Photon detector is one of the key units in the integrated optical devices. Due to the weak atom-photon interaction, the detection efficiency of a single photon by a bare atom (as a photon detector aside the waveguide) is relatively low (the most reachable value is shown as $50\%$, which has being approached experimentally). In order to overcome such a limit, a pair of spatially separated two-level atoms (the photon lenses) with adjustable inflection and transmission coefficients of the photon are introduced to generate an atomic cavity, in which the detected probability of the waveguide photon by the single detector can be significantly enhanced (even reach to $100\%$ under certain conditions). The proposal is demonstrated specifically by a circuit QED system for highly effective detection of a single microwave photon.

The Laguerre-Gauss modes are a class of fundamental and well-studied optical fields. These stable shape-invariant photons, exhibiting circular-cylindrical symmetry, are familiar from laser optics, micromechanical manipulation, quantum optics, communication, and foundational studies in both classical optics and quantum physics. They are characterized, chiefly, by two mode numbers: the azimuthal index indicating the orbital angular momentum of the beam, which itself has spawned a burgeoning and vibrant subfield, and the radial index, which up until recently has largely been ignored. In this paper we develop a differential operator formalism for dealing with the radial modes in both the position and momentum representations and, more importantly, give the meaning of this quantum number in terms of a well-defined physical parameter: the intrinsic hyperbolic momentum charge.

We have predicted theoretically and verified experimentally the occurrence of a giant asymmetry of the transmission of arbitrarily polarized light propagating through a linear nonmagnetic optical system that consists of a metal film with a two-dimensional array of nanoholes in it and that is deposited on the surface of a planar dielectric photonic crystal. The asymmetry of the light transmission is caused by two factors: (i) the excitation of an optical Tamm state in the system, and (ii) the existence of many secondary lobes in the diffraction pattern. Our results are of interest for the development of efficient planar optical diodelike systems and related nanophotonic devices.

We consider an optomechanical quantum system composed of a single cavity mode interacting with $N$ mechanical resonators. We propose a scheme for generating continuous-variable graph states of arbitrary size and shape, including the so-called cluster states for universal quantum computation. The main feature of this scheme is that, differently from previous approaches, the graph states are hosted in the mechanical degrees of freedom rather than in the radiative ones. Specifically, via a $2N$-tone drive, we engineer a linear Hamiltonian which is instrumental to dissipatively drive the system to the desired target state. The robustness of this scheme is assessed against finite interaction times and mechanical noise, confirming it as a valuable approach towards quantum state engineering for continuous-variable computation in a solid-state platform.

Networks constitute efficient tools for assessing universal features of complex systems. In physical contexts, classical as well as quantum networks are used to describe a wide range of phenomena, such as phase transitions, intricate aspects of many-body quantum systems, or even characteristic features of a future quantum internet. Random quantum networks and their associated directed graphs are employed for capturing statistically dominant features of complex quantum systems. Here, we develop an efficient iterative method capable of evaluating the probability of a graph being strongly connected. It is proven that random directed graphs with constant edge-establishing probability are typically strongly connected, i.e., any ordered pair of vertices is connected by a directed path. This typical topological property of directed random graphs is exploited to demonstrate universal features of the asymptotic evolution of large random qubit networks. These results are independent of our knowledge of the details of the network topology. These findings suggest that other highly complex networks, such as a future quantum internet, may also exhibit similar universal properties.

Fragmentation of ${\text{CO}}_2{}^{4+}$ induced by electron impact at the energy of 500 eV is studied using a momentum imaging spectrometer. It is found that ${\text{CO}}_2{}^{4+}$ decays mainly through two three-body-fragmentation channels: ${\mathrm{CO}}_2{}^{4+}\to {\mathrm{O}}^{+}+{\mathrm{C}}^{2+}+{\mathrm{O}}^{+}$ (121) and ${\mathrm{CO}}_2{}^{4+}\to {\mathrm{O}}^{2+}+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ (211). The fragmentation dynamics of these two channels are analyzed using Dalitz plots and Newton diagrams. In channel (121) ${\text{CO}}_2{}^{4+}$ dissociates mainly through linear and molecular bending fragmentation, while the asynchronous breakup mechanism dominates channel (211). The distributions of momentum correlation angles between ionic fragments and the kinetic energy releases are obtained. Based on the Coulomb explosion model, the bond angle and the bond length of ${\text{CO}}_2{}^{4+}$ before fragmentation are reconstructed. The experimental most probable values of the O-C-O bond angle are ${172}^{\circ }$ and ${171}^{\circ }$ for channel (121) and (211), which agree quite well with that of the neutral ${\text{CO}}_2$ molecule $(172.5^{\circ })$. The reconstructed values of C-O bond length, $1.20 \text{Å}$ and $1.13 \text{Å}$ from channel (121) and channel (211), are also inconsistent with the equilibrium value of the neutral ${\text{CO}}_2$ molecule (1.16 Å).

