Electron-induced dissociative ionization of ${\mathrm{O}}_2$ has been investigated by means of $\left(e,e+\mathrm{ion}\right)$ spectroscopy. The linear momenta of the fragment ions have been measured in coincidence with electron energy-loss spectra for ${\mathrm{O}}_2$ at scattering angles of $2.2^{\circ }, 4.2^{\circ }$, and $8.2^{\circ }$ using an incident energy of 1.4 keV. From the analysis of the data, partial ion yield spectra have been obtained for the $B{}^2\mathrm{\Sigma }, \{c{}^4\mathrm{\Sigma }+2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}, \{2{}^2\mathrm{\Sigma }+2{}^4\mathrm{\Sigma }\}$, and $3{}^2\mathrm{\Sigma }$ ionization, allowing us to assess state-specific momentum-transfer dependence of the relative ionization cross sections. It has been shown that the shape resonance in the $B{}^2\mathrm{\Sigma }$ channel is substantially suppressed by the contributions of electric nondipole transitions. To get insight into the stereodynamics in the electron-${\mathrm{O}}_2$ collision processes, the angular distributions of the fragment ions have been examined. It has been revealed that the $\left(e,e+\mathrm{ion}\right)$ cross section for the $2{\sigma }_{\mathrm{g}}\to 1{\pi }_{\mathrm{g}}$ autoionization band exhibits a characteristic angular distribution reflecting the anisotropic shape of the molecular orbital to which the target electron is excited.

The dissociative frustrated multiple ionization of hydrogen chloride (HCl) molecules in intense femtosecond laser fields is investigated by employing the coincidence measurement technique in cold target recoil ion momentum spectroscopy. Various dissociative frustrated multiple ionization channels caused by populating one or two electrons to highly excited Rydberg states can be clearly distinguished for HCl in linearly polarized intense laser fields. The excited charged fragmentations, depending on their occupied principle quantum numbers, can be either directly identified forming a “knee” structure in the kinetic energy release spectrum, or singly ionized by the weak electric field in the spectrometer, or by blackbody radiation after the conclusion of the laser pulse. The inherent feature of the potential energy curves of HCl plays an important role in the strong-field dissociative frustrated multiple ionization process.

We have studied dilute Bose-Bose mixtures of atoms with attractive interspecies and repulsive intraspecies interactions using quantum Monte Carlo methods at $T=0$. Using a number of models for interactions, we determine the range of validity of the universal equation of state of the symmetric liquid mixture as a function of two parameters: the $s$-wave scattering length and the effective range of the interaction potential. It is shown that the Lee-Huang-Yang correction is sufficient only for extremely dilute liquids with the additional restriction that the range of the potential is small enough. Based on the quantum Monte Carlo equation of state we develop a density functional which goes beyond the Lee-Huang-Yang term and use it together with the local density approximation to determine density profiles of realistic self-bound drops.

We study the instability of a ring Bose-Einstein condensate under a periodic modulation of interatomic interactions. The condensate exhibits temporal and spatial patterns induced by the parametric resonance, which can be characterized by Bogoliubov quasiparticle excitations in the Floquet basis. As the ring geometry significantly limits the number of excitable Bogoliubov modes, we are able to capture the nonlinear dynamics of the condensate using a three-mode model. We further demonstrate the robustness of the temporal and spatial patterns against disorder, which is attributed to the mode-locking mechanism under the ring geometry. Our results can be observed in cold atomic systems and are also relevant to physical systems described by the nonlinear Schrödinger equation.

We extend the concept of the Eberly-Wódkiewicz time-dependent physical spectrum of light to electromagnetic fields by considering the observable time dependence of four appropriately defined Stokes parameters. We also define the concept of time-dependent physical degree of polarization of light by means of these parameters, and discuss the measurement of these quantities using a tunable spectral filter and a detector with a finite response time. The concepts are illustrated by examples with spectrally phase-modulated fully and partially coherent model pulse trains.

High-dimensional degrees of freedom of photons can encode more quantum information than their two-dimensional counterparts. While the increased information capacity has advantages in quantum applications (such as quantum communication), controlling and manipulating these systems has been challenging. Here we show a method to perform deterministic arbitrary high-dimensional Pauli $X$ gates for single photons carrying orbital angular momentum. The $X$ gate consists of a cyclic permutation of qudit basis vectors and, together with the $Z$ gate, forms the basis for performing arbitrary transformations. The proposed experimental setups only use two basic optical elements such as mode sorters and mode shifters and thus could be implemented in any system where these experimental tools are available. Furthermore the number of involved interferometers scales logarithmically with the dimension, which is important for practical implementation.

Kordas et al. [Phys. Rev. A 90, 032104 (2014)] claim to have established a time-continuous formulation of the path integration in the coherent-state (CS) basis free from the inconsistency first reported by Wilson and Galitski [Phys. Rev. Lett. 106, 110401 (2011)]. Since a few recent investigations consider the approach advocated by Kordas et al. as providing a remedy for the CS path-integral anomaly, I feel it is appropriate to show that the formulation proposed by Kordas et al. does not eliminate the anomaly. The Reply does not change the status of my criticism.

