Sequential measurement-device-independent entanglement detection by multiple observers

TL;DR

This paper demonstrates that measurement-device-independent entanglement detection allows multiple observers to sequentially detect entanglement from a single shared pair, outperforming traditional methods in robustness and preserving entanglement.

Contribution

It introduces a sequential measurement scenario using measurement-device-independent witnesses, showing increased robustness and multiple successful detections compared to standard methods.

Findings

More observers can detect entanglement sequentially than with traditional methods.

Detection remains successful even with sharp measurements on nearly maximally entangled states.

The method is robust against detector loss and measurement imperfections.

Abstract

Violation of a Bell inequality certifies that the underlying state must be entangled in a device-independent way, although there may exist some entangled states which do not violate such an inequality. On the other hand, for every entangled state, it is possible to find a hermitian operator called entanglement witness that can detect entanglement through some local measurements in a device-dependent method. The methods are significantly fragile to lossy detectors. To avoid such difficulties, measurement-device-independent entanglement witness based on a semi-quantum nonlocal game was proposed which turns out to be robust against lossy detectors. We employ here such a measurement-device-independent entanglement witness to detect entanglement in a scenario where half of an entangled pair is possessed by a single observer while the other half is with multiple observers performing measurements sequentially, independently, and preserving entanglement as much as possible. Interestingly, we find that the numbers of successful observers who can detect entanglement measurement device-independently, both with equal and unequal sharpness quotients, are higher than that obtained with standard and Bell inequality-based entanglement detection methods, reflecting its robustness. The entanglement contents of the sequentially shared states are also analyzed. Unlike other scenarios, our investigations also reveal that in this measurement-device-independent situation, states having entanglement in proximity to maximal, remains entangled until two sequential observers even if they measure sharply.