An important new development in the field of ultracold atomic gases is the study of the properties of these gases in a so-called optical lattice. An optical lattice is a periodic trapping potential for the atoms that is formed by the interference pattern of a few laser beams. A reason for the interest in these systems is that the effects of the interatomic interactions can be strongly enhanced. More specifically, it has been shown in a beautiful experiment by Greiner et al. in 2002 that by loading a Bose-Einstein condensate into an optical lattice it is possible for the system to undergo a quantum phase transition to a new quantum phase of matter, the so-called Mott insulator phase. Within this Mott insulator phase each lattice site is occupied by exactly one atom. This makes the Mott insulator phase especially well suited for applications in the field of quantum computation and quantum information processing. We have theoretically investigated the above mentioned quantum phase transition and our formalism allows for a description of the Mott insulator phase at nonzero temperatures. Another important experimental development in the field of ultracold atomic gases is the use of Feshbach resonances to control the interatomic interactions. Such a resonance occurs whenever two colliding atoms form a long-lived molecule for some time. The crucial point of a Feshbach resonance is that the above mentioned molecule has a magnetic moment that is not equal to twice the magnetic moment of the atom. As a consequence the energy difference between the two atoms and the molecule and hence the interactions between the atoms can be controlled by using an external magnetic field. By combining these two techniques, i.e, by trapping ultracold atomic gases in an optical lattice and by tuning a magnetic field near a Feshbach resonance there can be a new quantum phase transition between two superfluid phases. We gave derived the the theory for the description of these Feshbach resonances in optical lattices and applied it to various systems. To be a bit more precise, if we tune the external magnetic field such that the energy difference between a molecule and two atoms is sufficiently negative, then the gas consists of a Bose-Einstein condensate of molecules. In contrast, if the energy difference is large enough and positive we have a gas that consists primarily of a Bose-Einstein condensate of atoms. It turns out that these two limits are separated by an Ising-like quantum phase transition. By using atomic Bose gases near a Feshbach resonance detailed experimental studies which test the theoretical predictions of the statistical and dynamical properties of these quantum phase transitions can be made.
An important new development in the field of ultracold atomic gases is the study of the properties of these gases in a so-called optical lattice. An optical lattice is a periodic trapping potential for the atoms that is formed by the interference pattern of a few laser beams. A reason for the interest in these systems is that the effects of the interatomic interactions can be strongly enhanced. More specifically, it has been shown in a beautiful experiment by Greiner et al. in 2002 that by loading a Bose-Einstein condensate into an optical lattice it is possible for the system to undergo a quantum phase transition to a new quantum phase of matter, the so-called Mott insulator phase. Within this Mott insulator phase each lattice site is occupied by exactly one atom. This makes the Mott insulator phase especially well suited for applications in the field of quantum computation and quantum information processing. We have theoretically investigated the above mentioned quantum phase transition and our formalism allows for a description of the Mott insulator phase at nonzero temperatures. Another important experimental development in the field of ultracold atomic gases is the use of Feshbach resonances to control the interatomic interactions. Such a resonance occurs whenever two colliding atoms form a long-lived molecule for some time. The crucial point of a Feshbach resonance is that the above mentioned molecule has a magnetic moment that is not equal to twice the magnetic moment of the atom. As a consequence the energy difference between the two atoms and the molecule and hence the interactions between the atoms can be controlled by using an external magnetic field. By combining these two techniques, i.e, by trapping ultracold atomic gases in an optical lattice and by tuning a magnetic field near a Feshbach resonance there can be a new quantum phase transition between two superfluid phases. We gave derived the the theory for the description of these Feshbach resonances in optical lattices and applied it to various systems. To be a bit more precise, if we tune the external magnetic field such that the energy difference between a molecule and two atoms is sufficiently negative, then the gas consists of a Bose-Einstein condensate of molecules. In contrast, if the energy difference is large enough and positive we have a gas that consists primarily of a Bose-Einstein condensate of atoms. It turns out that these two limits are separated by an Ising-like quantum phase transition. By using atomic Bose gases near a Feshbach resonance detailed experimental studies which test the theoretical predictions of the statistical and dynamical properties of these quantum phase transitions can be made.
