Within the quantum phase representation we derive Heisenberg limits, in closed form, for NOON states and two other classes of states that can perform better in terms of local performance metrics relevant for multiply peaked distributions. One of these can also enhance the superresolution factor beyond that of a NOON state of the same power, at the expense of diminished fringe visibility. An accurate phase estimation algorithm, which can be applied to the minimally resourced apparatus of a standard interferometer, is shown to be resilient in the presence of additive white-Gaussian noise (AWGN). In the limit of no AWGN the algorithm achieves over nine digits of accuracy for the case of a four-photon NOON state, orders of magnitude below its Heisenberg limit.

The high-order-harmonic generation (HHG) of graphene in an intense laser field is investigated using the strong-field approximation method. The initial wave function is presented by gaussian and gamess software. The molecular structure along the $x$ and $y$ axes represents different types of graphene: armchair and zigzag, respectively. The results show that the HHG intensity of the armchair type of graphene is two magnitudes higher than that of the zigzag type in the plateau area. The ionization yield and electron density distribution are also presented to further explain this difference. Finally, by superposing a properly selected range of harmonics, a main pulse with the duration of 91 and 99 attoseconds accompanied by weak satellite pulses will be generated for the case of armchair and zigzag graphene, respectively, and the corresponding intensity from armchair graphene is much higher than that from zigzag graphene.

We introduce three tunable parameters to optimize the fidelity of quantum teleportation with continuous variables in a nonideal scheme. By using the characteristic-function formalism, we present the condition that the teleportation fidelity is independent of the amplitude of input coherent states for any entangled resource. Then we investigate the effects of tunable parameters on the fidelity with or without the presence of the environment and imperfect measurements by analytically deriving the expression of fidelity for three different input coherent-state distributions. It is shown that, for the linear distribution, the optimization with three tunable parameters is the best one with respect to single- and two-parameter optimization. Our results reveal the usefulness of tunable parameters for improving the fidelity of teleportation and the ability against decoherence.

We discuss dynamical response theory of driven-dissipative quantum systems described by Markovian master equations generating semigroups of maps. In this setting thermal equilibrium states are replaced by nonequilibrium steady states, and dissipative perturbations are considered in addition to the Hamiltonian ones. We derive explicit expressions for the linear dynamical response functions for generalized dephasing channels and for Davies thermalizing generators. We introduce the notion of maximal harmonic response and compute it exactly for a single-qubit channel. Finally, we analyze linear response near dynamical phase transitions in quasifree open quantum systems. It is found that the effect of the dynamical phase transition shows up in a peak at the edge of the spectrum in the imaginary part of the dynamical response function.

Contextuality provides a unifying paradigm for nonclassical aspects of quantum probabilities and resources of quantum information. Unfortunately, most forms of quantum contextuality remain experimentally unexplored due to the difficulty of performing sequences of projective measurements on individual quantum systems. Here we show that two-point correlations between binary compatible observables are sufficient to reveal any form of contextuality. This allows us to design simple experiments that are more robust against imperfections and easier to analyze, thus opening the door for observing interesting forms of contextuality, including those requiring quantum systems of high dimensions. In addition, it allows us to connect contextuality to communication complexity scenarios and reformulate a recent result relating contextuality and quantum computation.

We present a quantum algorithm for computing the Ramsey numbers whose computational complexity grows superexponentially with the number of vertices of a graph on a classical computer. The problem is mapped to a decision problem on a quantum computer, and a probe qubit is coupled to a register that represents the problem and detects the energy levels of the problem Hamiltonian. The decision problem is solved by detecting the decay dynamics of the probe qubit.

Quantum repeater networks have attracted attention for the implementation of long-distance and large-scale sharing of quantum states. Recently, researchers extended classical network coding, which is a technique for throughput enhancement, into quantum information. The utility of quantum network coding (QNC) has been shown under ideal conditions, but it has not been studied previously under conditions of noise and shortage of quantum resources. We analyzed QNC on a butterfly network, which can create end-to-end Bell pairs at twice the rate of the standard quantum network repeater approach. The joint fidelity of creating two Bell pairs has a small penalty for QNC relative to entanglement swapping. It will thus be useful when we care more about throughput than fidelity. We found that the output fidelity drops below 0.5 when the initial Bell pairs have fidelity $F<0.90$, even with perfect local gates. Local gate errors have a larger impact on quantum network coding than on entanglement swapping.

