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Quantum-Atom Optics Downunder (QAO)

Single Atom - Single Photon Interaction: from Bad-Cavity QED to Remote Entanglement

Jürgen Eschner, Markus Hennrich, Felix Rohde, Carsten Schuck, Marc Almendros, Roger Gehr, Pavel A. Bushev, D. Rotter, Christoph Raab, François Dubin, Manas Mukherjee, Christoph Becher, Ferdinand Schmidt‐Kaler, R. Blatt

Published Jan 1, 2007• 0 citations
Publisher

Abstract

We describe a series of experiments showing how a single trapped ion, which interacts with its own resonance fluorescence via a distant mirror, exhibits cavity-QED effects such as modified spontaneous emission, level shifts, and even mechanical forces. The same interaction is used to establish feedback on the ion's quantum motion, providing additional cooling and phase stabilisation of its vibration in the trap. Based on these methods and results we present an experiment dedicated at creating entanglement between two distant ions by projective measurements.

DOI
10.1364/qao.2007.qmb2

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We describe a series of experiments showing how a single trapped ion, which interacts with its own resonance fluorescence via a distant mirror, exhibits cavity-QED effects such as modified spontaneous emission, level shifts, and even mechanical forces. The same interaction is used to establish feedback on the ion's quantum motion, providing additional cooling and phase stabilisation of its vibration in the trap. Based on these methods and results we present an experiment dedicated at creating entanglement between two distant ions by projective measurements.
Abstract The presence of mirrors modifies both the coherent coupling of an atom to a light mode and its spontaneous emission into the mode [1]. We study such cavity QED effects experimentally with single ions and optical cavities. We focus on two examples which are equally interesting as fundamental systems and for application in quantum information processing. (i) By retroreflecting the fluorescence of a single trapped Ba+ ion with a mirror 25 cm away, we observe inhibition and enhancement of the atom's spontaneous emission. When two ions are trapped, the distant mirror creates super‐ and subradiance. (ii) With a single trapped Ca+ ion we demonstrate coherent coupling of its narrow S1/2 – D5/2 “qubit” transition to a mode of a high‐finesse optical cavity. We also achieve deterministic coupling of the cavity standing wave to the ion's vibrational state by controlling the ion's position with nanometer‐precision and selectively exciting vibrational state‐changing transitions.
Experiments in quantum optics are motivated by fundamental questions about the quantum mechanical description of matter and light as well as by applications in quantum technologies such as quantum information processing. Exploiting the quantum character of both matter and light has turned out to be extremely rewarding for novel technological applications in the emerging fields of quantum communication and computation. Here we consider the implementation of quantum information technology with trapped ions and single photons, focusing on two key issues: the entanglement of distant atoms and the controlled interaction of a single photon with a single atom. This thesis describes the construction of a double ion trap apparatus including the necessary frequency-stabilized laser systems, and first experiments to investigate the generation of distant atomic entanglement, the implementation of a simple quantum network and controlled single-atom single-photon interaction. The experimental setup comprises two ion traps at a distance of one meter, which are independent but identical in construction. We introduce novel realizations of a photoionization method for calcium atoms, a laser frequency stabilization scheme and a radio frequency pulse sequencer. Linear Paul traps are used to load individual or strings of calcium ions. High-numerical aperture objectives mounted on sub-micrometer positioning systems allow us to address and manipulate a single ion with strongly focused light fields and collect the emitted fluorescence photons from a large solid angle. With these bidirectional single-atom single-photon interfaces the essential building blocks of a quantum network with atoms and photons are available. One of the main objectives we pursue with the double-trap apparatus is the generation of entanglement between ions in different traps, which is one of the key tasks in a quantum network. We compare single- and two-photon schemes for probabilistic entanglement creation and analyze how the performance of the schemes depends on photon detection efficiency. As a first step towards the experimental realization of such schemes we measure the second order correlation function of fluorescence photons emitted from two distant trapped ions. We observe high-contrast quantum (Hong-Ou-Mandel) interference, proving the indistinguishability of photons emitted from different ions, which is a prerequisite for the considered entanglement schemes. To investigate the interaction of single photons and a single atom in free space we combine the ion trap apparatus with a tunable, narrowband spontaneous parametric down-conversion source. The high-numerical aperture objectives allow us to efficiently couple bandwidth-tunable photons to a single trapped ion. Such atom-photon interaction at the single-particle level is the most fundamental process in quantum optics and lies literally at the focus of many experimental implementations of quantum information processing protocols and of studies of the (quantum) limits of matter -light interaction, which were so far restricted to weak coherent light fields. Here we observe individual atomic absorption events of one photon of an entangled pair by employing a quantum jump detection scheme. To characterize and control the interaction process we modify the spectral properties of the down conversion photons and perform single-photon spectroscopy of a single ion. To go beyond experiments with cohe rent fields we study temporal correlations between the absorption of photons from the spontaneous parametric down conversion source by a single ion and the detection of the corresponding partner photon. For the applied scheme we observe promising indications that the quantum correlations of photons produced in spontaneous parametric down conversion are preserved in the resonant interaction with a single atom. The implementation of more efficient atom-light coupling and entanglement transfer schemes with our apparatus is outlined.
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.
Combining optical cavities with trapped atomic ions is a powerful tool to investigate the interaction of light with matter on the most fundamental scale: a single atom with a single particle of light. Recent advances have not only demonstrated the emission of single photons, ion-photon entanglement and ion-ion entanglement but paved the way to use these systems in modern quantum technologies. Ion-cavity systems are now one of the most promising technologies for realising a scalable quantum computer and quantum networks. While the integration of optical cavities and ion traps is challenging, new techniques and technologies made it possible to demonstrate important building blocks for such systems.
A scheme for entangling distant atoms is realized, as proposed in the seminal paper by [C. Cabrillo et al., Phys. Rev. A 59, 1025 (1999)]. The protocol is based on quantum interference and detection of a single photon scattered from two effectively one meter distant laser cooled and trapped atomic ions. The detection of a single photon heralds entanglement of two internal states of the trapped ions with high rate and with a fidelity limited mostly by atomic motion. Control of the entangled state phase is demonstrated by changing the path length of the single-photon interferometer.