Cavity Quantum Electrodynamics of Systems from Few Atoms to Controlled Ensembles

Hungarian Scientiﬁc Research Fund NF proposal

Cavity quantum electrodynamics of systems from few atoms to controlled ensembles

Peter Domokos

Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences 1 INTRODUCTION 1

1 Introduction

The interaction of the electromagnetic field and atoms is significantly changed if, instead of being in free space, the field is limited by boundaries. The substantial modification of the radiative properties of single atoms or ensembles in a cavity is of fundamental interest, and has an impact on spectroscopy as well as on the current evolution of nanoscale technologies. Cavity quantum electrodynamics (cavity QED, CQED) has been studied for several decades now, however, it is in the last few years that this subject has undergone a breathtaking progress in the laboratories. Recent spectacular advances include, for example, the tracking of atomic trajectories [Hood et al., 2000; Pinkse et al., 2000], the realization of single-atom lasing [McKeever et al., 2003a] and that of deterministic single- photon source [Keller et al., 2004], the cooling of atomic motion by cavity fields [Maunz et al., 2004], the generation of Fock states of the cavity field [Varcoe et al., 2000], and a test of the complementarity principle [Bertet et al., 2001]. The prototype system of cavity QED is a single atomic dipole coupled to the electromagnetic radiation field in a tiny, high-finesse resonator (cavity) composed of mirrors with extremely high reflection coefficients. The interaction is strongly enhanced due to “recycling” the photons by reflections. Whereas lasers act on atoms merely as external fields, the resonator mode becomes a genuine dynamical variable of the coupled system. The atom exerts a significant back-action on the field while evolving under its influence. Cavity QED can be viewed as a model system extending the range of light-matter interaction. The usual scenario of an external field can be rendered with a parameter setting that yields a low number of photon round-trips. At the same time, the coupling can be continuously tuned to achieve a highly correlated atom-photon dynamics. This general- ization opens an avenue towards new applications based on the strong nonlinearities in the dynamics and on the resulting correlation between the subsystems. In this project, cavity QED will be connected to the subject of manipulating the gross motion of neutral massive particles by optical means. On the microscopic level, the mechanical effect of radiation becomes appreciable and gives rise to a wealth of interesting phenomena. With the use of spatially and temporally coherent light sources, i.e. lasers, the mechanical interaction can be designed and the center-of-mass degree of freedom can be controlled by light. As the most spectacular application we mention the various methods developed for the trapping and cooling of neutral particles, which have had an immense impact on today’s atomic, molecular, and optical physics [Chu, 1998; Cohen- Tannoudji, 1998; Phillips, 1998]. The importance of eliminating thermal noise cannot be overestimated since it allows to push measurement precision to the ultimate quantum noise limit given by the uncertainty principle. Owing to optical cooling methods, the quantum degenerate, Bose-condensed state of alkali atom gases has been reached [Cornell and Wieman, 2002; Ketterle, 2002], and later, that of many other species even including molecules [Jochim et al., 2003]. The project objectives are closely related to these outstanding scientific achievements, awarded by the Nobel prizes in 1997 (laser cooling and trapping), 2001 (Bose-Einstein condensation), and 2005 (optical coherence). The guiding principle of the project is that the manipulation of atoms by lasers and related effects must be revisited in the context of cavity QED. One example where this extension has already opened new directions of research is cavity cooling, a recently predicted [Horak et al., 1997] and demonstrated [Maunz et al., 2004] method of efficiently cooling atoms by means of dynamically varying cavity fields. Cavity cooling can poten- tially be applied to arbitrary massive particle with linear polarizability and is therefore a major research topic in several laboratories and theoretical groups today. 2 PROJECT OBJECTIVES 2

