We investigate the simulation of Jahn-Teller models with two nondegenerate vibrational modes using a circuit QED architecture. Typical Jahn-Teller systems are anisotropic and require at least a two-frequency description. The proposed simulator consists of two superconducting lumped-element resonators interacting with a common flux qubit in the ultrastrong coupling regime. We translate the circuit QED model of the system to a two-frequency Jahn-Teller Hamiltonian and calculate its energy eigenvalues and the emission spectrum of the cavities. It is shown that the system can be systematically tuned to an effective single-mode Hamiltonian from the two-mode model by varying the coupling strength between the resonators. The flexibility in manipulating the parameters of the circuit QED simulator permits the isolation of the effective single-frequency and pure two-frequency effects in the spectral response of Jahn-Teller systems.

We study the dynamics of dephasing in a quantum two-level system by modeling both $1/f$ and high-frequency noise by random telegraph processes. Our approach is based on a so-called spin-fluctuator model in which a noisy environment is modeled by a large number of fluctuators. In the continuous limit we obtain an effective random process (ERP) that is described by a distribution function of the fluctuators. In a simplified model, we reduce the ERP to the two (slow and fast) ensembles of fluctuators. Using this model, we study decoherence in a superconducting flux qubit and we compare our theoretical results with the available experimental data. We demonstrate good agreement of our theoretical predictions with the experiments. Our approach can be applied to many quantum systems, such as biological complexes, semiconductors, superconducting, and spin qubits, where the effects of interaction with the environment are essential.

We investigate quantum contextuality of one-dimensional quantum harmonic oscillator. Through the violation of a noncontextuality inequality, any eigenstate of quantum harmonic oscillator and some typical continuous-variable states, such as the coherent state and the squeezed state, exhibit quantum contextuality. The state-independent demonstration of quantum contextuality in continuous-variable states is also demonstrated.

The “speed” of unitary quantum evolution was recently shown to be connected to entanglement in multipartite quantum systems. Here, we discuss a tighter version of the Mandelstam-Tamm uncertainty relation that depends on the Fisher information. The passage time is estimated by a lower bound that depends inversely proportional to the square root of the Fisher information. This leads to a better understanding of the origin of a fast quantum time-evolution of entangled states.

We theoretically evaluate establishing remote entanglement between distinguishable matter qubits through interference and detection of two emitted photons. The fidelity of the entanglement operation is analyzed as a function of the temporal- and frequency-mode matching between the photons emitted from each quantum memory. With a general analysis, we define limits on the absolute magnitudes of temporal- and frequency-mode mismatches in order to maintain entanglement fidelities greater than 99$\%$ with two-photon detection efficiencies greater than 90$\%$. We apply our analysis to several selected systems of quantum memories. Results indicate that high fidelities may be achieved in each system using current experimental techniques, while maintaining acceptable rates of entanglement. Thus, it might be possible to use two-photon-mediated entanglement operations between distinguishable quantum memories to establish a network for quantum communication and distributed quantum computation.

We consider a sequential implementation of the optimal quantum cloning machine of Gisin and Massar and propose optimization protocols for experimental realization of such a quantum cloner subject to the real-life restrictions. We demonstrate how exploiting the matrix-product state (MPS) formalism and the ensuing variational optimization techniques reveals the intriguing algebraic structure of the Gisin-Massar output of the cloning procedure and brings about significant improvements to the optimality of the sequential cloning prescription of Delgado et al. [Phys. Rev. Lett. 98, 150502 (2007)]. Our numerical results show that the orthodox paradigm of optimal quantum cloning can in practice be realized in a much more economical manner by utilizing a considerably lesser amount of informational and numerical resources than hitherto estimated. Instead of the previously predicted linear scaling of the required ancilla dimension $D$ with the number of qubits $n$, our recipe allows a realization of such a sequential cloning setup with an experimentally manageable ancilla of dimension at most $D=3$ up to $n=15$ qubits. We also address satisfactorily the possibility of providing an optimal range of sequential ancilla-qubit interactions for optimal cloning of arbitrary states under realistic experimental circumstances when only a restricted class of such bipartite interactions can be engineered in practice.

The loss of quantum information due to interactions with external degrees of freedom, which is known as decoherence, remains one of the main obstacles for large-scale implementations of quantum computing. Accordingly, different measures are being explored for reducing its effect. One of them is dynamical decoupling (DD) which offers a practical solution because it only requires the application of control pulses to the system qubits. Starting from basic DD sequences, more sophisticated schemes were developed that eliminate higher-order terms of the system-environment interaction and are also more robust against experimental imperfections. A particularly successful scheme, called concatenated DD (CDD), gives a recipe for generating higher-order sequences by inserting lower-order sequences into the delays of a generating sequence. Here, we show how this scheme can be improved further by converting some of the pulses to virtual (and thus ideal) pulses. The resulting scheme, called ${\left(XY4\right)}^n$, results in lower power deposition and is more robust against pulse imperfections than the original CDD scheme.

