Cluster states can be used to perform measurement-based quantum computation. The cluster state is a useful resource, because once it has been generated only local operations and measurements are needed to perform universal quantum computation. In this paper, we explore techniques for quickly and deterministically building a cluster state. In particular, we consider generating cluster states on a qubus quantum computer, a computational architecture which uses a continuous variable ancilla to generate interactions between qubits. We explore several techniques for building the cluster, with the number of operations required depending on whether we allow the ability to destroy previously created controlled-phase links between qubits. In the case where we cannot destroy these links, we show how to create an $n\times m$ cluster using just $3nm-2n-\lceil 3m/2\rceil +3$ operations. This gives more than a factor of 2 savings over a naive method. Further savings can be obtained if we include the ability to destroy links, in which case we only need $\lceil \frac{1}{3}(8nm-4n-4m-8)\rceil$ operations. Unfortunately, the latter scheme is more complicated so choosing the correct order to interact the qubits is considerably more difficult. A halfway scheme, that keeps a modular generation but saves additional operations over never destroying links requires only $3nm-2n-2m+4$ operations. The first scheme and the last scheme are the most practical for building a cluster state because they split up the generation into the repetition of simple sections.

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\Delta E/E={10}^{-4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.

We present a classical, relativistic Monte Carlo calculation investigating the role of the magnetic field in bound-state dynamics and ionization for atoms in ultrastrong external radiation fields. Atoms with atomic numbers $1⩽Z⩽20$ in external fields from 0.01 to ${10}^3$ a.u. are studied. We show that, when calculating ionization rates, the dipole approximation yields accurate values compared to a full classical electromagnetic laser field for intensities up to ${10}^{23}$ W/cm${}^2$. The calculations indicate that the quasistatic approximation is valid for external-field frequencies less than 1/20 of the Kepler-orbit time for the ionizing state. For ionization at fields above 100 a.u., the magnetic field affects the atom by altering the portion of the bound state that ionizes and deflecting the mean emission angle of the photoelectron angular distributions, although no change in the width of the final-state angular emission about this mean occurs. The photoelectron distribution and change in the ionizing bound state are seen as an indication of nonperturbative magnetic field effects for atoms in ultrastrong radiation fields.

Modification of the resonance fluorescence spectrum of a two-level atom driven by a monochromatic field in the close proximity of a plasmonic nanostructure (metal sphere) is studied in detail. It is shown that one can control this spectrum by varying several key parameters: (i) the radius of the nanosphere, (ii) polarization of the incident radiation, and (iii) the atom's location around the nanosphere (its radial coordinate and polar angle in the spherical coordinate system). These parameters affect the local field enhancement and the modification of the radiative decay rate of the atom interacting with the nanosphere, which leads to modification of the resonance fluorescence spectrum of the atom (e.g., frequency shift of the satellite lines in the Mollow-type triplet, widths of the lines, and the spectrum intensity) by contrast with that in free space. The permittivity of the metal the nanosphere is made of is also an additional parameter, which defines the nonradiative decay. The latter in combination with other parameters allows continuous control of the transition from resonance fluorescence enhancement to its quenching.

We design an experimental system to test the equivalence between the canonical and Minkowski momentum of light, which can provide a judgment on the recent resolution of the century-old Abraham-Minkowski controversy by S. M. Barnett [Phys. Rev. Lett. 104, 070401 (2010)]. By measuring the recoil momentum of ultracold rubidium atoms in a rubidium Bose-Einstein condensate after the electromagnetically induced absorption of a monochromatic laser pulse, the momentum of the pulse in the ultracold atoms can be obtained. If the equivalence is valid, the measured results will coincide with the theoretical values of the canonical momentum of the pulse. Otherwise, if the equivalence is invalid, the measured results will coincide with the theoretical values of the Minkowski momentum, which are significantly different from that of the canonical momentum. Our scheme is amethod to test the equivalence between the canonical and Minkowski momentum of light. It can also be improved to distinguish between the Minkowski and Abraham momenta to contribute to the study of the Abraham-Minkowski controversy in the future.

The application of the transformation design method developed in transformation optics in a quantum system with the interaction of central force fields is explored. It is shown that altering an interaction from a specific type to a desired form is possible and that it may result in the possibility of creating a new quantum state to share the well-known energy spectrum and offer an alternative way of designing a new atomic system.