Cavity QED can greatly contribute to our knowledge of light-matter interaction con- cerning composite systems. Cavity-induced forces on a dilute gas of atoms are distinguished by the inherent role of many-body effects: the evolution cannot be interpreted in terms of independent single-atom processes as opposed to free space schemes. There is a cavity-mediated interaction between remote atoms, which can alter the dynamics dramat- ically. It is through the electromagnetic field that a collective behaviour of an ensemble of polarizable particles arises, and the back-reaction of those particles on the field is am- plified by the collectivity. For example, a gas of laser-driven atoms scattering light into a cavity has been shown to produce a phase transition between the homogeneous spatial distribution and a regular pattern bound by the electromagnetic field of the cavity. We found that the efficiency of cavity cooling in this scheme can significantly improve owing to the collective action of the atoms: they self-organize into a regular pattern and dissipate motional energy by superradiant scattering into the cavity [Domokos and Ritsch, 2002]. The novelty of the proposed research subject is that we study many-body phenomena in a cavity at a very fundamental level, including a precise modeling of the atom-atom interaction. This initiative is motivated by the fact that the collective effects in an atom cloud depend heavily on the properties of the atom-atom interaction. Therefore we need to complete the previous models with two additional interaction channels. In a dense cloud of cold atoms illuminated by quasi-resonant laser fields, the anisotropic dipole-dipole coupling is the dominant interaction, mediated by the electromagnetic vacuum field. This coupling amounts to a measurable modification of the optical properties of the cloud, e. g., a nonlinear density dependence of the refractive index [Morice et al., 1995]. As a consequence, slow diffusion of light [Labeyrie et al., 2003] and an instabil- ity of the homogeneous distribution [Sesko et al., 1991] have been observed in a dense atom cloud. Furthermore, as one of the major components of this project, we take into account the short range atom-atom interaction resulting from the electron exchange of overlapping atoms. Its effect on the optical properties of colliding atoms has never been explored before. This model will enable us to study photoassociation of atoms into bound molecules. Ultimately, equipped with the full model we can address the regime of ultracold atoms, where the material component manifests wavelike behavior. This is a research direction of fundamental interest in many-body physics. Quantum statistical properties can be investigated in well-controlled, weakly interacting atomic ensembles. A unique feature of the system is that the collisional properties are partially tunable. The necessary ultralow temperature regime can be achieved today, phase transition into a Bose-Einstein condensate is being routinely realized with alkali atoms. Various other manifestations of quantum statistics, superfluidity, etc., can be observed. The strongly coupled dynamics of electromagnetic radiation field and matter, where this latter is represented by a quantum degenerate gas has not yet been explored at all. Experiments are in construction, the first results have recently been reported on the coupling of Bose-Einstein condensates to a high-finesse resonator [Ottl et al., 2005], strongly motivating our effort on the theoretical description.

2 Project objectives

Our general objective is to study the dynamics of composite quantum systems with increasing degrees of complexity in their dimensionality, where the different degrees of free- 2 PROJECT OBJECTIVES 3 dom communicate via their common coupling to the same cavity field modes. Neither simple theoretical models, nor brute force numerical simulations are efficient to describe such systems of interest. Instead, sophisticated phase-space and Monte Carlo methods of quantum optics will be invoked, which enable us to properly treat the quantum noise in open, nonlinear systems. We aim at providing the theoretical ground for exploring new regimes of light-matter interaction in the case of few-atom systems, molecules, Bose- Einstein condensates and Fermi gases.

gf(x)

η(ω) κ γ

ηt(ω)

Figure 1: Schematic representation of the general system to be studied. Atoms moving in an optical resonator are strongly coupled to a sinusoidal mode of the cavity [gf(x)]. The system can be excited by injecting a laser field (η) into the resonator through the partially transmitting mirror or the atoms can be directly laser-driven from the side (ηt). The atoms radiate into free space as well, giving rise to an atom-atom coupling through the vacuum field modes. Depending on the temperature, the atoms can behave as pointlike small wavepackets (see Sect. 2.1), or can extend into large wavefunctions at ultracold temperatures (see Sect. 2.3). The contact interaction between colliding atoms will be studied in detail (see Sect. 2.3). The system has irreversible loss channels: beside the spontaneous emission into the free vacuum field, the resonator photons escape at rate κ.

