Distributing entangled pairs is a fundamental operation required for many quantum information science and technology tasks. In a general entanglement distribution scheme, a photonic pulse is used to entangle a pair of remote quantum memories. Most applications require multiple entangled pairs between remote users, which in turn necessitates several photonic pulses (single photons) being sent through the channel connecting those users. Here we present an entanglement distribution scheme using only a single photonic pulse to entangle an arbitrary number of remote quantum memories. As a consequence, the spatial temporal resources are dramatically reduced. We show how this approach can be simultaneously combined with an entanglement purification protocol to generate even-higher-fidelity entangled pairs. The combined approach is faster to generate those high-quality pairs and requires fewer resources in terms of both matter qubits and photons consumed. To estimate the efficiency of our scheme we derive a normalized rate taking into account the raw rate at which the users can generate purified entangled pairs divided by the total resources used. We compare the efficiency of our system with the Deutsch protocol in which the entangled pairs have been created in a traditional way. Our scheme outperforms this approach in terms of both generation rate and resources required. Finally, we show how our approach can be extended to more general error correction and detection schemes with higher normalized generation rates naturally occurring.

We investigate the collective spin dynamics of a self-rephasing bosonic ensemble of ${}^{87}\mathrm{Rb}$ trapped in a one-dimensional vertical optical lattice. We show that the combination of the frequency shifts induced by atomic interactions and inhomogeneous dephasing, together with the spin self-rephasing mechanism, leads to the existence of a “magic density”: i.e., a singular operating point where the clock transition is first-order insensitive to density fluctuations. This feature is very appealing for improving the stability of quantum sensors based on trapped pseudo-spin-1/2 ensembles. Ramsey spectroscopy of the $|5s{}^2{S}_{1/2},F=1,m_F=0\rangle \to |5s{}^2{S}_{1/2},F=2,m_F=0\rangle$ hyperfine transition is in qualitative agreement with a numerical model based on coupled Bloch equations of motion for energy-dependent spin vectors.

Matter wave interferometry is becoming an increasingly important technique in quantum metrology. However, unlike its photonic counterpart, this technique relies on the interference of particles possessing a nonzero rest mass and an electric charge. Matter waves can therefore experience alterations in their wavelike features while propagating through uniform fields to which a linear potential can be attributed, e.g., the Newtonian gravitational potential. Here, we derive the propagation kernel attributed to matter waves within such a potential. This kernel thereafter allows us to provide analytical formulations for structured matter waves subjected to a linear potential. Our formulations are in agreement with both the classical dynamics attributed to such waves and with previous interferometry experiments. Eigenbasis representations of structured matter waves are also introduced along with their application to enhanced interferometric measurements. Our results are not only relevant to matter wave interferometry, but also emphasize its fundamental differences with respect to photonic interferometry.

We discuss an original manifestation of Anderson localization for wave packets launched with a finite average velocity: after an initial ballistic motion, the center of mass of the wave packet experiences a retroreflection and slowly returns to its initial position, an effect that we call “quantum boomerang” and describe numerically and analytically in dimension 1. In dimension 3, we show numerically that the quantum boomerang is a genuine signature of Anderson localization: it exists if and only if the quantum dynamics is localized.

Future free-space optical clock networks will require optical links for time and frequency transfer. In many potential realizations of these networks, these links will extend over long distances and will span moving platforms, e.g., ground-to-air or ground-to-satellite. In these cases, the transverse platform motion coupled with spatial variations in atmospheric optical turbulence will lead to a breakdown in the time-of-flight reciprocity upon which optical two-way time-frequency transfer is based. Here, we report experimental measurements of this effect by use of comb-based optical two-way time-frequency transfer over two spatially separated optical links. We find only a modest degradation in the time synchronization and frequency syntonization between two sites, in good agreement with theory. Based on this agreement, we can extrapolate this 2-km result to longer distances, finding only a few-fs timing noise increase due to turbulence for a link from ground to a midearth orbit satellite.

