Universal quantum computation can be achieved by simply performing single-qubit measurements on a highly entangled resource state. Resource states can arise from ground states of carefully designed two-body interacting Hamiltonians. This opens up an appealing possibility of creating them by cooling. The family of Affleck-Kennedy-Lieb-Tasaki (AKLT) states are the ground states of particularly simple Hamiltonians with high symmetry, and their potential use in quantum computation gives rise to a new research direction. Expanding on our prior work [T.-C. Wei, I. Affleck, and R. Raussendorf, Phys. Rev. Lett. 106, 070501 (2011)], we give a detailed analysis to explain why the spin-3/2 AKLT state on a two-dimensional honeycomb lattice is a universal resource for measurement-based quantum computation. Along the way, we also provide an alternative proof that the 1D spin-1 AKLT state can be used to simulate arbitrary one-qubit unitary gates. Moreover, we connect the quantum computational universality of 2D random graph states to their percolation property and show that these states whose graphs are in the supercritical (i.e., percolated) phase are also universal resources for measurement-based quantum computation.

Linear Paul traps have been used recently to simulate the transverse-field Ising model with long-range spin-spin couplings. We study the intrinsic effects of phonon creation (from the initial phonon ground state) on the spin-state probability and spin entanglement for such quantum spin simulators. While it has often been assumed that phonon effects are benign because they play no role in the pure Ising model, they can play a significant role when a transverse field is added to the model. We use a many-body factorization of the quantum time-evolution operator of the system, adiabatic perturbation theory, and exact numerical integration of the Schrödinger equation in a truncated spin-phonon Hilbert space followed by a tracing-out of the phonon degrees of freedom to study this problem. We find that moderate phonon creation often makes the probabilities of different spin states behave differently from the static spin Hamiltonian. In circumstances in which phonon creation is minor, the spin dynamics state probabilities converge to the static spin Hamiltonian prediction at the cost of reducing the spin entanglement. We show how phonon creation can severely impede the observation of kink transitions in frustrated spin systems when the number of ions increases. Many of our results also have implications for quantum simulation in a Penning trap.

We develop a general approach for deriving the energy minima of biseparable states in chains of arbitrary spins $s$, and we report numerical results for spin values $s\le 5/2$ (with $N\le 8$). The minima provide a set of threshold values for exchange energy that allow us to detect different degrees of multipartite entanglement in one-dimensional spin systems. We finally demonstrate that the Heisenberg exchange Hamiltonian of $N$ spins has a nondegenerate $N$-partite entangled ground state, and it can thus witness such correlations in all finite spin chains.

We study single collisions between orthopositronium beams and targets with unpaired electrons, in which the conversion (quenching) of the long-lived orthopositronium into the short-lived parapositronium occurs as a result of electron exchange. We consider unpolarized and polarized targets with one and two unpaired electrons. Angular-momentum coupling and density-matrix techniques are used to calculate the probabilities of obtaining either para- or orthopositronium after the collisions. The probability of the initial orthopositronium being converted to parapositronium (quenching probability) is given in terms of complex scattering amplitudes labeled with total electron spin. Quenching probabilities are calculated for polarized and unpolarized targets, with and without detection of the spins of the targets after the collisions. Ratios of the probabilities of producing parapositronium to that of obtaining orthopositronium are given in terms of electron-spin amplitudes.

The electronic dephasing of benzene on the second excited state has been observed using a femtosecond time-resolved photoelectron imaging technique coupled with time-resolved mass spectroscopy. Upon absorption of two 400-nm photons, benzene is excited to the $S_2$ state, and the time evolution of the parent ion can be well described by a biexponential decay: a rapid dephasing pathway with 43 fs and a longer-lived channel with 1.06 ps. The fast dephasing is ascribed to the internal conversion through the $S_2-S_1$ conical intersections, and the slower one is attributed to the intersystem crossing from the $S_2$ state. It is possible that the vibrational excitation modes (${9^1}_0{{16}^1}_1,{5^1}_0$, and ${9^1}_0$) from the $S_2$ state enhance the spin-orbit coupling greatly, which leads to the $S_2-T_3$ intersystem crossing. The photoelectron kinetic energy distributions between 1.1 and 1.7 eV exhibit a rapid energy transfer, revealing nuclear motion on the $S_2$ adiabatic surface.

We present a complex-scaling (CS) generalized pseudospectral (GPS) method in hyperspherical coordinates (HSC) for $\mathit{ab}$ initio and accurate treatment of the resonance energies and autoionization widths of two-electron atomic systems in the presence of a strong dc electric field. The GPS method allows nonuniform and optimal spatial discretization of the two-electron Hamiltonian in HSC with the use of only a modest number of grid points. The procedure is applied for the first precision calculation of the energies and autoionization widths for the high-lying ${}^1{S}^e$, ${}^1{P}^o$, ${}^1{D}^e$, and ${}^1{F}^o$ ($n=10$–20) doubly excited resonance states of He atoms. In addition, we present a theoretical prediction of the energies and widths of high-lying doubly excited resonance states of ${}^1{P}^o$ ($n=$ 8–15) in external dc electric field strengths of 3.915–10.44 kV/cm. The calculated dc-field perturbed high-lying resonance energies are in good agreement with the latest experimental data.