We study experimentally and theoretically a dense ensemble of negatively charged nitrogen-vacancy centers in diamond coupled to a high-$Q$ superconducting coplanar waveguide cavity mode at low temperature. The nitrogen-vacancy centers are modeled as effective spin-one defects with inhomogeneous frequency distribution. For a large enough ensemble the effective magnetic coupling of the collective spin dominates the mode losses and inhomogeneous broadening of the ensemble and the system exhibits well-resolved normal-mode splitting in probe transmission spectra. We use several theoretical approaches to model the probe spectra and the number and frequency distribution of the spins. This analysis reveals an only slowly temperature-dependent $q$-Gaussian energy distribution of the defects with a yet unexplained decrease of effectively coupled spins at very low temperatures below $100\mathrm{m}\mathrm{K}$. Based on the system parameters we predict the possibility to implement an extremely stable maser by adding an external pump to the system.

We report a scheme for mode-locked biphoton generation by engineering quasi-phase-matching materials. An aperiodically poled lithium tantalate is designed to supply multiple reciprocal vectors for two-photon frequency comb generation. Temporally, such two-photon state exhibits mode-locked pulse train. The two-photon pulse duration reaches 4.2 fs when the frequency comb is centered at 800 nm and covers 442 nm. The HOM interference of the mode-locked biphoton approaches a 1.26-$\mu$m dip and exhibits a reappearance of dip and peak, which can be taken as a new scheme for engineering quantum light source. Furthermore, we also design another aperiodically poled crystal to generate two sets of frequency combs. The HOM interference will appear as a revival of spatial beatings. The mode-locked biphotons suggest applications in precise time synchronization, quantum metrology, quantum optical coherence tomography, and quantum computing.

We demonstrate that the long-lived bound states (supermolecules) can exist in the dilute limit when we tune the shape of the effective potential between polar molecules by an external microwave field. Binding energies, average sizes, and phase diagrams for both $s$-orbital (bosons) and $p$-orbital (fermions) dimers are studied, together with bosonic trimer states. We explicitly show that the nonadiabatic transition rate can be easily tuned small for such ground-state supermolecules, so that the system can be stable from collapse even near the associated potential resonance. Our results, therefore, suggest a feasible cold molecule system to investigate novel few-body and many-body physics (for example, the $p$-wave BCS–Bose-Einstein-condensate crossover for fermions and the paired condensate for bosons) that cannot be easily accessed in single species atomic gases.

We investigate experimentally parity-time ($\mathcal{PT}$) symmetric scattering using $LRC$ circuits in an inductively coupled $\mathcal{PT}$-symmetric pair connected to transmission line leads. In the single-lead case, the $\mathcal{PT}$-symmetric circuit acts as a simple dual device—an amplifier or an absorber depending on the orientation of the lead. When a second lead is attached, the system exhibits unidirectional transparency for some characteristic frequencies. This nonreciprocal behavior is a consequence of generalized (nonunitary) conservation relations satisfied by the scattering matrix.

With carrier-envelope-phase-stabilized pulses, the self-referencing technique is applied to evaluate the relative phase of high-order harmonics generated in atoms and aligned molecules. The Fourier transform analysis from the frequency domain to the time domain shows that the effective duration of the driving pulse, during which the specific orders of harmonics are efficiently generated, is decreased as the harmonic order is increased. In the case of aligned molecules, the interference fringes between the two adjacent odd-order harmonics from N${}_2$ are more distinctive than those from CO${}_2$, which may be explained by the difference in the complexity associated with the symmetry of highest occupied molecular orbitals between N${}_2$ and CO${}_2$.

We find that locally accessible information of multiparty quantum ensembles have properties akin to quantum as well as classical correlations of single multiparty quantum states. It satisfies monotonicity under local quantum operations and classical communication. However, we show that it does not follow monogamy, an important property usually satisfied by quantum correlations, and actually violates any such relation to the maximal extent. We find an operational sufficient criterion for violation of monogamy. Violation is obtained even for locally indistinguishable, but globally orthogonal, multisite ensembles. The results assert that while single multiparty quantum states are monogamous with respect to their shared quantum correlations, ensembles of multiparty quantum states may not be so.

In this work, we propose the nonlocal tunneling mechanism for high-fidelity state transfer between distant parties. The nonlocal tunneling follows from the overlap between the distant sending and receiving wave functions, which is indirectlymediated by the off-resonant normal modes of a quantum channel. This channel is made up of a network of dissipative quantum systems exhibiting the same bosonic or fermionic statistical nature as the sender and receiver. We demonstrate that the incoherence arising from quantum channel nonidealities is almost completely circumvented by the tunneling mechanism, which thus affords a high-fidelity transfer process.

We study the influence of a single Feshbach resonance on the near-threshold bound and continuum states in interatomic potentials with attractive tails. We present an energy-independent parametrization of the Feshbach resonance to reveal its influence on the bound state spectrum as well as on the scattering properties. The applicability of this parametrization is confirmed by comparison with numerical coupled-channel calculations. A formula for the scattering length is derived, which—in contrast to previous parametrizations—transparently exposes the dependence on the characteristic quantities of the uncoupled channels and on channel-coupling effects and is valid for all deep potentials falling off faster than $-1/r^3$ asymptotically.

