We study the Landau-Zener (LZ) dynamics in a setup of two Rydberg atoms with time-dependent detuning, both linear and periodic, using both the exact numerical calculations as well as the method of adiabatic impulse approximation (AIA). By varying the Rydberg-Rydberg interaction strengths, the system can emulate different three-level LZ models, for instance, bow-tie and triangular LZ models. The LZ dynamics exhibits nontrivial dependence on the initial state, the quench rate, and the interaction strengths. For large interaction strengths, the dynamics is well captured by AIA. In the end, we analyze the periodically driven case, and AIA reveals a rich phase structure involved in the dynamics. The latter may find applications in quantum state preparation, quantum phase gates, and atom interferometry.

The recent experimental observation of dissipation-induced structural instability provides new opportunities for exploring the competition mechanism between stationary and nonstationary dynamics [N. Dogra et al., Science 366, 1496 (2019)]. In that study, two orthogonal quadratures of cavity field were coupled to two different Zeeman states of a spinor Bose-Einstein condensate (BEC). Here we propose a scheme to couple two density-wave degrees of freedom of a BEC to two quadratures of the cavity field. Different from previous studies, the light-matter quadratures coupling in our model is endowed with a tunable coupling angle. Apart from the uniform and self-organized phases, we unravel a dynamically unstable state induced by the cavity dissipation. Interestingly, the dissipation defines a particular coupling angle, across which the instabilities disappear. Moreover, at this critical coupling angle, one of the two atomic density waves can be independently excited without affecting one another. It is also found that our system can be mapped into a reduced three-level model under the commonly used low-excitation-mode approximation. However, the effectiveness of this approximation is shown to be broken by the dissipative nature of some special system parameters, hinting that the low-excitation-mode approximation is insufficient in capturing some dissipation-sensitive physics. Our work enriches the quantum simulation toolbox in the cavity-quantum-electrodynamics system and broadens the frontiers of light-matter interaction.

One of the most intriguing topological features of open systems is that they exhibit exceptional point (EP) singularities. Apart from the widely explored second-order EPs (EP2s), the exploration of higher-order EPs in any system requires more complex topology, which is still a challenge. Here, we encounter a third-order EP (EP3) with the simultaneous presence of multiple second-order EPs in a simple fabrication feasible gain-loss assisted trilayer optical microcavity. Using the scattering-matrix formalism, we study the simultaneous interactions between three successive coupled states via avoided-resonance-crossing (ARC) phenomena, and we identify two EP2s near two ARC regimes. Such an occurrence of two EP2s inside a closed two-dimensional parametric space associated with an unbalanced gain-loss profile leads to the functionality of a cube-root branch point, i.e., an EP3. Following an adiabatic variation of two control parameters around the embedded EP3 in the presence of two identified EP2s, we present a robust successive-state-conversion mechanism among three coupled states. The proposed scheme indeed opens up a unique platform to manipulate light in integrated photonic devices.

Sorting spatial optical modes is a key challenge that underpins many applications from superresolved imaging to high-dimensional quantum key distribution. However, to date, implementations of optical mode sorters only operate on specific sets of modes, such as those carrying orbital angular momentum, and therefore lack versatility with respect to operation with an arbitrary spatial basis. Here we demonstrate an arbitrary spatial mode sorter by harnessing the random mode mixing process occurring during light propagation in a multimode fiber by wave-front shaping. By measuring the transmission matrix of the fiber, we show sorting of up to 25 transverse spatial modes of the Fourier, Laguerre-Gaussian, and random basis to an arbitrary set of positions at the output. Our approach provides a spatial mode sorter that is compact, easy to fabricate, programable, and usable with any spatial basis, which is promising for quantum and classical information science.

The single-photon cooperative dynamics of an assembly of two-level quantum emitters coupled by a bosonic bath is investigated. The bosonic bath is general and can be anything as long as the exchange of excitations between quantum emitters and bath is present. In these systems it is found that the population on the excited emitter keeps a simple and universal trapping law due to the existence of the system's dark states. Different from the trapping regime caused by photon-emitter dressed states, this type of trapping is only associated with the number of quantum emitters. According to the trapping law, cooperative spontaneous emission at the single-photon level in this kind of system is universally inhibited when the emitter number is large enough.

This paper shows how network coding and superdense coding can be combined to increase the classical data throughput by a factor $2-\epsilon$ (for some $\epsilon >0$) compared to the maximum that could be achieved using either network coding or superdense coding alone. Additionally, a general decomposition of a mixed network (i.e., consisting of classical and quantum links) is given, and it is reasoned that, owing to the inherent hardness of finding network codes, this may well lead to an increase in classical data throughput in practice, should a scenario arise in which quantum networks are used to transfer classical information.

