We propose an efficient numerical method to compute configuration averages of observables in disordered open quantum systems whose dynamics can be unraveled via stochastic trajectories. We prove that the optimal sampling of trajectories and disorder configurations is simply achieved by considering one random disorder configuration for each individual trajectory. As a first application, we exploit the present method to study the role of disorder on the physics of the driven-dissipative Bose-Hubbard model in two different regimes: (i) for strong interactions, we explore the dissipative physics of fermionized bosons in disordered one-dimensional chains; (ii) for weak interactions, we investigate the role of on-site inhomogeneities on a first-order dissipative phase transition in a two-dimensional square lattice.

Hypothesis testing is an important task in mathematics and physics. Hypothesis testing of two random variables is related to the Kullback-Leibler divergence of the two corresponding distributions. Similarly, quantum hypothesis testing of two quantum states is characterized by the quantum relative entropy. While a quantum state can be abstracted as a device that only has outputs but no inputs, the most general quantum device is a quantum channel that also has inputs. In this work, we extend hypothesis testing to general quantum channels. In both the one-shot and asymptotic scenario, we study several quantifiers for hypothesis testing under different assumptions of how the channels are used. As the quantifiers are analogs to the quantum relative entropy of states, we call them the quantum relative entropy of channels. Then, we define the entropy of channels based on relative entropies from the target channel to the completely depolarizing channel. We investigate the properties that the quantum relative entropy of channels should satisfy, and we study its interplay with entanglement. With the broad applications of the quantum relative entropy of states, our results can be useful for understanding general properties of quantum channels.

The success probability of a quantum algorithm constructed from noisy quantum gates cannot be accurately predicted from single-parameter metrics that compare noisy and ideal gates. We illustrate this concept by examining a system with coherent errors and comparing algorithm success rates for different choices of two-qubit gates that are constructed from composite pulse sequences, where the residual gate errors are related by a unitary transformation. As a result, all of the sequences have the same error relative to the ideal gate under any distance measure that is invariant under unitary transformations. However, the circuit success can vary dramatically by choosing error orientations that do not affect the final outcome and error orientations that cancel between conjugate controlled-nots, as demonstrated here with Clifford circuits, compiled Toffoli gates, and quantum simulation algorithms. The results point to the utility of both minimizing the error and optimizing the error direction and also to the advantages of using multiple control sequences for the same gate type within a single algorithm.

We explore the notion of generated entropy in open quantum systems. We focus on the study of the discrete-time quantum walk on the line, from the entropy production perspective. We argue that the evolution of the coin can be modeled as an open two-level system that exchanges energy with the lattice at some effective temperature that depends on the initial state. The entropy balance shows that there is a positive-entropy production during the evolution, in accordance with the second law of thermodynamics.

We investigate the relation between transport properties and entanglement between the internal (spin) and external (position) degrees of freedom in one-dimensional discrete time quantum walks. We obtain closed-form expressions for the long-time position variance and asymptotic entanglement of quantum walks whose time evolution is given by any balanced quantum coin, starting from any initial qubit and position states following $\delta$-like (local) and Gaussian distributions. We find out that the knowledge of the limit velocity of the walker together with the polar angle of the initial qubit provide the asymptotic entanglement for local states, while this velocity with the quantum coin phases give it for highly delocalized states.

We propose a method for generation of genuine multipartite entangled states in a short-range Ising spin chain with periodic global pulses of magnetic field. We consider an integrable and a nonintegrable Floquet system that is periodic in time and has constant quasienergy gaps with degeneracies. We start with all spins polarized along one direction and show that they evolve into states with high entanglement by calculating the average entanglement entropy and geometric measure of entanglement. We show that some of these states have a high number of parties involved in the entanglement by calculating the quantum Fisher information. Such controlled generation of multipartite entanglement has potential applications in quantum information processing.

We propose an efficient single-photon router comprising two resonator waveguide channels coupled by several sequential cavities with embedded three-level atoms. We show that the system can operate as a perfect four-way single-photon switch. We also demonstrate that an incident single photon propagating in one of the waveguides can be routed into one or the other output channels; such routing can be controlled by the external classical electromagnetic field driving the atoms. We argue that, under appropriate conditions, the efficiency of such routing can be close to 100% within a broad operational bandwidth, suggesting various applications in photonics.

