Abstract This chapter describes entanglement in bipartite systems. We define entanglement for pure states introducing the concept of Schmidt decomposition, and we show that entanglement can be quantified through the von Neumann entropy. We then present dense coding and teleportation, two quantum information protocols that use entanglement as a resource. We present the notions of locality and realism and derive the Clauser-Horne-Shimony-Holt inequality, an extended version of Bell’s inequalities. We then focus on mixed states and we present different techniques to solve the separability problem
Abstract This chapter describes entanglement in bipartite systems. We define entanglement for pure states introducing the concept of Schmidt decomposition, and we show that entanglement can be quantified through the von Neumann entropy. We then present dense coding and teleportation, two quantum information protocols that use entanglement as a resource. We present the notions of locality and realism and derive the Clauser-Horne-Shimony-Holt inequality, an extended version of Bell’s inequalities. We then focus on mixed states and we present different techniques to solve the separability problem
Chapter devoted to the basic quantum properties of entanglement and separability. Introduces the Schmidt decomposition for pure states and the positive partial transpose criterion for mixed states as entanglement witnesses. Introduces the famous Einstein–Podolsky–Rosen paradox and its implementation in terms of qubits, then the Bell inequality, quickly reviewing the experimental demonstrations that quantum mechanics violates this inequality. Gives examples of the use of entanglement in a quantum algorithm to accelerate an information task, namely a database search (Grover algorithm) and the possibility of teleportation of a quantum state.
In 1935, Einstein, Podolsky, and Rosen published the paper that introduced what would become quantum mechanics’ most studied phenomenon. Two systems prepared together and then separated remain correlated — perfectly correlated, in some cases — across arbitrary distances. The correlations exceed anything classical physics permits, are tested today across hundreds of kilometers via satellite‑mediated photon pairs, and form the resource that quantum cryptography, teleportation, and computation rely on. The standard QM account is mathematically clean. Two systems with Hilbert spaces H_A and H_B are described on H_A ⊗ H_B; states that cannot be written as |ψ_A⟩ ⊗ |ψ_B⟩ are entangled. Bell–Tsirelson bounds CHSH at 2√2. Schmidt decomposes any pure bipartite state biorthogonally. Coffman–Kundu–Wootters codifies monogamy. No‑signaling follows from linearity of partial trace. Von Neumann entropy S(ρ_A) = −Tr(ρ_A log ρ_A) measures entanglement strength. Each piece is a theorem of the postulated formalism — but the postulates themselves (tensor‑product joint structure, Hilbert space, Born rule) are not derived. This walkthrough shows how the Event Density framework derives the structural commitments themselves. Six results, each derived from the same substrate primitives: (1) Tensor‑product composition — the joint participation measure of two endpoints sharing one rule is carried on H_A ⊗ H_B with bilinearly induced inner product. Form forced; no auxiliary assumption. (2) Generic non‑factorizability — entangled states are dense and full‑measure; product states are the measure‑zero exception. (3) Schmidt decomposition — every pure bipartite state has a biorthogonal decomposition with non‑negative coefficients summing to one. Form forced; values inherited. (4) Monogamy — each substrate endpoint has a finite local cross‑chain bandwidth budget; full bipartite entanglement saturates it; no capacity remains for a third endpoint. CKW is the substrate bandwidth‑conservation law in concurrence units. (5) No‑signaling — over‑determined by three independent substrate locks: T18 forward‑cone‑only kernel structure, P11 commitment irreversibility, and the ED‑I‑06 no‑fundamental‑fields guardrail. (6) Von Neumann entropy — the form is uniquely picked by Shannon–Khinchin axioms each independently derived from named substrate primitives. This walkthrough has more substrate content than Bell–Tsirelson or Heisenberg. Those ran on standard mathematical‑physics arguments once the framework supplied the Hilbert space; the contribution was upstream. Here the framework is upstream for tensor‑product composition, Schmidt, and the density of entangled states — and substantively new for monogamy, no‑signaling, and entropy, each receiving a substrate‑mechanism account standard QM does not provide. The structural payoff: bipartite entanglement is what the substrate produces when two endpoints share one undeveloped participation rule. Tensor product, Schmidt structure, monogamy, no‑signaling, and entropy form are six projections of one substrate phenomenon.
Abstract This chapter presents a detailed discussion of Bell inequalities and their experimental tests. Starting from the Aharonov-Bohm version of the Einstein-Podolsky-Rosen (EPR) paradox, we review EPR’s criteria for realism and completeness and Bell’s locality condition, before deriving the Clauser-Horne-Shimony-Holt (CHSH) inequality. This is followed by an exposition on Bell’s original inequality and other Bell inequalities by Wigner, as well as Clauser and Horne. We then discuss the milestones in the experimental violation of Bell inequalities, from the experiment of Freedman and Clauser in the 1970s, to the efforts of Aspect et al. in the 1980s, the experiments by the teams of Zeilinger and Gisin in the 1990s, and culminating the loophole-free tests in 2015. Finally we turn to a brief discussion of the interpretations of quantum mechanics, focusing on contrasting the realist views held by, e.g., Einstein and Bell, and the informational views with proponents such as Brukner and Zeilinger
Abstract In this paper, we report an experimental activity suitable for undergraduate students that makes them question their ideas about locality and realism. The experiment here reported complements previous educational approaches to Bell inequalities, since the usually called S function, that quantifies correlations, is mapped by measuring it for different detection angles. The students themselves worked in a pre-aligned setup that allowed them to test and violate the Clauser–Horne–Shimony–Holt–Bell inequality for two distant photons entangled in polarization.
Abstract This chapter gives a detailed overview of entanglement for pure and mixed states. We begin with the Schmidt-decomposition theorem, which allows us to associate the subsystem mixedness of pure state with entanglement. We discuss the purification of mixed quantum states, the convexity of the set of separable states, and entanglement as a form of correlation. As an example we examine the Bloch decomposition for two qubits. For the detection of entanglement the Peres-Horodecki criterion is introduced, before we analyse the relation between entanglement and non-locality via the CHSH-Operator Criterion and the family of Werner states. We further consider Tsirelson’s bound and hidden non-locality. We then discuss important mathematical tools for entanglement detection, including the positive-map theorem, the Størmer-Woronowicz theorem, and the reduction criterion. In this context we examine the family of isotropic states. Finally, the Weyl states provide an example for which we study the geometry of two-qubit entanglement
Quantum information theory is a rapidly growing area of math and physics that combines two independent theories, quantum mechanics and information theory. Quantum entanglement is a concept that was first proposed in the EPR paradox. In quantum mechanics, particles can be in superposition, meaning they are in multiple different states at once. It is not until the particle is measured that it is forced into a single state. However, it is possible that particles can be tied to other particles, meaning that the measurement of one particle will determine the measurement of the other particle. Entanglement is at the very core of quantum information theory. It is one of the core pieces that allows for the massive increase in computing power. For this paper, we decided to focus on demonstrating the mathematical method (the Schmidt decomposition) for determining if a system is entangled, and a demonstration of quantum entanglement's use (quantum teleportation) as well as a quick look at how to extend the uses of the Schmidt decomposition.