The interaction of matter and light is one of the fundamental processes occurring in nature, and its most elementary form is realized when a single atom interacts with a single photon. Reaching this regime has been a major focus of research in atomic physics and quantum optics1 for several decades and has generated the field of cavity quantum electrodynamics2,3. Here we perform an experiment in which a superconducting two-level system, playing the role of an artificial atom, is coupled to an on-chip cavity consisting of a superconducting transmission line resonator. We show that the strong coupling regime can be attained in a solid-state system, and we experimentally observe the coherent interaction of a superconducting two-level system with a single microwave photon. The concept of circuit quantum electrodynamics opens many new possibilities for studying the strong interaction of light and matter. This system can also be exploited for quantum information processing and quantum communication and may lead to new approaches for single photon generation and detection.

https://arxiv.org/pdf/cond-mat/0407325.pdf

Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics

Quantum communication holds promise for absolutely secure transmission of secret messages and the faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for the physical implementation of quantum communication. However, owing to losses and decoherence in the channel, the communication fidelity decreases exponentially with the channel length. Here we describe a scheme that allows the implementation of robust quantum communication over long lossy channels. The scheme involves laser manipulation of atomic ensembles, beam splitters, and single-photon detectors with moderate efficiencies, and is therefore compatible with current experimental technology. We show that the communication efficiency scales polynomially with the channel length, and hence the scheme should be operable over very long distances.

https://ffden-2.phys.uaf.edu/103_fall2003.web.dir/ben_steiner/duan et al. 2001.pdf

Long-distance quantum communication with atomic ensembles and linear optics

Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.

Optical microcavities

We propose a realizable architecture using one-dimensional transmission line resonators to reach the strong-coupling limit of cavity quantum electrodynamics in superconducting electrical circuits. The vacuum Rabi frequency for the coupling of cavity photons to quantized excitations of an adjacent electrical circuit (qubit) can easily exceed the damping rates of both the cavity and qubit. This architecture is attractive both as a macroscopic analog of atomic physics experiments and for quantum computing and control, since it provides strong inhibition of spontaneous emission, potentially leading to greatly enhanced qubit lifetimes, allows high-fidelity quantum nondemolition measurements of the state of multiple qubits, and has a natural mechanism for entanglement of qubits separated by centimeter distances. In addition it would allow production of microwave photon states of fundamental importance for quantum communication.

https://arxiv.org/pdf/cond-mat/0402216.pdf

Cavity quantum electrodynamics for superconducting electrical circuits: An architecture for quantum computation

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

/pdf/strong-coupling-in-a-single-quantum-dot-semiconductor-48bgjfnp5v.pdf

Strong coupling in a single quantum dot–semiconductor microcavity system

The motion of individual cesium atoms trapped inside an optical resonator is revealed with the atom-cavity microscope (ACM). A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. An inversion algorithm then allows individual atom trajectories to be reconstructed from the record of cavity transmission and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In these initial experiments, the ACM yields 2-micrometer spatial resolution in a 10-microsecond time interval. Over the duration of the observation, the sensitivity is near the standard quantum limit for sensing the motion of a cesium atom.

https://science.sciencemag.org/content/sci/287/5457/1447.full.pdf

The atom-cavity microscope: single atoms bound in orbit by single photons

Two recent experiments are discussed which demonstrate real-time trapping and monitoring of single atoms within a high-finesse optical resonator. A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. These cavity QED signals are used to trigger ON a confining potential to trap the atom. In the first experiment, the confining potential is provided by the single-photon forces associated with the cavity QED interaction. An inversion algorithm allows individual atom trajectories to be reconstructed from the record of cavity transmission, and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In a second experiment, an additional classical standing-wave dipole trap provides the trapping, enabling a 28 ms trap lifetime for single strongly coupled atoms.