An optical lattice is a periodic potential for atoms, created using a standing wave pattern of light. Due to the interaction between the light and the atoms, the atoms are attracted to either the nodes or the anti-nodes of the standing wave, depending on the exact wave lenght of the light. This means that if such a lattice is loaded with a sufficiently high number of ultracold atoms, a periodic array of atoms is obtained, we an interatomic distance of a few tenths of a micrometer. In order to obtain such a high number of cold atoms, one first has to create a so-called Bose-Einstein condensate. When an optical lattice is loaded from a Bose-Einstein condensate, it is possible to create a system in which every atom is in the lowest band of the lattice and there is on average one atom in each lattice site. Because the lattice potential is created with laser light, the depth of the lattice can easily be tuned by changing the intensity of the laser. When the intensity of the laser light is low, the atoms can tunnel from one site to the next. Due to this tunneling, the gas of atoms in the lattice will remain superfluid. However, if the intensity of the laser light is increased to above a certain critical value, a quantum phase transition occurs to a so-called Mott insulator. In this state, the atoms can no longer tunnel due to the fact that the on-site interaction between atoms becomes more important then the tunneling probability. In this PhD thesis, a description is given of the experimental setup that is being constructed in our group to create these systems in our lab. Also, a theoretical description is given of these systems and several important quantities our derived, such as the gap of the Mott-insulating state. Furthermore, an experiment is proposed that can be used to accurately measure this gap.
Ultracold quantum gases in optical lattices form almost ideal conditions to analyze the physics of strongly correlated quantum phases in periodic potentials. Such strongly correlated quantum phases are of fundamental interest in condensed matter physics, because they lie at the heart of topical quantum materials, such as high-Tc superconductors and quantum magnets, which pose a challenge to our basic understanding of interacting many-body systems. Quite generally, such strongly interacting quantum phases arise when the interaction energy between two particles dominates over the kinetic energy of the two particles. Such a regime can either be achieved by increasing the interaction strength between the atoms via Feshbach resonances or by decreasing the kinetic energy, such that eventually the interaction energy is the largest energy scale in the system. The latter can, for example, simply be achieved by increasing the optical lattice depth. This chapter tries gives an introduction into the field of optical lattices and the physics of strongly interacting quantum phases. A prominent example hereof is the superfluid-to-Mott insulator transition, which transforms a weakly interacting quantum gas into a strongly correlated many body system. Dominating interactions between the particles are in fact crucial for the Mott insulator transition and also for the realization of controlled interaction-based quantum gates, of which several have been successfully realized experimentally.
The quantum phase transition from the Mott insulator to the superfluid phases of the bosonic atoms trapped in an optical lattice, in which the on-site interaction can be tuned by a Feshbach resonance, is investigated by a variational approach within mean-field theory. We derive an extended Bose-Hubbard model to describe this ultracold atomic system. By theoretical calculation and analysis, the phase diagram is shown clearly, and we find an exciting and novel phenomenon that is the appearance of the Mott insulator-sea (MI-sea). Meanwhile, the experimental feasibility of observing the MI-sea is discussed by analyzing the published data related to the Fashbach resonance at present. Finally, the potential application of the MI-sea for quantum information processing and quantum computation is also discussed in detail.
