Gaussian noise induced by loss on Gaussian states may be corrected by distributing Einstein-Podolsky-Rosen entanglement through the loss channel, purifying the entanglement using a noiseless linear amplifier (NLA), and then using it for continuous-variable teleportation of the input state. Linear optical implementations of the NLA unavoidably introduce small amounts of excess noise and detection, and source efficiency will be limited in current implementations. In this paper, we analyze the error-correction protocol with nonunit efficiency sources and detectors and show the excess noise may be partially compensated by adjusting the classical gain of the teleportation protocol. We present a strong case for the potential of demonstrable error-correction with current technology.

We calculate electron capture and ionization cross sections of ${\mathrm{N}}_2$ impacted by the ${\mathrm{H}}^{+}$ projectile at keV energies. To this end, we employ the time-dependent density-functional theory coupled nonadiabatically to molecular dynamics. To avoid the explicit treatment of the complex density matrix in the calculation of cross sections, we propose an approximate method based on the assumption of constant ionization rate over the period of the projectile passing the absorbing boundary. Our results agree reasonably well with experimental data and semi-empirical results within the measurement uncertainties in the considered energy range. The discrepancies are mainly attributed to the inadequate description of exchange-correlation functional and the crude approximation for constant ionization rate. Although the present approach does not predict the experiments quantitatively for collision energies below 10 keV, it is still helpful to calculate total cross sections of ion-molecule collisions within a certain energy range.

We report efficient microwave-optical two photon excitation of Rb Rydberg atoms in a magneto-optical trap. This approach allows the excitation of normally inaccessible states and provides a path toward excitation of high-angular-momentum states. The efficiency stems from the elimination of the Doppler width, the use of a narrow-band pulsed laser, and the enormous electric-dipole matrix element connecting the intermediate and final states of the transition. The excitation is efficient in spite of the low optical and microwave powers, of order 1 kW and 1 mW, respectively. This is an application of the large dipole coupling strengths between Rydberg states to achieve two-photon excitation of Rydberg atoms.

We study the mass imbalanced Fermi-Fermi mixture within the framework of a two-dimensional lattice fermion model. Based on the thermodynamic and species-dependent quasiparticle behavior, we map out the finite-temperature phase diagram of this system and show that unlike the balanced Fermi superfluid, there are now two different pseudogap regimes as PG-I and PG-II. While within the PG-I regime both the fermionic species are pseudogapped, PG-II corresponds to the regime where pseudogap feature survives only in the light species. We believe that the single-particle spectral features that we discuss in this paper are observable through the species-resolved radio-frequency spectroscopy and momentum-resolved photoemission spectroscopy measurements on systems such as $6_{\text{Li}}\text{−}{40}_{\text{K}}$ mixture. We further investigate the interplay between the population and mass imbalances and report that at a fixed population imbalance, the BCS-BEC crossover in a Fermi-Fermi mixture would require a critical interaction ($U_c$) for the realization of the uniform superfluid state. The effect of imbalance in mass on the exotic Fulde-Ferrell-Larkin-Ovchinnikov superfluid phase has been probed in detail in terms of the thermodynamic and quasiparticle behavior of this phase. It has been observed that in spite of the $s$-wave symmetry of the pairing field, a nodal superfluid gap is realized in the Larkin-Ovchinnikov regime. Our results on the various thermal scales and regimes are expected to serve as benchmarks for the experimental observations on $6_{\text{Li}}\text{−}{40}_{\text{K}}$ mixture.

We investigate the out-of-equilibrium dynamics of ultracold atoms trapped in an optical lattice and loaded into an optical resonator that is driven transversely. We derive an effective quantum master equation for weak atom-light coupling that can be brought into Lindblad form in both the bad- and good-cavity limits. In the so-called bad-cavity regime, we find that the steady state is always that of infinite temperature, but that the relaxation dynamics can be highly nontrivial. For small hopping, the interplay between dissipation and strong interactions generally leads to anomalous diffusion in the space of atomic configurations. However, for a fine-tuned ratio of cavity-mediated and on-site interactions, we discover a limit featuring normal diffusion. In contrast, for large hopping and vanishing on-site interactions, the system can be described by a linear rate equation leading to an exponential approach of the infinite-temperature steady state. Finally, in the good-cavity regime, we show that for vanishing on-site interactions, the system allows for optical pumping between momentum mode pairs enabling cavity cooling.

