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Improved entanglement detection with subspace witnesses

Won Kyu Calvin Sun, Alexandre Cooper, and Paola Cappellaro
Phys. Rev. A 101, 012319 – Published 13 January 2020

Abstract

Entanglement, while being critical in many quantum applications, is difficult to characterize experimentally. While entanglement witnesses based on the fidelity to the target entangled state are efficient detectors of entanglement, they in general underestimate the amount of entanglement due to local unitary errors during state preparation and measurement. Therefore, to detect entanglement more robustly in the presence of such control errors, we introduce a ‘subspace’ witness that detects a broader class of entangled states with strictly larger violation than the conventional state-fidelity witness at the cost of additional measurements, while remaining more efficient with respect to state tomography. We experimentally demonstrate the advantages of the subspace witness by generating and detecting entanglement with a hybrid, two-qubit system composed of electronic spins in diamond. We further extend the notion of the subspace witness to specific genuine multipartite entangled (GME) states detected by the state witness, such as GHZ, W, and Dicke states, and motivate the choice of the metric based on quantum information tasks, such as entanglement-enhanced sensing. In addition, as the subspace witness identifies the many-body coherences of the target entangled state, it facilitates (beyond detection) lower-bound quantification of entanglement via generalized concurrences. We expect that the straightforward and efficient implementation of subspace witnesses would be beneficial in detecting specific GME states in noisy, intermediate-scale quantum processors with a hundred qubits.

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  • Received 24 September 2019

DOI:https://doi.org/10.1103/PhysRevA.101.012319

©2020 American Physical Society

Physics Subject Headings (PhySH)

Quantum Information, Science & Technology

Authors & Affiliations

Won Kyu Calvin Sun1,*, Alexandre Cooper2, and Paola Cappellaro1

  • 1Research Lab of Electronics and Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  • 2Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

  • *wksun@mit.edu

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Issue

Vol. 101, Iss. 1 — January 2020

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