Peculiar features of quantum correlations

Speaker: Prof. Ryszard Horodecki
Affiliation: University of Gdańsk, Poland

Quantum theory can be viewed as a kind of inscription encoded in the language of Hilbert space, which is a set of rules defining how to predict the results of future measurements in laboratories. The discovery that the inscription predicted the existence of non-classical correlations shook our ideas about nature, inspiring unprecedented development of quantum technologies. During the lecture, I will discuss the peculiar features of quantum correlations discovered in 1935 by Einstein, Podolski and Rosen and Schrödinger, called entanglement, as well as correlations beyond entanglement, discovered in the early 1980s, which came to be called discord.
The far from understanding, subtle and complex nature of quantum correlations. it still reveals new, fascinating features, observable in increasingly sophisticated experiments. During the lecture I will discuss, among others: discovered in Gdańsk, unusual features of quantum entanglement and methods of its detection. Let me start with a reference to the famous reading from the inscription by Erwin Schrödinger (1935), with three question marks: “The best possible knowledge of the whole does not include the best possible knowledge of its parts – and this is what keeps coming back to haunt us.” The importance of this profound observation was clarified and formalized in the mid-1990s and confirmed in a pioneering experiment in 2005, which provided the first nonlinear witness of entanglement.
Next, I will briefly present the basics of noisy entanglement detection, based on the concept of a linear entanglement witness, the Peres criterion and the so-called nonlinear entanglement witnesses currently used in laboratories. As it soon turned out, these criteria were unable to detect the so-called bound entanglement (1998). This new type of entanglement obtained in a series of spectacular experiments (2009-2020) implies the existence of a new type of irreversibility in Nature. This phenomenon surprised physicists “….bound entangled states test the limits of our understanding and puzzle us with their inherent irreversibility” (Physics Today (B. Terhal et al., 2003). Along the way, I will discuss some superadditive effects such as activation and superactivation of bound entanglement , and the nonadditivity effect of entanglement in quantum metrology.

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