Self-accelerating shape-invariant beams are attracting major attention, presenting applications in many areas such as laser manipulation and patterning, light-sheet microscopy, and plasma channels. Moreover, optical lattices are offering many applications, including quantum computation, quantum phase transition, spin-exchange interaction, and realization of magnetic fields. We report observation of a class of accelerating and self-healing beams which covers the features required by all the aforementioned applications. These beams are accelerating, shape invariant, and self-healing for more than several tens of meters, have numerous phase anomalies and unprecedented patterns, and can be feasibly tuned. Diffraction of a plane wave from radial phase gratings generates such beams, and due to their beauty and structural complexity we have called them “carpet” beams. By tuning the value of phase variations over the grating, the resulting carpet patterns are converted into two-dimensional optical lattices with polar symmetry. Furthermore, the number of spokes in the radial grating, phase variation amplitude, and wavelength of the impinging light beam can also be adjusted to obtain additional features. We believe that radial carpet beams and lattices might find more applications in optical micromanipulation, optical lithography, super-resolution imaging, lighting design, optical communication through atmosphere, etc.

We demonstrate the spin and extrinsic orbital-angular-momentum interaction of light in a few-mode fiber. The interaction of spin and extrinsic orbital angular momentum splits the degeneracy of a degenerate subspace of the higher-order solution of Maxwell's equation and creates correction to the propagating constants of its degenerate eigenmodes that results in the formation of transmitting-length-dependent beat along the propagating trajectory. We examine the interaction of spin and extrinsic orbital angular momentum by the observation of the rotation of the polarization or intensity pattern of the propagating modes which are the combinations of these degenerate eigenmodes when altering the length of this transmitting-length-dependent beat. In experiment, the length of this transmitting-length-dependent beat is altered by the alteration of the pitch of a fiber helix which is made for the production of the extrinsic angular momentum and where the space inversion or mirror symmetry of the mode is broken and the interaction between the spin and extrinsic orbital angular momentum is realized. We demonstrate that the spin and extrinsic orbital-angular-momentum interaction induce the rotation of the polarization or intensity pattern of propagating modes in a way related with the order index of spin and orbital angular momentum carried by their constituent eigencomponents, which are circular vortex modes. This work may contribute to understanding the physical mechanism underlying the evolution of the modes in few-mode fiber or other complex systems for their various applications.

We study coherent and radiative interactions induced among two or more quantum units by coupling them to two-dimensional (2D) lattices acting as structured environments. This model can be representative of atoms trapped near photonic crystal slabs, trapped ions in Coulomb crystals, or to surface acoustic waves on piezoelectric materials, cold atoms on state-dependent optical lattices, or even circuit QED architectures, to name a few. We compare coherent and radiative contributions for the isotropic and directional regimes of emission into the lattice, for infinite and finite lattices, highlighting their differences and existing pitfalls, e.g., related to long-time or large-lattice limits. We relate the phenomenon of directionality of emission with linear-shaped isofrequency manifolds in the dispersion relation, showing a simple way to disrupt it. For finite lattices, we study further details such as the scaling of resonant number of lattice modes for the isotropic and directional regimes, and relate this behavior with known van Hove singularities in the infinite lattice limit. Furthermore, we export the understanding of emission dynamics with the decay of entanglement for two quantum, atomic or bosonic, units coupled to the 2D lattice. We analyze in some detail completely subradiant configurations of more than two atoms, which can occur in the finite lattice scenario, in contrast with the infinite lattice case. Finally, we demonstrate that induced coherent interactions for dark states are zero for the finite lattice.

We study the adiabatic-impulse approximation (AIA) as a tool to approximate the time evolution of quantum states when driven through a region of small gap. Such small-gap regions are a common situation in adiabatic quantum computing and having reliable approximations is important in this context. The AIA originates from the Kibble-Zurek theory applied to continuous quantum phase transitions. The Kibble-Zurek mechanism was developed to predict the power-law scaling of the defect density across a continuous quantum phase transition. Instead, here we quantify the accuracy of the AIA via the trace norm distance with respect to the exact evolved state. As expected, we find that for short times or fast protocols, the AIA outperforms the simple adiabatic approximation. However, for large times or slow protocols, the situation is actually reversed and the AIA provides a worse approximation. Nevertheless, we found a variation of the AIA that can perform better than the adiabatic one. This counterintuitive modification consists in crossing the region of small gap twice. Our findings are illustrated by several examples of driven closed and open quantum systems.

