Quantum algorithms are usually described as monolithic circuits, becoming large at modest input size. Near-term quantum architectures can only manage a small number of qubits. We develop an automated method to distribute quantum circuits over multiple agents, minimizing quantum communication between them. We reduce the problem to hypergraph partitioning and then solve it with state-of-the-art optimizers. This makes our approach useful in practice, unlike previous methods. Our implementation is evaluated on five quantum circuits of practical relevance.

We report on a systematic investigation of wavelength scaling strong-field double ionization of Mg in intense laser fields. A significant decrease of nonsequential double ionization (NSDI) yield with increasing wavelength from 800–2000 nm is observed. Our data is well reproduced by a three-dimensional Monte Carlo simulation considering recollision impact excitation cross section. We demonstrate that the NSDI of Mg mainly occurs via the first ionic excited state ${\mathrm{Mg}}^{+*}(3p{}^{2}P_{3/2,1/2})$ pumped by returning electron impact process. The recollision impact direct ionization pathway plays a minor role here. The wavelength dependence of the NSDI ratio is due to the recollision energy-dependent excitation cross section as well as the electron wave packet diffusion effects, both sensitively depending on the wavelength. Our work represents a step towards strong-field double ionization experiments on Mg in the long wavelength limit and sheds light on the NSDI mechanism of alkaline-earth metal atoms.

We study theoretically the propagation of a guided atom laser across an Aharonov-Bohm ring which is exposed to a synthetic gauge field. The presence of disorder within the ring gives rise to Al'tshuler-Aronov-Spivak oscillations, seen in the disorder average of the transmission as a function of the effective gauge flux that is contained within the ring. Those oscillations are induced by coherent backscattering and represent a manifestation of weak localization. Through analytical and numerical calculations that are based on the mean-field Gross-Pitaevskii approximation for the propagating Bose-Einstein condensate, we show that the presence of a very weak atom-atom interaction within the ring leads to an inversion of the Al'tshuler-Aronov-Spivak oscillations, in a very similar manner as for the coherent backscattering of Bose-Einstein condensates within two-dimensional disorder potentials. Numerical simulations based on the truncated Wigner method reveal that this signature of weak antilocalization becomes washed out if the interaction strength is increased.

We demonstrate a hybrid method based on field ionization and state-selective deexcitation capable of measuring the lifetimes of high-lying Rydberg levels. For $nS$ Rydberg states of Rb atoms with principal quantum number $60\le n\le 88$, we measure both the lifetimes of single target states and those of the ensembles of Rydberg states populated via blackbody-radiation-induced transitions. We find good overall agreement with numerical calculations of the expected lifetimes in both cases. However, for the target-state lifetimes, we find a local deviation towards shorter lifetimes for states around $n=72$, which we interpret as a signature of a modified blackbody spectrum in the finite volume in which our experiments take place.

Boson sampling is a well-defined scheme for demonstrating quantum supremacy with photons in the near term. However, relying only on multiphoton interference in nonadaptive linear-optical networks, the scheme is difficult to simulate classically. Here we study boson sampling using matrix product states, a powerful method from quantum many-body physics. This method stores the instantaneous quantum state during the evolution of photons in the optical quantum circuit, which allows us to reveal the dynamical features of single photons in boson sampling devices, such as entanglement entropy growth. We show the flexibility of this method by also applying it to dissipative optical quantum circuits, as well as circuits with fermionic particles. Our paper shows that matrix product states provide a powerful platform to simulate optical quantum circuits, which is readily extended to study quantum dynamics in multiparticle quantum walks beyond boson sampling.

We describe a quantum algorithm for preparing states that encode solutions of nonhomogeneous linear partial differential equations. The algorithm is a continuous-variable version of matrix inversion: it efficiently inverts differential operators that are polynomials in the variables and their partial derivatives. The output is a quantum state whose wave function is proportional to a specific solution of the nonhomogeneous differential equation, which can be measured to reveal features of the solution. The algorithm consists of three stages: preparing fixed resource states in ancillary systems, performing Hamiltonian simulation, and measuring the ancilla systems. The algorithm can be carried out using standard methods for gate decompositions, but we improve this in two ways. First, we show that for a wide class of differential operators, it is possible to derive exact decompositions for the gates employed in Hamiltonian simulation. This avoids the need for costly commutator approximations, reducing gate counts by orders of magnitude. Additionally, we employ methods from machine learning to find explicit circuits that prepare the required resource states. We conclude by studying two example applications of the algorithm: solving Poisson's equation in electrostatics and performing one-dimensional integration.

