Through frequency up-conversion of polarization-shaped, femtosecond laser pulses nonlinearly interacting with xenon atoms, energetic, broadband, coherent, XUV continuum radiation is generated. By exploiting the thus-formed short-duration XUV pulses, all the optically allowed excited states of H${}_2$ are coherently populated. Nuclear and electronic 1-fs-scale dynamics are subsequently investigated by means of XUV-pump–XUV-probe measurements, which are compared to the results of ab initio calculations. The revealed dynamics reflects the intrinsic molecular behavior, as the XUV probe pulse hardly distorts the molecular potential.

We study a model describing $N$ identical bosonic atoms trapped in a double-well potential together with a single-impurity atom, comparing and contrasting it throughout with the Dicke model. As the boson-impurity coupling strength is varied, there is a symmetry-breaking pitchfork bifurcation which is analogous to the quantum phase transition occurring in the Dicke model. Through stability analysis around the bifurcation point, we show that the critical value of the coupling strength has the same dependence on the parameters as the critical coupling value in the Dicke model. We also show that, like the Dicke model, the mean-field dynamics goes from being regular to chaotic above the bifurcation and macroscopic excitations of the bosons are observed. Although the boson-impurity system behaves like a poor man's version of the Dicke model, we demonstrate a self-trapping phenomenon which thus far has not been discussed in the realm of the Dicke model.

The practical construction of scalable quantum-computer hardware capable of executing nontrivial quantum algorithms will require the juxtaposition of different types of quantum systems. We analyze a modular ion trap quantum-computer architecture with a hierarchy of interactions that can scale to very large numbers of qubits. Local entangling quantum gates between qubit memories within a single register are accomplished using natural interactions between the qubits, and entanglement between separate registers is completed via a probabilistic photonic interface between qubits in different registers, even over large distances. We show that this architecture can be made fault tolerant, and demonstrate its viability for fault-tolerant execution of modest size quantum circuits.

There is now a significant body of results on quantum interactions with closed timelike curves (CTCs) in the quantum information literature, for both the Deutsch model of CTC interactions (D-CTCs) and the projective model (P-CTCs). As a consequence, there is a prima facie argument exploiting entanglement that CTC interactions would enable superluminal and, indeed, effectively instantaneous signaling. In cases of spacelike separation between the sender of a signal and the receiver, whether a receiver measures the local part of an entangled state or a disentangled state to access the signal can depend on the reference frame. We propose a consistency condition that gives priority to either an entangled perspective or a disentangled perspective in spacelike-separated scenarios. For D-CTC interactions, the consistency condition gives priority to frames of reference in which the state is disentangled, while for P-CTC interactions the condition selects the entangled state. Using the consistency condition, we show that there is a procedure that allows Alice to signal to Bob in the past via relayed superluminal communications between spacelike-separated Alice and Clio, and spacelike-separated Clio and Bob. This opens the door to time travel paradoxes in the classical domain. Ralph [T. C. Ralph, arXiv:1107.4675 [quant-ph].] first pointed this out for P-CTCs, but we show that Ralph's procedure for a “radio to the past” is flawed. Since both D-CTCs and P-CTCs allow classical information to be sent around a spacetime loop, it follows from a result by Aaronson and Watrous [S. Aaronson and J. Watrous, Proc. R. Soc. A 465, 631 (2009)] for CTC-enhanced classical computation that a quantum computer with access to P-CTCs would have the power of PSPACE, equivalent to a D-CTC-enhanced quantum computer.

When a harmonic oscillator is under the influence of a Gaussian process such as linear damping, parametric gain, and linear coupling to a thermal environment, its coherent states are transformed into states with Gaussian Wigner function. Qubit states can be encoded in the $|0\rangle$ and $|1\rangle$ Fock states of a quantum harmonic oscillator, and it is relevant to know the fidelity of the output qubit state after a Gaussian process on the oscillator. In this paper we present a general expression for the average qubit fidelity in terms of the first and second moments of the output from input coherent states subjected to Gaussian processes.