Wave-packet scattering from a stationary potential is significantly modified when the wave packet is subject to an external time-dependent force during the interaction. In the semiclassical limit, wave-packet motion is simply described by Newtonian equations, and the external force can, for example, cancel the potential force, making a potential barrier transparent. Here we consider wave-packet scattering from reflectionless potentials, where in general the potential becomes reflective when probed by an accelerated wave packet. In the particular case of the recently introduced class of complex Kramers-Kronig potentials we show that a broad class of time-dependent forces can be applied without inducing any scattering, while there is a breakdown of the reflectionless property when there is a broadband distribution of initial particle momentum, involving both positive and negative components.

For phase estimation using qubits, we show that for some noise channels, the optimal entanglement-assisted strategy depends on the noise level. We note that there is a nontrivial crossover between the parallel-entangled strategy and the ancilla-assisted strategy: in the former the probes are all entangled; in the latter the probes are entangled with a noiseless ancilla but not among themselves. The transition can be explained by the fact that separable states are more robust against noise and therefore are optimal in the high-noise limit, but they are in turn outperformed by ancilla-assisted ones.

The $X{}^2\mathrm{\Sigma }_{1/2}^{+}$, $A{}^2\mathrm{\Pi }_{1/2}$, $A{}^2\mathrm{\Pi }_{3/2}$, and $B^2{\mathrm{\Sigma }}_{1/2}^{+}$ potential-energy curves for Rb+He are computed at the spin-orbit multireference configuration interaction level of theory using a hierarchy of Gaussian basis sets at the double-zeta (DZ), triple-zeta (TZ), and quadruple-zeta (QZ) levels of valence quality. Counterpoise and Davidson-Silver corrections are employed to remove basis-set superposition error and ameliorate size-consistency error. An extrapolation is performed to obtain a final set of potential-energy curves in the complete basis-set (CBS) limit. This yields four sets of systematically improved $X{}^2\mathrm{\Sigma }_{1/2}^{+}$, $A{}^2\mathrm{\Pi }_{1/2}$, $A{}^2\mathrm{\Pi }_{3/2}$, and $B^2{\mathrm{\Sigma }}_{1/2}^{+}$ potential-energy curves that are used to compute the $A{}^2\mathrm{\Pi }_{3/2}$ bound vibrational energies, the position of the $D_2$ blue satellite peak, and the $D_1$ and $D_2$ pressure broadening and shifting coefficients, at the DZ, TZ, QZ, and CBS levels. Results are compared with previous calculations and experimental observation.

We investigate laser-cooled atoms periodically driven by pulsed standing waves of light tuned close to an open atomic transition. This nonunitary system displays survival resonances for certain driving frequencies. The survival resonances emerge as a result of the matter-wave Talbot-Lau effect, similar to the Talbot effect causing quantum resonances in the atom optics $\delta$-kicked rotor. Since the Talbot-Lau effect occurs for incoherent waves, the survival resonances can be observed using thermal atoms. A microlensing effect can enhance the height and incisiveness of the resonances. This may find applications in precision measurements.

By using the thin-layer approach, we derive the effective equation for the electromagnetic wave propagating along a space curve. We find intrinsic spin-orbit, extrinsic spin-orbit, and extrinsic orbital angular-momentum and intrinsic orbital angular-momentum couplings induced by torsion, which can lead to geometric phase, spin, and orbital Hall effects. And we show the helicity inversion induced by curvature that can convert a right-handed circularly polarized electromagnetic wave into a left-handed polarized one, vice versa. Finally, we demonstrate that the gauge invariance of the effective dynamics is protected by the geometrically induced gauge potential.

