A chip-scale optomechanical system with extremely low dissipation might enable the observation of quantum behavior in tangible objects visible to the naked eye. At the beginning of the last century Heisenberg formulated the theory of quantum mechanics. It dictates that mechanical motion—from that of electrons around nuclei to the macroscopic behavior of everyday objects—is quantized. It was only in 1986, more than 60 years after Heisenberg’s initial work, that scientists could directly observe quantum jumps of individual electrons.1 Ten years later, advances in laser techniques and optics enabled the observation of nonclassical motional states of trapped ions,2 which can be 100,000 times heavier than electrons. Yet a fun-damental question has remained: Why do larger objects behave classically, rather than following the rules that govern the small-est particles?
A chip-scale optomechanical system with extremely low dissipation might enable the observation of quantum behavior in tangible objects visible to the naked eye. At the beginning of the last century Heisenberg formulated the theory of quantum mechanics. It dictates that mechanical motion—from that of electrons around nuclei to the macroscopic behavior of everyday objects—is quantized. It was only in 1986, more than 60 years after Heisenberg’s initial work, that scientists could directly observe quantum jumps of individual electrons.1 Ten years later, advances in laser techniques and optics enabled the observation of nonclassical motional states of trapped ions,2 which can be 100,000 times heavier than electrons. Yet a fun-damental question has remained: Why do larger objects behave classically, rather than following the rules that govern the small-est particles?
Quantum effects usually manifest themselves only in the microscopic world of atoms, whereas macroscopic objects of the everyday world follow the classical laws of Newtonian mechanics. But can large objects also behave quantum mechanically? In his Perspective,
Blencowe
discusses work reported in the same issue by
LaHaye
et al. on attempts to observe the phenomenon of Heisenberg9s uncertainty principle in a relatively large vibrating beam. Although still small by everyday standards, the microfabricated beam consists of about 10atoms. The measurements indicate that the apparatus is within striking distance of direct observation of quantum fluctuations.
In the classic Chinese novel Journey to the West, the Monkey King has a special power that enables him to be in two or more places simultaneously. Normally, an amazing thing such as this could never happen in real life. However, in the quantum world, where the motions of objects obey the rules of quantum mechanics, this could happen. These weird quantum rules usually only apply in the microscopic world to atoms and molecules, because quantum properties are generally very fragile in the macroscopic world. However, scientists have recently been making efforts to push macroscopic and mesoscopic mechanical systems into the quantum world, i.e. they are putting conventional mechanical systems into the quantum mechanical regime. Spring oscillators, swings, and pendulum clocks are typical mechanical systems that we often see in daily lives, and they can be described by a basic physical model: the harmonic oscillator. In the quantum regime, harmonic oscillators have some quantum properties, including discrete energy spectra and zero-point motion. To observe these quantum properties, the environmental thermal noise must be suppressed so that the energy of the quantum motion dominates over the energy of the thermal motion. To this end, a generic model of a cavity optomechanical system is used, as shown in Fig. 1. The system consists of an optical cavity with a fixed mirror and a movable mirror, where the latter is attached to a spring and undergoes harmonic oscillation. A laser beam is launched into the cavity and is reflected multiple times between the two mirrors, and the cavity field thus builds up, resulting in a greatly enhanced optical field, which then exerts a considerable force on the movable mirror. The interaction between the optical field and the mechanical motion can reduce the thermal motion noise to enable the fragile quantum properties to be observed. The study of such systems has produced an emerging field called cavity optomechanics [1–3]. The pioneering work was conducted by Braginsky and co-workers in the 1960s [4]. In recent years, various experimental systems have been proposed and investigated, including Fabry–Pérot cavities [by groups including MIT/Caltech LIGO Laboratory, the Mavalvala group at MIT, the Aspelmeyer group at the University of Vienna, the Heidmann group at the University of Pierre and Marie Curie, and the Bouwmeester group at the University of California, Santa Barbara (UCSB)], whispering-gallery cavities (by groups including the Vahala group at Caltech, the Kippenberg group at École Polytechnique Fédérale de Lausanne, the Wang group at the University of Oregon, the Lipson group at Cornell University, the Carmon group at the University of Michigan, the Bowen group at the University of Queensland, and the Xiao group at Peking University), photonic crystal cavities (by groups including the Painter group at Caltech, the Wong group at Columbia University, and the Tang group at Yale University), separated mechanical oscillators inside or near a cavity (by groups including the Harris