We present a protocol for generating multipartite quantum correlations across a quantum network with a continuous-variable architecture. An arbitrary number of users possess two-mode entangled states, keeping one mode while sending the other to a central relay. Here a suitable multipartite detection is implemented, by multiple homodyne detections at the outputs of the interferometer, to conditionally generate a cluster state on the retained modes. This cluster state can be suitably manipulated by the parties and used for tasks of quantum communication in a fully optical scenario. More interestingly, the protocol can be used to create a purely-mechanical cluster state starting from a supply of optomechanical systems. We show that detecting the optical parts of optomechanical cavities may efficiently swap entanglement into their mechanical modes, creating cluster states up to five modes under suitable cryogenic conditions.

We study confined scattering of a quantum particle by two centers fixed on the longitudinal axis of a harmonic-waveguide-like trap. The conditions of confinement-induced resonances (CIRs) appearing in these systems, when scattering cross section approaches the unitary limit, are derived for a regularized pseudopotential describing particle interaction with scattering centers. In the limit of a single center, the position of CIR for an even state tends to the well-known result obtained by Olshanii [Phys. Rev. Lett. 81, 938 (1998)]. Our result can be applicable to confined atomic scattering by fixed impurities, such as ions or Rydberg atoms, with possible extension to $N$ impurities, or by two-atomic molecules.

We have studied the $K$-shell excitation of He-like uranium (${\mathrm{U}}^{90+}$) in relativistic collisions with hydrogen and argon atoms. Performing measurements with different targets, as well as with different collision energies, enabled us to explore the proton- (nucleus-) impact excitation as well as the electron-impact excitation process for the heaviest He-like ion. The large fine-structure splitting in uranium allowed us to partially resolve excitation into different $L$-shell levels. State-of-the-art relativistic calculations which include excitation mechanisms due to the interaction with both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the electron-impact excitation process.

We report on the investigation of the scattering properties between the ground state ${}^{1}S_0$ and the metastable state ${}^{3}P_0$ of the fermionic isotope of ${}^{171}\mathrm{Yb}$. We measure the $s$-wave scattering lengths in the two-orbital collision channels as $a_{eg}^{+}=225\left(13\right)a_0$ and $a_{eg}^{-}=355\left(6\right)a_0$, using clock transition spectroscopy in a three-dimensional optical lattice. The results show that the interorbital spin-exchange interaction is antiferromagnetic, indicating that ${}^{171}\mathrm{Yb}$ is a promising isotope for the quantum simulation of the Kondo effect with the two-orbital system.

A method for diffracting the weak probe beam into unidirectional and higher-order directions is proposed via a Rydberg electromagnetically induced grating, providing a way for the implementations of quantum devices with cold Rydberg atoms. The proposed scheme utilizes a suitable position-dependent adjustment to the two-photon detuning besides the modulation of the standing-wave coupling field, producing an in-phase modulation which can change the parity of the dispersion. We observe that when the modulation amplitude is appropriate, a perfect unidirectional diffraction grating can be realized. In addition, due to the mutual effect between the van der Waals (vdW) interaction and the atom-field interaction length that deeply improves the dispersion of the medium, the probe energy can be counterintuitively transferred into higher-order diffractions as increasing the vdW interaction, leading to the realization of a controllable higher-order diffraction grating via a strong blockade.

Previous theoretical works showed that all pure two-qubit entangled states can generate one bit of local randomness and can be self-tested through the violation of proper Bell inequalities. We report an experiment in which nearly pure partially entangled states of photonic qubits are produced to investigate these tasks in a practical scenario. We show that small deviations from the ideal situation make low entangled states impractical to self-testing and randomness generation using the available techniques. Our results show that in practice lower entanglement implies lower randomness generation, recovering the intuition that maximally entangled states are better candidates for device-independent quantum information processing.

We introduce modulational instability in non-Hermitian systems to study state conversion of topological edge states. We show that state conversion in non-Hermitian systems leads to topological pumping, which is a way of transferring the topological edge state from one edge to the opposite edge. In contrast to Hermitian systems, topological pumping can occur spontaneously in non-Hermitian systems. We show that topological edge state transfer occurs fast.

It is known that secondary nonstoquastic drivers may offer speedups or catalysis in some models of adiabatic quantum computation accompanying the more typical transverse field driver. Their combined intent is to raze potential barriers to zero during adiabatic evolution from a false vacuum to a true minimum; first-order phase transitions are softened into second-order transitions. We move beyond mean-field analysis to a fully quantum model of a spin ensemble undergoing adiabatic evolution in which the spins are mapped to a variable mass particle in a continuous one-dimensional potential. We demonstrate that the necessary criterion for enhanced mobility or “speedup” across potential barriers is actually a quantum form of the Rayleigh criterion. Quantum catalysis is exhibited in models where previously thought not possible, when barriers cannot be eliminated. For the 3-spin model with a secondary antiferromagnetic driver, catalyzed time complexity scales between linear and quadratic with the number of qubits. As a corollary, we identify a useful resonance criterion for quantum phase transition that differs from the classical one, but converges on it, in the thermodynamic limit.