We study the effect of resonances associated with complex molecular interaction of Rydberg atoms on Rydberg blockade. We show that densely spaced molecular potentials between doubly excited atomic pairs become unavoidably resonant with the optical excitation at short interatomic separations. Such molecular resonances limit the coherent control of individual excitations in Rydberg blockade. As an illustration, we compute the molecular interaction potentials of Rb atoms near the $100s$ states asymptote to characterize such detrimental molecular resonances and determine the resonant loss rate to molecules and inhomogeneous light shifts. Techniques to avoid the undesired effect of molecular resonances are discussed.

The interaction of single-electron atoms with a strong laser field is studied in the Wigner representation. The Wigner function is a quasiprobability function in phase space that allows one to study position-momentum correlations. These correlations give a physical interpretation of the emergence of the above-threshold-ionization (ATI) energy spectrum. Conversely, the quantum-mechanical interference between electrons from neighboring photon orders can explain the spatial bunching of the electron density by the laser field. Furthermore, the Wigner function offers one a rather accurate and relatively efficient quasiclassical estimate of the bound-state population. This method is applied to laser-induced electron-ion scattering and the stationary regime of the bound-state population can be determined. The present calculations are performed for a one-dimensional Rosen-Morse potential. Extensions to general spherically symmetric atomic potentials are indicated.

In this paper, we study Bose-Hubbard models on square and honeycomb lattices with complex hopping amplitudes, which are feasible by recent experiments of cold atomic gases in optical lattices. To clarify phase diagrams, we use extended quantum Monte Carlo simulations (eQMC). For the system on the square lattice, the complex hopping is realized by an artificial magnetic field. We found that vortex-solid states form for certain set of magnetic field, i.e., the magnetic field with the flux quanta per plaquette $f=p/q$, where $p$ and $q$ are co-prime natural numbers. For the system on the honeycomb lattice, we add the next-nearest-neighbor complex hopping. The model is a bosonic analog of the Haldane-Hubbard model. By means of eQMC, we study the model with both weak and strong onsite repulsions. Numerical study shows that the model has a rich phase diagram. We also found that in the system defined on the honeycomb lattice of the cylinder geometry, an interesting edge state appears.

To investigate the physical meaning of pulse peak in fast and slow light media, we investigated propagation of differently shaped pulses experimentally, controlling the sharpness of the pulse peak. Symmetric behavior with respect to fast and slow light was observed in traditional Gaussian pulses; that is, propagated pulses were advanced or delayed, respectively, whereas the pulse shape remained unchanged. This symmetry broke down when the pulse peak was sharpened; in the fast light medium, the sharp pulse peak propagated with luminal velocity, and the transmitted pulse deformed into a characteristic asymmetric profile. In contrast, in the slow light medium, a time-delayed smooth peak appeared with a bending point at $t=0$. This symmetry breaking with respect to fast and slow light is a universal characteristic of pulse propagation in causal dispersive systems. The sharp pulse peak can be recognized as a bending nonanalytical point and may be capable of transferring information.

The spin-flip model describing optically pumped spin-polarized vertical-cavity surface-emitting lasers is considered. The steady-state solutions of the model for elliptically polarized fields are studied. Asymptotic analysis for the existence and stability of the steady-state solutions is developed, particularly in the presence of pump polarization ellipticity. The expansion is with respect to small parameters representing the ellipticity and the difference between the total pump power and the lasing threshold. The analytical results are then confirmed numerically, where it is obtained that generally one of the steady-state solutions is stable while the other is not. The theoretical results are shown to be in qualitative agreement with the experiments.

