Detection and Observables

Nisticò, Giuseppe, Angela Sestito

In Quantum Physics it is not always possible to directly perform the measurement of an obsevable; in some of these cases, however, its value can be {\sl detected}, i.e. it can be inferred by measuring {\sl another} observable characterized by perfect correlation with the observable of interest. Though a {\sl detection} is often interpreted as a {\sl measurement} of the detected observable, we prove that the two concepts cannot be identified in Quantum Physics. Furthermore, we establish what meaning and role can be ascribed to detections coherently with Quantum Theory.

Herbert S. Green

John P. Ralston

Chirag Srivastava, Mahasweta Pandit, Ujjwal Sen

Experimental detection of entanglement certainly disturbs the underlying shared state. It is possible that the entanglement content of the system is lost in the process of its detection. This observation has led to the study of sequential detection properties of various quantum correlations. Here, we investigate the sequential detection of genuinely multipartite entanglement of quantum systems composed of an arbitrary number of qubits. In order to detect genuine multipartite entanglement sequentially, observers can recycle any fixed subset of all the qubits, thus leading to a hierarchy of scenarios, categorized according to the number of qubits which are recycled by the observers. We show that the sequential detection of genuine multipartite entanglement, for every scenario in the hierarchy, leads to an unboundedly long sequence. This is shown to be possible if the initial state shared among the observers is the multipartite generalized Greenberger-Horne-Zeilinger state and is a class of mixed states. A comparison among different hierarchical scenarios is drawn based on the number of sequential detections of genuine multipartite entanglement for a specific measurement strategy employed by observers.

Stephen J. Gustafson, Israel Michael Sigal

Mao-Sheng Li, Wen Xu, Shao-Ming Fei +2 more

Quantum coherence is the key resource in quantum technologies, including faster computing, secure communication, and advanced sensing. Its quantification and detection are, therefore, paramount within the context of quantum information processing. Having certain prior knowledge of the observables may enhance the efficiency of coherence detection. In this work, we posit that the trace of the observables is a known quantity. Our investigation confirms that this assumption indeed extends the scope of coherence-detection capabilities. Utilizing this prior knowledge of the trace of the observables, we establish a series of coherence-detection criteria. We investigate the detection capabilities of these coherence criteria from diverse perspectives and ultimately ascertain the existence of four distinct and inequivalent criteria. These findings contribute to the deepening of our understanding of coherence-detection methodologies, thereby potentially opening new avenues for advancements in quantum technologies.