Detection loophole in measurement-device-independent entanglement witnesses

There always exists an entanglement witness for every entangled quantum state. Negativity of the expectation value of an entanglement witness operator guarantees entanglement of the corresponding state, given that the measurement devices involved are perfect, i.e., the performed measurements actually constitute the witness operator for the state under consideration. In a realistic situation, there are two possible ways of measurements to drive the process away from the ideal one. First, wrong measurements may be performed, and, second, while the measurement operators are implemented correctly, the detection process may be noisy. Entanglement witnesses are prone to both of these imperfections. The concept of measurement-device-independent entanglement witnesses was introduced to remove the first problem. We analyze the detection loophole in the context of measurement-device-independent entanglement witnesses, which deal with the second problem of imprecise measurements. We obtain an upper bound, a threshold value, on the entanglement witness function in the measurement-device-independent entanglement witness scenario, below which entanglement is guaranteed for given nonideal detector efficiencies, that can involve both lost events and dark counts. We also present a threshold value on the function in the worst-case scenario where the threshold of the witness operator for guaranteeing entanglement assumes its maximal negative value.

Automated summary

There exists an entanglement witness for every entangled state where negativity of the expectation value guarantees entanglement, with measurement-device-independent entanglement witnesses introduced to remove imperfections. Through analysis, upper bounds and threshold values can be determined for guaranteeing entanglement in nonideal detector efficiency scenarios.