We use quantum optimal control to identify fast collision-based two-qubit $\sqrt{\mathrm{SWAP}}$ gates in ultracold atoms. We show that a significant speedup can be achieved by optimizing the full gate instead of separately optimizing the merge-wait-separate sequence of the trapping potentials. Our optimal strategy does not rely on the atoms populating the lowest eigenstates of the merged potential, and it crucially includes the accumulation of quantum phases before the potentials are fully merged. Our analyses transcend the particular trapping geometry, but for comparison with previous works, we present systematic results for an optical lattice and find greatly improved gate durations and fidelities.

Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in barium monofluoride (BaF) molecules has been performed in order to determine the lifetimes of the $A{}^2{\mathrm{\Pi }}_{1/2}$ and $A{}^2{\mathrm{\Pi }}_{3/2}$ states. The method permits control over experimental parameters such that systematic biases in the interpretation of the data can be controlled to below ${10}^{-3}$ relative accuracy. The statistically limited values for the lifetimes of the $A{}^2{\mathrm{\Pi }}_{1/2}(\nu =0)$ and $A{}^2{\mathrm{\Pi }}_{3/2}(\nu =0)$ states are 57.1(3) and 47.9(7) ns, respectively. Although discrepancies between presently and previously measured lifetimes and theoretical values persist, the ratio of these values is in good agreement with scaling for the different excitation energies. The results serve as a benchmark for current molecular structure calculations. The investigated molecular states are of relevance for an experimental search for a permanent electric dipole moment of the electron in BaF.

The three-body parameter (3BP) $a_{-}^{\left(1\right)}$ is crucial for understanding Efimov physics, and a universal 3BP has been shown in experiments and theory in ultracold homonuclear gases. The 3BP of heteronuclear systems has been predicted to possess much richer properties than its homonuclear counterparts due to the larger parameter space. In this work, we investigate the universal properties of $a_{-}^{\left(1\right)}$ for heavy-heavy-light (HHL) systems with a negative intraspecies scattering length $a_{HH}$. We find that $a_{-}^{\left(1\right)}$ follows a universal behavior determined by the van der Waals (vdW) interaction and the mass ratio. A linear fit of $a_{-}^{\left(1\right)}$ for large $|a_{HH}|$ leads to a simple formula, which gives the general dependence of $a_{-}^{\left(1\right)}$ on the mass ratio and the vdW length $r_{vdW}$. In a special case, when the two heavy atoms are in resonance, $a_{-}^{\left(1\right)}$ is approximately constant: $a_{-}^{\left(1\right)}=-(6.3\pm 0.6)r_{vdW,HL}$.

We propose a type of Feshbach resonance occurring when two different ultracold atoms in their ground state undergo an $s$-wave collision in the presence of a laser. The collisional levels of the atom pair are coupled by the laser to a rovibrational molecular level of the same electronic ground state: We name it a laser-assisted self-induced Feshbach resonance. This mechanism, valid for all heteronuclear quantum gas mixtures, is analyzed on the example of ultracold ${}^{87}\mathrm{Rb}$ and ${}^{84}\mathrm{Sr}$ atoms for which the resonant laser frequency falls in the subterahertz range. The interspecies scattering length can be controlled with the laser frequency and intensity without atom loss. Moreover, chirping slowly, the frequency allows for the adiabatic formation of ultracold ${}^{87}\mathrm{Rb}{}^{84}\mathrm{Sr}$ molecules in a manner very similar to a magnetic Feshbach resonance. A stimulated Raman adiabatic passage follows for stabilizing the molecules in their rovibronic ground state.

We show how the motional fluctuations of two systems of very different sizes—a trapped ion and a mirror—can be coupled coherently, via their common interaction with an optical cavity. Specifically, we show that the fluctuations can be transferred from one system to the other by using pulsed excitation of the cavity mode, and vibrational fluctuations can exhibit entanglement in steady state. We numerically display a sudden death of this entanglement at certain pulse parameters and ambient equilibrium temperature. More interestingly, the state of the driving pulse can also be coherently mapped, in a selective way, into the vibrational mode of the trapped ion or the mirror, which acts as a quantum memory for the optical pulse. We further present a detailed analysis to show that it is also possible to map the state of the pulse into a binomial entangled state of its vibrational fluctuations. We finally investigate how to sympathetically cool the mirror by cooling the ionic vibration, thanks to an effective coupling between the two modes.

