Memory-assisted measurement-device-independent quantum key distribution (MA-MDI-QKD) is a promising scheme that aims to improve the rate-versus-distance behavior of a QKD system by using the state-of-the-art devices. It can be seen as a bridge between current QKD links to quantum repeater based networks. While, similar to quantum repeaters, MA-MDI-QKD relies on quantum memory (QM) units, the requirements for such QMs are less demanding than that of probabilistic quantum repeaters. Here we present a variant of MA-MDI-QKD structure that relies on only a single physical QM: a nitrogen-vacancy center embedded into a cavity where its electronic spin interacts with photons and its nuclear spin is used for storage. This enables us to propose a simple but efficient MA-MDI-QKD scheme resilient to memory errors and capable of beating, in terms of rate and reach, existing QKD demonstrations. We also show how we can extend this setup to a quantum repeater system, reaching, thus, larger distances.

It is a computationally hard task to certify genuine multipartite entanglement (GME). We investigate the relation between the norms of the correlation vectors and the detection of GME for tripartite quantum systems. A sufficient condition for GME and an effective lower bound for the GME concurrence are derived. Several examples are considered to show the effectiveness of the criterion and the lower bound of GME concurrence.

One way to study the physical plausibility of closed timelike curves (CTCs) is to examine their computational power. This has been done for Deutschian CTCs (D-CTCs) and postselection CTCs (P-CTCs), with the result that they allow for the efficient solution of problems in PSPACE and PP, respectively. Since these are extremely powerful complexity classes, which are not expected to be solvable in reality, this can be taken as evidence that these models for CTCs are pathological. This problem is closely related to the nonlinearity of this models, which also allows, for example, cloning quantum states, in the case of D-CTCs, or distinguishing nonorthogonal quantum states, in the case of P-CTCs. In contrast, the process matrix formalism allows one to model indefinite causal structures in a linear way, getting rid of these effects and raising the possibility that its computational power is rather tame. In this paper, we show that process matrices correspond to a linear particular case of P-CTCs, and therefore that its computational power is upperbounded by that of PP. We show, furthermore, a family of processes that can violate causal inequalities but nevertheless can be simulated by a causally ordered quantum circuit with only a constant overhead, showing that indefinite causality is not necessarily hard to simulate.

Nonadiabatic geometric quantum computation provides a means to perform fast and robust quantum gates. It has been implemented in various physical systems, such as trapped ions, nuclear magnetic resonance, and superconducting circuits. Another system being adequate for implementation of nonadiabatic geometric quantum computation may be Rydberg atoms, since their internal states have very long coherence time and the Rydberg-mediated interaction facilitates the implementation of a two-qubit gate. Here, we propose a scheme of nonadiabatic geometric quantum computation based on Rydberg atoms, which combines the robustness of nonadiabatic geometric gates with the merits of Rydberg atoms.

Cat-state qubits (qubits encoded with cat states) have recently drawn intensive attention due to their enhanced lifetimes with quantum error correction. We here propose a method to implement a universal controlled-phase gate of two cat-state qubits via two microwave resonators coupled to a superconducting transmon qutrit. During the gate operation, the qutrit remains in the ground state; thus decoherence from the qutrit is greatly suppressed. This proposal requires only two basic operations and neither classical pulse nor measurement is needed; therefore the gate realization is simple. Numerical simulations show that high-fidelity implementation of this gate is feasible with current circuit QED technology. The proposal is quite general and can be applied to implement the proposed gate with two microwave resonators or two optical cavities coupled to a single three-level natural or artificial atom.

It is known that no quantum process can produce a predetermined superposition of unknown arbitrary states. It has already been shown that with some partial information about the states, one can produce with some probability such superpositions. In general the success probability of these machines, even for orthogonal states, are less than unity. Here we show that there are specific machines that can produce superpositions of orthogonal qubit states with unit probability.

The electron-capture processes in ${\mathrm{C}}^{4+}$ + He collisions have been studied theoretically using a two-active-electron semiclassical atomic-orbital close-coupling method in a wide energy domain. The results of the present calculations are compared with available theoretical predictions and experimental measurements: very good agreements are found for both total and state-selective single-electron-capture (SEC) and double-electron-capture (DEC) cross sections. We extend the understanding on that system to high energies for which only a single series of data exists. Furthermore, the mechanisms responsible for SEC and DEC processes have been investigated by additional restricted two-active-electron and single-active-electron calculations. The role of electronic correlations in the collisions is also discussed.