It is demonstrated that the effect of a static cage potential on the photoionization of endohedrally enclosed atoms can be incorporated into standard $R$-matrix calculations using one of two independent methods. For photoionization processes occurring entirely within the fullerene, the outer-region solutions can be modified by the additional cage potential to yield phase-shifted Coulomb functions that are matched to the inner-region $R$ matrix. Alternatively, if the cage potential is contained within the $R$-matrix “box,” it can be directly incorporated into the formalism via simple one-electron integral contributions to the Hamiltonian, yielding a modified $R$ matrix. Both methods are applied to the photoionization of Xe@C${}_{60}$ in the vicinity of the giant $4d\to \epsilon f$ shape resonance and are found to be in excellent agreement with each other.

We present time-resolved ab initio study of the attosecond dynamics of electron-electron correlation during single-electron, single XUV-photon ionization of an excited helium atom. We identify multiple time scales in these dynamics, caused by the interplay of the initial shake-up excitation with the excitation cascades induced by the postionization interaction of the outgoing electron with the core. The time scales and the structure of these excitation cascades are directly linked to the time it takes the “active” continuum electron, lifted from a deeper bound orbital, to traverse the orbit of the “passive” electron residing in the outer orbit.

We propose a device whose reflectivity can be controlled optically over a wide frequency band. To this objective, we propose a Fabry-Pérot cavity filled with a three-level atomic gas such that the effective susceptibility can be adjusted through incoherent pump and coherent driving fields. Adopting the concepts of electromagnetic-induced transparency and the white light cavity, we can control the susceptibility to break or satisfy the resonance condition over a wide frequency range, resulting in a wide-band optical switch. In addition to numerical calculation, we first give the analytical parameters, such as pump rate, and the Rabi frequency of the driven field, required for white light cavity in this paper. Using such a cavity as a wall of another cavity, we can, optically, realize a wide-band $Q$-switched cavity. Such a device can also be used as a new type of optical switch for quantum information.

Understanding what can be inferred about a multiparticle quantum system given only the knowledge of its subparts is a highly nontrivial task. Clearly, if a global system does not contain an information resource of some kind, neither do its subparts. For the case of entanglement as an information resource, it is known that the converse of this last statement is not true: Some nonentangled reduced states are only compatible with global states which are entangled. We extend this result to correlations and provide local marginal correlations that are only compatible with global genuinely tripartite nonlocal correlations. Quantum nonlocality can thus be deduced from the mere observation of local marginal correlations.

The rate of electron-positron pair production in linearly polarized counterpropagating lasers is evaluated from a recently discovered solution of the time-dependent Dirac equation. The latter is solved in momentum space, where it is formally equivalent to the Schrödinger equation describing a strongly driven two-level system. The solution is found from a simple transformation of the Dirac equation and is given in compact form in terms of the doubly confluent Heun's function. By using the analogy with the two-level system, it is shown that for high intensity lasers pair production occurs through periodic nonadiabatic transitions when the adiabatic energy gap is minimal. These transitions give rise to an intricate interference pattern in the pair spectrum, reminiscent of the Landau-Zener-Stückelberg phenomenon in molecular physics: the accumulated phase result in constructive or destructive interference. The adiabatic-impulse model is used to study this phenomenon and shows an excellent agreement with the exact result.

The common spin Hamiltonians such as the Ising, $XY$, and Heisenberg models do not have eigenstates that are suitable resources for measurement-based quantum computation. Various highly entangled many-body states have been suggested as a universal resource for this type of computation; however, it is not easy to preserve these states in solid-state systems due to their short coherence times. To solve this problem, we propose a scheme for generating a Hamiltonian that has a cluster state as a ground state. Our approach employs a series of pulse sequences inspired by established NMR techniques and holds promise for applications in many areas of quantum information processing.

The multiple ionization of Ar-core–Ne-shell clusters in intense extreme-ultraviolet laser pulses ($\lambda \sim 62$ nm) from the free-electron laser in Japan was investigated utilizing a momentum imaging technique. The Ar composition dependence of the kinetic energies and the yields of the fragment ions give evidence for charge transfer from the Ar core to the Ne shell. We have extended the uniformly charged sphere model originally applied to pristine clusters [Islam et al., Phys. Rev. A 73, 041201(R) (2006)] to the core-shell heterogeneous clusters to estimate the amounts of charge and energy transfers.