group at Yale University, the Regal group at Joint Institute for Laboratory Astrophysics (JILA), the Favero group at Paris Diderot University, and the Weig group at the University of Munich), superconducting circuits (by groups including the Schwab group at Caltech, the Lehnert group at JILA, the Cleland group at UCSB, and the Teufel group at National Institute of Science and Technology, Boulder), and cold atoms (by groups including the Kimble group at Caltech, the Stamper-Kurn group at the University of California, Berkeley, and the Treutlein group at the University of Basel). Illustration of a generic cavity optomechanical system. The left mirror is fixed and the right mirror is attached to a spring. Cavity optomechanics has been featured as the most recent milestone in photon history in Nature. The research in cavity optomechanics is important to both fundamental physics studies and the applied sciences. First, cavity optomechanics provides a unique platform for the study of fundamental quantum physics, including macroscopic quantum phenomena, decoherence, and quantum–classical transitions. The field offers the best test bed to study effects like gravity-induced decoherence, which is important to the understanding of macroscopic quantum phenomena. Second, cavity optomechanics is promising for high-precision measurements of small forces, masses, displacements, and accelerations, and is considered to be capable of surpassing the standard quantum limit. Third, cavity optomechanics provides resources for both classical and quantum information processing. For instance, optomechanical devices can serve as information storage devices and act as interfaces between light beams with different wavelengths or even microwaves. In the last few years, researchers have made considerable efforts to put mechanical systems into their quantum ground states [5]. Recent efforts have demonstrated optomechanically induced transparency [6], normal-mode splitting [7], quantum-coherent coupling [8], wavelength conversion [9], and measurements performed below the standard quantum limit [10]. Future developments will aim to integrate different quantum systems to form hybrid quantum devices, e.g. hybrid optomechanical and electromechanical systems. In this way, we can enable phonons, photons, and electrons to work together in the quantum world. Recent years have seen rapidly growing interest in the field of cavity optomechanics. As shown in Fig. 2, the publications and citations in this field have grown exponentially. The now booming development in this field will turn the dream of manipulating macroscopic mechanical systems in a quantum manner into reality. Numbers of publications (a) and citations (b) per year in the period from 2005 to 2013 with the keyword ‘cavity optomechanics’, retrieved from the Web of Science.
The smaller they make their devices, the closer electronics engineers come to entering the world of quantum mechanics, that wonderland where the rules of physics differ drastically from those of the macroscopic world people experience with their senses. In one example, physicists and engineers are creating so-called quantum dots and are investigating how to use them in new forms of electronics. They aspire to transmit data not by tiny currents, but by single electrons that hop from one quantum dot to the next. Probably the one thing generally known about quantum mechanics is that matter-despite its particulate nature-may behave like a wave. This idea was emphasized once again last year when a team at the Massachusetts Institute of Technology (MIT), in Cambridge, produced coherent beams of matter in what the group refers to as an atom laser. (The same group just two years ago produced an entirely new state of matter known as Bose-Einstein condensates.) Elsewhere, scientists in France have succeeded in detecting decoherence, the transition from the quantum world into the macroscopic world, by contriving systems that can be in two quantum states at the same time, albeit briefly. Also in France, physicists have shown how to exploit chaos to encrypt optically transmitted data. This finding might eventually be useful in ultrafast applications, such as the scrambling and unscrambling of television signals transmitted over optical fibers.
Ever since the advent of quantum mechanics, it has been clear that the atoms composing matter do not obey Newton's laws. Instead, their behavior is described by the Schroedinger equation. Surprisingly though, until recently, no clear explanation was given for why everyday objects, which are merely collections of atoms, are observed to obey Newton's laws. It would seem that, if quantum mechanics explains all the properties of atoms accurately, they, too, should obey quantum mechanics. This reasoning led some scientists to believe in a distinct macroscopic, or ``big and complicated,'' world in which quantum mechanics fails and classical mechanics takes over, although there has never been experimental evidence for such a failure. Even those who insisted that Newtonian mechanics would somehow emerge from the underlying quantum mechanics as the system became increasingly macroscopic were hindered by the lack of adequate experimental and theoretical tools. In the last decade, however, this quantum-to-classical transition has become accessible to experimental study and quantitative description, and the resulting insights are the subject of this article.
