If an excited atom is put in between high-reflectance mirrors, the light emitted by the atom can be bounced back and be re-absorbed and be re-emitted, again and again until the light leaks out of the mirrors or is re-emitted into a wrong direction. This coherent oscillation between atom and light, called vacuum Rabi oscillation, results from interference between two superposition states of the atom and the photon, which have different energies or frequencies, or vacuum Rabi splitting. The vacuum Rabi oscillation and splitting demonstrate the quantum nature of light. They also provide single-photon non-linearity for photonic applications. In recent years, these effects have been considered particularly useful for quantum information processing [1] since the oscillation presents a natural mechanism for information exchange between a stationary quantum bit carried by the atom and a flying quantum bit carried by the photon, making scalable and distributed quantum computing possible. Realization of the vacuum Rabi oscillation and splitting, however, is highly non-trivial. It requires the so-called strong coupling condition, that is, the exchange between the atom and the photon must be faster than the photon leaks out of the space enclosed by the mirrors, i.e. the cavity, and faster than the atom emits the light into a wrong direction. The technique to realize the strong coupling condition is to make the cavity small so that the photon is bounced back more frequently to the atom and to make the mirrors more reflective so that the photon can be bounced more times. That means to decrease the cavity volume V and to increase the quality factor Q. Remarkable progresses have been made to make optical microcavities with small V and high Q, facilitating observation of vacuum Rabi oscillation and splitting in highly dissipative solid-state systems [2,3]. However, there is no systematic method to reduce V/Q—the key factor for realizing strong coupling (see Fig. 1a). The reason appears to be fundamental. When the size of a cavity is made as small as comparable to the light wavelength, small deformation of the cavity would result in large evanescent wave and hence photon leakage [4]. (a) An atom and a small cavity have strong coupling but the cavity leakage is also strong. A large cavity has small photon leakage but its coupling to the atom is small (so emission goes to free space). The atom and the large cavity can have strong coupling, with photons exchanged through but not occupying the small cavity—using the quantum tunneling effect. (b) In the classical world, two lower players have to provide sufficient energy to pass the ball through the higher player. (c) In the quantum world, the two lower players can pass balls through the wall via the higher players but with energy of the balls insufficient to reach the higher player. A recent proposal by [5] provides a general solution, using a divide-and-conquer strategy. A small cavity with relatively low Q is used to enhance the interaction between the emitter and the photon, while the photon stays in a large cavity with high Q. [5] demonstrate the strong coupling by showing Rabi splitting, Rabi oscillation, and single-photon non-linearity of the system. The scheme at the first sight does not seem to work since the emitter has no contact with the high-Q large cavity and if the photon enters the small cavity, it would leak out. The clever point of the scheme is: in the quantum mechanical world, the photon can interact with the emitter without occupying the small cavity! Similar to the ball game in Fig. 1b, a classical particle needs to have sufficient energy to pass a barrier. The small cavity is designed to have a frequency higher than those of the emitter and the large cavity, which would prevent the emitted photons entering the large cavity. However, quantum ball games allow tunneling of a barrier (Fig. 1c). So the emitter and the large cavity can indeed exchange photons. The small cavity would not be occupied since its frequency is not right, which prevents photon leakage. It looks the scheme applies generally to all types of photonic systems—photonic crystals, microspheres, microtoroids, etc. Considering the simplicity of the scheme, we expect applications be found soon.