Introducing correlations between the spatial and temporal degrees of freedom of a pulsed optical beam (or wave packet) can profoundly alter its propagation in free space. Indeed, appropriate spatiotemporal spectral correlations can render the wave packet propagation-invariant: the spatial and temporal profiles remain unchanged along the propagation axis. The spatiotemporal spectral locus of any such wave packet lies at the intersection of the light cone with tilted spectral hyperplanes. We investigate $(2+1)\mathrm{D}$ propagation-invariant “space-time” light sheets and identify ten classes categorized according to the magnitude and sign of their group velocity and the nature of their spatial spectrum—whether the low spatial frequencies are physically allowed or forbidden according to their compatibility with causal excitation and propagation. We experimentally synthesize and characterize all ten classes using an experimental strategy capable of synthesizing space-time wave packets that incorporate arbitrary spatiotemporal spectral correlations.

The recent criticism of Vaidman's proposal for the analysis of the past of a particle in the nested interferometer is refuted. It is shown that the definition of the past of the particle adopted by Englert et al. [B. G. Englert et al., Phys. Rev. A 96, 022126 (2017)] is applicable only to a tiny fraction of photons in the interferometer which indeed exhibit different behavior. Their proof that all pre- and postselected particles behave this way, i.e., follow a continuous trajectory, does not hold, because it relies on the assumption that it is intended to prove.

We stand by our findings in Phys. Rev. A 96, 022126 (2017). In addition to refuting the invalid objections raised by Peleg and Vaidman, we report a retrocausation problem inherent in Vaidman's definition of the past of a quantum particle.

We investigate the boundary condition dependence of non-Hermitian topological systems. We consider a complex extension of the Su-Schrieffer-Heeger model. We discuss that topological zero-energy modes exist if the system has open edges. We show that robust zero-energy states appear for the closed lattice too. We also consider a complex extension of the Bernevig-Hughes-Zhang Hamiltonian. We show that the translationally invariant system has a complex-valued band structure. We discuss that $\mathcal{PT}$ symmetry is restored for helical edge states if the translational invariance is lost in one direction.

We investigate quantum teleportation between users located in noninertial frames and analyze how the acceleration can improve the fidelity of quantum teleportation between two relatively accelerating partners. We see that the fidelity increases with respect to the acceleration as a consequence of the Unruh effect. In our scheme, Alice and Rob initially share a special $\mathsf{X}$-type state, Alice stays stationary, while Rob is uniformly accelerated with respect to Alice. We additionally study the system-environment dynamics in the noninertial frames, i.e., the $\mathsf{X}$-type state shared between Alice and Rob interacts with the environment via amplitude damping channel. Our results demonstrate a monogamy relation of the fidelity with acceleration and amplitude damping channel, these result suggest that the fidelity of quantum teleportation cannot be enhanced simultaneously by both the acceleration and amplitude damping channel. Such a property may have potential applications in protecting the fidelity of quantum teleportation performed by observers in relative motion.

We present a phase control method for a general three-mode system with closed-loop coupling that drives the system into an entangled steady state and produces directional steering between two completely symmetric modes via quantum interference effects. In the scheme, two modes are coupled with each other both by a direct binary interaction and by an indirect interaction through a third intermediate damping mode, creating interference effects determined by the relative phase between the two physical interaction paths. By calculating the populations and correlations of the two modes, we show that, depending on the phase, two modes can be prepared into an entangled steady state with asymmetric and directional steering even if they possess completely symmetric decoherence properties. Meanwhile, entanglement and steering can be significantly enhanced due to constructive interference and thus are more robust to thermal noises. This provides an active method to manipulate the asymmetry of steering instead of adding asymmetric losses or noises to subsystems at the cost of reducing steerability. Moreover, we show that the interference effects can also enhance and control the correlations between other pairs of modes in the loop with opposite phase-dependent behavior, indicating monogamy constraints for distributing correlations among multiple parties. The present model could be applied in cavity optomechanical systems or in antiferromagnets where all components can mutually interact.