We describe how continuous-variable Abelian anyons, created on the surface of a continuous-variable analog of Kitaev's lattice model can be utilized for quantum computation. In particular, we derive protocols for the implementation of quantum gates using topological operations. We find that the topological operations alone are insufficient for universal quantum computation, which leads us to study additional nontopological operations such as offline squeezing and single-mode measurements. It is shown that these in conjunction with a non-Gaussian element allow for universal quantum computation using continuous-variable Abelian anyons.

The cross-Kerr nonlinearity (XKNL) effect can induce efficient photon interactions in principle with which photonic multiqubit gates can be performed using far fewer physical resources than linear optical schemes. Unfortunately, it is extremely challenging to generate giant cross-Kerr nonlinearities. In recent years much effort has been made to perform multiqubit gates via weak XKNLs. However, the required nonlinearity strengths are still difficult to achieve in an experiment. We here propose an XKNL-based scheme for realizing a two-photon polarization-parity gate, a universal two-qubit gate, in which the required strength of the nonlinearity could be orders of magnitude weaker than those required for previous schemes. The scheme utilizes a ring cavity fed by a coherent state as a quantum information bus which interacts with a path mode of the two polarized photons (qubits). The XKNL effect makes the bus pick up a phase shift dependent on the photon number of the path mode. Even when the potential phase shifts are very small they can be effectively measured using photon-number resolving detectors, which accounts for the fact that our scheme can work in the regime of tiny XKNL. The measurement outcome reveals the parity (even parity or odd parity) of the two polarization qubits.

We use analytical and numerical calculations to obtain speed limits for various unitary quantum operations in multiqubit systems under typical experimental conditions. The operations that we consider include single-, two-, and three-qubit gates, as well as quantum-state transfer in a chain of qubits. We find in particular that simple methods for implementing two-qubit gates generally provide the fastest possible implementations of these gates. We also find that the three-qubit Toffoli gate time varies greatly depending on the type of interactions and the system's geometry, taking only slightly longer than a two-qubit controlled-not (cnot) gate for a triangle geometry. The speed limit for quantum-state transfer across a qubit chain is set by the maximum spin-wave speed in the chain.

We theoretically show that six-partite continuous-variable entanglement can be generated by nonlinear optical processes conducted in a designed photonic crystal-like periodically poled lithium niobat (PPLN). The processes consist of a nonlinear parametric down-conversion and four sum-frequency processes. The characteristics of the six-partite entanglement among the six beams are analyzed both above and below the threshold by applying a sufficient inseparability criterion for multipartite continuous-variable entanglement. We investigated our proposal for a specific case of a PPLN optical superlattice with fixed light and material properties.

The multipartite Greenberger-Horne-Zeilinger (GHZ) state is a paradigmatic example of a highly entangled multipartite state with distinct quantum features. However, the GHZ state is very sensitive to generic decoherence processes, where its quantum features and, in particular, its entanglement diminish rapidly, thereby hindering possible practical applications. In this paper, we discuss GHZ-like quantum states with a block-local structure and show that they exhibit a drastically increased stability against noise for certain choices of block-encoding, thereby extending results of [Phys. Rev. Lett. 106, 110402 (2011)]. We analyze in detail the decay of the interference terms, the entanglement in terms of distillable entanglement, and negativity as well as the notion of macroscopicity as measured by the so-called index $q$ and provide general bounds on these quantities. We focus on an encoding where logical qubits are themselves encoded as GHZ states, which leads to so-called concatenated GHZ (C-GHZ) states. We compare the stability of C-GHZ states with other types of encodings, thereby showing the superior stability of the C-GHZ states. Analytic results are complemented by numerical studies, where tensor network techniques are used to investigate the quantum properties of multipartite entangled states under the influence of decoherence.

In the framework of density-functional theory, several popular density functionals for exchange and correlation have been constructed to satisfy a local form of the Lieb-Oxford bound. In its original global expression, the bound represents a rigorous lower limit for the indirect Coulomb interaction energy. Here we employ exact-exchange calculations for the G2 test set to show that the local form of the bound is violated in an extensive range of both the dimensionless gradient and the average electron density. Hence, the results demonstrate the severity in the usage of the local form of the bound in functional development. On the other hand, our results suggest alternative ways to construct accurate density functionals for the exchange energy.

We demonstrate that the polarizability anisotropy of the acetylene molecule could be experimentally determined by molecular alignment-induced spatial focusing and defocusing effects under femtosecond impulsive excitation. The molecular alignment signals of oxygen and acetylene were measured under identical experimental conditions and the polarizability anisotropy of acetylene was quantitatively determined with oxygen as the reference molecule. Meanwhile, the Kerr coefficients of oxygen and acetylene were estimated. This intuitive and simple method is also applicable to other molecules.

In this paper we describe an approach to Casimir force problems that is ultimately generalizable to all fields, boundary conditions, and cavity geometries. This approach utilizes locally defined reflection amplitudes to express the energy per unit area of any Casimir interaction. To demonstrate this approach we solve a number of Casimir Force problems including the case of uniaxial boundary conditions in a parallel-plate cavity.