If an excited atom is put in between high-reflectance mirrors, the light emitted by the atom can be bounced back and be re-absorbed and be re-emitted, again and again until the light leaks out of the mirrors or is re-emitted into a wrong direction. This coherent oscillation between atom and light, called vacuum Rabi oscillation, results from interference between two superposition states of the atom and the photon, which have different energies or frequencies, or vacuum Rabi splitting. The vacuum Rabi oscillation and splitting demonstrate the quantum nature of light. They also provide single-photon non-linearity for photonic applications. In recent years, these effects have been considered particularly useful for quantum information processing [1] since the oscillation presents a natural mechanism for information exchange between a stationary quantum bit carried by the atom and a flying quantum bit carried by the photon, making scalable and distributed quantum computing possible. Realization of the vacuum Rabi oscillation and splitting, however, is highly non-trivial. It requires the so-called strong coupling condition, that is, the exchange between the atom and the photon must be faster than the photon leaks out of the space enclosed by the mirrors, i.e. the cavity, and faster than the atom emits the light into a wrong direction. The technique to realize the strong coupling condition is to make the cavity small so that the photon is bounced back more frequently to the atom and to make the mirrors more reflective so that the photon can be bounced more times. That means to decrease the cavity volume V and to increase the quality factor Q. Remarkable progresses have been made to make optical microcavities with small V and high Q, facilitating observation of vacuum Rabi oscillation and splitting in highly dissipative solid-state systems [2,3]. However, there is no systematic method to reduce V/Q—the key factor for realizing strong coupling (see Fig. 1a). The reason appears to be fundamental. When the size of a cavity is made as small as comparable to the light wavelength, small deformation of the cavity would result in large evanescent wave and hence photon leakage [4]. (a) An atom and a small cavity have strong coupling but the cavity leakage is also strong. A large cavity has small photon leakage but its coupling to the atom is small (so emission goes to free space). The atom and the large cavity can have strong coupling, with photons exchanged through but not occupying the small cavity—using the quantum tunneling effect. (b) In the classical world, two lower players have to provide sufficient energy to pass the ball through the higher player. (c) In the quantum world, the two lower players can pass balls through the wall via the higher players but with energy of the balls insufficient to reach the higher player. A recent proposal by [5] provides a general solution, using a divide-and-conquer strategy. A small cavity with relatively low Q is used to enhance the interaction between the emitter and the photon, while the photon stays in a large cavity with high Q. [5] demonstrate the strong coupling by showing Rabi splitting, Rabi oscillation, and single-photon non-linearity of the system. The scheme at the first sight does not seem to work since the emitter has no contact with the high-Q large cavity and if the photon enters the small cavity, it would leak out. The clever point of the scheme is: in the quantum mechanical world, the photon can interact with the emitter without occupying the small cavity! Similar to the ball game in Fig. 1b, a classical particle needs to have sufficient energy to pass a barrier. The small cavity is designed to have a frequency higher than those of the emitter and the large cavity, which would prevent the emitted photons entering the large cavity. However, quantum ball games allow tunneling of a barrier (Fig. 1c). So the emitter and the large cavity can indeed exchange photons. The small cavity would not be occupied since its frequency is not right, which prevents photon leakage. It looks the scheme applies generally to all types of photonic systems—photonic crystals, microspheres, microtoroids, etc. Considering the simplicity of the scheme, we expect applications be found soon.
A single two-level emitter coupled to a single mode of an electromagnetic resonator constitutes the most fundamental instance of light--matter interaction. Realizations of this type of system are investigated in the field of cavity quantum electrodynamics (cavity QED). Particularly important are schemes in which the coupling is strong enough to exceed the spontaneous emission rate as well as cavity damping. In this case, the coherent exchange of excitation between source and photon is the dominating process, providing the unique possibility to deterministically control the quantum evolution of the system. Great progress has recently been accomplished, technologically as well as conceptually, and the focus section in this issue of Journal of Optics B: Quantum and Semiclassical Optics features five articles highlighting important recent developments in the field. Strong coupling between single atoms and the electromagnetic cavity field have been