Historically, the scientific exploration of new phenomena has often been guided by systematic studies of observations, i.e., experimentally verifiable facts, which can be used as the basis to construct the underlying physical laws. As the apex of the investigations, one tries to identify the minimal set of fundamental assumptions – referred to as the axioms – needed to describe correctly the experimental observations. Even though the axioms form the basis to predict the system's behavior completely, they themselves have no rigorous derivation or interpretation. Thus, the axioms must be viewed as the elementary postulates that allow us to formulate a systematic description of the studied system based on well-defined logical reasoning. Even though it might seem unsatisfactory that axioms cannot be “derived,” one has to acknowledge the paramount power of well-postulated axioms to predict even the most exotic effects. As is well known, the theory of classical mechanics can be constructed using only the three Newtonian axioms. On this basis, an infinite variety of phenomena can be explained, ranging from the cyclic planetary motion all the way to the classical chaos. In this book, we are mainly interested in understanding how the axioms of classical and quantum mechanics can be applied to obtain a systematic description for the phenomena of interest. Especially, we want to understand how many-particle systems can be modeled, how quantum features of light emerge, and how these two aspects can be combined and utilized to explore new intriguing phenomena in semiconductor quantum optics.
Historically, the scientific exploration of new phenomena has often been guided by systematic studies of observations, i.e., experimentally verifiable facts, which can be used as the basis to construct the underlying physical laws. As the apex of the investigations, one tries to identify the minimal set of fundamental assumptions – referred to as the axioms – needed to describe correctly the experimental observations. Even though the axioms form the basis to predict the system's behavior completely, they themselves have no rigorous derivation or interpretation. Thus, the axioms must be viewed as the elementary postulates that allow us to formulate a systematic description of the studied system based on well-defined logical reasoning. Even though it might seem unsatisfactory that axioms cannot be “derived,” one has to acknowledge the paramount power of well-postulated axioms to predict even the most exotic effects. As is well known, the theory of classical mechanics can be constructed using only the three Newtonian axioms. On this basis, an infinite variety of phenomena can be explained, ranging from the cyclic planetary motion all the way to the classical chaos. In this book, we are mainly interested in understanding how the axioms of classical and quantum mechanics can be applied to obtain a systematic description for the phenomena of interest. Especially, we want to understand how many-particle systems can be modeled, how quantum features of light emerge, and how these two aspects can be combined and utilized to explore new intriguing phenomena in semiconductor quantum optics.
The behavior of classical macroscopic particles – from baseballs to planets – is governed by Newtonian mechanics as we have described in the first chapters of this book. The behavior of very small particles, on the other hand, is governed by an entirely different collection of equations and rules that go under the name of “quantum mechanics” and these particles are referred to as quantum particles. These rules have analogies to classical mechanics, but are fundamentally different and somewhat mysterious as we discussed in Chapter 8. The fundamental principles of quantum mechanics cannot be proven to be correct. They were developed based on the results of experimental observation, by analogy with classical mechanics, and through the consideration of logical implications of the formalisms used. There is no way to derive the fundamental elements of quantum mechanics, any more than one could predict from first principles that Newtonian forces should add as vectors. On the other hand, quantum mechanics is among the most rigorously tested theories in science, and the predictions of quantum mechanics have, so far, always been in agreement with experiment. In this chapter, we will present quantum mechanics as a series of postulates and examine their implications in application to simple one-dimensional systems. In the next chapter we will examine quantum effects in real systems including atoms, molecules, crystals, and artificially engineered nano-structures. Postulates of quantum mechanics QM Postulate 1: A quantum particle is described by a wave function Ψ( x , y , z , t ) which is a function of spatial coordinates and time. All physically determinable quantities can be derived from the wave function Ψ( x , y , z , t ) ; quantities which cannot be found from the wave function are not physically meaningful. Note that some things that one could ask about a quantum particle, for example the exact position and simultaneous exact momentum of a particle, are not derivable from the wave function and are therefore considered indeterminate (or uncertain ). These are things which cannot ever be known about a particle, and, as such, they are meaningless to contemplate.