In this Reply we briefly clarify the main points of our method to construct coherent-state path integrals in the continuum and we reply to the critique raised by in the preceding Comment by Kochetov [Phys. Rev. A 99, 026101 (2019)]. By using definite examples, we prove that our approach is capable of resolving the inconsistencies accompanying the standard coherent-state path-integral representation of interacting systems.

Measurement incompatibility is the most basic resource that distinguishes quantum from classical physics. Contextuality is the critical resource behind the power of some models of quantum computation and is also a necessary ingredient for many applications in quantum information. A fundamental problem is thus identifying when incompatibility produces contextuality. Here, we show that, given a structure of incompatibility characterized by a graph in which nonadjacent vertices represent incompatible ideal measurements, the necessary and sufficient condition for the existence of a quantum realization producing contextuality is that this graph contains induced cycles of size larger than three.

We give a careful proof that a parallelized version of adiabatic quantum computation can efficiently simulate universal gate model quantum computation. The proof specifies an explicit parameter-dependent Hamiltonian $H\left(\lambda \right)$ that is based on ground state quantum computation [Mizel et al., Phys. Rev. A. 63, 040302 (2001)]. We treat both a 1-dimensional configuration in which qubits on a line undergo nearest-neighbor 2-qubit gates and an all-to-all configuration in which every qubit undergoes 2-qubit gates with every other qubit in the system.

Path-entangled $N$-photon states are key resources for quantum enhanced metrology and quantum imaging, as well as quantum computation. However, the quantum tomography of path-entangled indistinguishable photons is still in its infancy. We propose and implement a quantum tomographical method to characterize path-entangled two-photon NOON states, which can be extended to arbitrary $N$. To access both the populations and the coherences of the path-encoded density matrix, a single ancilla spatial mode is introduced, and photon correlations are performed as a function of a single phase within an interferometer. We characterize a two-photon state generated through the Hong-Ou-Mandel interference of indistinguishable single photons emitted by a semiconductor quantum dot and show that an overcomplete data set reveals spatial coherences that could be otherwise hidden due to limited statistics. We finally extract the truly indistinguishable part of the density matrix and identify the main origin for the reduced degree of entanglement.

We compare the performance of quantum error correcting codes when memory errors are unitary with the more familiar case of dephasing noise. For a wide range of codes, we analytically compute the effective logical channel that results when the error correction steps are performed noiselessly. Our examples include the entire family of repetition codes, the five-qubit, Steane, Shor, and surface codes. When errors are measured in terms of the diamond norm, we find that the error correction is typically much more effective for unitary errors than for dephasing. We observe this behavior for a wide range of codes after a single level of encoding, and in the thresholds of concatenated codes using hard decoders. We show that this holds with great generality by proving a bound on the performance of any stabilizer code when the noise at the physical level is unitary. By comparing the diamond norm error $D_{\diamond }^{\prime }$ of the logical qubit with the same quantity at the physical level $D_{\diamond }$, we show that $D_{\diamond }^{\prime }\le c{D}_{\diamond }^d$ where $d$ is the distance of the code and $c$ is a constant that depends on the code but not on the error. This bound compares very favorably to the performance of error correction for dephasing noise and other Pauli channels, where an error correcting code of odd distance $d$ will exhibit a scaling $D_{\diamond }^{\prime }\sim D_{\diamond }^{(d+1)/2}$.

Experiments with quantum gas microscopes have started to explore the antiferromagnetic phase of the two-dimensional Fermi-Hubbard model and effects of doping with holes away from half filling. In this work we show how direct measurements of the system averaged two-spin density matrix and its full counting statistics can be used to identify different correlated magnetic phases with or without long-range order. We discuss examples of phases which are potentially realized in the Hubbard model close to half filling, including antiferromagnetically ordered insulators and metals, as well as insulating spin liquids and metals with topological order. For these candidate states we predict the doping- and temperature dependence of local correlators, which can be directly measured in current experiments.

We present a scheme to generate steady-state atomic spin squeezing in a cavity QED system using cavity-mediated Raman transitions to engineer effective atom-photon interactions, which include both linear and nonlinear (dispersive) atom-cavity couplings, on a potentially equal footing. We focus on a regime where the dispersive coupling is very large and find that the steady state of the system can in fact be a strongly spin-squeezed Dicke state $|N/2,0\rangle$ of the atomic ensemble. These states offer Heisenberg-limited metrological properties and feature genuine multipartite entanglement among the entire atomic ensemble.