We present a twofold contribution to the numerical simulation of Lindblad equations. First, an adaptive numerical approach to approximate Lindblad equations using low-rank dynamics is described: a deterministic low-rank approximation of the density operator is computed, and its rank is adjusted dynamically, using an on-the-fly estimator of the error committed when reducing the dimension. On the other hand, when the intrinsic dimension of the Lindblad equation is too high to allow for such a deterministic approximation, we combine classical ensemble averages of quantum Monte Carlo trajectories and a denoising technique. Specifically, a variance reduction method based on the consideration of a low-rank dynamics as a control variate is developed. Numerical tests for quantum collapse and revivals show the efficiency of each approach, along with the complementarity of the two approaches.

We evaluate the spectral properties of the Goldstino in a Bose-Fermi mixture of cold atoms and molecules whose dynamics is governed by a supersymmetric Hamiltonian. Model-independent results are obtained from sum rules obeyed by the spectral function. We carry out specific calculations, at zero and finite temperature, using the random-phase approximation, and obtain in particular analytic expressions for the dispersion relation at low momentum. These explicit calculations allow us to pin down more precisely the features of the Goldstino that can be attributed to the supersymmetry alone, together with its spontaneous breaking. The anomalous large effect of the Fermi sea at moderate and large momenta is emphasized.

The implementation of quantum gates with fidelities that exceed the threshold for reliable quantum computing requires robust gates whose performance is not limited by the precision of the available control fields. The performance of these gates also should not be affected by the noisy environment of the quantum register. Here we use randomized benchmarking of quantum gate operations to compare the performance of different families of gates that compensate errors in the control field amplitudes and decouple the system from the environmental noise. We obtain average fidelities of up to 99.8%, which exceeds the threshold value for some quantum error correction schemes as well as the expected limit from the dephasing induced by the environment.

The minimum probability of error (MPE) measurement discriminates between a set of candidate quantum states with the minimum average error probability allowed by quantum mechanics. Conditions for a measurement to be MPE were derived by Yuen, Kennedy, and Lax [H. P. Yuen et al., IEEE Trans. Info. Theory IT-21, 125134 (1975)]. MPE measurements have been found for states that form a single orbit under a group action, i.e., there is a transitive group action on the states in the set. For such state sets, termed geometrically uniform (GU) previously, it was shown that the “pretty good measurement” attains the MPE. Even so, evaluating the actual probability of error (and other performance metrics) attained by the pretty good measurement on a GU set involves inverting large matrices and is not easy in general. Our first contribution is a formula for the MPE and conditional probabilities of GU sets, using group representation theory. Next, we consider sets of pure states that have multiple orbits under the group action. Such states are termed compound geometrically uniform (CGU). MPE measurements for general CGU sets are not known. In this paper, we show how our representation-theoretic description of optimal measurements for GU sets naturally generalizes to the CGU case. We show how to compute the MPE measurement for CGU sets by reducing the problem to solving a few simultaneous equations. The number of equations depends on the sizes of the multiplicity space of irreducible representations. For many common group representations (such as those of several practical good linear codes), this is much more tractable than solving large semidefinite programs—which is what is needed to solve the Yuen-Kennedy-Lax conditions numerically for arbitrary state sets. We show how to evaluate MPE measurements for CGU states in some examples relevant to quantum-limited classical optical communication.

Quantum computation can be achieved by preparing an appropriate initial product state of qudits and then letting it evolve under a fixed Hamiltonian. The readout is made by measurement on individual qudits at some later time. This approach is called the Hamiltonian quantum computation and it includes, for example, the continuous-time quantum cellular automata and the universal quantum walk. We consider one spatial dimension and study the compromise between the locality $k$ and the local Hilbert space dimension $d$. For geometrically 2-local (i.e., $k=2$), it is known that $d=8$ is already sufficient for universal quantum computation but the Hamiltonian is not translationally invariant. As the locality $k$ increases, it is expected that the minimum required $d$ should decrease. We provide a construction of a Hamiltonian quantum computer for $k=3$ with $d=5$. One implication is that simulating one-dimensional chains of spin-2 particles is BQP-complete (BQP denotes “bounded error, quantum polynomial time”). Imposing translation invariance will increase the required $d$. For this we also construct another 3-local $(k=3)$ Hamiltonian that is invariant under translation of a unit cell of two sites but that requires $d$ to be 8.