2.1 Cavity cooling and dipole-dipole interaction in a cavity We consider cold neutral atoms that can be described by small wave-packets, well localized on the radiation wavelength scale. Besides1 being commonly coupled to selected modes of a resonator field, they interact through the broadband reservoir of vacuum field modes: a dipole-dipole coupling arises from the reabsorption of spontaneously emitted photons. This interaction has previously been neglected, assuming a relatively low density of the atom gas. However, thanks to the efficient cavity cooling schemes, the gas may become significantly compressed in the vicinity of antinodes of the resonator mode. The semiclassical model developed to describe the dissipative atomic center-of-mass (CM) motion in a strongly coupled cavity field for single atoms and for non-interacting gas will be completed by this long range (∝ 1/r) atom-atom interaction terms. The model is based on an adiabatic elimination of the atomic excited state, and hence describes the system properly if the pumping field is far detuned from the atomic resonance, and the Doppler effect can be neglected. From another perspective, this model treats a very general class of material components, namely, linearly polarisable particles. The resulting effective master equation accounts for the coupled atomic CM and field mode dynamics to 2 PROJECT OBJECTIVES 4 a high accuracy, even for large forces and large velocities (e.g. those occuring in a firmly trapped situation). Quantum Optics is equipped with powerful phase-space methods to solve such master equations in the semiclassical limit. By use of a truncated Wigner function expansion, one can systematically derive coupled stochastic equations well suited for numerical sim- ulation. The integration of stochastic differential equations of the Ito-type yields random trajectories in phase space. Physical quantities can be extracted by ensemble-averaging over many runs. We resorted to this method for modeling many-atom systems in three- dimensional space in several problems, simulating the motion of up to 10000 atoms at a time. Inclusion of the direct dipole interaction will lead to significant slowing down of the numerical simulations. However, up to hundred atoms, it should be possible to calculate the corresponding stochastic trajectories. We are going to develop a mean-field model in the spirit of the self-consistent field method (Hartree-Fock method). The first step along this road has already been taken: in our recent work [Asbóth et al., 2005] we set up and solved numerically the self-consistent mean field equations for a gas of polarizable particles interacting with a high-finesse cavity in the absence of the direct dipole-dipole interaction. Including this latter coupling is a highly non-trivial task. Nevertheless, this approach is suitable to find the stationary states of the system and their stability as a function of the system parameters. Problems to be studied:

• Eﬀects of the atomic density on the cavity transmission spectrum from high to ultralow temperatures of the interacting atom cloud.

• Revealing density limitations of the cavity cooling scheme, i.e. stability regions of a single-peaked, localised cloud in thermal equilibrium.

• Determination of stable three-dimensional patterns due to the anisotropic dipolar interaction.

• Characterisation of a quantum gas (e.g. condensed fraction) and searching for un- ambiguous signal of a phase transition into the quantum degenerate gas, e.g., into Bose-Einstein condensed state.

2.2 Atom-atom collisions and diatomic molecules in a cavity As a complementary study of the one described in the previous subsection, we will consider exact molecular potentials in the full range of the interatomic distance. In this way we can describe the collision of ultracold atoms in the presence of exciting laser fields, which deserves the detailed investigations in connection with the widely studied effect of photoassociation [Weiner et al., 1999]. In the regime of the molecular bond distances, the direct atom–atom interaction resulting from the electron exchange between overlapping atoms has to be taken into account. This is a problem that has never been addressed in quantum electrodynamics. The standard description of atom-field interaction relies on the Power-Zienau-Woolley gauge which assumes non-overlapping clusters of charges (corresponding to atoms). Our idea is that we consider molecular states approximated by the valence bond method, which renders exactly the “separated atoms” limit, meanwhile it accounts for the exchange interaction terms for nuclei with overlapping electron clouds. This approach leads to an effective Hamiltonian which should give a qualitative description of the dynamics and some novel 2 PROJECT OBJECTIVES 5 effects. A moderate level of precision can be expected, however, the quantitative calculations can be performed using numerical ab initio calculations of molecular states. Several numerical packages (COLUMBUS, DALTON) are freely available for this purpose. The main effect the effective model should include is that the electron exchange term inherently modifies the coupling to optical fields. It gives rise to a variation of the transition dipole moment as a function of the interatomic distance. In other words, we search for effects beyond the Franck-Condon approximation usually applied for the calculation of molecular spectra. We can do so because colliding atoms explore a much wider range of distances than those characteristic of deeply bound vibrational states. Unexpected effects arise from the dependence of the dipole moment on the distance. For example, in an inhomogeneous field, the center-of-mass coordinate of the molecule couples to the vibration, allowing for cooling the translational motion by manipulating vibrations. The same coupling can yield a dissipative dynamics of the relative coordinate of two atoms. This can be the basis of cavity-induced inelastic collisions, which yields the formation of cold molecules in a photoassociation process. The detailed study of atomic collisions in a cavity field is a new research direction within cavity QED. From another point of view, this is relevant to photoassociation which seems to be the most promising way to produce an ensemble of cold molecules, starting from an ensemble of cold atoms. Moreover, the study of laser assisted atomic collisions is a central issue in quantum information processing based on neutral atoms, e.g. on atom chips, where the gating transformations (two qubit transformations like CNOT gates) are implemented via controlled collisions of atoms [Cirac and Zoller, 2003]. Problems to be studied:

