The beginning of the 20th century is marked by tremendous new scientific developments and specifically by the birth of quantum mechanics. It starts with Max Planck and Max Einstein, who introduce and apply the concept of quanta to characterize the energy of light. Quantum computers can considerably decrease the time it takes to complete calculations and solve extremely complex problems. Until the 18th century, very little is understood about light. According to the philosophers Euclid and Plato, light is produced by our eyes. Light is clearly capable of traveling through an interstellar void, while a wave needs a medium for material propagation to exist. However, because of Isaac Newton’s reputation, the corpuscular theory persists until the 19th century. When light is beamed on the top layer, it frees and pushes an electron that moves towards the bottom layer, creating a current. This current is then used to power engines for electric cars.
The beginning of the 20th century is marked by tremendous new scientific developments and specifically by the birth of quantum mechanics. It starts with Max Planck and Max Einstein, who introduce and apply the concept of quanta to characterize the energy of light. Quantum computers can considerably decrease the time it takes to complete calculations and solve extremely complex problems. Until the 18th century, very little is understood about light. According to the philosophers Euclid and Plato, light is produced by our eyes. Light is clearly capable of traveling through an interstellar void, while a wave needs a medium for material propagation to exist. However, because of Isaac Newton’s reputation, the corpuscular theory persists until the 19th century. When light is beamed on the top layer, it frees and pushes an electron that moves towards the bottom layer, creating a current. This current is then used to power engines for electric cars.
You have probably heard of quantum computing or quantum in general; otherwise, you would not have bought this book. Quantum is a concept from the world of physics, where it goes by the name quantum mechanics. Its development started in the beginning of the 20th century by well-known physicists like Max Planck, Niels Bohr, Werner Heisenberg, Albert Einstein, Erwin Schrödinger, Richard Feynman, and many others. Quantum mechanics studies the behavior and properties of nature on an atomic scale. Thanks to the discoveries in the field of quantum mechanics, we can leverage these discoveries and create a new tenor in computer science.
At the beginning of the twentieth century, it became clear that microscopic particles do not obey Newton's laws of classical mechanics. The idea of light quanta was at odds with the established wisdom that light is a wave phenomenon, of course, but no other explanation of the photoelectric effect seemed possible. The next step in the development of quantum mechanics was taken by Niels Bohr, who introduced postulates about the discreteness of the energy of the bound electrons in an atom similar to the discreteness of the energy of light quanta. Modern quantum mechanics was founded by Werner Heisenberg, Max Born, Erwin Schroedinger, Paul Adrian Maurice Dirac, Wolfgang Pauli, Enrico Fermi, and others. In quantum mechanics, it is in general fundamentally impossible to determine the exact outcome of an experiment; one can only determine the probability of any given outcome.
At the turn of the 20th century, Max Planck and Albert Einstein introduced the quantum theory which revolutionized our understanding of the nature of energy, in particular the nature of light energy. According to Planck, energy is absorbed or emitted from hot objects in tiny discrete bundles called quanta, and each quantum carries a precise quantity of energy which is related to the frequency of light wave. Einstein generalized Planck’s quantum theory. Einstein’s quantum theory removed a grating asymmetry in nature, between energy and matter. Louis DeBroglie said proposed that if light waves behave as particles, electrons, normally thought of as particles, ought to reciprocally behave as waves. DeBroglie provided a crucial missing piece in the quantum puzzle of discrete Bohr orbitals. Erwin Schroedinger seized DeBroglie’s brain wave. He developed a “wave mechanics” to describe the quantum world. For Schroedinger, particles are just an “illusion” of many waves; he turned waves into particles.
In the 20th century, two new terms were added to the scientific lexicon: relativistic mechanics and quantum mechanics. The challenge to the concept of the absoluteness of space and time started at the end of the 19th century and gave birth to Einstein's special relativity theory in 1905. This unified space and time to a single entity, space–time, and we have been introduced to many new concepts such as time dilation, space contraction, mass–energy relation, and the space–time continuum in relativity theory. We have also learned that the highest possible velocity in this universe is that of light and that nothing in the universe can move faster than light. Einstein's general theory of relativity took into consideration gravitation and established the chronogeometric concept of gravity. Quantum mechanics developed by Bohr, de Broglie, Heisenberg, Schrödinger, and others gave us knowledge in the atomic and subatomic domain and led to the wave–particle duality of de Broglie, the uncertainty principle of Heisenberg, the wave mechanics of Schrödinger, and so forth. We have learned that three distinct chords are at the heart of quantum mechanics—granularity, relational, and indeterminism. Although quantum mechanics is a weird subject, as we will discuss, the modern physics is totally based on it.
The quantum revolution in physics of the 20th century has completely transformed the nature not only of physics and science in general, but also of philosophy and our worldview. Quantum revolution began in the early 19th century with the discovery of ‘quantum’ (plural ‘quanta’) principally by Max Planck, and with theories and discoveries in the field of relativity, linked inseparably with the name of Albert Einstein. Max Planck came across the quanta when he was studying what is known as the ‘blackbody radiation’. In classical physics in particular and the classical world in general, there was no serious problems with objects, objectivity and ontology. In classical physics, particles and fields have an existence independent of the observer. In broader view, everything, the world, exists independent of the observer and of the consciousness. The first quantum-mechanical theories on electron in mid-1920s did not take into account the effect of velocity of light, and were therefore non-relativistic.
Quantum optics was born in the first years of the 20th century. The first papers leading to further ideas of quanta of light were presented by Max Planck and Albert Einstein. Their pioneering papers laid the foundations for the theory of quantum optics. However, one should remember that for the birth of the formal theory of quantisation of light, the development of quantum mechanics was necessary. Interestingly, the word ’photon’ appeared for the first time in a paper by a Gilbert Lewis, a chemist, in 1926 and was later used by P.A.M. Dirac, a physicist, the next year. The modern sense of the term quantum optics was established in 1956 when Hanbury Brown and Twiss built the intensity interferometr and performed their famous experiment in which correlations between two light beams were measured. Although the results of their experiment could be explained on the basis of the classical theory of light, with some quantumcomponent to the process of photodetection, their result is commonly accepted as the beginning of the modern quantum optics era. The next milestone in the development of quantum optics was the invention of laser in 1960. The physical properties of the light generated by lasers were considerably different from those characterising the light generated by classical thermal sources. This fact was a great impulse leading to further development of quantum optics’ ideas. The first of them were presented in the fundamental papers written by Glauber, describing new states of light quantum coherent states.