam may be significantly altered without reducing the contextuality witness value below the Tsirelson bound, meaning that maximum entanglement is preserved.We also show that two paths may be considered distinguishable even when the path states significantly overlap.Therefore, we have shown that our experimental results are consistent with the distinguishable subsystem assumption down to a separation of less than 100 nm, proving entanglement and the contextual nature of reality on short length scales.This work is the key step in the realization of the modular, robust technique of entangled neutron scattering, which can extract entanglement information from a sample without the knowledge of the microscopic sample Hamiltonian: only semiquantitative knowledge of the correlation lengths of the relevant degrees of freedom and the timescales of the characteristic dynamics is required.