• Construction of an eﬀective diatomic Hamiltonian based on valence bond molecular wavefunctions to describe the collision of identical atoms in the presence of quasi- resonant electromagnetic radiation.

• Photoassociation schemes in a cavity, i.e. create molecules from inelastic collisions of ground state atoms using the cavity ﬁeld as a catalysator to dissipate the molecular binding energy.

• Quantitative study of the laser-assisted collision of atoms by numerically calculating molecular potential surfaces, transition dipole moments.

2.3 Ultracold atoms in the optical lattice of a high-finesse cavity A spatially modulated laser field far red detuned from an atomic resonance creates a designable optical potential to trap and manipulate ultra-cold atoms [Kasevich, 2002]. For a periodic lattice potential the system realizes a toy model of solids with a perfect crystal (no defects, tunable depth and lattice vectors). This enables tailored implementations of the Bose-Hubbard Hamiltonian [Jaksch et al., 1998]. E.g., the quantum phase transitions between Mott insulator and superfluid phases has been observed recently [Greiner et al., 2002]. Our atom-cavity system can be regarded as a dynamical optical lattice where the atomic configuration acts back on the lattice potential due to the strong coupling. The other interesting aspect of the dynamical potential is the weak intensity limit: even a single-photon cavity field exerts significant forces on atoms and the quantum properties of the field can no longer be ignored [Maschler and Ritsch, 2005; Vukics et al., 2005]. One striking consequence of the dynamical nature of the lattice potential is the spatial self-organization of a laser-illuminated atomic ensemble, which was first anticipated by 3 TIMELINESS AND NOVELTY OF THE PROPOSED RESEARCH 6

[Domokos and Ritsch, 2002]. Above a threshold pump intensity, the atoms, spontaneously breaking the continuous translational symmetry of the cloud, form one of two regular patterns in a phase transition [Asbóthet al., 2005]. These patterns scatter the maximum field from the pump into the cavity, which has one of the two possible phases, as observed experimentally by [Black et al., 2003]. The atoms find their stable configuration by a feedback mechanism: the potential is created or at least modified by the cavity field which is composed of the interference of components scattered from the pump by different atoms. Accumulating around every second antinode the scattering into the cavity mode is enhanced by constructive interference (superradiance), and the potential at the lattice sites occupied by atoms is maximally deepened. The extension of this classical phase-transition effect to ultracold atoms having a kinetic energy less than the recoil energy, i.e., their wave function is flat on the wavelength scale, is a problem of fundamental interest. This is a generic example of a system where the particle is subject to a dynamically varying potential (feedback). However, at this temperature range the cavity field realizes a quantum feedback [Lloyd, 2000] for the atomic motion, in which entanglement is a crucial element. Scattered field amplitudes with opposite phases do not cancel but entangle to different atomic wave functions. The quantum average of the field amplitude may be zero, whereas the photon number is not, which is clearly incompatible with the mean-field description. Field components of the superposition create different forces, which pull the atomic wave functions towards the corresponding self-organized configurations. Hence self-organization is started immediately even at T = 0 and without measurement induced projections — in fact, no spontaneous symmetry breaking occurs. The effect can be generic for a wide class of quantum phase transitions whenever the quantum state acts back on its control. We still need to approach the thermodynamic limit with the coupled quantum atom-field dynamics taken sufficiently into account. It remains to be answered if there is an abrupt change in the structure of the ground state of the system at some critical parameter setting, i.e., quantum phase transition. In summary, the optical CQED systems is a suitable ground to study various quantum many-body effects in an elucidating depth. Problems to be solved:

• Constructing generalized Hubbard-type models of many-atom physics in periodic optical potentials. Drawing analogy to models of solids with defect.