Cross-section measurements are reported for single and double photoionization of ${\mathrm{C}}_{60}^{+}$ ions in the photon energy range 18–150 eV accompanied by the loss of zero to seven pairs of carbon atoms, as well as for fragmentation without ionization resulting in loss of two to eight pairs of C atoms in the photon energy range 18–65 eV. Absolute measurements were performed by merging a beam of ${\mathrm{C}}_{60}^{+}$ molecular ions with a beam of monochromatized synchrotron radiation. Product channels involving dissociation yielding smaller fullerene fragment ions account for nearly half of the total measured oscillator strength in this energy range. The sum of cross sections for the measured product channels is compared to a published calculation of the total photoabsorption cross section of neutral ${\mathrm{C}}_{60}$ based on time-dependent density-functional theory. This comparison and an accounting of oscillator strengths indicate that with the exception of ${\mathrm{C}}_{58}^{+}$, the most important product channels resulting from photoabsorption were accounted for in the experiment. Threshold energies for the successive removal of carbon atom pairs accompanying photoionization are also determined from the measurements.

Phase transitions can modify quantum behavior on mesoscopic scales and give access to new and unusual quantum dynamics. Here we investigate the superfluid properties of a rotating two-component Bose-Einstein condensate as a function of changes in the interaction energy and in particular through the phase transition from miscibility to immiscibility. We show that the breaking of one of the hallmarks of superfluid flow, namely, the quantization condition on circulation, is continuous throughout an azimuthal phase-separation process and displays intriguing flow dynamics. We find that the resulting currents are stable for long times and possess a boundary between the two condensate components that exhibits classical solid-body rotation, despite the quantum nature of superfluid flow. To support this coexistence of classical and quantum behavior the system develops a unique velocity flow profile, which includes unusual radial flow in regions near the boundary.

We study the two-body bound and scattering states of two particles in a one-dimensional optical lattice in the presence of a coherent coupling between two internal atomic levels. Due to the interplay between periodic potential, interactions, and coherent coupling, the internal structure of the bound states depends on their center-of-mass momentum. This phenomenon corresponds to an effective momentum-dependent magnetic field for the dimer pseudospin, which could be observed in a chirping of the precession frequency during Bloch oscillations. The essence of this effect can be easily interpreted in terms of an effective bound-state Hamiltonian. Moreover for indistinguishable bosons, the two-body eigenstates can present simultaneously attractive and repulsive bound-state natures or even bound and scattering properties.

We study the stability of persistent currents in a coherently coupled quasi-two-dimensional Bose-Einstein condensate confined in a ring trap at $T=0$. By numerically solving Gross-Pitaevskii equations and by analyzing the excitation spectrum obtained from diagonalization of the Bogoliubov–de Gennes matrix, we describe the mechanisms responsible for the decay of the persistent currents depending on the values of the interaction coupling constants and the Rabi frequency. When the unpolarized system decays due to an energetic instability in the density channel, the spectrum may develop a rotonlike minimum, which gives rise to the finite wavelength excitation necessary for vortex nucleation at the inner surface. When decay in the unpolarized system is driven by spin-density excitations, the finite wavelength naturally arises from the existence of a gap in the excitation spectrum. In the polarized phase of the coherently coupled condensate, there is a hybridization of the excitation modes that leads to complex decay dynamics. In particular, close to the phase transition, a state of broken rotational symmetry is found to be stationary and stable.

Electron shelving gives rise to bright and dark periods in the resonance fluorescence of a three-level atom. The spectral signature of such blinking is a very narrow inelastic peak on top of the two-level atom spectrum. Here, we investigate theoretically phase-dependent fluctuations (e.g., squeezing) of intermittent resonance fluorescence in the frameworks of balanced and conditional homodyne detection (BHD and CHD, respectively). In BHD, the squeezing is reduced significantly in size and Rabi frequency range compared to that for a two-level atom. The sharp peak is found only in the spectrum of the squeezed quadrature, splitting the negative broader squeezing peak for weak fields. CHD correlates the BHD signal with the detection of emitted photons. It is thus sensitive to third-order fluctuations of the field, produced by the atom-laser nonlinearity, that cause noticeable deviations from the second-order BHD results. For weak driving, the third-order spectrum is negative, enlarging the squeezing peak but also reducing the sharp peak. For strong driving, the spectrum is dominated by third-order fluctuations, with a large sharp peak and the sidebands becoming dispersive. Finally, the addition of third-order fluctuations makes the integrated spectra of both quadratures equal in magnitude in CHD, in contrast to those by BHD. A simple mathematical approach allows us to obtain very accurate analytical results in the shelving regime.

Everyday experience supports the existence of physical properties independent of observation in strong contrast to the predictions of quantum theory. In particular, the existence of physical properties that are independent of the measurement context is prohibited for certain quantum systems. This property is known as contextuality. This Rapid Communication studies whether the process of decay in space-time generally destroys the ability of revealing contextuality. We find that in the most general situation the decay property does not diminish this ability. However, applying certain constraints due to the space-time structure either on the time evolution of the decaying system or on the measurement procedure, the criteria revealing contextuality become inherently dependent on the decay property or an impossibility. In particular, we derive how the context-revealing setup known as Bell's nonlocality tests changes for decaying quantum systems. Our findings illustrate the interdependence between hidden and local hidden parameter theories and the role of time.