Mutually unbiased bases (MUBs) constitute the canonical example of incompatible quantum measurements. One standard application of MUBs is the task known as quantum random access code (QRAC), in which classical information is encoded in a quantum system, and later part of it is recovered by performing a quantum measurement. We analyze a specific class of QRACs, known as the $2^d\to 1$ QRAC, in which two classical dits are encoded in a $d$-dimensional quantum system. It is known that among rank-1 projective measurements MUBs give the best performance. We show (for every $d$) that this cannot be improved by employing nonprojective measurements. Moreover, we show that the optimal performance can only be achieved by measurements which are rank-1 projective and mutually unbiased. In other words, the $2^d\to 1$ QRAC is a self-test for a pair of MUBs in the prepare-and-measure scenario. To make the self-testing statement robust we propose measures which characterize how well a pair of (not necessarily projective) measurements satisfies the MUB conditions and show how to estimate these measures from the observed performance. Similarly, we derive explicit bounds on operational quantities like the incompatibility robustness or the amount of uncertainty generated by the uncharacterized measurements. For low dimensions the robustness of our bounds is comparable to that of currently available technology, which makes them relevant for existing experiments. Last, our results provide essential support for a recently proposed method for solving the long-standing existence problem of MUBs.

A laser-power-dependent shift has been observed in various atomic beam spectroscopy measurements, which was often treated by extrapolating the results to the zero-field limit. Here we present an experimental and theoretical study of this effect in the measurements of the $2{}^{3}S-2{}^{3}P$ transition frequency of ${}^4\mathrm{He}$. The light-force-induced shift was attributed as a result of modulated atomic trajectories induced by the standing-wave laser field and consequent distortions in the lineshape. Modulation of the beam spatial distributions was detected by imaging atoms at high laser power and was also simulated by the Monte Carlo wave-function approach. The nonlinear behavior of the light-force-shift was observed experimentally and reproduced by the simulations. The systematic shift in the extrapolated result at the zero-field limit was analyzed. As a consequence of this effect, a correction of +0.50(80) kHz was added to our previous result on the $2{}^3{S}_1-2{}^3{P}_1$ transition frequency [Phys. Rev. Lett. 119, 263002 (2017)], and the reevaluated value is 276 734 477 704.3(1.6) kHz.

We present a theoretical investigation of the single ionization of He in collisions with ${\mathrm{H}}^{+}$ projectile ions at 75-keV impact energy. Using the frameworks of the independent-electron model and the impact parameter picture, fully differential cross sections (FDCS) are evaluated in the continuum distorted-wave with eikonal initial-state approximation (CDW-EIS). Comparisons are made to the recent measurements of Schulz et al. [Phys. Rev. A 73, 062704 (2006)] and Arthanayaka et al. [J. Phys. B: At. Mol. Opt. Phys. 49, 13LT02 (2016)]. Strong influence of the internuclear interaction and effects of target core polarization due to the presence of the projectile ion are observed. Comparing the present results to experimental data and other theoretical predictions, it has been found that the CDW-EIS method qualitatively reproduces structures in the FDCS. Projectile coherence effects are investigated by representing the projectile beam as a Gaussian wave packet. Evidence of interference effects due to projectile-electron and projectile-target core interactions are discussed and the need for further theoretical investigations is proposed.

We theoretically investigate the selection of electron quantum trajectories in the macroscopic high harmonic generation (HHG) with few-cycle, 1200-nm near-infrared lasers by changing gas pressure. The selected continuous HHG spectra are able to support the isolated attosecond pulses (IAPs) in the soft x rays. The time-frequency analysis of high harmonics indicates that the long-trajectory electron emissions are dominant at lower pressure and the short-trajectory emissions are suffered at higher and optimal pressure. The quantitative analysis of phase mismatch uncovers the physical origin of dynamic changes in the harmonics fields after propagation in the gas medium. Furthermore, we check the dependency of IAP on the carrier-envelope phase (CEP) of laser pulses and find that IAP can still be obtained even if the CEP changes.