In the present work we theoretically analyze the emergence appearance of entanglement between electronic and vibrational states in a system of two coupled molecules after switching on the interaction between them. In the framework of the adiabatic approach this effect appears because the equilibrium position of one of the molecules is sensitive to its electron occupation. The time evolution of the system shows multiple changes between its entangled and unentangled states. If this evolution starts from a pure coherent vibrational mode state and a state of a single electron localized in one of the molecules, the Schrödinger-cat states can appear during the system's time evolution. The Schrödinger-cat states can arise as a quantum superposition of distinct coherent states of molecule vibrational mode. Such states are entangled with empty and singly occupied electronic states of the molecule. The presence of entanglement between electronic and vibrational molecule states can be revealed by measuring the electron occupation (charge) of a particular molecule.

We study the effect of different open boundary conditions on the insulating ground states of the one-dimensional extended Bose-Hubbard model at and near unit filling. To this end, we employ the density-matrix renormalization-group method with system sizes up to 250 sites. To characterize the system, various order parameters and entanglement entropies are calculated. When opposite edge potentials are added to the two ends of the chain, the inversion symmetry is explicitly broken, and the regular bulk phases appear. On the other hand, simple open boundary conditions often exhibit nondegenerate ground states with a domain wall in the middle of the chain, which induces a sign flip of an order parameter. Such a domain wall can lead to an algebraic behavior of the off diagonals of the single-particle density matrix. We show that this algebraic behavior adds only a finite contribution to the entanglement entropy, which does not diverge as the system size increases. Therefore, it is not an indication of a superfluid phase. We confirm this picture by analytical calculations based on an effective Hamiltonian for a domain wall.

Quantum machine learning is a research discipline intersecting quantum algorithms and machine learning. While a number of quantum algorithms with potential speedups have been proposed, it is quite difficult to provide evidence that quantum computers will be useful in solving real-world problems. Our work makes progress towards this goal. In this work we design quantum algorithms for dimensionality reduction and for classification and combine them to provide a quantum classifier that we test on the MNIST data set of handwritten digits. We simulate the quantum classifier, including errors in the quantum procedures, and show that it can provide classification accuracy of $98.5\%$. The running time of the quantum classifier is only polylogarithmic in the dimension and number of data points. Furthermore, we provide evidence that the other parameters on which the running time depends scale favorably, ascertaining the efficiency of our algorithm.

The techniques of low-rank matrix recovery were adapted for quantum state tomography (QST) previously by Gross et al. [Phys. Rev. Lett. 105, 150401 (2010)] where they consider the tomography of $n$ spin-1/2 systems. For the density matrix of dimension $d=2^n$ and rank $r$ with $r\ll 2^n$, it was shown that randomly chosen Pauli measurements of the order $O[drlog{\left(d\right)}^2]$ are enough to fully reconstruct the density matrix by running a specific convex optimization algorithm. The result utilized the low operator norm of the Pauli operator basis, which makes it “incoherent” to low-rank matrices. For quantum systems of dimension $d$ not a power of two, Pauli measurements are not available, and one may consider using $\mathrm{SU}\left(d\right)$ measurements. Here, we point out that the $\mathrm{SU}\left(d\right)$ operators, owing to their high operator norm, do not provide a significant savings in the number of measurement settings required for successful recovery of all rank-$r$ states. We propose an alternative strategy in which the quantum information is swapped into the subspace of a power-two system using only $\mathrm{poly}[log{\left(d\right)}^2]$ gates at most with QST being implemented, subsequently, by performing $O[drlog{\left(d\right)}^2]$ Pauli measurements. We show that, despite the increased dimensionality, this method is more efficient than the one using $\mathrm{SU}\left(d\right)$ measurements.

We completely characterize the condition when a tile structure provides an unextendible product basis (UPB) and construct UPBs of different large sizes in ${\mathbb{C}}^m\otimes {\mathbb{C}}^n$. In particular, we show that there exists a UPB of size $(mn-4\lfloor \frac{m-1}{2}\rfloor )$ in ${\mathbb{C}}^m\otimes {\mathbb{C}}^n$ for any $n\ge m\ge 3$, which solves an open problem [S. Halder et al., Phys. Rev. A 99, 062329 (2019)]. As an application, we show that this class of UPBs can be perfectly distinguished by local operations and classical communications assisted with a $\lceil \frac{m}{2}\rceil \otimes \lceil \frac{m}{2}\rceil$ maximally entangled state.

The $W$ state is a symmetrically entangled multipartite state where a single excitation is shared by all parties. It is an important resource for various quantum algorithms and communication systems, and hence, its preparation is of immense interest to the quantum information community. We examine here a deterministic scheme to prepare a $W$ state of an $n$-qubit system with all-to-all pairwise exchange interaction between $n$ qubits. This relies on sharing superposed excitations of a smaller number of $q$ qubits among others. We present a bound on the maximal jumps from $q$ to $n$ and formalize a scheme to generate the $W_n$ state in $O(logn)$ stages. We demonstrate this scheme in the context of spin-torque-based quantum computing architecture that is characterized by repeated interactions between static and flying qubits.