Single atoms bound in orbit by single photons

The combination of cold atoms and large coherent coupling enables investigations in a new regime in cavity QED with single-atom trajectories monitored in real time with high signal-to-noise ratio. The underlying ``vacuum-Rabi'' splitting is clearly reflected in the frequency dependence of atomic transit signals recorded atom by atom, with evidence for mechanical light forces for intracavity photon number $l1$. The nonlinear optical response of one atom in a cavity is observed to be in accord with the one-atom quantum theory but at variance with semiclassical predictions.

/pdf/real-time-cavity-qed-with-single-atoms-2rj8d1mkou.pdf

Real-Time Cavity QED with Single Atoms

Two recent experiments have reported the trapping of individual atoms inside optical resonators by the mechanical forces associated with single photons [Hood et al., Science 287, 1447 (2000); Pinkse et al., Nature (London) 404, 365 (2000)]. Here we analyze the trapping dynamics in these settings, focusing on two points of interest. First, we investigate the extent to which light-induced forces in these experiments are distinct from their free-space counterparts, and whether or not there are qualitatively different effects of optical forces at the single-photon level within the setting of cavity QED. Second, we explore the quantitative features of the resulting atomic motion, and how these dynamics are mapped onto experimentally observable variations of the intracavity field. Toward these ends, we present results from extensive numerical simulations of the relevant forces and their fluctuations, as well as a detailed derivation of our numerical simulation method, based on the full quantum-mechanical master equation. Not surprisingly, qualitatively distinct atomic dynamics arise as the coupling and dissipative rates are varied. For the experiment of Hood et al., we show that atomic motion is largely conservative and is predominantly in radial orbits transverse to the cavity axis. A comparison with the free-space theory demonstrates that the fluctuations of the dipole force are suppressed by an order of magnitude. This effect is based upon the Jaynes-Cummings eigenstates of the atom-cavity system and represents distinct physics for optical forces at the single-photon level within the context of cavity QED. By contrast, even in a regime of strong coupling in the experiment of Pinkse et al., there are only small quantitative distinctions between the potentials and heating rates in the free-space theory and the quantum theory, so it is not clear that a description of this experiment as a novel single-quantum trapping effect is necessary. The atomic motion is strongly diffusive, leading to an average localization time comparable to the time for an atom to transit freely through the cavity, and to a reduction in the ability to infer aspects of the atomic motion from the intracavity photon number.

https://espace.library.uq.edu.au/view/UQ:109145/UQ109145_OA.pdf

Trapping of single atoms with single photons in cavity QED

Publisher Summary This chapter discusses applications of optical cavities in modern atomic, molecular, and optical physics. For many contemporary physics experiments, the use of an optical cavity has become a powerful tool for enhancement in detection sensitivities, nonlinear interactions, and quantum dynamics. Indeed, an optical cavity allows one to extend the interaction length between matter and field, to build up the optical power, to impose a well-defined mode structure on the electromagnetic field, to enable extreme nonlinear optics, and to study manifestly quantum mechanical behavior associated with the modified vacuum structure and/or the large field associated with a single photon confined to a small volume. Experimental activities that have benefited from the use of optical cavities appear in such diverse areas as ultra-sensitive detection for classical laser spectroscopy, nonlinear optical devices, optical frequency metrology and precision measurement, and cavity quantum electrodynamics (cavity QED). One of the important themes in laser spectroscopy is to utilize an extended interaction length between matter and field inside a high finesse cavity for increased detection sensitivity. Two key ingredients are needed to achieve the highest sensitivity possible in detection of atomic and molecular absorptions: enhancement of the absorption signal and elimination of technical noise. The chapter discusses exploration of quantum dynamics associated with the enhanced interaction between atoms and cavity field; where the structure of the cavity enables a large field amplitude associated with single intracavity photons, the system dynamics can become manifestly quantum and nonlinear.

/pdf/applications-of-optical-cavities-in-modern-atomic-molecular-1z255wqhws.pdf

T. W. Lynn

Papers

The atom-cavity microscope: single atoms bound in orbit by single photons

Single atoms bound in orbit by single photons

Real-Time Cavity QED with Single Atoms

Trapping of single atoms with single photons in cavity QED

Applications of optical cavities in modern atomic, molecular, and optical physics