We discuss the effect of a sharp variation in the input signal temporal envelope upon the output at destructive interference in Fano resonances. The interference takes place between the resonant and off-resonant partitions canceling each other. For high-Q resonances, the characteristic relaxation time for the former may be much larger than that for the latter. Then, if the variation of the input is faster than the relaxation of the resonant partition, it destroys the balance between the partitions, i.e., the conditions for the destructive interference. Therefore, the variation (whether an increase or a decrease) must give rise to a sharp spike in the uncompensated output. To illustrate these general arguments, we study numerically the scattering of a linearly polarized laser pulse of a rectangular envelope by a dielectric cylinder. The simulation reveals pronounced spikes in the scattered light, situated just behind the leading edge of the incident pulse and, counterintuitively, behind its trailing edge, when the incident pulse is already complete. The second spike is explained by the rapid release of the electromagnetic energy confined in the cylinder in the steady-state scattering owing to the destructive Fano interference. The spikes have narrow scattering diagrams so that the effect may be employed to design new nanodevices, e.g., a single-nanoparticle laser generating ultrashort pulses.

We propose a quantum metrology scheme in a cavity QED setup to achieve the Heisenberg limit. In our scheme, a series of identical two-level atoms randomly pass through and interact with a dissipative single-mode cavity. Different from the entanglement-based Heisenberg limit metrology scheme, we do not need to prepare the atomic entangled states before they enter into the cavity. We show that the initial atomic coherence will induce an effective driving to the cavity field, whose steady state is an incoherent superposition of orthogonal states, with the superposition probabilities being dependent on the atom-cavity coupling strength. By measuring the average photon number of the cavity in the steady state, we demonstrate that the root mean square of the fluctuation of the atom-cavity coupling strength is proportional to $1/N_c^2$ ($N_c$ is the effective atom number interacting with the photon in the cavity during its lifetime). It implies that we have achieved the Heisenberg limit in our quantum metrology process. We also discuss the experimental feasibility of our theoretical proposal. Our findings may find potential applications in quantum metrology technology.

We report on the observation of dipolar splitting in ${}^6\mathrm{Li}p$-wave Feshbach resonances by high-resolution atom-loss spectroscopy. The Feshbach resonances at 159, 185, and $215\mathrm{G}$ exhibit a doublet structure of 10, 6, and $13\mathrm{mG}$, respectively, associated with different projections of the orbital angular momentum. The observed splittings agree well with coupled-channel calculations. We map out the temperature dependence of the atom-loss spectrum and extrapolate resonance positions as well as the corresponding widths to zero temperature. The observed dipolar splitting in fermionic lithium might be useful for the realization of the quantum phase transition between the polar and axial $p$-wave superfluid phases.

The robust phase estimation (RPE) protocol was designed to be an efficient and robust way to calibrate quantum operations. The robustness of RPE refers to its ability to estimate a single parameter, usually gate amplitude, even when other parameters are poorly calibrated or when the gate experiences significant errors. Here we demonstrate the robustness of RPE to errors that affect initialization, measurement, and gates. In each case, the error threshold at which RPE begins to fail matches quantitatively with theoretical bounds. We conclude that RPE is an effective and reliable tool for calibration of one-qubit rotations and that it is particularly useful for automated calibration routines and sensor tasks.

We investigate the coherence of quantum channels and establish a resource theory for quantifying the coherence of quantum channels. To this aim, we define the incoherent channels and incoherent superchannels. This theory recovers the case of quantum states when we view quantum states as a special case of quantum channels and also allows some analytical expressions for coherence measures.