A recently developed model to describe proton collisions from molecules involving basic atoms such as hydrogen, carbon, nitrogen, oxygen, and phosphorus (H, C, N, O, P) is extended to treat collisions with multiply charged ions. The ion-atom collisions are computed using the two-center basis generator method (TC-BGM), which has a proven track record of yielding accurate total cross sections for electron capture and ionization. The atomic net ionization cross sections are then used to assemble two models for ion-molecule collisions: an independent-atom model (IAM) that follows the Bragg additivity rule (labeled IAM-AR), and also the so-called pixel-counting method (IAM-PCM). The latter yields reduced cross sections relative to IAM-AR near the maximum, since it takes into account the overlapping nature of effective cross-sectional areas. The IAM-PCM for higher-charge projectiles leads to strong reductions of net ionization cross sections relative to the IAM-AR method, and is computed directly for projectile charges $Q=1,2,3$. The scaling behavior of the IAM-PCM is investigated over a wide range of energies $E$, and at high $E$ it converges towards the IAM-AR. An empirical scaling rule based on the IAM-PCM results is established which allows one to reproduce these results based on proton impact calculations. Detailed comparisons are provided for the uracil target (${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2{\mathrm{O}}_2$), for which other theoretical as well as experimental results are available. Data are also shown for targets such as water (${\mathrm{H}}_2\mathrm{O}$), methane (${\mathrm{CH}}_4$), adenine (${\mathrm{C}}_5{\mathrm{H}}_5{\mathrm{N}}_5$), L-valine (${\mathrm{C}}_5{\mathrm{H}}_{11}{\mathrm{NO}}_2$), and the nucleotide dAMP (${\mathrm{C}}_{10}{\mathrm{H}}_{14}{\mathrm{N}}_5{\mathrm{O}}_6\mathrm{P}$). Based on the scaling model derived from the IAM-PCM cross sections it is shown how the experimental data for uracil and water bombarded by multiply charged ions can be reduced to effective $Q=1$ cross sections, respectively, and these are compared to proton impact data.

An orthogonally polarized two-color (OTC) field of frequency ratio $1:2$ is known to generate both odd and even harmonic spectra in the mutually orthogonal directions of the fundamental and the second-harmonic fields, respectively. Recollision angles of an ionized electron driven back to its parent ion by an OTC field are inferred from the intensity ratio of odd and even harmonics, where the relative phase delays between the two-color fields can be used in time-resolved measurements on a subcycle time scale. In this work, we present the ab initio, many-electron calculation of time-resolved recollision angles of noble-gas atoms driven by an OTC field, based on the time-dependent density-functional theory. Our results agree well with experimental data by Shafir et al. in Nat. Phys. 5, 412 (2009) of helium atom and New J. Phys. 12, 073032 (2010) of neon atom.

We present designs for the augmentation “mirror” pulses of large-momentum-transfer atom interferometers that maintain their fidelity as the wave-packet momentum difference is increased. These biselective pulses, tailored using optimal control methods to the evolving bimodal momentum distribution, should allow greater interferometer areas and hence increased inertial measurement sensitivity, without requiring elevated Rabi frequencies or extended frequency chirps. Using an experimentally validated model, we have simulated the application of our pulse designs to large-momentum-transfer atom interferometry using stimulated Raman transitions in a laser-cooled atomic sample of ${}^{85}\mathrm{Rb}$ at 1 $\mu \mathrm{K}$. After the wave packets have separated by 42 photon recoil momenta, our pulses maintain a fringe contrast of 90%, whereas, for adiabatic rapid passage and conventional $\pi$ pulses, the contrast is less than 10%. Furthermore, we show how these pulses may be adapted to be robust to laser intensity variations between pulses and to suppress the detrimental off-resonant excitation that limits other broadband pulse schemes.

We study the formation of off-axis optical vortices propagating inside a double-Raman gain atomic medium. The atoms interact with two weak probe fields as well as two strong pump beams which can carry orbital angular momentum (OAM). We consider a situation when only one of the strong pump lasers carries an OAM. A particular superposition of probe fields coupled to the matter is shown to form specific optical vortices with shifted axes. Such off-axis vortices can propagate inside the medium with sub- or superluminal group velocity depending on the value of the two-photon detuning. The superluminal optical vortices are associated with the amplification as the energy of pump fields is transferred to the probe fields. The position of the peripheral vortices can be manipulated by the OAM and intensity of the pump fields. We show that the exchange of optical vortices is possible between individual probe beams and the pump fields when the amplitude of the second probe field is zero at the beginning of the atomic cloud. The model is extended to a more complex double Raman doublet interacting with four pump fields. In contrast to the double-Raman-singlet, now the generation of the off-axis sub- or superluminal optical vortices is possible even for zero two-photon detuning.