Distributed quantum sensing uses quantum correlations between multiple sensors to enhance the measurement of unknown parameters beyond the limits of unentangled systems. We describe a sensing scheme that uses continuous-variable multipartite entanglement to enhance distributed sensing of field-quadrature displacement. By dividing a squeezed-vacuum state between multiple homodyne-sensor nodes using a lossless beam-splitter array, we obtain a root-mean-square (rms) estimation error that scales inversely with the number of nodes (Heisenberg scaling), whereas the rms error of a distributed sensor that does not exploit entanglement is inversely proportional to the square root of the number of nodes (standard quantum limit scaling). Our sensor's scaling advantage is destroyed by loss, but it nevertheless retains an rms-error advantage in settings in which there is moderate loss. Our distributed sensing scheme can be used to calibrate continuous-variable quantum key distribution networks, to perform multiple-sensor cold-atom temperature measurements, and to do distributed interferometric phase sensing.

We consider three-partite pure states in the Hilbert space ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^n$ and investigate to which states a given state can be locally transformed with a nonvanishing probability. Whenever the initial and final states are elements of the same Hilbert space, the problem can be solved via the characterization of the entanglement classes which are determined via stochastic local operations and classical communication (SLOCC). In the particular case considered here, the matrix pencil theory can be utilized to address this point. In general, there are infinitely many SLOCC classes. However, when considering transformations from higher to lower dimensional Hilbert spaces, an additional hierarchy among the classes can be found. This hierarchy of SLOCC classes coarse grains SLOCC classes which can be reached from a common resource state of higher dimension. We first show that a generic set of states in ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^n$ for $n=m$ is the union of infinitely many SLOCC classes, which can be parameterized by $m-3$ parameters. However, for $n\ne m$ there exists a single SLOCC class which is generic. Using this result, we then show that there is a full-measure set of states in ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^n$ such that any state within this set can be transformed locally to a full measure set of states in any lower dimensional Hilbert space. We also investigate resource states, which can be transformed to any state (not excluding any zero-measure set) in the smaller dimensional Hilbert space. We explicitly derive a state in ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^{2m-2}$ which is the optimal common resource of all states in ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^m$. We also show that for any $n<2m$ it is impossible to reach all states in ${\mathbb{C}}^2\otimes {\mathbb{C}}^m\otimes {\mathbb{C}}^{\tilde{n}}$ whenever $\tilde{n}>m$.

Fault-tolerant quantum computation (FTQC) schemes that use multiqubit large block codes can potentially reduce the resource overhead to a great extent. A major obstacle is the requirement for a large number of clean ancilla states of different types without correlated errors inside each block. These ancilla states are usually logical stabilizer states of the data-code blocks, which are generally difficult to prepare if the code size is large. Previously, we have proposed an ancilla distillation protocol for Calderbank-Shor-Steane (CSS) codes by classical error-correcting codes. It was assumed that the quantum gates in the distillation circuit were perfect; however, in reality, noisy quantum gates may introduce correlated errors that are not treatable by the protocol. In this paper, we show that additional postselection by another classical error-detecting code can be applied to remove almost all correlated errors. Consequently, the revised protocol is fully fault tolerant and capable of preparing a large set of stabilizer states sufficient for FTQC using large block codes. At the same time, the yield rate can be boosted from $O\left(t^{-2}\right)$ to $O\left(1\right)$ in practice for an $\left[\right[n,k,d=2t+1\left]\right]$ CSS code. Ancilla preparation for the [[23,1,7]] quantum Golay code is numerically studied in detail through Monte Carlo simulation. The results support the validity of the protocol when the gate failure rate is reasonably low. To the best of our knowledge, this approach is one of the first attempts to prepare general large block stabilizer states free of correlated errors for FTQC in a fault-tolerant and efficient manner.

The optimal quantum control theory is employed to determine electric pulses capable of producing quantum gates with a fidelity higher than 0.9997, when noise is not taken into account. Particularly, these quantum gates were chosen to perform the permutation algorithm in hybrid qubits in double quantum dots (DQDs). The permutation algorithm is an oracle based quantum algorithm that solves the problem of the permutation parity faster than a classical algorithm without the necessity of entanglement between particles. The only requirement for achieving the speedup is the use of a one-particle quantum system with at least three levels. The high fidelity found in our results is closely related to the quantum speed limit, which is a measure of how fast a quantum state can be manipulated. Furthermore, we model charge noise by considering an average over the optimal field centered at different values of the reference detuning, which follows a Gaussian distribution. When the Gaussian spread is of the order of 5 $\mu \text{eV}$ (10% of the correct value), the fidelity is still higher than 0.95. Our scheme also can be used for the practical realization of different quantum algorithms in DQDs.