In this thesis I report on experiments that enter a new regime in the many body physics of ultracold atomic gases. A Bose-Einstein condensate is loaded into a three-dimensional optical lattice potential formed by a standing wave laser light field. In this novel quantum system we have been able to both realize a quantum phase transition from a superfluid to a Mott insulator, and to observe the collapse and revival of a macroscopic matter wave field. Quantum phase transitions are driven by quantum fluctuations and occur, even at zero temperature, as the relative strength of two competing energy terms in the underlying Hamiltonian is varied across a critical value. In the first part of this work I report on the observation of such a quantum phase transition in a Bose-Einstein condensate with repulsive interactions, held in a three-dimensional optical lattice potential. In the superfluid ground state, each atom is spread-out over the entire lattice, whereas in the Mott insulating state, exact numbers of atoms are localized at individual lattice sites. We observed the reversible transition between those states and detected the gap in the excitation spectrum of the Mott insulator. A Bose-Einstein condensate is usually described by a macroscopic matter wave field. However, a quantized field underlies such a "classical" matter wave field of a Bose-Einstein condensate. The striking behavior of ultracold matter due to the field quantization and the nonlinear interactions between the atoms is the focus of the second part of this work. The matter wave field of a Bose-Einstein condensate is observed to undergo a series of collapses and revivals as time evolves. Furthermore, we show that the collisions between individual pairs of atoms lead to a fully coherent collisional phase shift in the corresponding many-particle state, which is a crucial cornerstone of proposed novel quantum computation schemes with neutral atoms. With these experiments we enter a new field of physics with ultracold quantum gases. In this strongly correlated regime, interactions between atoms dominate the behavior of the many-body system such that it can no longer be described by the usual theories for weakly interacting Bose gases. This novel quantum system offers the unique possibility to experimentally address fundamental questions of modern solid state physics, atomic physics, quantum optics, and quantum information.
Imagine you could produce an artificial crystal for quantum matter, defect free and with complete control over the periodic crystal potential. The shape of the periodic potential, its depth and the interactions between the underlying particles could be changed at will and the particles could be moved around in a highly controlled way, at essentially zero temperature. This sounds almost too good to be true, but it is in fact what optical lattices have made possible for cold and ultracold atoms. How does this work? Neutral atoms can be trapped in the periodic intensity pattern of light, which is formed when two laser beams interfere. The varying light intensity creates a periodic potential landscape – the optical lattice – for the atoms via an AC-Stark shift interaction. Such optical lattices have recently turned into a major research field, especially with the advent of degenerate bosonic and fermionic quantum gases. There, the artificial crystal of light is used to create and investigate fundamental quantum phases of bosonic and fermionic many-body systems. One of the great advantages of these systems is that the physical behaviour of the atoms is usually perfectly described by a simple underlying Hamiltonian. They can thus can be used as a testbed for modern condensed matter theories with unique manipulation and control possibilities. Furthermore, novel and exotic many body quantum phases have also been predicted to be realizable in such 'quantum simulators as Richard P Feynman might have called them. Some of the most famous Hamiltonians that have been proposed in this respect include the Bose–Hubbard and Hubbard Hamiltonians for interacting bosons and fermions in periodic potentials, respectively. The physics within the Hubbard Hamiltonian is believed to contain an explanation for the origin of High- T c superconductivity, but so far even the exact phase diagram of this basic Hamiltonian is still under discussion after decades of theoretical research on this problem. Another research focus in this respect has been to use ultracold atoms in optical lattices for the investigation