We study the effects of field fluctuations on the total yields of Auger electrons, obtained in the excitation of neutral atoms to a core-excited state by means of short-wavelength free-electron-laser pulses. Beginning with a self-contained analysis of the statistical properties of fluctuating free-electron-laser pulses, we analyze separately and in detail the cases of single and double Auger resonances, focusing on fundamental phenomena such as power broadening and ac Stark (Autler-Townes) splitting. In certain cases, field fluctuations are shown to influence dramatically the frequency response of the resonances, whereas in other cases the signal obtained may convey information about the bandwidth of the radiation as well as the dipole moment between Auger states.

We report the realization of a Bose-Einstein condensate of ${}^{39}$K atoms without the aid of an additional atomic coolant. Our route to Bose-Einstein condensation comprises sub-Doppler laser cooling of large atomic clouds with more than ${10}^{10}$ atoms and evaporative cooling in an optical dipole trap where the collisional cross section can be increased using magnetic Feshbach resonances. Large condensates with almost ${10}^6$ atoms can be produced in less than 15 s. Our achievements eliminate the need for sympathetic cooling with Rb atoms, which was the usual route implemented until now due to the unfavorable collisional property of ${}^{39}$K. Our findings simplify the experimental setup for producing Bose-Einstein condensates of ${}^{39}$K atoms with tunable interactions, which have a wide variety of promising applications, including atom interferometry to studies on the interplay of disorder and interactions in quantum gases.

The dependence of the number of Rydberg excitations in a laser-driven cloud of atoms on the interaction strength is found to be affected by the finite size of the system. A theoretical model predicting a simple power-law dependence is compared with results extracted from a numerical many-body simulation. We find that for finite-size systems the numerical results do not agree with the analytical prediction. By individually testing the assumptions leading to the theoretical prediction using the results from the numerical analysis, we identify the origin of the deviations, and explain it as arising from the finite size of the system. Furthermore, finite-size effects in the pair correlation function $g^{\left(2\right)}$ are predicted. Finally, in larger ensembles, we find that the theoretical predictions and the numerical results agree, provided that the system is sufficiently homogeneous.

The rate of spontaneous emission is known to depend on the environment of a emitter, and the enhancement of one-photon emission in a resonant cavity is known as the Purcell effect. Here we develop a theory of spontaneous two-photon emission for a general electromagnetic environment including inhomogeneous dispersive and absorptive media. This theory is used to evaluate the two-photon Purcell enhancement in the vicinity of metallic nanoparticles and demonstrates that the surface-plasmon resonances supported by these particles can increase the emission rate by more than two orders of magnitude. The control over two-photon Purcell enhancement given by tailored nanostructured environments could provide an emitter with any desired spectral response and may serve as an ultimate route for designing light sources with novel properties.

Multi-longitudinal-mode lasers have been believed to be good candidates as pump sources for optical frequency conversion. However, we present a semiclassical model for the frequency conversion of optical signals with a multimode pump laser, which shows that fluctuations of the instantaneous pump power limit the conversion efficiency. In an experiment, we up-converted a 1.55-$\mu$m optical signal in a periodically poled lithium niobate waveguide using a multi-longitudinal-mode laser and observed a maximum conversion efficiency of 68%, in good agreement with our theoretical model. Compared to single-mode pumping, multimode pumping is not a suitable technique for attaining stable near-unity-efficiency frequency conversion. However, our results could find application in the characterization of the spectral or temporal structure of multi-longitudinal-mode lasers.

This paper considers the extension of the non-Markovian stochastic approach for quantum open systems strongly coupled to a fermionic bath to the models in which the system operators commute with the fermionic bath. This technique can also be a useful tool for studying open quantum systems coupled to a spin-chain environment, which can be further transformed into an effective fermionic bath. We derive an exact stochastic Schrödinger equation (SSE), called the fermionic quantum state diffusion (QSD) equation, from the first principle by using the fermionic coherent state representation. The reduced density operator for the open system can be recovered from the stochastic average of the solutions to the QSD equation over the Grassmann-type noise. By employing the exact fermionic QSD equation, we can derive the corresponding exact master equation. The power of our approach is illustrated by the applications of our stochastic approach to several models of interest including the one-qubit dissipative model, the coupled two-qubit dissipative model, the quantum Brownian motion model, and the $N$-fermion model coupled to a fermionic bath. Different effects caused by the fermionic and bosonic baths on the dynamics of open systems are also discussed.

We study the entanglement properties of a quantum lattice-gas model for which we can find the exact ground state (of the Rokhsar-Kivelson type). The ground state can be expressed as a superposition of states, each of which is characterized by a particle configuration with nearest-neighbor exclusion. We show that the reduced density matrix of the model on a ladder is intimately related to the transfer matrix of the classical hard-square model. The entanglement spectra of the model on square and triangular ladders are critical when parameters are chosen so that the corresponding classical hard-square models are critical. A detailed analysis reveals that the critical theories for the entanglement Hamiltonians are $c<1$ minimal conformal field theories. We further show that the entanglement Hamiltonian for the triangular ladder is integrable despite the fact that the original quantum lattice-gas model is nonintegrable.