We study the directional phase-sensitive amplification between microwave and optical photons in a triple-cavity optomechanical system consisting of three cavities and one mechanical resonator. When the microwave cavity is pumped on both red and blue sidebands but the directly coupled optical cavities are only pumped on the red sidebands, directional phase-sensitive amplification can be realized by tuning the effective optomechanical couplings and the coupling strength between the optical cavities. We show that both gain and bandwidth of the amplifier can be enhanced by increasing the optomechanical cooperativities, indicating an unlimited gain-bandwidth product. Meanwhile, the proposed phase-sensitive amplifier can approach the quantum limit of zero added noise in the large cooperativity limit.

We establish an exact mapping between the positions of $N$ noninteracting fermions in a two-dimensional rotating harmonic trap in its ground state and the eigenvalues of the $N\times N$ complex Ginibre ensemble of random matrix theory (RMT). Using RMT techniques, we make precise predictions for the statistics of the positions of the fermions, both in the bulk as well as at the edge of the trapped Fermi gas. In addition, we compute exactly, for any finite $N$, the Rényi entanglement entropy and the number variance of a disk of radius $r$ in the ground state. We show that while these two quantities are proportional to each other in the (extended) bulk, this is no longer the case very close to the trap center nor at the edge. Near the edge, and for large $N$, we provide exact expressions for the scaling functions associated with these two observables.

We study quantum information aspects of the fermionic entanglement negativity recently introduced [H. Shapourian et al., Phys. Rev. B 95, 165101 (2017)] based on the fermionic partial transpose. In particular, we show that it is an entanglement monotone under the action of local quantum operations and classical communications, which preserves the local fermion-number parity, and satisfies other common properties expected for an entanglement measure of mixed states. We present fermionic analogs of tripartite entangled states such as $W$ and Greenberger-Horne-Zeilinger states and compare the results of bosonic and fermionic partial transpose in various fermionic states, where we explain why the bosonic partial transpose fails in distinguishing separable states of fermions. Finally, we explore a set of entanglement quantities which distinguish different classes of entangled states of a system with two and three fermionic modes. In doing so, we prove that vanishing entanglement negativity is a necessary and sufficient condition for separability of $N\ge 2$ fermionic modes with respect to the bipartition into one mode and the rest. We further conjecture that the entanglement negativity of inseparable states which mix local fermion-number parity is always nonvanishing.

Vacuum radiation has been the subject of theoretical study in both cosmology and condensed matter physics for many decades. Recently there has been impressive progress in experimental realizations as well. Here we study vacuum radiation when a field mode is driven both parametrically and by a classical source. We find that in the Heisenberg picture the field operators of the mode undergo a Bogoliubov transformation combined with a displacement; in the Schrödinger picture the oscillator evolves from the vacuum to a squeezed coherent state. Whereas the Bogoliubov transformation is the same as would be obtained if only the parametric drive were applied, the displacement is determined by both the parametric drive and the force. If the force is applied well after the parametric drive, then the displacement is essentially the same as would be obtained by the action of the force alone. If the force is applied well before, then the displacement is found to oscillate as a function of the time lag between the forcing and the parametric driving; the oscillations can be understood in terms of quantum interference. A rich variety of behavior is found when the force and parametric drive act simultaneously. The displacement depends only on the Fourier component of the force at a single resonant frequency when the forcing and parametric drive are well separated in time. However, for a weak parametric drive that is applied concurrently with the force we show that the displacement responds to a broad range of frequencies of the force spectrum. We apply our results to a superconducting circuit consisting of a Josephson junction-terminated transmission line. Implications for other experimental systems are also briefly discussed.

We investigate the 1140 nm magnetic-dipole transition of thulium atoms trapped in solid argon and neon. These solids can be straightforwardly grown on any substrate at cryogenic temperatures, making them prime targets for surface-sensing applications. Our data are well described by a splitting of the single vacuum transition into three components in both argon and neon, with each component narrower than the 0.8 nm spectrometer resolution. The lifetime of the excited states is 14.6(0.5) ms in argon and 27(3) ms in neon, shorter than in vacuum or in solid helium. We also collected visible laser-induced fluorescence spectroscopy showing broader emission features in the range of 580–600 nm. The narrow infrared features in particular suggest a range of possible applications.

Soft-x-ray free-electron-laser pulses were used to create highly charged molecular tetrabromothiophene species by sequential multiphoton ionization from bromine $3d$ orbitals. The experiment was performed at the SACLA facility in Japan and the products of molecular dissociation were analyzed by means of multicoincidence momentum-resolved ion time-of-flight spectroscopy. Total charge states up to +13 atomic units were produced, creating a particular dissociation pattern for the carbon ions, a Coulomb implosion, due to the concerted forces by the surrounding heavy bromine ions. This behavior was explored both experimentally and by numerical molecular-dynamics simulations and the fingerprints of the Coulomb implosion were identified in both. In simulations, Coulomb implosion was predicted to be highly sensitive to the initial (thermal) motion of the atoms and, after including vibrational motion for several temperatures, good general agreement between the experiment and simulations was found. The agreement with the experiment was further improved by adding charge dynamics to the simulation, according to our point-charge dynamics model with empirical rate constants.