Strong dipole-exchange interactions due to spontaneously produced contaminant states can trigger rapid dephasing in many-body Rydberg ensembles [E. A. Goldschmidt et al., Phys. Rev. Lett. 116, 113001 (2016)]. Such broadening has serious implications for many proposals to coherently use Rydberg interactions, particularly Rydberg dressing proposals. The dephasing arises as a runaway process where the production of the first contaminant atoms facilitates the creation of more contaminant atoms. Here we study the time dependence of this process with stroboscopic approaches. Using a pump-probe technique, we create an excess “pump” Rydberg population and probe its effect with a different “probe” Rydberg transition. We observe a reduced resonant pumping rate and an enhancement of the excitation on both sides of the transition as atoms are added to the pump state. We also observe a time scale for population growth that is significantly shorter than predicted by homogeneous mean-field models, as expected from a clustered growth mechanism where high-order correlations dominate the dynamics. These results support earlier works and confirm that the time scale for the onset of dephasing is reduced by a factor which scales as the inverse of the atom number. In addition, we discuss several approaches to minimize these effects of spontaneous broadening, including stroboscopic techniques and operating at cryogenic temperatures. It is challenging to avoid the unwanted broadening effects, but under some conditions they can be mitigated.

We present estimates of the attosecond streaking delay in photoionization of atoms based on an analytical formula. In the derivation of the formula we use that the streaking delay depends on the propagation of the photoelectron over a finite range in space. We find that the analytical estimates agree well with results of ab initio calculations. Application of the formula provides insights into the influence of the streaking field on the field-free time delay in the analysis of streaking measurements.

The accurate determination of atomic final states following nuclear $\beta$ decay plays an important role in several experiments. In particular, the charge state distributions of ions following nuclear $\beta$ decay are important for determinations of the $\beta \text{−}\nu$ angular correlation with improved precision. Beyond the hydrogenic cases, the decay of neutral ${}^6\mathrm{He}$ presents the simplest case. Our measurement aims at providing benchmarks to test theoretical calculations. The kinematics of ${\mathrm{Li}}^{n+}$ ions produced following the $\beta$ decay of ${}^6\mathrm{He}$ within an electric field were measured using ${}^6\mathrm{He}$ atoms in the metastable $(1s2s,{}^{3}S_1)$ and $(1s2p,{}^{3}P_2)$ states confined by a magneto-optical trap. The electron shakeoff probabilities were deduced, including their dependence on ion energy. We find significant discrepancies on the fractions of Li ions in the different charge states with respect to a recent calculation.

A two-step photoionization strategy of an ultracold rubidium beam for application in a focused ion beam instrument is analyzed and implemented. In this strategy the atomic beam is partly selected with an aperture after which the transmitted atoms are ionized in the overlap of a tightly cylindrically focused excitation laser beam and an ionization laser beam whose power is enhanced in a build-up cavity. The advantage of this strategy, as compared to without the use of a build-up cavity, is that higher ionization degrees can be reached at higher currents. Optical Bloch equations including the photoionization process are used to calculate what ionization degree and ionization position distribution can be reached. Furthermore, the ionization strategy is tested on an ultracold beam of ${}^{85}\mathrm{Rb}$ atoms. The beam current is measured as a function of the excitation and ionization laser beam intensity and the selection aperture size. Although details are different, the global trends of the measurements agree well with the calculation. With a selection aperture diameter of $52\mu \mathrm{m}$, a current of $\left(170\pm 4\right)$ pA is measured, which according to calculations is 63% of the current equivalent of the transmitted atomic flux. Taking into account the ionization degree the ion beam peak reduced brightness is estimated at $1\times {10}^7$ A/(${\mathrm{m}}^2\mathrm{sr}\mathrm{eV}$).