achieved in two distinct regimes, the optical and the microwave region. The latter is represented by the micromaser, a high-Q microwave cavity pumped by a beam of highly excited Rydberg atoms. It has proved to be an almost ideal system for the generation of non-classical field states. A constraint in these experiments is the fact that the field inside the cavity can only be examined indirectly, by analysing the occupation of atoms leaving the cavity. While only two atomic states were previously identified for this purpose, V A Reshetov shows in his article (pages 119--126) that more information on the field may be extracted by taking advantage of the Zeeman substructure of the atomic levels. In the optical domain of cavity QED, by contrast, the state of the resonator field may be probed directly by measuring the light leaking from the cavity using photodetectors. In order to minimize damping, high-quality cavities must be employed. In most optical experiments these are Fabry--Perot resonators with ultra-high reflectivity mirrors, with losses as low as a few parts per million being state of the art. At the same time, the coupling of atoms and photons is enhanced by reducing the length of the cavity, so that strong coupling conditions are reached. A very attractive application of this type of cavity QED setup is the possibility of exerting control over the quantum dynamics of the system. An impressive example is the implementation of quantum feedback, reported in the paper by W P Smith and L A Orozco (pages 127--134). Using information gained from detecting a single photon escaping from the cavity, active feedback is applied to halt the dynamical evolution of the system. This is the first time active feedback has been used to modify the quantum evolution of a system at the single-photon level. The authors discuss how detunings between the subsystems affect the quantum feedback, an issue particularly relevant for ensembles of atoms with a spread in detuning. Perfect quantum feedback in this case requires the use of additional parameters to control the system. The majority of optical cavity QED experiments performed so far have used as a medium atoms traversing the cavity on random trajectories, such as, for example, thermal atomic beams. In this case, the observation of quantum effects is degraded in two ways. The Doppler effect and the finite transit time lead to line broadening, while the spatial structure of the cavity mode results in atoms in different locations experiencing different coupling with the photonic field. J E Reiner et al (pages 135--142) investigate the impact of these effects on quantum fluctuations in their cavity QED system. In order to overcome line broadening, they have developed a continuous cold atomic beam, extracted from a magneto-optical trap. An even better control of broadening effects and fluctuating coupling strength is provided by trapping the atoms inside the cavity and cooling them to temperatures at which their position spread is smaller than the optical wavelength. Much recent activity has been devoted to developing such an ultimate cavity QED system. A Vukics et al (pages 143--153) present a theoretical proposal for dynamically trapping and cooling atoms inside a cavity. By driving the atoms with a standing wave injected from the side of the cavity and by weakly exciting the cavity field itself, they predict three-dimensional trapping of the atom for periods around 1 second, an extremely long time on the typical scale of cavity QED dynamics. The essential component of their scheme is the interference between the two driving fields, determining the temperature of the particle and the stable trapping positions. In the quest for better confinement of radiation, lately, an appealing alternative to the Fabry--Perot cavity has been investigated -- the optical microsphere resonator. A sphere with a diameter of a few tens of microns, it supports so-called whispering gallery modes near its circumference, for which quality factors up to 10 10 have been measured. In order to exploit the correspondingly low field damping rates for cavity QED, a method must be found to couple atomic or solid state emitters to these modes. S Göotzinger et al (pages 154--158) report the coupling of single nanocrystals on the surface of a microsphere through the evanescent field of the mode. Selective excitation of a single nano-emitter was accomplished with a scanning confocal microscope. The system is a promising candidate for cavity QED with single-quantum emitters. The ability to fully control the quantum dynamics of atoms and light is beginning to spawn early applications. The most notable example is quantum information processing. This requires the coherent manipulation of single-quantum systems, which is ideally provided by cavity QED interaction in the strong-coupling limit. The development of practical devices will be among the principal future tasks for research in the field. This will include physical systems, which have not been covered in this issue, such as semiconductors, quantum dots as emitters, or photonic crystals as resonator structures. Cavity QED is therefore guaranteed to remain a vigorous field in the years to come.