Benchmark calculations of the total and transition energies of the four lowest ${}^{1}S$ states of the beryllium atom are performed. The computational approach is based on variational calculations with finite mass of the nucleus. All-particle explicitly correlated Gaussian (ECG) functions are used to expand the total non-Born-Oppenheimer nonrelativistic wave functions and the ECG exponential parameters are optimized using the standard variational method. The leading relativistic and quantum electrodynamics energy corrections are calculated using the first-order perturbation theory. A comparison of the experimental transition frequencies with the ones calculated in this work shows excellent agreement. The deviations of $0.02–0.09{\mathrm{cm}}^{-1}$ are well within the estimated error limits for the experimental values.

The Schrödinger equation for the $2S$ state and $2P$ state of the lithiumlike ion ${\mathrm{O}}^{5+}$ is solved by using the Rayleigh-Ritz variational method in Hylleraas coordinates. The leading-order relativistic and QED corrections are calculated perturbatively and higher-order corrections are estimated approximately. The transition frequencies between the $2S_{1/2}$ and $2P_J$ ($J=1/2,3/2$) states, as well as the fine-structure splitting between the $2P_{1/2}$ and $2P_{3/2}$ states, are determined and compared with experimental results.

We report electron-atom Compton scattering experiments on the ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ molecules. Energy-loss spectra of electrons quasielastically backscattered at an angle of 135° are measured as a function of azimuthal angle at an incident electron energy of 2.0 keV. Momentum distributions of the H and D atoms due to molecular vibration are extracted from the experimental data by using a protocol that we propose here. The results are successfully compared with theoretical ones predicted by the molecular vibrational wave functions. It is shown that electron-atom Compton scattering has a unique ability to provide direct information about intramolecular motion of each atom with different mass numbers.

Coherence among rotational ion channels during photoionization is exploited to control the anisotropy of the resulting photoelectron angular distributions at specific photoelectron energies. The strategy refers to a robust and single parameter control using two ultrashort light pulses delayed in time. The first pulse prepares a superposition of a few ion rotational states, whereas the second pulse serves as a probe that gives access to a control of the molecular asymmetry parameter $\beta$ for individual rotational channels. This is achieved by tuning the time delay between the pulses, leading to channel interferences that can be turned from constructive to destructive. The illustrative example is the ionization of the $E{(}^1{\mathrm{\Sigma }}_g^{+}$) state of ${\mathrm{Li}}_2$. Quantum wave-packet evolutions are conducted including both electronic and nuclear degrees of freedom to reach angle-resolved photoelectron spectra. A simple interference model based on coherent phase accumulation during the field-free dynamics between the two pulses is precisely exploited to control the photoelectron angular distributions from almost isotropic to marked anisotropic.

We describe a procedure for engineering beams of cold atoms by selectively draining particles from a trapped gas based on momenta. Atoms escape through a filter potential that only transmits atoms with the desired momenta. We outline an algorithm that outputs a filter potential that produces a prespecified beam momentum profile. We illustrate this procedure for the case of a narrow band-pass (NBP) quantum filter. Lastly, we discuss the application of the NBP filter for probing the self-energy and effective mass of Bose polarons, as well as the corresponding Landau criterion.

We theoretically demonstrate that an external magnetic field can be used to control quantum reflection of matter waves in graphene due to its extraordinary magneto-optical properties. We calculate the quantum reflection probabilities in graphene for three experimentally relevant atomic species (He, Na, and Rb) using the full Casimir-Polder potential computed by the Lifshitz formula valid at all distance regimes, going beyond the traditional approach to quantum reflection, based on power-law potentials, which are known to be valid only in the short-distance (nonretarded van der Waals) or in the large-distance (retarded) regimes. We predict the energy range for which quantum reflection is more likely to occur as a function of the magnetic field and show that the quantum reflection probabilities exhibit discontinuities that reflect the structure of Landau levels in graphene. Altogether our findings suggest an alternative way to control quantum reflection at the nanoscale and pave the way for the design of alternative, magnetically tuned reflective diffraction elements for matter waves.