The field of cavity optomechanics is based on the coupling of optical cavities to mechanical degrees of freedom. Over the past two decades, it has led to remarkable and surprising discoveries. A cavity optomechanical system enabled the first observation of gravitational waves, a discovery awarded with the 2017 Nobel prize, confirming Einstein's 100-year-old prediction. It was also a cavity optomechanical system that has for the first time enabled ground state cooling of the motion of an object containing ~1012 atoms, and entanglement of mechanical oscillators of similar size. Moreover, cavity optomechanical systems have facilitated significant progress in precision-sensing technologies, with diverse applications such as biosensing, magnetometry, and many more.But this may only be the beginning. The field is still in its early days and experimental capabilities are improving fast. For instance, cavity optomechanical systems have the potential to test predicted signatures of quantum gravity such as modifications to the canonical commutator, short-distance modifications to Newtonian gravity, dark matter and dark energy, and even extra dimensions. In this sense, they form an economical platform with a short experimental turnover time, that can compete in its predictive power with mega-scale, multi-billion-dollar experiments such as the Large Hadron Collider.This thesis is dedicated to utilizing cavity optomechanics to explore the maybe most challenging and unexplored frontier in quantum mechanics: The dynamics of macroscopic quantum systems. Quantum physics, since its discovery in the early 20th century, has revolutionized our understanding of the microworld, but has long been thought to apply exclusively to extremely small objects. However, over the past decades, ever-larger quantum systems have emerged in laboratories around the world. On one hand, these are collective, macroscopic quantum phenomena such as superfluidity, superconductivity, and Bose-Einstein condensation. On the other hand, they are quantum states of relatively massive mechanical objects. While our understanding of the interplay of quantum mechanical and classical systems has greatly improved over the past decades, as described by theories of decoherence and open quantum systems, many basic questions about macroscopic quantum systems still remain unanswered. Quantum fluids such as Bose-Einstein condensates and superfluid helium are inherently hard to understand due to their complex dynamics, and difficult to probe experimentally due to the weakness of their interactions with the environment and their fragile nature. This is especially true in the strongly-interacting regime, which is, for instance, found in superfluid helium.Another long-standing, unanswered set of questions in quantum mechanics is related to the quantum measurement problem. What constitutes a quantum measurement? What happens if we engineer a quantum state that is large enough to be, in principle, part of our everyday world? What if we attempt to create a quantum superposition of a living being? Do the laws of physics even permit either of these scenarios?Cavity optomechanics is a unique and powerful platform to investigate macroscopic quantum systems. The extreme sensitivity of cavity optomechanical sensors in readout of mechanical motion makes them susceptible to minute quantum effects. The strong coupling between single photons and phonons in these systems allows engineering and measurement of quantum states of mechanical systems. Furthermore, most cavity optomechanical experiments are cryo-compatible and small in size, which allows integration with other quantum systems to be analyzed, such as superfluid helium.
This paper takes a look at the two cornerstones of modern physics--Quantum Mechanics and The Theory of Relativity (Both Special and General). The differences and similarities for the two subjects are presented. More of their differences and similarities will emerge with new developments in physics. One can't rule out the possibility of the two being unified into one one fine day. That will herald another triumphant moment in physics. Introduction In 1905, Albert Einstein at the age of twenty-six formulated single-handedly, among others, the Theory of Special Relativity together with his other seminal papers on photoelectricity, Brownian motions of atoms and molecular dimensions. The year has been universally declared the Miraculous Year [1]. Ten years later, he came up with the theory of General Relativity--a new and revolutionary framework on gravity. Meanwhile in Europe a group of young physicists played pivotal role in the birth of Quantum Mechanics. Many of them went on to win Nobel prizes for their monumental contributions. Einstein also won the coveted prize not for Relativity but ironically for Photoelectric Effect. Irony has been part of physics in its history. Relativity with its weird 'Twin Paradox', constancy of the speed of light and the interchangebility between mass and energy (E=[mc.sup.2]) enthralls the public and scientists alike. It provides a way to measure particle masses [2]. The Quantum Theory has its fair share of weirdness and bewilderment. Both Quantum Mechanics and the Theory of Relativity bring forth new ideas in human thoughts. They are among Joanne Baker's 50 physics ideas [3]. Quantum Mechanics and the Theory of Relativity The quantum world where counterintuition reigns supreme is couched in uncertainty as epitomised by Heisenberg's uncertainty principle. Things do not behave in a deterministic way as enshrined and expected in classical Newtonian physics. The microscopic world of the electrons was dictated by randomness and indeterminacy. Albert Einstein was appalled by such dictates that he famously declared: 'God does not play dice with the universe.' The famous Danish physicist Neils Bohr sprang to his feet by saying: 'Stop telling God what to do with his dice.' The sparring between the two stalwarts in physics is legendary. According to Trefil [4], there are a couple of things that need to be said about the uncertainty principle: it does not imply that we cannot know either the position or the velocity of a particle, and it does not require the presence of a conscious mind. Marcus Chown [5] wraps up by saying 'The bitter irony, not lost on Einstein, was that he was the one who, by postulating the existence of the photon, had inadvertently set loose the genie of randomness of physics. The two theories of relativity by Einstein are not couched in randomness however. According to Smoot [6], both implied a radically new cosmology. Many people are of the opinion that with Einstein's theory of relativity, old classical physics is no more in vogue. According to Angier [7], Albert Einstein did not prove that Isaac Newton was wrong. Instead, he showed that Newton's theories of motion and gravity were incomplete, and that new equations were needed to explain the behavior of objects under extreme circumstances, such as when tiny particles travel at or near the speed of light. …