chap1 - Chapter 1 Introduction Nature at the sub-atomic...

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Chapter 1 Introduction Nature at the sub-atomic scale behaves totally diferently From anything that our experience with the physical world prepares us For. Quantum mechanics is the name oF the branch oF physics that governs the world oF elementary particles such as electrons and photons, and it is paradoxical, unintuitive, and radically strange. Below is a sampling oF a Few such odd Features: Complete knowledge oF a system’s state is Forbidden - A measurement reveals only a small amount oF inFormation about the quantum state oF the system. The act oF measuring a particle Fundamentally disturbs its state. Quantum entities do not have trajectories. All that we can say For an elementary particle is that it started at A and was measured later at B. We cannot say anything about the trajectory or path it took From A to B. Quantum Mechanics is inherently probabilistic. IF we prepare two ele- mentary particles in identical states and measure them, the results may be diferent For each particle. Quantum entities behave in some ways like particles and in others like waves. But they really behave in their own unique way, neither particles nor waves. These Features are truly strange, and difficult to accept. To quote the great physicist Niels Bohr, “Anyone who is not shocked by quantum theory has not understood it.” We start by describing a simple experiment that highlights many diferences between quantum mechanics and our classical intuition. It 1
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2 CHAPTER 1. INTRODUCTION is the famous double slit experiment. The intuition gained in the process will help us as we deFne qubits, and more generally in the study of quantum computing. 1.1 The Double Slit Experiment What is the nature of light? You may have learned light is electromagnetic waves propagating through space. Also, you may have learned that light is made of a rain of individual particles called photons. But these two notions seem contradictory, how can it be both? The debate over the nature of light goes deep into the history of science. The eminent physicist Isaac Newton believed that light was a rain of particles, called corpuscles. At the beginning of the nineteenth century, Thomas Young demonstrated with his famous double-slit interference experiment that light propagates as waves. With Maxwell’s formulation of electromagnetism at the end of the nineteenth century, it was generally accepted that light is propa- gated as electromagnetic waves, and the debate seemed to be over. However, in 1905, Einstein was able to explain the photoelectric e±ect, by using the idea of light quanta, or particles which we now call photons. Similar confusion reigned over the nature of electrons, which behaved like particles, but then it was discovered in electron di±raction experiments, per- formed in 1927, that they exhibit wave behavior. So do electrons behave like particles or waves? And what about photons? This great challenge was re- solved with the discovery of the equations of quantum mechanics. But the
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chap1 - Chapter 1 Introduction Nature at the sub-atomic...

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