This paper addresses the problem of two-photon double ionization (TPDI) of He($1s^2$) and He($1s2s^{1}S$). First, we reconsider TPDI of He($1s^2$) with a photon energy of 2.1 a.u.; it is well known that TPDI is mainly a sequential process, resulting in an electron spectrum dominated by two peaks. In the past, we have noticed that the peaks are shifted as the pulse duration shortens; they move toward each other. The first objective of the present work is to clarify the origin of the shift and to evaluate it quantitatively. In parallel with the resolution of the time-dependent Schrödinger equation (TDSE), we have developed a model calculation based on lowest-order perturbation theory for the laser-atom interaction and for the atomic structure representation. The model agrees with TDSE calculations, and it also explains the physical origin of the shifts. Furthermore, it provides a quantitative evaluation of these shifts. The second objective of the present work is to extend our study to TPDI of He($1s2s^{1}S$), which has received much less attention until now. We have investigated the cases of photon energies of 1.8 and 2.5 a.u. We show that TPDI is dominated by a sequential process where the first ionization leaves a remaining He${}^{+}\left(2s\right)$ ion, which is ionized by a second photon. As in the case of He($1s^2$), the model agrees with TDSE calculations, and it leads to better insights into the physics underlying the double-ionization process.

We introduce, analyze, and compare two methods of single-photon cooling that generically cool and compress molecular gases. The first method compresses the molecular gas density by 3 orders of magnitude and increases collision frequency in trapped samples. The second method compresses the phase-space density of the gas by at least 2 orders of magnitude. Designed with combinations of electric and magnetic fields, these methods cool the molecules from $\sim$100 to 1 mK using a single irreversible state change. They can be regarded as generic cooling schemes applicable to any molecule with a magnetic and electric dipole moment. The high efficiency calculated, compared to schemes involving cycling, is a result of cooling the molecules in a single step.

We study excitations of the spin-1 Bose gas at finite temperatures and in the presence of a not-so-strong magnetic field or, equivalently, when the gas sample is partially polarized. Motivated by the success of two-fluid hydrodynamics of scalar superfluids we develop a three-fluid hydrodynamic description to treat the low-frequency and long-wavelength excitations of the spin-1 Bose gas. We derive coupled linear hydrodynamic equations of the three sounds and evaluate them numerically in a self-consistent mean-field approximation valid for dilute gas in the intermediate and critical temperature regions. In this latter region we identify the critical mode.

We have theoretically and numerically investigated an analog of electromagnetically induced transparency (EIT) in plasmonic systems consisting of multiple cascaded nanodisk resonators, aperture-side-coupled to metal-insulator-metal bus waveguides. A simplified theoretical model is established to study spectral features in the plasmonic waveguide-resonator systems, and the calculated results are in good agreement with finite-difference time-domain simulations. The main dependent factors of EIT-like spectral response, namely, the resonance detuning, intrinsic Drude loss, and especially cavity-cavity separation, are discussed in detail. Similar to multiple EIT in quantum systems, multiple induced-transparency peaks are found in the areas of strong dispersion generated in our plasmonic system. The group indices and quality factors of transparency resonances with high transmission can reach levels of ∼35 and ∼200, respectively. These results pave a way toward dynamic control of light in the nanoscale domain, which can actualize some new devices for fundamental study and applications of plasmonic nanostructures.

We show how to control spatial quantum correlations in a multimode, degenerate, type-I optical parametric oscillator below threshold by introducing a spatially inhomogeneous medium, such as a photonic crystal, in the plane perpendicular to light propagation. We obtain the analytical expressions for all of the correlations in terms of the relevant parameters of the problem and study the number of photons, entanglement, squeezing, and twin beams. Considering different regimes and configurations, we show that it is possible to tune the instability thresholds as well as the quantumness of correlations by breaking the translational invariance of the system through a photonic-crystal modulation.

This paper derives master equations for an atomic two-level system for a large set of unitarily equivalent Hamiltonians without employing the rotating-wave and certain Markovian approximations. Each Hamiltonian refers to physically different components as representing the “atom” and as representing the “field” and hence results in a different master equation, when assuming a photon absorbing environment. It is shown that the master equations associated with the minimal-coupling and the multipolar Hamiltonians predict enormous stationary state narrow-band photon emission rates, even in the absence of external driving, for current experiments with single quantum dots and color centers in diamond. These seem to confirm that the rotating-wave Hamiltonian identifies the components of the atom-field system most accurately.