We report the homonuclear photoassociation (PA) of ultracold ${}^{52}\mathrm{Cr}$ atoms in an optical dipole trap. This constitutes the measurement of PA in an element with total electron spin $\tilde{S}>1$. Although Cr, with its ${}^{7}S_3$ ground and ${}^{7}P_{4,3,2}$ excited states, is expected to have a complicated PA spectrum we show that a spin-polarized cloud exhibits a remarkably simple PA spectrum when circularly polarized light is applied. Over a scan range of 20 GHz below the ${}^{7}P_3$ asymptote we observe two distinct vibrational series each following a LeRoy-Bernstein law for a $C_3/R^3$ potential with excellent agreement. We determine the $C_3$ coefficients of the Hund's case (c) relativistic adiabatic potentials to be $-1.83\pm 0.02$ and $-1.46\pm 0.01$ a.u.. Theoretical nonrotating Movre-Pichler calculations enable a first assignment of the series to $\mathrm{\Omega }=6_u$ and $5_g$ potential energy curves. In a different set of experiments we disturb the selection rules by a transverse magnetic field which leads to additional PA series.

We consider macroscopic correlations in a bipartite system consisting of $2N$ particles described by a generalized probabilistic theory. In particular, we discuss a case of $N$ Popescu-Rohrlich boxes shared between two parties. We characterize macroscopic measurements as collective measurements of the same property on all the boxes in the same region. Such measurements are assumed to reveal only the average value of the measured collective property. We show that for two measurements per observer and $N\ge 2$ there always exists a joint probability distribution explaining all observable data and therefore the system admits local hidden variables. Next, we generalize this result to include measurement of fluctuations and additional measurement settings. Finally, we discuss our result in the context of previous works in which contradictory results were presented.

We study numerically a disordered transverse-field Ising Hamiltonian with long-range couplings. This model was recently investigated experimentally in a trapped-ion quantum simulator and was found to exhibit features of many-body localization at strong disorder. We use exact diagonalization to study the collective state preservation and the eigenstate entanglement structure as a function of both disorder strength and interaction range. Our numerical results, using the same system sizes as the experiment, verify the observation of many-body localization reported in the recent quantum simulation experiment, and point to directions for future experiments.

Two-party xor games (correlation Bell inequalities with two outcomes per party) are the most studied Bell inequalities, and one of the few classes for which the optimal quantum value is known to be exactly calculable. We study a natural generalization of the binary xor games to the class of linear games with $d>2$ outcomes, and propose an easily computable bound on the quantum value of these games. Many interesting properties such as the impossibility of a quantum strategy to win these games, and the quantum bound on the CHSH game generalized to $d$ outcomes are derived. We also use the proposed bound to prove a large-alphabet generalization of the principle of no quantum advantage in nonlocal computation, showing that quantum theory provides no advantage in the task of nonlocal distributed computation of a class of functions with $d$ outcomes for prime $d$, while general no-signaling boxes do. This task is one of the information-theoretic principles attempting to characterize the set of quantum correlations from amongst general no-signaling ones.

We report our detailed analysis of a tabletop system for the measurement of the weak-force-induced electric dipole moment of a ground-state hyperfine transition carried out in an atomic beam geometry. We describe an experimental configuration of conductors for application of orthogonal rf and static electric fields, with cavity enhancement of the rf field amplitude, that allows confinement of the rf field to a region in which the static fields are uniform and well characterized. We carry out detailed numerical simulations of the field modes and analyze the expected magnitude of statistical and systematic limits to the measurement of this transition amplitude in atomic cesium. The combination of an atomic beam with this configuration leads to strong suppression of magnetic dipole contributions to the atomic signal. The application of this technique to the measurement of extremely weak transition amplitudes in other atomic systems, especially alkali metals, seems very feasible.

The ac Stark shift (also known as the “light shift”) is one of the most important physical processes that arises in precision spectroscopy, affecting the basic understanding of field-matter interactions, measurements of fundamental constants, and even the atomic clocks onboard GPS satellites. Although the theory of the ac Stark shift was fully developed by the 1960s and 1970s, precision tests of theory have, for the most part, been few. Taking advantage of recent developments in atomic clock technology, specifically the pulsed approach to atomic signal generation, which allows frequency measurements with a resolution of $\sim {10}^{-15}$, we demonstrate a methodology for measuring the ac Stark shift. Here, we report results from a precision examination of the ac Stark shift in a vapor phase system, examining the resonant frequency of the ${}^{87}\text{Rb}$ 0-0 hyperfine transition for a perturbing laser tuned over a broad optical frequency range (18 GHz) around the $D_1$ absorption resonance. Over the full frequency range the agreement between semiclassical theory and experiment is very good (better than $5\times {10}^{-2}$), and in our experiments we test both the frequency dependence of the scalar and tensor components of the light shift.