One of the most counterintuitive consequences of quantum mechanics is that particles can behave like waves. This wavelike nature is seen not only for elementary particles, such as electrons as they orbit the nucleus, but also for free particle beams, as shown by many experiments in electron, neutron and atom optics. Interference effects provide unambiguous proof of wave-like behaviour and in the last five years such effects have been widely used for beams of neutral atomic particles. Researchers are now starting to exploit the wave nature of these beams further by using many tricks already established for optics.
One of the most counterintuitive consequences of quantum mechanics is that particles can behave like waves. This wavelike nature is seen not only for elementary particles, such as electrons as they orbit the nucleus, but also for free particle beams, as shown by many experiments in electron, neutron and atom optics. Interference effects provide unambiguous proof of wave-like behaviour and in the last five years such effects have been widely used for beams of neutral atomic particles. Researchers are now starting to exploit the wave nature of these beams further by using many tricks already established for optics.
The interference pattern formed when a beam of electrons passes through a double slit is clear evidence that electrons can behave as both waves and particles. This wave-particle duality lies at the heart of quantum mechanics, but physicists remain intrigued by the boundary between the quantum and classical worlds. Neutrons, atoms and small molecules have all shown quantum-interference effects. Now Markus Arndt and co-workers at the University of Vienna have observed the wave-like behaviour of carbon-60 molecules. The "bucky-ball" molecules are at least an order of magnitude more massive than any other object where wave properties have been observed (Nature 1999 401 680).
Newton thought that light was made up of particles, but then it was discovered, as we have seen here, that it behaves like a wave. Later, however (in the beginning of the twentieth century) it was found that light did indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So it really behaves like neither. Now we have given up. We say: “it is like neither.” ere is one lucky break, however-electrons behave like light. e quantum behaviour of atomic objects (electrons, protons, neutron, photons, and so on) is the same for all, they are all ‘particle waves’ or whatever you want to call them.
At the atomic scale, the distinction between particles and waves breaks down. The realization, early in the twentieth century, that light consists of discrete packages, or “photons,” prompted the converse suggestion that particles could behave like waves. Confirmation of this idea came in 1927, from an experiment involving the scattering of electrons from a crystal. Today, “wave-particle duality” is central to quantum mechanics, and the wave properties of subatomic particles are widely used in microscopy and crystallography. Electron diffraction techniques are widely used to characterize atomic arrangements on solid surfaces and inside thin films, and to investigate the structures of molecules in gases. The dual nature of waves and particles is part of the “spookiness” of quantum mechanics, which cannot be explained in any classical way. It used to be that one could only imagine an experiment that would simultaneously demonstrate the particle and wave nature of electrons.
Most scientists immediately associate optics with photons. Indeed optics, defined as the study of the properties of light, is one of the oldest fields of physics – dating back to the earliest civilisations – and is still a thriving area of research. But there is more to optics than light and lasers. “Matter wave” optics, where electrons and neutrons take the place of photons, began with the demonstration of the wave nature of the electron by G P Thomson, Davisson and Germer n the 1920s. Since then electron and neutron interferometers have been used in many beautiful experiments that have improved our understanding of quantum mechanics. Beams of electrons and neutrons have also become indispensable tools in the study of solids and surfaces. Therefore it was perhaps inevitable that one day physicists would be able to do optics with atoms – to coherently split, focus and refract beams of neutral atoms.
Optical interference is proof of the wave nature of light and matter–wave interferometers have also been shown to work for electrons and neutrons (by exploiting the de Broglie wavelength of the particles). Now a working interferometer for atoms has been demonstrated at the Massachusetts Institute of Technology(MIT) for the first time, in the sense that it uses “atomic beam splitters” to separate and recombine atomic beams (David W Keith et al. Phys. Rev. Lett. (1991) 66 2693). A simpler experiment at Konstanz University, Germany (O Carnal and J Mlynek Phys. Rev. Lett. (1991) 66 2689) has also demonstrated interference phenomena in atomic beams but a detailed description of the apparatus is not yet published. Atom interferometers will some day be useful in precision metrology, mass tomography, precision gyroscopes, general relativity, fundamental quantum mechanics and atomic physics.
The years of the third decade of the present century were heady times. Old social orders were being swept away, monarchies becoming republics and the United States of America was emerging as a world power. In physics too, revolutions were taking place, none more potent than that which resulted in the emergence of quantum mechanics as a model for the behaviour of sub-atomic particles. It was Newton who first suggested that light was particulate but during the nineteenth century this view had fallen into neglect. Indeed a number of experiments, for example the classic Young's slits experiment, demonstrated quite conclusively that light was a wave motion. How else could interference fringes occur? However, the experiments on the photoelectric effect demonstrated just as conclusively that light energy was carried in packets, or quanta, and that a continuous wave description was not applicable. This ‘wave or particle’ dilemma was not unique to the behaviour of light. J. J. Thompson showed that cathode rays were charged, had a well defined mass and had all the properties expected of a beam of particles. Nevertheless, Davisson and Germer showed that diffraction of electrons could take place, an effect made visually much more dramatic by G. P. Thompson's transmission electron diffraction patterns. Nowadays, electron diffraction is used as a routine analytical tool in all advanced metallurgical and materials science laboratories (Fig. 2.1). Clearly new axioms were required.