A single-center coupled pseudostate approximation originally used by McGovern et al. [Phys. Rev. A 79, 042707 (2009)] to study differential ionization under ion impact is extended to take account of electron exchange. The approximation is fully quantal and includes the interaction between the projectile and target nuclei as well as post-collisional interactions between the ionized electron and the two nuclei. Calculations are presented for proton-impact ionization of atomic hydrogen at 75 keV. These include fully (triple) differential cross sections which illustrate how the post-collisional interaction switches from being dominated by the interaction between the ejected electron and the target nucleus to being dominated by the electron interaction with the projectile nucleus (charge exchange to the continuum). Comparison is made with some measurements of the double differential cross section with respect to ejected electron energy and proton scattering angle. While overall agreement between theory and the experimental data is encouraging, detailed agreement is lacking. The need for more experimental work is emphasized.

Nonresonant dressing fields can make the transition frequency between two circular Rydberg states insensitive to second-order variations in the dc electric field. Perturbation theory can be used to establish the required dressing field amplitude and frequency. The same perturbative approach may be used to understand removal of the first-order dependence of the transition frequency on electric field about a bias dc electric field [Hyafil et al., Phys. Rev. Lett. 93, 103001 (2004)]. The directional alignment of the dressing and dc fields is critical in determining the electric field sensitivity of the dressed transition frequencies. This sensitivity is significantly larger for circular Rydberg states compared to low-angular momentum Rydberg states of Rb.

Inspired by recent experiments with cold atoms in optical lattices, we consider a Stückelberg interferometer for a particle performing Bloch oscillations in a tight-binding model on the honeycomb lattice. The interferometer is made of two avoided crossings at the saddle points of the band structure (i.e., at M points of the reciprocal space). This problem is reminiscent of the double Dirac cone Stückelberg interferometer that was recently studied in the continuum limit [Phys. Rev Lett. 112, 155302 (2014)]. Although the two problems share similarities—such as the appearance of a geometric phase shift—lattice effects, not captured by the continuum limit, make them truly different. The particle dynamics in the presence of a force is described by the Bloch Hamiltonian $H\left(\mathit{k}\right)$ defined from the tight-binding Hamiltonian and the position operator. This leads to many interesting effects for the lattice Stückelberg interferometer: a twisting of the two Landau-Zener tunnelings, saturation of the interband transition probability in the sudden (infinite force) limit, and extended periodicity or even nonperiodicity beyond the first Brillouin zone. In particular, Stückelberg interferometry gives access to the overlap matrix of cell-periodic Bloch states thereby allowing one to fully characterize the geometry of Bloch states, as, e.g., to obtain the quantum metric tensor.

We present an ab initio calculation of the shear viscosity as a function of interaction strength in a two-component unpolarized Fermi gas near the unitary limit, within a finite temperature quantum Monte Carlo (QMC) framework and using the Kubo linear-response formalism. The shear viscosity decreases as we tune the interaction strength $\frac{1}{a{k}_F}$ from the Bardeen-Cooper-Schrieffer side of the Feshbach resonance towards Bose-Einstein condensation limit and it acquires the smallest value for $\frac{1}{a{k}_F}\approx 0.4$, with a minimum value of ${\left.\frac{\eta }{s}\right|}_{\text{min}}\approx 0.2\frac{\hslash }{k_B}$, which is about twice as small as the value reported for experiments in quark-gluon plasma ${\left.\frac{\eta }{s}\right|}_{\text{QGP}}\lesssim 0.4\frac{\hslash }{k_B}$. The Fermi gas near unitarity thus emerges as the most “perfect fluid” observed so far in nature. The clouds of dilute Fermi gas near unitarity exhibit the unusual attribute that, for the sizes realized so far in the laboratory, or larger (less than ${10}^9$ atoms), can sustain quantum turbulence below the critical temperature, but at the same time the classical turbulence is suppressed in the normal phase.