• Solving lattice models by variational ansatz, renormalization group, and other methods well-established in solid state physics.

• Study the role of quantum statistics (fermion vs. boson) and degeneracy in the transmission spectrum of the cavity.

3 Timeliness and novelty of the proposed research

The proposed research is motivated in large part by the recent experimental progress that has demonstrated the ability of coherently controlling the interaction between light and matter. Studying the mechanical effects of light in a resonator is relatively new direction, and proliferating these days. The modification of light forces in a high-finesse resonator was first observed in 1998 [Hood et al., 1998]. It was soon followed by pioneering experiments [Hood et al., 2000; Pinkse et al., 2000] on the path to a systematic exploration 3 TIMELINESS AND NOVELTY OF THE PROPOSED RESEARCH 7 of the effects of cavity cooling [Maunz et al., 2004] and trapping [McKeever et al., 2003b]. Many of the selected publications appeared in the Nature and Science, indicating that these achievements are considered by the general scientific community as breakthroughs in scientific development. The dynamics of a single atom in a cavity is quite well understood today, although surprises can show up. For example, in 2004, we predicted the anomalous behaviour of the Doppler shift due to an interference effect in a resonator [Domokos et al., 2004]. It was shown to have an effect in considerably extending the trapping times due to an efficient cooling mechanism. Very recently, the Rempe group in Garching has reported the observation of the predicted long trapping times [Nussmann et al., 2005a,b]. The need for developing the theory in this subject is evident: the many-body effects have hardly been explored so far. The cavity-induced long range forces between atoms have also been observed [Münstermann et al., 2000]. Very recently, the effect of collective cooling accompanied by laser emission [Chan et al., 2003], and then the demonstration of self-organization, that we predicted in 2002, have been reported [Black et al., 2003]. This experiment performed by the group of Vladan Vuletic (MIT, Cambridge) has been selected by the Physical Review Focus (vol. 12, story 17, 7 November 2003, “Atoms Team Up to Slow Down”). It is considered a potential breakthrough in getting around the limitations of standard laser cooling techniques developed in the past two decades (recognized by the 1997 Physics Nobel Prizes). Similar experiments with ring cavities have been recently set up at the universities of Tübingenand Hamburg, where the effect of collective atomic recoil lasing has been observed [Kruse et al., 2003; Nagorny et al., 2003]. In these many-atom experiments the resonator has a somewhat larger mode volume that could cover the atom cloud loaded from a magneto-optical trap. The single atom coupling to the field is weaker, nevertheless the effect of the cavity has been demonstrated because a gas of atoms enhance it by their collective action [Klinner et al., 2006]. The very fact of observing many-atom effects makes the system even more interesting. However, the theoretical description of as many as 106 atoms is rather challenging and certainly requires massive numerical calculations to account for the collective atomic effects. It is the recent progress in our computational facilities which gives us a chance to simulate these systems and, in the future, with codes better exploiting the power of parallelism in PC-clusters we hope to considerably push the limits of theoretical approaches. In the last two years, several laboratories joined this field with a broad spectrum of schemes and methods. Therefore, we need to extend our existing models to cover a wider scope of experimental possibilities. Recently, there have been groundbreaking experiments with optical lattices, such as the observation of the transition from superfluid to Mott-insulator phase [Greiner et al., 2002]. When Bose-condensates are loaded into a Fabry-Perot resonator, as it has been done at ETH Zürich, such experiments can be performed with dynamically varying potential, a system which comprises even more complexity and evidently deserves to be studied right now. The description of the interaction between a condensate and light is challenging in the sense that many parts of modern physics have to be invoked. We intend to treat this system on the ground of first principles of quantum mechanics. Finally let us mention that this proposed research programme addresses the problem of single-atom detection, which is a central issue of other international cooperations we participate in. In particular, this is a hot topic for the so-called “atom chips”, being an astonishing possibility of a completely new technology that is similar to microelectronics but is based on the controlled flow and interaction of cold atoms. Owing to the minia- 4 INTERNATIONAL CONTEXT OF THE RESEARCH IN THIS FIELD 8 turization of atom traps above microfabricated surfaces, atoms can be confined within quantum dots that are just a hundred nanometres across, or made to flow in long narrow tubes, rather like electrons in a very thin wire. Various types of resonators are being in- tegrated on these atom chips to construct a non-destructive detection system for neutral atoms.