A two-step, two-color laser spectroscopy technique has been used to measure the hyperfine splitting of the $7p_{1/2}$ excited state in ${}^{203}$Tl and ${}^{205}$Tl, as well as the isotope shift within the $7s_{1/2}$-$7p_{1/2}$ transition. Our measured values for the hyperfine splittings, 2153.2(7) MHz (in ${}^{203}$Tl) and 2173.3(8) MHz (in ${}^{205}$Tl), each differ by 20 MHz from previously published values which quoted comparable precision. The transition isotope shift of ${}^{203}$Tl relative to ${}^{205}$Tl was measured to be 534.4(9) MHz. In our experiment, one laser was locked to the thallium $6p_{1/2}$-$7s_{1/2}$ 378-nm transition, while a second, spatially overlapping laser was scanned across the $7s_{1/2}(F=1)$-$7p_{1/2}(F=0,1)$ infrared transitions. To facilitate accurate frequency calibration, radio-frequency modulation of the laser was used to create sidebands in the absorption spectrum.

We present a paradigm for capturing the complementarity of two observables. It is based on the entanglement created by the interaction between the system observed and the two measurement devices used to measure the observables sequentially. Our main result is a lower bound on this entanglement and resembles well-known entropic uncertainty relations. Besides its fundamental interest, this result directly bounds the effectiveness of sequential bipartite operations—corresponding to the measurement interactions—for entanglement generation. We further discuss the intimate connection of our result with two primitives of information processing, namely, decoupling and coherent teleportation.

We analyze three different post-processing methods applied to a single-shot qubit readout: the average-signal (boxcar filter), peak-signal, and maximum-likelihood methods. In contrast to previous work, we account for a stochastic turn-on time $t_i$ associated with the leading edge of a pulse signaling one of the qubit states. This model is relevant to spin-qubit readouts based on spin-to-charge conversion and would be generically reached in the limit of large signal-to-noise ratio $r$ for several other physical systems, including fluorescence-based readouts of ion-trap qubits and nitrogen-vacancy center spins. We derive analytical closed-form expressions for the conditional probability distributions associated with the peak-signal and boxcar filters. For the boxcar filter, we find an asymptotic scaling of the single-shot error rate $\varepsilon \sim lnr/\sqrt{r}$ when $t_i$ is stochastic, in contrast to the result $\varepsilon \sim lnr/r$ for deterministic $t_i$. Consequently, the peak-signal method outperforms the boxcar filter significantly when $t_i$ is stochastic, but is only marginally better for deterministic $t_i$ (a result that is consistent with the widespread use of the boxcar filter for fluorescence-based readouts and the peak signal for spin-to-charge conversion). We generalize the theoretically optimal maximum-likelihood method to stochastic $t_i$ and show numerically that a stochastic turn-on time $t_i$ will always result in a larger single-shot error rate. Based on this observation, we propose a general strategy to improve the quality of single-shot readouts by forcing $t_i$ to be deterministic.

We calculate the isotope shift of $2{}^1{P}_0-2{}^1{S}_0$, $3{}^1{S}_0-2{}^1{S}_0$ transitions and of the $2{}^1{S}_0$ ionization potential in the four-electron beryllium atom. The achieved precision is high enough to make possible the accurate determination of the nuclear charge radii from the experimental isotope shifts. This calls for corresponding measurements and opens the window for the determination of charge radii of heavier nuclei.

We present kinematically complete measurements of the photo-double-ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double-ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photoelectrons and the nuclear kinetic-energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels. The reason for such a striking difference is explained in part on the basis of a propensity rule that influences the population of states in the photo-double-ionization of a centrosymmetric closed-shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential-energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

We investigate a hybrid electromechanical system consisting of a pair of charged macroscopic mechanical oscillators coupled to a small ensemble of Rydberg atoms. The resonant dipole-dipole coupling between an internal atomic Rydberg transition and the mechanics allows cooling to its motional ground state with a single atom despite the considerable mass imbalance between the two subsystems. We show that the rich electronic spectrum of Rydberg atoms, combined with their high degree of optical control, paves the way towards implementing various quantum-control protocols for the mechanical oscillators.

