Session T3: Rydberg-atom Many-body Systems and Applications

Dynamical crystallisation of a low-dimensional Rydberg gas

Rydberg atoms provide a way to investigate long-range interacting many-body systems in a very controlled way. In our setup we implemented an optical detection technique for Rydberg atoms with sub-micron resolution, which allows for the measurement spatial correlations in strongly interacting collective states of Rydberg atoms. We prepare a well-defined configuration of ground state atoms in an optical lattice and laser couple them to a Rydberg state. The Rydberg atoms interact via the van der Waals force, which extends over approximately half the system size, thereby leading to strong correlations. Using numerically optimized pulse shapes for coupling strength and detuning, we deterministically prepare the crystalline state in this long-range interacting many-body system. Control of the spatial configuration of the initial state is of great importance for the investigation of the phase diagram. To achieve this, we developed an experimental scheme based on single site addressing which allows for the preparation of initial states with sub-Poisson number fluctuations. The developed techniques might allow for the detailed characterisation of the phases in Rydberg gases and their coherence properties.   

The effects of light-shift and temporal evolution on collective Rydberg excitations

Optical dipole traps are widely used to trap and manipulate cold ground-state and Rydberg atoms. Here, we present a first study of the effects of dipole-trap-induced light shifts on the spatial pair-correlation function of Rydberg excitations generated in clouds of cold $^{85}$Rb atoms. We use ion imaging techniques to obtain the Rydberg pair-correlation functions. We measure and interpret the effects of excitation-laser detuning and dipole-trap-induced light shifts on the Rydberg excitation blockade. We also observe an enhancement of the probability of exciting two Rydberg atoms at a particular separation, which we explain as direct two-photon excitation of Rydberg-atom pairs. In a second experiment, we investigate the evolution of collective Rydberg excitations by adding a time delay between the excitation pulse and the read-out sequence. The observed time dependence of the pair-correlation signal reflects the van-der-Waals forces between the Rydberg atoms.   

Non-equilibrium dynamics and magnetic correlations with atomic chains in optical lattices

We study the non-equilibrium dynamics of one dimensional chains of ultracold atoms in an optical lattice. By using tilted lattices or excitations to Rydberg levels, it is possible to generate effective spin models, which can produce interesting ordered states either in coherent dynamics or through dissipative driving. In the case of a tilted lattice, the spin models are based on the location of the atoms relative to an initial Mott Insulator state for Bosons, and interactions are induced by tunnelling. Using analytical techniques and time-dependent density matrix renormalization group methods, we study the dynamics of these systems in 1D, both when the tilt is varied adiabatically, and after a sudden quench. We especially consider how magnetic correlations and buildup of entanglement change as we manipulate the system with extra elements, including phonon- and photon-dressed Bloch Bands.   

Creation and characterization of a Rydberg excited superatom

We have prepared and studied a single superatom consisting of a mesoscopic ultracold atomic sample with several hundred atoms. The sample is excited to a collective Rydberg state and probed by photoionization. For resonant excitation the created blockade results in an anti-bunched ion emission. We determine an effective blockade radius and demonstrate the saturation of the superatom. The rich internal level structure of the superatom can be further exploited to create multiple excitations for an off-resonant driving. The resulting ion signal shows strong bunching with record values up to $g^{(2)}(t=0)=60$. Varying the coupling strength and the detuning, we observe a significant change in the excitation dynamics indicating a transition between a regime of saturated single and fluctuating pair excitations. Our experiment represents the first realization of an isolated superatom and opens new possibilities for quantum optical experiments with Rydberg blockaded samples.   

A Controlled-Phase Gate via Adiabatic Rydberg Dressing of Neutral Atoms

The dipole blockade effect between Rydberg atoms is a promising tool for quantum information processing in neutral atoms. So far, most efforts to perform a quantum logic gate with this effect have used resonant laser pulses to excite the atoms, which makes the system particularly susceptible to decoherence through thermal motional effects. We explore an alternative scheme in which the atomic ground states are adiabatically ``dressed'' by turning on an off-resonant laser. We analyze the implementation of a CPHASE gate using this mechanism and find that fidelities of $>$99\% should be possible with current technology, owing primarily to the suppression of motional errors. We also discuss how such a scheme could be generalized to perform more complicated, multi-qubit gates; in particular, a simple generalization would allow us to perform a Toffoli gate in a single step.   

Towards a controlled-phase gate using Rydberg-dressed atoms

We are implementing a controlled-phase gate based on singly trapped neutral atoms whose coupling is mediated by the dipole-dipole interaction of Rydberg states. An off-resonant laser field dresses ground state cesium atoms in a manner conditional on the Rydberg blockade mechanism [1,2], providing the required entangling interaction. We will present our progress [3] toward implementing the controlled-phase gate with an analysis of possible sources of decoherence such as RF radiation from wireless communication devices. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. \\[4pt] [1] S. Rolston, et al. Phys. Rev. A, 82, 033412 (2010)\\[0pt] [2] T. Keating, et al. Phys. Rev. A, 87, 052314 (2013)\\[0pt] [3] A. Hankin, et al. arXiv:1401.2191   

Atomic Fock State Preparation and Rydberg Dynamics

We present a method for preparing atomic ensembles in an optical lattice with sub-Poissonian number fluctuations using Rydberg blockade. Experimental results demonstrating preparation of N=1, 2 atom Fock states are shown, along with observation of coherent dynamics of ensemble qubits in a Rydberg blockaded ensemble. This work is supported by the NSF and the AFOSR Quantum Memories MURI.   

Spin squeezing and supersolids using Rydberg-dressed strontium atoms

Coherent excitation of cold atoms to Rydberg states provides a new platform for quantum many-body physics. We present new perspectives provided by divalent atoms such as strontium. We show that laser excitation of the second valence electron enables spatially, resolved state-selective detection of Rydberg atoms with single-atom sensitivity.\footnote{J. Millen et al. Phys. Rev. Lett. {\bf 105} 213004 (2010); G.Lochead et al., Phys. Rev. A {\bf 87} 053409 (2013)} Narrow intercombination lines enable two-photon excitation to the Rydberg state with low decoherence, providing an ideal system to investigate ``Rydberg dressing''. Here, a strong, off-resonant coupling to the Rydberg state introduces a new tunable, soft-core interaction between the atoms, with potential for the formation of a Rydberg supersolid phase.\footnote{N. Henkel et al., Phys. Rev. Lett. {\bf 104} 195302 (2010)} With the MPIPKS Dresden we show that applying this dressed interaction to strontium lattice clocks can also lead to the generation of significant squeezing that could be used to improve the signal-to-noise ratio.\footnote{L. Gil et al., {arXiv:1306.6240} (2013) accepted for Phys. Rev. Lett.} We present our experiments seeking to observe Rydberg dressing via intercombination lines in ultracold Sr atoms.   

Mapping the dipole-dipole interaction among ultracold Rydberg atoms

A long-range dipole-dipole interaction couples the atoms in an ultracold Rydberg gas. This can lead to changes in the spatial configuration of states over an extended region. We discuss the use of selective field ionization with a spatially sensitive ion detector to directly map dipole-dipole interaction induced level shifts and energy exchange over large distances in a MOT. Experimental and simulation results will be presented.