The destruction of high-$n, n\sim 300$, strontium atoms contained in a hot atomic beam through Rydberg-Rydberg collisions is examined. The Rydberg atoms are initially created, under blockade conditions, in a localized volume and their subsequent motions lead to creation of a string of Rydberg atoms. The Rydberg atoms, however, are formed with a thermal distribution of velocities resulting in Rydberg-Rydberg collisions which lead to their destruction. The experimental data are interpreted using classical trajectory Monte Carlo techniques that simulate the excitation of the Rydberg atoms together with their subsequent motions. Using calculated collisional ionization cross sections and blockade radii, the model yields results in good agreement with experiment. The results highlight the important role collisional destruction can play when attempting to study (long-range) Rydberg-Rydberg interactions in a hot atomic beam.

We report the observation of a $4.1\pm 0.2$ THz quantum beat in the photoelectron spectrum of molecular nitrogen following coherent optical excitation by 14 eV photons and probed by two photons at 800 nm. The intermediate states in the two pathway interference are the valence state $b^{\prime }{}^1\mathrm{\Sigma }_u^{+}v=13$ and Rydberg state $c_4^{\prime }{}^1\mathrm{\Sigma }_u^{+}v=4$. The amplitude of the coherent oscillations decays in about 3 ps and does not revive. Opposite phases in the photoelectron yield corresponding to different vibrational levels of the final cationic states $X{}^2\mathrm{\Sigma }_g^{+}v=0,A{}^2{\mathrm{\Pi }}_{u}v=1$, and $X{}^2\mathrm{\Sigma }_g^{+}v=2,3,4$ are reported. Simulation results show that the interference originates from the strongly mixed character of the two simultaneously excited eigenstates, causing large population oscillations in the zeroth-order Born-Oppenheimer diabatic basis. No quantum beat was observed when probing with single photon at 400 nm, suggesting a resonance with a state just below the ionization potential which acts as a filter, improving the quantum beat contrast.

The laser cooling of atoms with a narrow-line optical transition, i.e., in regimes of quantum nature of laser-light interactions resulting in a significant recoil effect, is studied. It is demonstrated that a minimum laser-cooling temperature for two-level atoms in a standing wave reached for red detuning close to three recoil frequencies is vastly different from the theory used for a semiclassical description of Doppler cooling. A set of dimensionless parameters uniquely characterizing the time evolution and the steady state of different atoms with narrow-line optical transitions in the laser field is introduced. The results can be used for analysis of optimal conditions for laser cooling of atoms with narrow lines such as Ca, Sr, and Mg, which are of great interest for atomic clocks.

Photoionization microscopy (PM) is a photoelectron imaging method based on the measurement of the electron current probability density in the case of photoionization of an atom in an external uniform static electric field. The resolution of PM is high enough for observing the quantum oscillatory spatial structure of the outgoing electron flux. We examine the Stark states of the magnesium atom and focus on PM features which are specific to the energy range just above the saddle-point energy. Particularly, the existence of $m$-dependent saddle-point thresholds is clearly demonstrated by following the evolution of the recorded angular distributions with energy. Furthermore, the outer inflection point radii of the radial distributions of the images are generally found to increase with energy monotonically but occasionally abruptly, signaling parabolic channel openings (above which these channels are converted to continua). This observation is in accord with recently reported quantum mechanical PM calculations for hydrogenic systems. More importantly, there are a number of nonmonotonic variations (local maxima) of the outer inflection point. The latter are attributed to the local emergence of an intensity halo at the outer part of the images, a feature relevant to the excitation of quasibound Stark states (resonances). The emergence of the halo for a given resonance is found to depend on the static electric field strength. These observations along with the difficulties in observing resonant effects on the recorded images of nonhydrogenic atoms, such as the medium-size magnesium atom, are discussed in detail.

