We present a novel system for the simulation of quantum phase transitions of collective internal qubit and phononic states with a linear crystal of trapped ions. The laser-ion interaction creates an energy gap in the excitation spectrum, which induces an effective phonon-phonon repulsion and a Jaynes-Cummings-Hubbard interaction. This system shows features equivalent to phase transitions of polaritons in coupled cavity arrays. Trapped ions allow for easy tunabilty of the hopping frequency by adjusting the axial trapping frequency, and the phonon-phonon repulsion via the laser detuning and intensity. We propose an experimental protocol to access all observables of the system, which allows one to obtain signatures of the quantum phase transitions even with a small number of ions.Comment: 4 pages, 3 figure

A vital requirement for a quantum computer is the ability to locally address, with high fidelity, any of its qubits without affecting their neighbors. We propose an addressing method using composite sequences of laser pulses, which reduces dramatically the addressing error in a lattice of closely spaced atoms or ions, and at the same time significantly enhances the robustness of qubit manipulations. To this end, we design novel high-fidelity composite pulses for the most important single-qubit operations. In principle, this method allows one to beat the diffraction limit, for only atoms situated in a small spatial region around the center of the laser beam are excited, well within the laser beam waist. c 2017 Optical Society of America Scalable quantum computers depend critically on the ability to perform local addressing of their individual qubits [1]. In a Paul ion trap, which is one of the most promising scalable platforms for the future quantum computer [2], local addressing is the ability to operate on a single ion by focused laser light while keeping the neighboring ions unaffected. When the number of ions increases, the distance between them diminishes and local addressing becomes one of the principal experimental challenges. For example, in a recent experimental demonstration of the Toffoli gate [3] most of the error was attributed to addressing error, as the neighboring ions were seeing 7% of the central Rabi frequency.In this Letter, we propose a method for high-fidelity local addressing applicable to various types of atomic qubits: trapped ions, atoms in optical lattices, quantum dots, etc. To this end, we present new narrowband (NB) and passband (PB) composite pulses 1 , which are specially designed for local addressing. The excitation profiles of such pulses allow one to manipulate only a single qubit, as the outer parts of the spatial laser beam profile practically do not excite its neighbors, although the latter may be subjected to significant laser intensity. Moreover, with a PB pulse one enhances the robustness of qubit manipulations, thereby eliminating errors due to imperfectly calibrated and fluctuating laser intensity and laser beam pointing instability.The technique of composite pulses was introduced in nuclear magnetic resonance (NMR) [4-6] as a powerful tool for manipulation of spins by magnetic fields. A composite pulse compensates the imperfections of a single pulse, which is the traditional tool used to drive a quantum transition, and it consists of a sequence of pulses, each with a well-defined phase. The composite phases are determined from the conditions imposed on the desired overall excitation profile. In particular, in a NB composite pulse only the qubits seeing pulse areas within a narrow range around some value A are subjected to trans-1 We follow the usual NMR terminology, which is related to the features of the excitation profile, rather than the radiation.formation, while qubits seeing areas outside this range remain unaffected in the end of the composite sequence.Most known...

We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f , 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2+1+1 Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly driven two-photon transition 4s ←← 3s, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3+1 REMPI via the states 5f and 6f . In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark-shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state "bow-tie" level crossing allows the preparation of coherent superposition states.

Generating entanglement by simply cooling a system into a stationary state which is highly entangled has many advantages. Schemes based on this idea are robust against parameter fluctuations, tolerate relatively large spontaneous decay rates, and achieve high fidelities independent of their initial state. A possible implementation of this idea in atom-cavity systems has recently been proposed by Kastoryano et al. [Phys. Rev. Lett. 106, 090502 (2011)]. Here we propose an improved entanglement cooling scheme for two atoms inside an optical cavity which achieves higher fidelities for comparable single-atom cooperativity parameters C. For example, we predict fidelities above 90% even for C as low as 20 without requiring individual laser addressing and without having to detect photons.Comment: 12 pages, 11 figure