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Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences

Non-classical light 20 years later: an assessment of the voyage into Hilbert space

Harry J. Kimble

Published Dec 15, 1997• 11 citations
Publisher

Abstract

Although diverse manifestations of the quantum or non–classical character of the electromagnetic field have arisen over the past two decades in quantum optics, almost without exception these observations have been made in a domain of weak coupling for which dissipation is dominant over the coherent dynamical processes associated with single quanta. By contrast, research in the area of cavity quantum electrodynamics has achieved the exceptional circumstance of strong coupling for the interaction of individual atoms with the quantized field of a high–quality resonator. The research programs in the Quantum Optics Group at Caltech attempt to explore quantum dynamical processes in this domain of strong coupling, and include investigations of photon antibunching due to quantum–state reduction, of the implementation of quantum logic with single photons, of new avenues for quantum–state synthesis and of atomic centre–of–mass motion for single atoms falling one–by–one through the field of a high–finesse optical cavity.

DOI
10.1098/rsta.1997.0130

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Although diverse manifestations of the quantum or non–classical character of the electromagnetic field have arisen over the past two decades in quantum optics, almost without exception these observations have been made in a domain of weak coupling for which dissipation is dominant over the coherent dynamical processes associated with single quanta. By contrast, research in the area of cavity quantum electrodynamics has achieved the exceptional circumstance of strong coupling for the interaction of individual atoms with the quantized field of a high–quality resonator. The research programs in the Quantum Optics Group at Caltech attempt to explore quantum dynamical processes in this domain of strong coupling, and include investigations of photon antibunching due to quantum–state reduction, of the implementation of quantum logic with single photons, of new avenues for quantum–state synthesis and of atomic centre–of–mass motion for single atoms falling one–by–one through the field of a high–finesse optical cavity.
An important development in modern physics is the emerging capability for investigations of dynamical processes for open quantum systems in a regime of strong coupling for which individual quanta play a decisive role. Of particular significance in this context is research in cavity quantum electrodynamics which explores quantum dynamical processes for individual atoms strongly coupled to the electromagnetic field of a resonator. An overview of the research activities in the Quantum Optics Group at Caltech is presented with an emphasis on strong coupling in cavity QED which enables exploration of a new regime of nonlinear optics with single atoms and photons.
Cavity quantum electrodynamics (QED) in the strong-coupling regime provides the opportunity to monitor and control the dynamics of a simple quantum system. A single Cesium atom interacts strongly with single-photon fields in the mode of a high-finesse optical cavity. When the resulting coherent coupling rate dominates dissipation in the system, strong coupling is realized and the system displays distinctively quantum behavior. The coupling between atomic internal states and the quantized cavity field allows for diverse protocols in quantum state preparation, quantum communication, and quantum logic. However, the atom's external or motional state must also be taken into account. My research develops cavity QED in the limit where the coherent atom-field coupling dominates the atomic kinetic energy and thus significantly affects the atomic center-of-mass motion. In this regime, the interaction of the atom with the cavity field provides both a means of controlling atomic motion and a signal for detecting that motion in real time with high signal-to-noise. The sensing capability of the ``atom-cavity microscope" is exploited to trap single atoms with single-photon fields and to monitor their orbits in real time as they are bound in the cavity. Such real-time position sensing is the basis for a detailed strategy and ongoing experiment to actively stabilize select aspects of an atom's motion within the cavity. As the cavity-enabled position measurement approaches the standard quantum limit, this work begins to realize a quantum servo for atomic position and to address questions of optimal state estimation and state preparation. In combination with other progress in cavity QED, it furthers the goal of controlled atom-field interactions for quantum information science.
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Previous work in cavity quantum electrodynamics has focused principally on achieving the condition of strong coupling, such that the single-photon Rabi frequency for one atom in a cavity dominates the rates associated with all forms of dissipation and dephasing. Although strong coupling was convincingly demonstrated in these experiments, the use of thermal atomic beams as a source of atoms invariably limited the intracavity dwell time of any given atom to values marginally longer than the system's characteristic dynamical timescales. Accordingly, the observation of single-atom effects in these experiments required integration over the transits of many successive atoms through the optical resonator. In such ensemble-averaged experiments, the quantitative effects of dissipation and measurement could always be understood from the perspective of Master Equation formalisms. By contrast, our most recent work has incorporated laser cooling techniques to accomplish a thousand-fold increase in the dwell time of individual atoms while maintaining conditions of strong coupling. With this extreme separation of timescales, the effects of individual atoms on the cavity transmission can clearly be resolved. Moreover, detailed information on each individual atom's spatial trajectory through the cavity can be inferred from the time-varying amplitude and phase of the transmitted light. Understanding the exact nature of such correlations in real laboratory measurements requires not only a careful characterization of excess technical noise, but also an in-depth study of how (and when) aspects of continuous quantum measurement theory should properly be applied. This thesis primarily contains a detailed description of an experiment in which we used broadband heterodyne spectroscopy to record the complete time-evolution of interaction energy between one atom and a high-finesse optical cavity, during individual scattering events of [...] duration. With a characteristic interaction energy scale [...], we achieve a shot-noise limited measurement sensitivity [...] over a bandwidth that covers the dominant rates of variation in [...]. I also include two brief studies in quantum measurement theory, which highlight the significance of the experimental work for continuing research on conditional quantum dynamics. The first investigates quantum limits to broadband measurement of position and force, and the second proposes a novel trajectory-based approach to quantum system identification.
The radiative properties of atoms inside an optical cavity can be dramatically changed with respect to those in free space. In the strong-coupling limit, the reversible dipole interaction between an atom and the cavity field dominates the irreversible decay resulting from spontaneous emission and cavity damping. In this limit the atom and field can no longer be regarded as separate quantities but instead form a composite dynamic system. To enhance the dipole coupling constant and increase the photon storage time, our experiments are performed with a small cavity 1 mm long, with a finesse of 8 × 104. On average, 0.4 photons are sufficient to saturate the resonance transition at 852 nm for the cesium atoms used, while the cooperativity parameter for a single atom is 2. Optical bistability is studied in this system over the range 3 < N < 65 atoms inside the cavity, with the semiclassical theory predicting bistability for N > 5. We report observations of manifestly quantum effects in this regime, including photon antibunching and sub-Poissonian photon statistics, with the magnitude of these nonclassical effects observed to be largely independent of the number of intracavity atoms.
This research program investigated the dynamics of open quantum systems in manifestly quantum or nonclassical domains. The research program exploited recently discovered possibilities in the microscopic realm of quantum mechanics to accomplish tasks that would otherwise be impossible by traditional classical means, including new capabilities for precision measurement and for the processing and distribution of information. Within this setting, the particular investigations related to strong coupling in optical physics whereby nonlinear interactions require only single atoms and photons. The research achieved capabilities beyond traditional nonlinear optics and laser physics and moved into a new regime with dynamical processes involving atoms and photons taken one by one. The research program addressed fundamental issues related to quantum metrology, to quantum communications, and to the general development of quantum information sciences. The most important accomplishments of this work have been the following: (1) The first demonstration of a laser that operates with a single atom; (2) The first realization of a deterministic source for single photons that generates single photons "on demand" from one atom trapped in a cavity; (3) The development of a protocol for the real-time determination of the number of trapped atoms inside an optical cavity in a regime of strong coupling, with N = 0,1,2,3...atoms; and (4) The first experiment to achieve manifestly quantum or nonclassical photon correlations suitable for the implementation of scalable quantum communications networks.
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.