We use the $R$-matrix with time dependence method to study detachment from ${\mathrm{F}}^{-}$ in circularly polarized laser fields of infrared wavelength. By decomposing the photoelectron momentum distribution into separate contributions from detached $2p_1$ and $2p_{-1}$ electrons, we demonstrate that the detachment yield is distributed asymmetrically with respect to these initial orbitals. We observe the well-known preference for strong-field detachment of electrons that are initially counter-rotating relative to the field, and calculate the variation in this preference as a function of photoelectron energy. The wavelengths used in this work provide natural grounds for comparison between our calculations and the predictions of analytical approaches tailored for the strong-field regime. In particular, we compare the ratio of counter-rotating electrons to corotating electrons as a function of photoelectron energy. We carry out this comparison at two wavelengths, and observe good qualitative agreement between the analytical predictions and our numerical results.

We develop an effective field theory to understand collective modes of a three-dimensional two-component Fermi superfluid with dipolar interparticle interactions, which are modeled by an idealized separable potential. We first examine the phase transition of the system at zero temperature, as the fermionic superfluidity is known to be characterized by two competing order parameters. We find that for strong interactions there exists a regime where the two order parameters are out of phase and coupled, giving rise to an undamped massive Leggett mode. This is in addition to the well-known gapless phonon mode. We show that the Leggett mode can be seen in the spectral function of the in-medium Cooper pairs, and in principle could be measured through Bragg spectroscopy.

We report an optomechanical system with two dielectric membranes inside a Fabry-Pérot cavity. The cavity resonant frequencies are measured in such a two-membrane-in-the-middle system, which shows an interesting band-structure-like diagram. This system exhibits great controllability on the parameters of the system. The positions and angles of each membrane can be manipulated on demand by placing two membranes inside the cavity separately. The eigenfrequencies of the vibrational modes of the membranes can also be tuned individually with piezoelectricity. This scheme could be straightforwardly extended to multiple-membrane-in-the-middle systems, where more than two membranes are involved. Such a well-controllable multiple-membrane optomechanical system provides a promising platform for studying nonlinear and quantum dynamical phenomena in multimode optomechanics with distinct mechanical oscillators.

Subvacuum effects arise in quantum field theory when a classically positive quantity, such as the local energy density, acquires a negative renormalized expectation value. Here we investigate the case of states of the quantized electromagnetic field with negative mean-squared electric field and their effects on the propagation of light pulses in a nonlinear dielectric material with a nonzero third-order susceptibility. We identify two distinct signatures of the subvacuum effect in this situation. The first is an increase in the speed of the pulse, which is analogous to the superluminal light propagation in gravity which can arise from negative energy density. This increase in speed leads to a phase shift which might be large enough to observe. The second effect is a change in the frequency and power spectra of the pulse. We identify a specific measure of the modified spectra which can signal the presence of a negative mean-squared electric field. These ideas are implemented in the particular example of a wave guide filled with a nonlinear dielectric material.

In theoretical and experimental work, we consider the process in which an entangled photon pair recombines into a single photon in a nonlinear crystal, with particular interest in the amplitude distribution and coherence properties of the up-converted light. It is found that the up-converted state has perfect mutual coherence with the pump that originally produced the photon pair in another nonlinear crystal and that the amplitude has the spatiotemporal structure of the original pump mode, filtered by a linear transfer function. These conclusions are drawn from leading-order calculations performed in the interaction picture, with the multimodal nature of all fields being fully accounted for. In our experimental results, we observe the spatial similarity between the up-converted light and the original pump mode; their mutual coherence is demonstrated through observation of fringes with near-unit visibility. The effects studied here have potential applications in quantum information processing and also confirm that the principle of path indistinguishability holds even under unusually extreme conditions.