of quantum magnetism. Here the spin state of an atom does not have to be restricted to two possible values, as for electrons, but can cover several possible magnetic substates, yielding rich and novel quantum phases. In the latest proposals, it has also been shown how quantum spin models could be robustly and efficiently simulated with polar molecules trapped in optical lattices. Next to exploring the fundamental behaviour of quantum matter in periodic potentials, optical lattices have also shown to be very useful for the generation of ultracold molecules. Imagine for example having two atoms at each lattice site of an optical lattice. Then photoassociation or Feshbach resonances can be used to convert these atom pairs into stable molecules in a defined rotational-vibrational quantum state. One might consider such a coherent conversion of atoms into molecules to be the ultimate quantum limit that we can reach in the control over a 'chemical reaction'. Next to being micro-laboratories for molecular physics, optical lattices also play an important role in quantum information processing. Atoms localized in the nodes of the optical potential can be seen as a natural quantum register that opens powerful possibilities for quantum computing. In this Focus Issue, it is shown, for example, how atoms can be sorted with an arrangement of two crossed optical standing waves. The topics covered by the articles in this Focus Issue of New Journal of Physics reflect almost the whole breadth of topics discussed above and in fact the great interest in this field has required us to split off some of the articles to be published at a later date (still as part of this focus issue). We hope that you will find detailed new research results in this collection that are of interest to you and furthermore hope that the issue can increase the interest of the non-expert for this exciting and interdisciplinary research field. The articles below represent the first contributions and further additions will appear. Focus on Cold Atoms in Otical Lattices Contents Theory of Feshbach molecule formation in a dilute gas during a magnetic field ramp J E Williams, Nicolai Nygaard and Charles W Clark Dressed molecules in an optical lattice K B Gubbels, D B M Dickersheid and Henk T C Stoof Precision measurement of spin-dependent interaction strengths for spin-1 and spin-2 87 Rb atoms Artur Widera, Fabrice Gerbier, Simon Folling, Tatjana Gericke, Olaf Mandel and Immanuel Bloch The Fermi–Hubbard model at unitarity Evgeni Burovski, Nikolay Prokof'ev, B Svistunov and M Troyer Interaction broadening of wannier functions and mott transitions in atomic BEC Jinbin Li, Yue Yu, A M Dudarev and Qian Niu Hanbury Brown–Twiss interferometry for fractional and integer Mott phases Ana Maria Rey, I I Satija and Charles W Clark Quantum random walk with Rydberg atoms in an optical lattice Robin Cote, Alexander Russell, Edward E. Eyler and Phillip Gould Raman Spectroscopy of Mott insulator states in optical lattices P Blair Blakie Interacting Bose gases in quasi-one dimensional optical lattices M A Cazalilla, A F Ho and Thierry Giamarchi Long distance transport of ultracold atoms using a 1D optical lattice Stefan Schmid, Gregor Thalhammer, Klaus Winkler, Florian Lang and J H Denschlag Signatures of the superfluid to Mott-insulator transition in the excitation spectrum of ultracold atoms S R Clark and Dieter Jaksch Extended fermionization of 1D bosons in optical lattices Guido Pupillo, Ana Maria Rey, Carl J Williams and Charles W Clark One-dimensional description of a Bose–Einstein condensate in a rotating closed-loop waveguide Sylvain Schwartz, Marco Cozzini, Chiara Menotti, I Carusotto, Philippe Bouyer and Sandro Stringari Photo-ionization in far-off resonant optical lattices R M Potvliege and Charles S Adams Cooling toolbox for atoms in optical lattices Markus Popp, J J Garcia-Ripoll, K G Vollbrecht and J I Cirac Experimental study of the transport of coherent interacting matter-waves in a 1D random potential induced by laser speckle David Clement, A F Varon, J A Retter, L Sanchez-Palencia, Alain Aspect and P Bouyer Quantum dynamics, particle localization and instability of Mott states: the effect of fermion-boson conversion on Mott states Fei Zhou and Congjun Wu Off-diagonal correlations of lattice impenetrable bosons in one dimension D M Gangardt and Gora V Shlyapnikov Collective mode damping and viscosity in a 1D unitary Fermi gas Matthias Punk and W Zwerger Coherent matter waves emerging from Mott-insulators