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Foundations of Quantum Mechanics in the Light of New Technology

QUANTUM OPTICS WITH SINGLE ATOMS AND PHOTONS

Harry J. Kimble

Published Jun 1, 2006• 0 citations
Publisher

Abstract

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.

DOI
10.1142/9789812773210_0004

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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.
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.
Recent experimental advances in the field of cavity quantum electrodynamics (QED) have opened new possibilities for control of atom‐photon interactions. A laser with “one and the same atom” demonstrates the theory of laser operation pressed to its conceptual limit. The generation of single photons on demand and the realization of cavity QED with well defined atomic numbers N = 0, 1, 2,… both represent important steps toward realizing diverse protocols in quantum information science. Coherent manipulation of the atomic state via Raman transitions provides a new tool in cavity QED for in situ monitoring and control of the atom‐cavity system. All of these achievements share a common point of departure: the regime of strong coupling. It is thus interesting to consider briefly the history of the strong coupling criterion in cavity QED and to trace out the path that research has taken in the pursuit of this goal.
Summary form only given. In the quantum optics group at Caltech, we are attempting to lay the foundations for quantum information science by way of advances on several fronts in optical physics. Within the setting of cavity QED, single atoms are strongly coupled to the field of a high finesse optical cavity at the single photon level, with current work directed toward trapping and localization of atoms inside the cavity. Although there are daunting technical problems to overcome, a principal scientific objective is the creation of quantum networks to implement fundamental quantum communication protocols and for distributed quantum computation. Beyond quantum information processing with internal atomic states and photons serving as qubits, we are also investigating algorithms for continuous quantum variables. A recent example is our realization of quantum teleportation for the quadrature amplitudes of a beam of light. The experiment utilizes squeezed-state entanglement to achieve unconditional quantum teleportation.
Cavity quantum electrodynamics (QED), the system of a single atom interacting with a single mode of a high finesse optical resonator, has tremendous potential for use in quantum logic, quantum information processing, and in enabling observation of quantum mechanical effects in the laboratory. The work of this thesis demonstrates a number of steps toward realizing these goals, by employing an experimental system with the strongest atom-field coupling achieved to date in optical cavity QED. The effects of strong coupling in cavity QED are studied for single cold atoms measured one at a time, in real time. The passage of single atoms through the cavity is employed to map out the frequency response of the system, and to demonstrate that quantum rather than semiclassical theory provides the correct theoretical model. The mechanical effects of strong coupling on single atoms are explored, these experiments demonstrating the first trapping of single atoms with single photons. Additionally, strong coupling enables high signal-to-noise ratio for monitoring atomic position at single photon field strengths; this capability is employed to investigate the motion of atoms trapped within the cavity. The transmitted cavity field is used to reconstruct the trajectories of single atoms, thereby realizing a new form of microscopy - the Atom-Cavity Microscope. Technical developments necessary to create the cavities used in this experiment are detailed. Ideas for future extensions to the experiment are proposed; a zero light-shift dipole force trap, and measurement of photon statistics of a single strongly coupled atom (which realizes a photon blockade device) are discussed.
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.