Stabilizing vortex solitons with high values of the topological charge $S$ is a challenging issue in optics, studies of Bose-Einstein condensates (BECs), and other fields. To develop an approach to the solution of this problem, we consider a two-dimensional dipolar BEC under the action of an axisymmetric radially periodic lattice potential, $V\left(r\right)\sim cos(2r+\delta )$, with dipole moments polarized perpendicular to the system's plane, which gives rise to isotropic repulsive dipole-dipole interactions. Two radial lattices are considered, with $\delta =0$ and $\pi$, i.e., a potential maximum or minimum at $r=0$, respectively. Families of vortex gap soliton (GSs) with $S=1$ and $S\ge 2$, the latter ones often being unstable in other settings, are completely stable in the present system (at least up to $S=11$), being trapped in different annular troughs of the radial potential. The vortex solitons with different $S$ may stably coexist in sufficiently far separated troughs. Fundamental GSs, with $S=0$, are found too. In the case of $\delta =0$, the fundamental solitons are ring-shaped modes, with a local minimum at $r=0.$ At $\delta =\pi$, they place a density peak at the center.

We numerically study a rotating Bose-Einstein condensate placed transiently over the critical rotation frequency, i.e., in a regime where the rotation frequency is larger than the radial frequency of the confinement. We study the reversibility of this process depending on the strength of the interactions and the presence of vortices. We find that the reversibility is broken by the interactions in the absence of vortices but systematically quasirestored in the presence of a single vortex.

Synthetic spin-orbit coupling in ultracold atomic gases can be taken to extremes rarely found in solids. We study a two-dimensional Hubbard model of bosons in an optical lattice in the presence of spin-orbit coupling strong enough to drive direct transitions from Mott insulators to superfluids. Here we find phase-modulated superfluids with finite momentum that are generated entirely by spin-orbit coupling. We investigate the rich phase patterns of the superfluids, which may be directly probed using time-of-flight imaging of the spin-dependent momentum distribution.

We propose a scheme to achieve a strong photon antibunching with a degenerate optical parametric amplifier coupled to a cavity via unconventional photon blockade. The photon blockade occurs as a result of the suppression of two-photon excitation due to a quantum interference effect between different transition paths. Via analytical calculations, we find out the conditions for optimized photon antibunching in terms of parametric gain and the parametric amplifier pump phase. Calculations of second-order correlation function demonstrate strong photon antibunching. The numerical results are compared with the analytical results and both are in complete agreement. Under the optimal parameters, the system can be used to generate sub-Poissonian light.

In an optically absorbing or amplifying linear medium, the energy flow density of interfering optical waves is in general periodically modulated in space. This makes the wave transmission through a material boundary, as described by the Fresnel transmission coefficients, nonreciprocal and apparently violating the energy conservation law. The modulation has been previously described in connection to ordinary homogeneous nonmagnetic materials. In this work, we extend the description to nanomaterials with designed structural units that can be magnetic at optical frequencies. We find that in such a “metamaterial” the modulation in energy flow can be used to enhance optical far-field emission in spite of the fact that the material is highly absorbing. We also demonstrate a nanomaterial design that absorbs light, but simultaneously eliminates the power flow modulation and returns the reciprocity, which is impossible to achieve with a nonmagnetic material. We anticipate that these unusual optical effects can be used to increase the efficiency of nanostructured light emitters and absorbers, such as light-emitting diodes and solar cells.

Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states.

We study the output entanglement in a three-mode optomechanical system via reservoir engineering by shifting the center frequency of filter function away from resonant frequency. We find the bandwidth of the filter function can suppress the entanglement in the vicinity of resonant frequency of the system, while the entanglement will become strong if the center frequency departs from the resonant frequency. We obtain the approximate analytical expressions of the output entanglement, from which we give the optimal center frequency at which the entanglement takes the maximum. Furthermore, we study the effects of time delay between the two output fields on the output entanglement, and obtain the optimal time delay for the case of large filter bandwidth.

We present an approach to achieve efficient single-photon frequency conversion in the microwave domain based on coherent control in superconducting quantum circuits, which consist of a driven artificial atom coupled to a semi-infinite transmission line. Using the full quantum-mechanical method, we analyze the single-photon scattering process in this system and find that single-photon frequency up- or down-conversion with efficiency close to unity can be achieved by adjusting the parameters of the control field applied to the artificial atom. We further show that our approach is experimentally feasible in currently available superconducting flux qubit circuits.