4 International context of the research in this ﬁeld

In the field of cavity QED there is strong competition between laboratories with very high reputation, such as, e.g., Caltech, Berkeley, MIT, Georgia Tech in the United States, and the Max Planck Institute (Garching), Ecole Normale Superieure (Paris), University of Tübingen,Hamburg, Bonn, etc., in Europe. At the time of 1997-2000, only two experiments were running with operating condi- tions that allowed for the observation of mechanical effects on atoms within a high-Q optical resonator. In these experiments the relevant cavity mode has an extremely tiny mode volume leading to the so-called “strong coupling” regime of cavity quantum electrodynamics. Both groups, the one of Prof. G. Rempe at the Max Planck Institute in Garching, and the other one of J. Kimble at Caltech, produced spectacular experiments of fundamental importance on the interaction of single atoms with single photons. Since then, the number of experimental groups interested in cavity cooling and related effects has multiplied. Cavity QED experiments in the strong-coupling regime with a controlled atomic transport into the cavity can be performed on a new setup at the Georgia Institute of Technology, in the group of M. Chapman [Sauer et al., 2004], in Bonn in the group of Prof. Meschede [Schrader et al., 2004] and also in the Garching group. H. Mabuchi at Caltech is currently mounting an experiment where the optical resonator will be represented by a nanofabricated photonic bandgap structure [Vuckovic et al., 2002]. Two groups at the University of Hamburg (Prof. A. Hemmerich) and at the University of Tübingen(Prof. C. Zimmermann) set up an experiment with a high-finesse ring cavity coupled to an atomic cloud released from a magneto-optical trap (MOT). It is to be noted that the theory for ring cavity predicts rather different effects compared to a standing-wave cavity. A highly degenerate Fabry-Perot type resonator is used in the experiments of V. Vuletic (M.I.T.), this setup has the possibility to pump the atomic cloud from the side, being an important element of our calculations. Another current tendency is the combination of other experiments with the use of a high-finesse cavity. For example, single ion traps are surrounded by mirrors in the group of R. Blatt (Innsbruck) to manipulate the ion vibration [Eschner et al., 2001], as well as in the group of H. Walther and W. Lange in Garching [Keller et al., 2004]. There is an initiative taken at the University of Aarhus by the group of M. Drewsen to place a linear ion chain trap within a high finesse resonator. On the theoretical side, the main group performing calculations on the atomic motion in cavities are the one in Innsbruck, led by prof. H. Ritsch, and ours at the Research Institute for Solid state Physics and Optics. Besides, there is a close collaboration between us. Furthermore, there are important theoretical contributions from the experimental groups of Garching, Hamburg and Caltech, and also from the quantum optics groups at in Ulm, in the Imperial College (London), and in Camerino. The project aims at the creation of a theoretical group with outstanding activity in the field of cold atoms in cavities, with very strong link to the experimental work. As some of our predictions have already been accepted, subsequently demonstrated and used by 5 MANAGEMENT OF THE PROJECT 9 different experimental groups, we believe that the proposed research programme will also attract further experimental efforts and advance the field into the proposed directions. It is likely that the ability of controlling the dynamics of a single atom in a cavity will be followed by experiments systematically increasing the complexity of the system and studying two, three, etc. number of atoms. Our key issues address this scenario, and we propose to explore and understand these systems prior to experiments.

5 Management of the project

In this research programme the material component of the light-matter interaction is considered on a wide range: from independent atoms, through dipole-dipole coupled atoms, to diatomic bound states. This yields completely new prospects in cavity QED researches. The full description of the interacting system is a formidable task, however, deep understanding of the problem can be gained by separately studying these interaction channels. Since the three lines of research are intimately related, we can exploit the specific group resources and can benefit from the parallelism. This justifies the application for an NF project. The present theoretical project is based on analytical calculations and numerical simulations. On the technical level, our research objectives heavily rely on the possibility of high-performance computation. It is the recent progress in the computational facilities with cluster-type architectures which gives a chance to simulate these systems. We can benefit from setting up codes exploiting the power of parallelism in PC-clusters. The pur- chase of such equipments would allow us to considerably push out the limits of theoretical approaches. Each of the main research directions will be pursued by the project leader and one PhD student or PostDoc. The participating researchers will be DávidNagy Diplome student (Technical University of Budapest) His work entitled “Study on the self-organization of laser-cooled atoms in a ring cavity” was awarded at the Scientific Conference of Students in 2005. He will work on the dipole-dipole coupling related problems.

