Chapter 33. A Strange Aspect of Intrinsic Spin

Dennis Morris

Nahid Binandeh Dehaghani, A. Pedro Aguiar, Rafał Wiśniewski

This chapter provides an in-depth analysis of the properties and phenomena associated with neutral K-mesons. Kaons are quantum systems illustrating strange behaviors. We begin by examining the significance of strangeness and charge parity violation in understanding these particles. The concept of strangeness oscillations is then introduced, explaining oscillations between K0 and K¯0 states. The regeneration of KS is investigated, uncovering the underlying mechanisms involved. The discussion moves on to quasi-spin space, exploring its bases and their implications. The entangled states of kaon pairs K0K¯0 are considered, with a focus on maximally entangled neutral kaons and nonmaximally entangled states. Decoherence effects on entangled kaons are examined, utilizing the density matrix description to capture the dynamics. A dedicated decoherence parameter is introduced to quantify the impact of decoherence. Furthermore, the chapter investigates the loss of entanglement through measures such as von Neumann entanglement entropy, entanglement of formation, and concurrence. These measures provide insights into quantifying and characterizing entanglement in the context of neutral kaons. Through this comprehensive exploration of properties, phenomena, and entanglement dynamics, this chapter aims to pointing out recent works on neutral kaons, contributing to advancements in particle physics.

Bahram Mashhoon, Helmut Kaiser

S. Haroche, J. M. Raimond

This chapter describes the non-classical aspects of the quantum theory. The strangeness of the quantum can be traced back to the superposition principle rooted in the linearity of quantum theory. Section 2.1 recalls how this principle conditions the properties of the wave function of a quantum particle. Section 2.2 discusses the ubiquitous quantum interference phenomenon and its relations with complementarity. Section 2.3 is about the subtleties of the superposition principle when applied to a system of identical particles. Section 2.4 shows that entanglement is a direct consequence of the superposition principle applied to composite systems. It analyses the non-local nature of quantum correlations in an entangled bipartite system and relates entanglement to complementarity. Section 2.5 discusses the quantum-classical boundary and analyses the decoherence phenomenon. Finally, Section 2.6 shows how simple quantum systems can be used as elementary qubits for information processing.

Mirjana Božić, Z. Marić

Sönke Johnsen

This concluding chapter explains that the modern theory of light falls within the field of quantum mechanics. At first glance, quantum mechanics does not seem that strange—its name is based on the fact that light comes in units and that electrons have discrete energy states. It also includes the uncertainty principle, which states that one cannot know certain pairs of physical properties with perfect precision. Moreover, quantum mechanics involves the wave-particle duality of photons. The chapter then explores two of the most unusual aspects of quantum mechanics: two-slit interference and quantum entanglement. Both violate the most fundamental notions about how the world works.