K Rodriguez, S R Manmana, Madrazo Marcos Rigol, R M Noack and A Muramatsu Phase-matched matter wave collisions in periodic potentials Klaus Moelmer Fermion pairing with spin-density imbalance in an optical lattice T Koponen, J Kinnunen, J-P Martikainen, L M Jensen and P Törmä Macroscopic superpositions of superfluid flows David W Hallwood, Keith Burnett and Jacob Dunningham Mean-field phase diagram of disordered bosons in a lattice at nonzero temperature K V Krutitsky, A Pelster and R Graham A primary noise thermometer for ultracold Bose gases R Gati, J Esteve, B Hemmerling, T B Ottenstein, J Appmeier, A Weller and M K Oberthaler Precision preparation of strings of trapped neutral atoms Y Miroshnychenko, W Alt, I Dotsenko, L Förster, M Khudaverdyan, A Rauschenbeutel and D Meschede Cold Fermi gases: a new perspective on spin-charge separation Corinna Kollath and Ulrich Schollwöck Analysis of localization phenomena in weakly interacting disordered lattice gases T Schulte, S Drenkelforth, J Kruse, R Tiemeyer, K Sacha, J Zakrzewski, M Lewenstein, W Ertmer and J J Arlt Localization and anomalous transport in a 1D soft boson optical lattice A K Tuchman, W Li, H Chien, S Dettmer and M A Kasevich Quantum switches and quantum memories for matter-wave lattice solitons V Ahufinger, A Mebrahtu, R Corbalán and A Sanpera Quantum noise analysis of spin systems realized with cold atoms Robert W Cherng and Eugene Demler Effective-range description of a Bose gas under strong one- or two-dimensional confinement Pascal Naidon, Eite Tiesinga, William F Mitchell and Paul S Julienne Dissipative dynamics of atomic Hubbard models coupled to a phonon bath: dark state cooling of atoms within a Bloch band of an optical lattice A Griessner, A J Daley, S R Clark, D Jaksch and P Zoller Inhomogeneous broadening of a Mott insulator spectrum V Guarrera, L Fallani, J E Lye, C Fort and M Inguscio Immanuel Bloch, Johannes Gutenberg-Universität, Mainz, Germany Peter Zoller, Universität Innsbruck, Austria
Ultracold atoms in optical lattices provide an extremely clean and controllable setting to explore quantum many-body phases of matter. Here we give a brief review of the strong-correlation physics that has been realized using such ultracold atoms in optical lattices ranging from the realization of Hubbard models to studies of quantum magnetism and the detection of single atoms with lattice site resolution. All this has opened up fundamentally new opportunities for the investigation of quantum many-body systems. Introduction Over the past years, ultracold atoms in optical lattices have emerged as versatile new systems to explore the physics of quantum many-body systems. On the one hand, they can be helpful in gaining a better understanding of known phases of matter and their dynamical behavior; on the other hand, they allow one to realize completely novel quantum systems that have not been studied before in nature [1, 2, 3]. Commonly, the approach of exploring quantum many-body systems in such a way is referred to as “quantum simulations.” Examples of some of the first strongly interacting many-body phases that have been realized both in lattices and in the continuum include the quantum phase transition from a superfluid to a Mott insulator [4, 5, 6], fermionic Mott insulators [7, 8], the achievement of a Tonks- Girardeau gas [9, 10], and the realization of the Bose-Einsten condensate (BEC)– Bardeen-Cooper-Schrieffer (BCS) crossover (see also Chapter 12) in Fermi gas mixture [11] using Feshbach resonances [12]. In almost all of these experiments, detection was limited to time-of-flight imaging or more refined derived techniques that mainly characterized the momentum distribution of the quantum gas [2]. For several years, researchers in the field have therefore aspired to employ in situ single-particle detection methods for the analysis of ultracold quantum gases. Only recently has it become possible to implement such imaging techniques, marking a milestone for the characterization and manipulation of ultracold quantum gases [13, 14, 15, 16, 17]. In our discussion, we will focus on one of the most successful of these techniques based on high-resolution fluorescence imaging. Despite being a rather new technique, such quantum gas microscopy has already proven to be an enabling technology for probing and manipulating quantum many-body systems. For the first time, controllable and strongly interacting many-body systems, as realized with ultracold atoms, could be observed on a local scale [17, 16].