Elementary atom-photon interactions are strongly modified when the atom is confined to a small cavity. Interesting application in clued the generation and non-destructive measurement of microwave fields containing only a few photons and the production of “non-classical” radiation fields with reduced intensity fluctuations. This should lead to new fundamental tests of quantum theory at very low energies.
Elementary atom-photon interactions are strongly modified when the atom is confined to a small cavity. Interesting application in clued the generation and non-destructive measurement of microwave fields containing only a few photons and the production of “non-classical” radiation fields with reduced intensity fluctuations. This should lead to new fundamental tests of quantum theory at very low energies.
Cavity quantum electrodynamics studies the behaviour of atoms inside a low-loss cavity. The composite atom–cavity system constitutes a new ‘molecule’ with radiative properties which differ radically from those of the individual components. For example, spontaneous emission becomes reversible, the spectrum splits into several peaks and the intensity fluctuations of the light transmitted by this novel ‘molecule’ are below the classical shot noise limit. In terms of practical applications, the system renders possible, first, the detection of single atoms with a high sensitivity and, second, optical bistability for a few atoms and photons only.
Experiments involving so-called elastic collisions between an atom and a cavity should allow the non-destructive detection of very small numbers of photons and the direct measurement of cavity field quantum jumps.An atom and a small cavity with highly reflecting walls can couple and correlate strongly to each other, resulting in pheno mena which may lead to the development of new kinds of field sensors to detect photons without destroying them; the same pheno mena also illustrate basic concepts of quan tum mechanics.The study of these effects, at first theoretical and now entering an acti ve experimental phase, forms a part of cav ity quantum electrodynamics [1], a growing field spanning atomic physics and quantum optics with many potential applications. The Atom-Cavity "Molecule"Consider an atom in a cavity whose size matches exactly the wavelength of a photon the atom can emit in a transition between two of its energy levels.If the atom Is initially excited and the resonator empty, a photon will be radiated within a short period of time in the cavity.Since this photon is trapped in the atom's vicinity by the reflecting cavity walls, it will be absorbed at a later time, the atom jumping back to the upper level, com mencing an ideally endless cycle of photon emission and absorption processes.Re markably, the exchanged photon also me diates a small force between the atom and the cavity walls [2], in the same way as charged particles exert a force on each other by exchanging a photon, or atoms attract each other in a molecule by trading electrons.The molecule analogy can help us under stand some basic features of the atom-cavi ty system.Two atoms with chemical affinity exchange an electron at a rate which de pends on their distance apart.As a result, they experience a force, which either ties them together in a molecular bound state, or repels them from each other in an "antibind ing" state.The same physics goes on in the atom-cavity system, albeit at a quite diffe rent energy scale.This system has two energy eigenstates, namely a binding one in which the excitation is shared symmetrically between the atom and the cavity (sum of the excited atom with no photon and the deexcited atom with one photon), and an anti-
Starting with early experiments about lifetimes of excited atoms near metallic boundaries, I review the evolution of Cavity Quantum Electrodynamics in the microwave and in the optical domains. The coherent coupling of single atoms with photons in high-Q cavities has led to the demonstration of fundamental quantum effects in light-matter interaction processes and to the generation of micromasers and microlasers emitting radiation with non-classical properties. It has made possible the development of single photon light sources and efficient detectors of single atoms as well as the implementation of quantum information and communication procedures in which atoms and photons play the roles of quantum bits. Mesoscopic superpositions of field states have been generated by entangling single atoms to fields made of many photons and the decoherence of these states has been studied, shedding new light on the quantum to classical boundary. Cavity Quantum Electrodynamics has also led recently to the quantum non-demolition counting of photons in a cavity, opening new perspectives for studying non-classical states of radiation. These atomic physics experiments are now extended - with promising applications in perspective - to the study of solid-state devices with microcavities built on substrates and coupled to artificial atoms such as quantum dots or superconducting junctions.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ‘‘meter’’ measuring the other. We have performed a ‘‘quantum nondemolition’’ measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (‘‘Schrodinger cat’’) gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ``meter'' measuring the other. We have performed a ``quantum nondemolition'' measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (``Schr\"odinger cat'') gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schr\"odinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.
Introduction Ever since 1916/17 when Einstein argued [45] [46] that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have believed that spontaneous emission is an inherent quantum mechanical property of atoms and that excited atoms inevitably radiate. This view, however, overlooks the fact that spontaneous emission is a consequence of the coupling of quantized energy states of atoms with the quantized radiation field, and is a manifestation of quantum noise or of emission ‘stimulated’ by ‘vacuum fluctuations’. An infinity of vacuum states is available to the photon radiated by an excited atom placed in free space, leading to the effective irreversibility of such emissions. If these vacuum states are modified, as for example by placing an excited atom between closely spaced mirrors or in a small cavity (essentially the Casimir effect), spontaneous emission can be greatly inhibited or enhanced or even made reversible. Recent advances in atomic and optical techniques have made it possible to control and manipulate spontaneous emission. A whole new branch of quantum optics called ‘cavity QED’ has developed since 1987 utilizing these dramatic changes in spontaneous emission rates in cavities to construct new kinds of microscopic masers or micromasers that operate with a single atom and a few photons or with photons emitted in pairs in a two-photon transition.