We propose a new system for implementing quantum logic gates: neutral atoms trapped in a very far-off-resonance optical lattice. Pairs of atoms are made to occupy the same well by varying the polarization of the trapping lasers, and then a near-resonant electric dipole is induced by an auxiliary laser. A controlled-NOT can be implemented by conditioning the target atomic resonance on a resolvable level shift induced by the control atom. Atoms interact only during logical operations, thereby suppressing decoherence.Comment: Revised version, To appear in Phys. Rev. Lett. Three separate postscript figure

Controlling the quantum entanglement between parts of a many-body system is key to unlocking the power of quantum technologies such as quantum computation, high-precision sensing, and the simulation of many-body physics. The spin degrees of freedom of ultracold neutral atoms in their ground electronic state provide a natural platform for such applications thanks to their long coherence times and the ability to control them with magneto-optical fields. However, the creation of strong coherent coupling between spins has been challenging. Here we demonstrate a strong and tunable Rydberg-dressed interaction between spins of individually trapped caesium atoms with energy shifts of order 1 MHz in units of Planck's constant. This interaction leads to a ground-state spin-flip blockade, whereby simultaneous hyperfine spin flips of two atoms are inhibited owing to their mutual interaction. We employ this spin-flip blockade to rapidly produce single-step Bell-state entanglement between two atoms with a fidelity ≥81(2)%. P ristine quantum control of many-body correlations is fundamental to realizing the power of quantum information processors. Steady progress has continued in various platforms ranging from solid-state spintronics 1 and superconductors 2,3 to nanophotonics 4 and ultracold trapped atoms, both ionic 5-7 and neutral 8-10 . Cold neutral atoms are particularly attractive as the ability to create entanglement between atoms would allow for greatly increased precision of interferometers for applications in clocks 11-13 , and force sensors 14-16 . In addition, cold atoms provide a natural platform for quantum simulation of condensed-matter physics 17,18 and scalable digital quantum computers 19-21 . Controlled entanglement of neutral atoms, however, has been challenging, particularly if one seeks tunable interactions that are strong, coherent and long-range (∼µm).One mechanism to achieve strong, long-range coupling is the Rydberg blockade 22 . This has been successfully employed for implementing controlled entangling interactions between atoms 9,10,23 and quantum logic gates 24 . In the standard protocol, short pulses excite the population of one atom to the Rydberg state and optical excitation of a second atom is blockaded because of the electric dipole-dipole interaction 21 (EDDI). An alternative protocol is to adiabatically dress the ground state with the excited Rydberg state 25-27 . This Rydberg-dressed interaction enables tunable, anisotropic interactions that open the door to quantum simulations of a variety of exotic quantum phases 26,28,29 . In addition, it allows for quantum control of interacting atoms based solely on microwave/radiofrequency fields whose phase coherence is easily maintained. Applications include spin-squeezing for metrology 13,25 , and quantum computing 30,31 . Although the promise of Rydbergdressed interactions is great, experimental demonstration has been elusive. We present here a clear measurement of this interaction between two Rydberg-dressed atoms and employ coherent control in the ...

Various fundamental phenomena of strongly correlated quantum systems such as high-T(c) superconductivity, the fractional quantum-Hall effect and quark confinement are still awaiting a universally accepted explanation. The main obstacle is the computational complexity of solving even the most simplified theoretical models which are designed to capture the relevant quantum correlations of the many-body system of interest. In his seminal 1982 paper (Feynman 1982 Int. J. Theor. Phys. 21 467), Richard Feynman suggested that such models might be solved by 'simulation' with a new type of computer whose constituent parts are effectively governed by a desired quantum many-body dynamics. Measurements on this engineered machine, now known as a 'quantum simulator,' would reveal some unknown or difficult to compute properties of a model of interest. We argue that a useful quantum simulator must satisfy four conditions: relevance, controllability, reliability and efficiency. We review the current state of the art of digital and analog quantum simulators. Whereas so far the majority of the focus, both theoretically and experimentally, has been on controllability of relevant models, we emphasize here the need for a careful analysis of reliability and efficiency in the presence of imperfections. We discuss how disorder and noise can impact these conditions, and illustrate our concerns with novel numerical simulations of a paradigmatic example: a disordered quantum spin chain governed by the Ising model in a transverse magnetic field. We find that disorder can decrease the reliability of an analog quantum simulator of this model, although large errors in local observables are introduced only for strong levels of disorder. We conclude that the answer to the question 'Can we trust quantum simulators?' is … to some extent.

We study the means to prepare and coherently manipulate atomic wave packets in optical lattices, with particular emphasis on alkali atoms in the far-detuned limit. We derive a general, basis independent expression for the lattice operator, and show that its off-diagonal elements can be tailored to couple the vibrational manifolds of separate magnetic sublevels. Using these couplings one can evolve the state of a trapped atom in a quantum coherent fashion, and prepare pure quantum states by resolved-sideband Raman cooling. We explore the use of atoms bound in optical lattices to study quantum tunneling and the generation of macroscopic superposition states in a double-well potential. Far-off-resonance optical potentials lend themselves particularly well to reservoir engineering via well controlled fluctuations in the potential, making the atom/lattice system attractive for the study of decoherence and the connection between classical and quantum physics.