JánosAsbóth PhD student (University of Pécs); He prepared his diplome thesis under my supervision, entitled Correlated motion of atoms in an optical resonator (Roland EötvösUniversity of Budapest, 2003, qualified “excellent”). He will work on the exchange interaction related problems.

AndrásVukics PhD student (University of Szeged); He prepared his diplome thesis under my supervision, entitled Three-dimensional dissipative motion of atoms in optical resonators (Roland EötvösUniversity of Budapest, 2003 qualified “excellent”). He has recently submitted his PhD thesis “Mobile atoms in a cavity field : statistical and quantum aspects”. He will work on the quantum statistical aspects of many-body systems.

AurélGábris PhD student (University of Szeged); He prepared his diplome thesis under Prof. J. Janszky’s supervision, entitled EPR pairs in coherent-state basis (Roland EötvösUniversity of Budapest, 2003, qualified “excellent”). He is currently writing his PhD thesis entitled “Multi-partite quantum optical systems for quantum information processing”. 6 SCIENTIFIC IMPACT AND PROMOTING INNOVATION 10

He will assist in any of the above directions, furthermore, he is responsible for the problems related to quantum information processing in high-Q cavities.

One full-time researcher position is requested within this application. This post will be filled either by one of the above-listed researchers or by an external candidate. All posts will be announced on the usual channels to the European scientific community of this field. We aim at involving more students in the scientific program, this will be partially financed by the NF project. Another important component of the management is the necessity to establish and maintain a strong link to the leading European experimental groups of this field (especially the one of Prof. G. Rempe at Max Planck Institute, Garching; and also the ones of Prof. C. Zimmermann at the UniversitätTübingenand that of Prof. A. Hemmerich at the UniversitätHamburg) as well as the as the theory group of Prof. Helmut Ritsch in Innsbruck.

6 Statement of scientiﬁc impact and potential for promoting innovation

Quantum Optics is a field which touches upon the foundations of physics. Simultaneously, it has a strong influence on the development of current technology. Its attractive feature is the balance of theory and experiment, and that new ideas can be implemented in table- top experiments on a relatively short timescale. Research in quantum optics contributes to the knowledge which, as industry is approaching the atomic lengthscale, is necessary for a new generation of optomechanical devices whose operation heavily relies on the laws of quantum mechanics. A very important aspect of novel quantum effects appearing in future “quantum devices” is the mechanical effect of photons, i.e., the mechanical kick that is imparted to atoms during a given operation. This interaction has to be understood and can then be employed, for example, to make operations with the atomic-scale building blocks by controlled collisions preserving quantum coherence. We envisage that strong coupling between light and matter and the consequent nanoscale control could be realized by various nanofabricated resonant structures, such as fused silica spheres or fiber arrays, which would greatly amplify the significance of this field in today’s technological revolution. The reduction of noise, i.e. cooling, is a crucial issue if we want to push measurement precision towards the ultimate limit allowed by the inevitable quantum noise. The impact of research on ultracold atoms has clearly been demonstrated as optical cooling methods are used in the latest generation of atomic clocks. The general purpose of enhancing our ability of coherently manipulating atomic systems, especially Bose-condensates is still a strong motivation of our work, and each achievement penetrates through various channels into applications and new technologies. Neutral atoms are distinguished in this respect because they couple weakly to the environment. In fact, exactly this made it difficult to tame and manipulate them. However, being equipped now with various optical methods, the property that they interact weakly with the environment makes them an ideal candidate for measuring fundamental physical constants and also for decoherence studies. The potential for manipulating systems with increasing but controlled degree of complexity leads to the systematic exploration of the limits between the classical and quantum worlds. REFERENCES 11

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