The spectroscopic behavior of 5$d_{3/2}nd$ (${}^1$$D_0$ and ${}^1$$D_2$) autoionizing Rydberg series of barium was studied under collision with rare gases. The series members from $n$ $=$ 8 to $n$ $=$ 64 were observed using two-photon excitation of the two valence electrons in the 6$s^{21}$$S_0$ ground state of barium. The barium vapor was produced in a heat-pipe-like oven, and a tunable dye laser pumped by an excimer laser was used as the excitation source. The obtained spectral data have Beutler-Fano profiles. These spectral lines were investigated when inert gases Ar, Kr, and Xe at different pressures were introduced into the oven as perturbing gases. The collision-induced line shifts were measured and the shift parameters for the even-parity 5$d_{3/2}nd$ ${}^1$$D_0$ and 5$d_{3/2}nd$ ${}^1$$D_2$ ($n$ $=$ 8–35) autoionizing states were extracted from the data. The collision-induced change in the spectral line shape at different Xe pressure was also explored.

The fragmentation scheme of the doubly charged anthracene molecule (C${}_{14}$H${}_{10}$${}^{2+}$) has been studied via monocharged fluorine impact at 3 keV using the CIDEC method (collision-induced dissociation under energy control). Doubly or singly charged fragments resulting from the loss of neutrals (H, C${}_2$H${}_2$) or C${}_x$H${}_y$${}^{+}$ ($x$ $=$ 1–5; $y$ $=$ 1–5) have been measured versus the excitation energy of the parent molecule C${}_{14}$H${}_{10}$${}^{2+}$. The branching ratio of the C${}_x$H${}_y$${}^{+}$ emission process was found to be 16$\%$. For the major neutral evaporation channels via $n$H or $n$C${}_2$H${}_2$ ($n$ $=$ 1, 2) emission, the measured population distributions have been fitted using a Rice-Ramsperger-Kassel (RRK) statistical dissociation and cascade model. The simulated rate-energy dependences of the two primary competing pathways, i.e., the loss of H or C${}_2$H${}_2$ from C${}_{14}$H${}_{10}$${}^{2+}$, present a crossing point at 13 eV. This feature is characteristic of two different fragmentation mechanisms: a direct cleavage for the loss of H for energy above the crossing point and a rearrangement process for the loss of C${}_2$H${}_2$ bellow the crossing point.

A recent measurement of the dielectronic recombination (DR) of W${}^{20+}$ [Schippers et al., Phys. Rev. A 83, 012711 (2011)] found an exceptionally large contribution from near-threshold resonances ($\lesssim$1 eV). This still affected the Maxwellian rate coefficient at much higher temperatures. The experimental result was found to be higher by a factor of 4 or more than that currently in use in the 100- to 300-eV range, which is of relevance for modeling magnetic fusion plasmas. We have carried out DR calculations with autostructure which include all significant single-electron promotions. Our intermediate-coupling (IC) results are more than a factor of 4 larger than our $LS$-coupling ones at 1 eV but still lie a factor of 3 below experiment here. If we assume complete (chaotic) mixing of near-threshold autoionizing states, then our results come into agreement (to within 20$\%$) with experiment below $\lesssim$2 eV. Our total IC Maxwellian rate coefficients are 50$\%$–30$\%$ smaller than those based on experiment over 100–300 eV.

Cross sections for elastic scattering of low-energy electrons by tetrahydrofuran, a prototype for the furanose ring found in the backbone of DNA, have been measured and calculated over a wide energy range, with an emphasis on energies below 6 eV, where previous data are scarce. The measurements employ a thin-aperture version of the relative-flow method, while the calculations employ the Schwinger multichannel method with an extensive treatment of polarization effects. Comparisons with earlier results, both experimental and theoretical, are presented and discussed. A proper accounting for the strong permanent electric dipole of tetrahydrofuran is found to be essential to obtaining reliable cross sections, especially at energies below 5 eV.

The optical properties of matter can be manipulated by coupling a strong field to an optical transition. The concepts of Autler-Townes splitting and electromagnetically induced transparency, where a control field manipulates the transmission of a probe field, have recently been extended to control the transmission of x rays. The use of a strong femtosecond pulse to control the transmission of an isolated attosecond extreme ultraviolet or soft x-ray pulse opens a new field of investigation, since the duration of the probe pulse is only a fraction of one optical cycle of the control pulse. In this paper a three-state model beyond the rotating wave approximation is formulated that treats the attosecond pulse as an initial value problem in the time domain. Two effects for spectroscopic measurements are elucidated: first the absorption profile of the attosecond pulse is split into multiple lines rather than a doublet in the high-field regime, and second the transmission of the attosecond pulse depends on the relative timing within the optical cycle of the control pulse. An analytical treatment is carried out in the time domain in contrast to the more common approach in the frequency domain. This is appropriate for the field of attosecond physics where the main interest lies in the time evolution of the fields, rather than spectroscopic studies.