0.0 Context: Why Quantum Physics Is Being Discussed

0.0.1 In June 1925, a breakthrough in understanding the microscopic world laid the foundation of modern quantum physics.
0.0.2 Over the last 100 years, quantum physics has reshaped science, technology, and human understanding of nature.
0.0.3 To mark this centenary, the United Nations declared 2025 as the International Year of Quantum Science and Technology.

0.1 Origin of Quantum Theory (1900–1925)

0.1.1 In 1900, Max Planck proposed that light is emitted in discrete packets (quanta) while explaining black-body radiation.
0.1.2 In 1905, Albert Einstein used this idea to explain the photoelectric effect, linking light to particle behaviour.
0.1.3 In 1913, Niels Bohr applied quantum ideas to explain the structure of the hydrogen atom.
0.1.4 These ideas solved isolated problems but lacked a single, unified theoretical framework.

0.2 Heisenberg’s Breakthrough (1925)

0.2.1 In June 1925, Werner Heisenberg, while recuperating on the island of Helgoland, attempted to describe atomic behaviour mathematically.
0.2.2 His aim was to unify scattered quantum ideas into a consistent theoretical system.
0.2.3 This work marked the birth of modern quantum mechanics.

0.3 Matrix Mechanics and Mathematical Formalism

0.3.1 Heisenberg shared his work with his mentor Max Born.
0.3.2 Born realised that the theory could be expressed using matrices, a powerful mathematical tool.
0.3.3 Along with Pascal Jordan, they published seminal papers during 1925–26.
0.3.4 These papers are regarded as foundational milestones of 20th-century science.

0.4 Parallel Developments in the 1920s

0.4.1 In 1924, Louis de Broglie proposed that matter behaves like waves, not just particles.
0.4.2 Though initially doubted, the idea gained acceptance after Einstein’s endorsement.
0.4.3 In 1925, Erwin Schrödinger formulated the wave equation, making quantum calculations more accessible.
0.4.4 Satyendra Nath Bose introduced a new method of counting photons, later extended by Einstein.

0.5 Bose–Einstein Condensate

0.5.1 Bose’s work predicted a new state of matter, later called the Bose–Einstein condensate.
0.5.2 This exotic state occurs when particles behave as a single quantum entity at extremely low temperatures.
0.5.3 It was experimentally observed decades later, confirming the predictive power of quantum theory.

0.6 Experimental Confirmation

0.6.1 In 1929, C V Raman demonstrated quantum effects in light–matter interaction, now known as the Raman Effect.
0.6.2 This discovery earned him the 1930 Nobel Prize in Physics.
0.6.3 Such experiments provided direct empirical validation of quantum principles.

0.7 Quantum Theory Consolidation

0.7.1 By 1927, Paul Dirac described quantum mechanics as a “complete theory of dynamics”.
0.7.2 This marked the formal consolidation of quantum physics as a mature scientific framework.

0.8 Technological Impact Across Decades

0.8.1 Semiconductors emerged in the 1950s, enabling modern electronics.
0.8.2 Lasers, developed in the 1960s, revolutionised communication and medicine.
0.8.3 High-density hard disks appeared in the 1990s, enabling digital data storage.
0.8.4 Highly sensitive electronic sensors emerged in the 2000s.
0.8.5 Today, computers, smartphones, medical imaging, and fibre-optic communication all rely on quantum physics.

---