The interaction of light with matter is at the heart of quantum optics, which itself enables insight into the fundamental aspects of quantum mechanics. First experimental access to this research field has been realized by coupling atoms to light. Here, the transition between discrete energy states of the atom is associated with the absorption and emission of a photon, a single quantum of the electromagnetic field. As a central aspect of quantum optics, the light-emitter interaction can be significantly enhanced by placing the emitter in optical cavity that is on resonance with the emitter. In recent years, this has led to a rapidly evolving research field known as cavity quantum electrodynamics (CQED). In CQED two different regimes are distinguished: the strong and the weak coupling regimes. In the strong coupling regime, the emitted photon is reflected from the cavity mirrors and eventually reabsorbed by the emitter. In contrast, the weak coupling regime describes the irreversible emission, where the photon leaks out of the cavity before it can be reabsorbed. Both the weak and strong coupling regimes enabled fundamental experiments for a better understanding of quantum optical phenomena. \n \n \n \nCQED grants access to the quantum world and hence offers potentially revolutionizing applications, particularly in the field of quantum information processing. A central aspect for the successful implementation of quantum applications is the system's scalability. Unfortunately, placing atoms deterministically inside a cavity remains technologically elaborate and hence minimizes the prospect of scaling a atom-CQED system. \n \nA possibility to address this issue is to implement CQED in the solid state, where sophisticated fabrication strategies allow miniaturization and scalability of the system. Particularly the development of self-assembled quantum dots (QD) in semiconductors represent a promising route. QDs can be considered as artificial atoms that mimic the atomic two-level system. These structures interact strongly with light and therefore have the potential for replacing atoms in CQED. As a central advantage, QDs are naturally trapped, which greatly simplifies the deterministic incorporation into the cavity. \n \nIn recent years, many efforts have been made to couple self-assembled QDs to microcavities. Generally, the successful implementation of CQED requires a cavity with a high quality factor Q and a low mode volume. In a majority of the approaches, the high Q/small mode volume cavities were monolithically defined around the QD, embedding the QD at a fixed position inside the cavity. Both the weak and strong coupling regimes have been reached with these systems. However, for future applications they suffer from several disadvantages. The fixed position of the QD inside the cavity minimizes the prospects for spectral tunability and spatial positioning the QD inside the cavity. Furthermore, prospects for further increasing of the cavity Q-factor and minimization of the mode volume remain limited in these systems. \n \nIn this thesis the mentioned disadvantages are addressed by developing a fully tunable miniaturized Fabry-Pérot microcavity with low mode volume. The design enables both spatial positioning of the emitter inside the cavity and spectral tunability. Successful coupling of a single QD to the microcavity is demonstrated including the strong coupling regime. Further a new approach to decrease the cavity mode volume is presented, where we demonstrate weak coupling is achieved
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-
When an atom is strongly coupled to a cavity, the two systems can exchange a single photon through a coherent Rabi oscillation. This process enables precise quantum-state engineering and manipulation of atoms and photons in a cavity, which play a central role in quantum information and measurement. Recently, a new regime of cavity QED was reached experimentally where the strength of the interaction between light and artificial atoms (qubits) becomes comparable to the atomic transition frequency or the resonance frequency of the cavity mode. Here we show that this regime can strongly modify the concept of vacuum Rabi oscillations, enabling multiphoton exchanges between the qubit and the resonator. We find that experimental state-of-the-art circuit-QED systems can undergo two- and three-photon vacuum Rabi oscillations. These anomalous Rabi oscillations can be exploited for the realization of efficient Fock-state sources of light and complex entangled states of qubits.
We predict a large cooperative effect involving two-atom two-photon vacuum Rabi oscillations in a high-quality cavity. The two-photon emission occurs as a result of simultaneous deexcitation of both atoms with two-photon resonance condition ${\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}\ensuremath{\approx}{\ensuremath{\omega}}_{a}+{\ensuremath{\omega}}_{b}$, where ${\ensuremath{\omega}}_{1}$,${\ensuremath{\omega}}_{2}$ are the atomic transition frequencies and ${\ensuremath{\omega}}_{a}$,${\ensuremath{\omega}}_{b}$ are the frequencies of the emitted photons. The actual resonance condition depends on the vacuum Rabi couplings. The effect can be realized either with identical atoms in a bimodal cavity or with nonidentical atoms in a single-mode cavity.
A promising approach to merge atomic systems with scalable photonics has emerged recently, which consists of trapping cold atoms near tapered nanofibers. Here, we describe a novel technique to achieve strong, coherent coupling between a single atom and photon in such a system. Our approach makes use of collective enhancement effects, which allow a lattice of atoms to form a high-finesse cavity within the fiber. We show that a specially designated 'impurity' atom within the cavity can experience strongly enhanced interactions with single photons in the fiber. Under realistic conditions, a 'strong coupling' regime can be reached, wherein it becomes feasible to observe vacuum Rabi oscillations between the excited impurity atom and a single cavity quantum. This technique can form the basis for a scalable quantum information network using atom–nanofiber systems.