We present a proposal to prepare the macroscopic superposition states of a mechanical oscillator in an optomechanical system with quadratic coupling. By coherently driving the optomechanical cavity with an external field, a Kerr-type interaction among phonons is induced. The macroscopic mechanical superposition can be generated by exploiting the Kerr nonlinearity in the mechanical oscillator, just as the generation of a nonclassical state of light in a Kerr medium. One merit of our proposal is that the Kerr nonlinearity can be strongly enhanced by adjusting the amplitude of the driving field. The influence of the system damping on the realization of mechanical non-Gaussian state is discussed by the negativity of the Wigner function. The results show that the macroscopic superposition states are robust against the mechanical thermal noise.

We experimentally address emergent spectrospatial correlations in a one-dimensional amplifying system that sustains Anderson-localized lasing modes. The structure is based on a periodic-on-average random template, and hence follows the frequency behavior determined by passbands and band gaps of a photonic crystal. A spectrospatial correlation map constructed for weak disorder reveals strong correlations and anticorrelations that extend over multiple band gaps. The origin of the correlation is traced to two parameters, namely the probability of generation of modes, and the congruence in the mode profiles of corresponding modes. Specific locations on the correlation map are identified that reflect the relative contribution of the two parameters. The experimental setup with a fine control on disorder strength allowed us to capture the systematic evolution of the correlation map with disorder, which uncovers an interesting variation in spectrospatial correlations in such Anderson-localized lasing systems.

Over the last few years it was pointed out that certain observables of time-evolving quantum systems may have singularities at certain moments in time, mimicking the singularities physical systems have when undergoing phase transitions. These were given the name of dynamical phase transitions. They were shown to exist in certain integrable (exactly solvable) quantum systems, and were seen numerically and experimentally in some models which were not integrable. The “universality classes” of such singularities were not yet convincingly established, however. We argue that random field Ising models feature singularities in time which may potentially be present in a wider variety of quantum systems, in particular in those which are many-body localized, and describe these singularities in detail analytically.

We experimentally investigate the non-Markovianity problems in a quantum channel implemented by an all-optical setup. The quantum channel simulates the dynamics of a two-level (qubit) system surrounded by bosonic modes, with the system-environment interaction treated without applying the rotating-wave approximation (RWA). Therefore, it allows us to demonstrate the important role of counterrotating-wave terms in influencing the memory effects of the environment. The experimental and theoretical results are in good agreement and show quite different behaviors of the non-Markovianity measure between RWA and non-RWA channels. One can find the nonmonotonic dependence of the non-Markovianity measure on the coupling and detuning parameters in the non-RWA case, which is attributed to the competing mechanisms caused by the counterrotating-wave terms. We emphasize that the non-RWA channel provides a more clear description of the open-system dynamics and the counterrotating-wave terms are indispensable in the study of non-Markovianity.

A non-Hermitian topological insulator with a real spectrum is interesting in the theory of non-Hermitian extension of topological systems. We find an experimentally realizable example of a two-dimensional non-Hermitian topological insulator with a real spectrum. We consider the two-dimensional Su-Schrieffer-Heeger (SSH) model with gain and loss. We introduce a non-Hermitian polarization vector to explore topological phase and show that topological edge states in the band gap exist in the system.

Polarization of photons is the key physical quantity in measurements of quantum spin correlations. According to a variant of quantum and classical electrodynamics formulated by us on the grounds of a hypothesis of existence of a preferred frame of reference (PF) a modification of the Malus law is expected if photonic states are frame dependent. A search for a quantum PF of the photon has been conducted by way of dedicated measurements of optical rotation using a high-precision commercial polarimeter. The data yield an upper limit of the PF velocity with respect to the Earth of $3.0\mathrm{km}/\mathrm{s}$ at the time of the measurements, allowing us to reject the hypothesis that the cosmic microwave background radiation frame might be a PF for the photon.

We generalize the approach by Braunstein and Caves, [Phys. Rev. Lett. 72, 3439 (1994)]. to quantum multiparameter estimation with general states. We derive a matrix bound of the classical Fisher information matrix due to each measurement operator. The saturation of all these bounds results in the saturation of the matrix Helstrom Cramér-Rao bound. Remarkably, the saturation of the matrix bound is equivalent to the saturation of the scalar bound with respect to any given positive definite weight matrix. Necessary and sufficient conditions are obtained for the optimal measurements that give rise to the Helstrom Cramér-Rao bound associated with a general quantum state. To saturate the Helstrom bound with separable measurements or collective measurement entangling only a small number of identical states, we find it is necessary for the symmetric logarithmic derivatives to commute on the support of the state. As an important application of our results, we construct several local optimal measurements for the problem of estimating the three-dimensional separation of two incoherent optical point sources.