Basic scientific theories usually start with a set of hypotheses or “postulates.” There is generally no logical reason, apart from internal consistency, that can be given to justify such postulates absolutely. They come from ‘revelations’ in the minds of ‘geniuses,’ most likely with hints from Nature based on extensive careful observations. Their general validity can only be established through experimental verification. If numerous rigorously derived logical consequences of a very small set of postulates all agree with experimental observations without exception, one is inclined to accept these postulates as correct descriptions of the laws of Nature and use them confidently to explain and predict other natural phenomena. Quantum mechanics is no exception. It is based on a few postulates. For the purpose of the present discussion, we begin with three basic postulates involving: the “state functions,” “operators,” and “equations of motion.” In this chapter, this set of basic postulates and some of the corollaries and related definitions of terms are introduced and discussed. We will first simply state these postulates and introduce some of the related mathematical tools and concepts that are needed to arrive at their logical consequences later. To those who have not been exposed to the subject of quantum mechanics before, each of these postulates taken by itself may appear puzzling and meaningless at first. It should be borne in mind, however, that it is the collection of these postulates as a whole that forms the foundations of quantum mechanics.
Quantum physics is the golden child of modern science. It is the basis of our understanding of atoms, radiation, and so much else, from elementary particles and basic forces to the behavior of materials. But for a century it has also been the problem child of science as it has been overwhelmed by intense disagreements between its inventors, strange paradoxes, and implications that seem like the stuff of fantasy. To understand the universe, quantum physics explores the behavior of the smallest particles in existence like quarks, electrons, and photons and which revealing a world that defies our everyday experiences and expectations. On the other hand, quantum mechanics (QM) is a fundamental theory in physics that describes the behavior of matter and energy at the atomic and subatomic levels, where classical physics fails to apply. However, the phenomena observed in QM can seem nothing short of miraculous. Physicists are trying to explain miracles of physics of universe meticulously which governed by precise scientific principles and mathematical equations. It is serious to modern technology, as its principles as well as wave-particle duality, superposition, and entanglement form the basis for electronics, computing, and medical technologies. The truth of QM in a scientific context refers to its exceptional ability to accurately describe and predict experimental results, not a philosophical ‘absolute truth’. Interestingly, the scientific elucidation of creation through quantum physics does not refute and contradict spiritual or religious narratives, rather augments our gratitude of the universe’s complexity and towards Almighty or Creator. The Bible’s or Quran’s account of creation provides a spiritual and philosophical outlook, accentuating purpose and intentionality. The precise explanations of particle behavior and cosmic formation in quantum physics, offers a balancing view grounded in empirical evidence. The purpose of this investigative study is to explore the scientific reality of QM and its purpose to depict the technical advancement for the development of modern civilization along with understand of universe in more logical way.
A note to the reader Quantum optics has grown from a sub-discipline in atomic, molecular, and optical physics to a broad research area that bridges several branches of physics and that captures the imagination of the public. Quantum information science has put quantum optics into the spotlight of modern physics, as has the physics of ultracold quantum gases with its many spectacular connections to condensed-matter physics. Yet through all these exciting developments quantum optics has maintained a characteristic core of ideas that I try to explain in this slim volume. Quantum optics focuses on the simplest quantum objects, usually light and few-level atoms, where quantum mechanics appears in its purest form without the complications of more complex systems, often demonstrating in the laboratory the thought experiments that the founders of quantum mechanics dreamed of. Quantum optics has been, and will be for the foreseeable future, quintessential quantum mechanics, the quantum mechanics of simple systems, based on a core of simple yet subtle ideas and experiments. One of the strengths of quantum optics is the close connection between theory and experiment. Although this book necessarily is theoretical, many of the theoretical ideas I describe are guided by experiments or, in turn, have inspired experiments themselves. Another strength of quantum optics is that it is done by individuals or small teams. One single person or a small group can build and perform an entire experiment.
The quantum of action was found more then a century ago, but until now the meaning of quantum theory is often claimed to be “unclear” or even “not truly understandable.” Another prejudice is that quantum theory is only relevant for the range of atomic and sub-atomic phenomena. After a century of successful applications without any evidence at odds with the predictions of quantum theory, and in view of the fact that at present quantum theory is at the economic bottom of about one third of the Gross National Product, the need for a comprehensive and even popular understanding of the principles of quantum theory, preferably dispensing with the burden of the full mathematical apparatus, is more urgent than ever. Interestingly, there is a connection between quantum theory and your daily-life experience, suggesting to extend and generalize the Copenhagen interpretation in such a way that the observer and his consciousness is no longer excluded from scientific description. Moreover, Quantum theory is easier to understood, if one realizes that its range of applicability surpasses widely that of quantum mechanics, comparable to the way classical physics surpasses classical mechanics. The essential new aspect is the role of abstract quantum information, referred to as Protyposis. The idea that quantum information can have an ontological status equivalent to matter or energy was developed more than half a century ago, but it is only rather recently that, in view of the success of experimental work, the relevance of quantum information has been widely acknowledged. The concept of abstract quantum information is especially helpful in the understanding of the measuring process.