We theoretically study a superradiant laser, deriving both the steady-state behaviors and small-amplitude responses of the laser's atomic inversion, atomic polarization, and light field amplitude. Our minimum model for a three-level laser includes atomic population accumulating outside of the lasing transition and dynamics of the atomic population distribution causing cavity frequency tuning, as can occur in realistic experimental systems. We show that the population dynamics can act as real-time feedback to stabilize or destabilize the laser's output power, and we derive the cavity frequency tuning for a Raman laser. We extend the minimal model to describe a cold-atom Raman laser using ${}^{87}$Rb, showing that the minimal model qualitatively captures the essential features of the more complex system [Bohnet et al., Phys. Rev. Lett. 109, 253602 (2012)]. This work informs our understanding of the stability of proposed millihertz linewidth lasers based on ultranarrow optical atomic transitions and will guide the design and development of these next-generation optical frequency references.

The proton radius conundrum [Pohl et al., Nature 466, 213 (2010) and , Science 339, 417 (2013)] highlights the need to revisit any conceivable sources of electron-muon nonuniversality in lepton-proton interactions within the standard model. Superficially, a number of perturbative processes could appear to lead to such a nonuniversality. One of these is a coupling of the scattered electron into an electronic vacuum-polarization loop as opposed to a muonic one in the photon exchange of two valence quarks, which is present only for electron projectiles as opposed to muon projectiles. However, we show that this effect actually is part of the radiative correction to the proton's polarizability contribution to the Lamb shift, equivalent to a radiative correction to double scattering. We conclude that any conceivable genuine nonuniversality must be connected with a nonperturbative feature of the proton's structure, e.g., with the possible presence of light sea fermions as constituent components of the proton. If we assume an average of roughly $0.7\times {10}^{-7}$ light sea positrons per valence quark, then we can show that virtual electron-positron annihilation processes lead to an extra term in the electron-proton versus muon-proton interaction, which has the right sign and magnitude to explain the proton radius discrepancy.

Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is analyzed theoretically. This theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated numerically in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. The scattering patterns and their temporal behaviors are shown to differentiate the two kinds of target electronic motion. Moreover, our simulations show that inelastic processes contribute significantly to the diffraction patterns. Thus, although the diffraction patterns clearly distinguish different modes of target electronic motion, they cannot be easily related to the time-dependent target charge density.

We use direct spatial ion imaging of cold ${}^{85}$Rb Rydberg atom clouds to measure the Rydberg-Rydberg correlation function, with and without light-shift potentials generated by an optical dipole trap. We find that the blockade radius depends on laser detunings and spatially varying light shifts. At certain laser detunings the probability of exciting Rydberg atoms at particular separations is enhanced, which we interpret to be a result of direct two-photon excitation of Rydberg atom pairs. The results are in accordance with predictions [F. Robicheaux and J. V. Hernández, Phys. Rev. A 72, 063403 (2005)] and a model we develop that accounts for a one-dimensional dipole-trap potential.

We show that there exist sets of three mutually orthogonal $d$-dimensional maximally entangled states which cannot be perfectly distinguished using one-way local operations and classical communication (LOCC) for arbitrarily large values of $d$. This contrasts with several well-known families of maximally entangled states, for which any three states can be perfectly distinguished. We then show that two-way LOCC is sufficient to distinguish these examples. We also show that any three mutually orthogonal $d$-dimensional maximally entangled states can be perfectly distinguished using measurements with a positive partial transpose (PPT) and can be distinguished with one-way LOCC with high probability. These results circle around the question of whether there exist three maximally entangled states which cannot be distinguished using the full power of LOCC; we discuss possible approaches to answer this question.