Characterizing secret communication over noisy quantum channels is an interesting problem from both a practical and theoretical perspective. Suppose Alice and Bob wish to communicate secret information so that an eavesdropper Eve will not suspect any type of encoded communication between the two. Classical or quantum cryptography will not suffice since it is always clear secret communication is taking place. Therefore Alice and Bob must execute what is known as a quantum steganographic protocol. Assuming Eve only has partial knowledge of the channel connecting Alice and Bob, we show that for the bit-flip and depolarizing channels Alice can use Eve's lack of knowledge of the channel parameter to encode quantum information steganographically. We give an explicit encoding procedure and calculate the rate at which Alice and Bob can communicate secretly. We also show that our encoding is optimal for nondegenerate quantum codes. We calculate the rate at which a secret key must be consumed. Finally, we discuss the possibility of steganographic communication over more general quantum channels and conjecture a general formula for the steganographic rate.

A quantum secret sharing scheme is a cryptographic protocol by which a dealer can share a secret among a group of $n$ players so that only certain subsets of players can recover the secret by collaboration. In this paper we propose communication efficient quantum threshold secret sharing schemes. They minimize the amount of quantum communication required to reconstruct the secret when more than the necessary number of players collaborate. They are based on a class of staircase codes proposed by Bitar and El Rouayheb. In a standard $\left(\right(k,n\left)\right)$ quantum threshold scheme, any subset of $k$ or more players can recover the secret. The quantum communication cost for reconstruction in such schemes is $k$ qudits for each secret qudit. Using the proposed construction, any subset of $d\ge k$ players can also collaborate to recover the secret with a communication cost of $d$ qudits for $d-k+1$ secret qudits. In other words, for the proposed schemes the quantum communication cost is only $\frac{d}{d-k+1}$ qudits for every secret qudit. For $d>k$, proposed schemes are communication efficient with respect to standard schemes; and when $d=2k-1$, the quantum communication cost is reduced by a factor $O\left(k\right)$. Further, when $n=2k-1$, the proposed schemes have optimal communication cost for secret reconstruction.

Selective bond breaking in a molecule with the use of photons opens the way to control chemical reactions. We demonstrate here that dissociation of a molecule can be efficiently achieved by first photoexciting a neighboring atom or molecule. On the example of the giant $\mathrm{He}-{\mathrm{H}}_2$ dimer, we show that simultaneous ionization and excitation of the helium atom induces ${\mathrm{H}}_2$ dissociation with a high probability. The excited ${\mathrm{He}}^{+}$ ion transfers its excess energy via interatomic Coulombic decay (ICD) or electron transfer mediated decay (ETMD) to ${\mathrm{H}}_2$ which is then singly or doubly ionized, respectively. In both cases, the molecular ion dissociates effectively within a few tens of femtoseconds. Molecular-bond breaking induced by ICD and ETMD are expected to be general phenomena, which provide alternatives to standard photochemistry.

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to $3\times {10}^{23}\mathrm{W}/{\mathrm{cm}}^2$ are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the $f$/1 focal geometry required to reach such intensity. At intensities exceeding ${10}^{23}\mathrm{W}/{\mathrm{cm}}^2$, highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply bound states. We consider the interaction of a tightly focused, $f$/1 laser pulse with krypton at an intensity of $3\times {10}^{23}\mathrm{W}/{\mathrm{cm}}^2$ and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultrarelativistic above-threshold ionization electrons produced by tunneling ionization from the $K$-shell states of krypton but does not change their energy spectrum.

The three-state Majorana model in the presence of dissipation is considered. Different models of system-environment interaction are explored, ranging from a situation where dissipation is the main effect to regimes where dephasing is mainly produced. It is shown that the detrimental effects of the noise are stronger in the presence of dissipation than in the presence of dephasing. The role of temperature is also discussed.

Dynamical kicking systems possess rich topological structures. In this work we study Floquet states of matter in a non-Hermitian extension of the double-kicked rotor model. Under the on-resonance condition, we find various non-Hermitian Floquet topological phases, each being characterized by a pair of topological winding numbers. A generalized mean chiral displacement is introduced to detect these winding numbers dynamically in two symmetric time frames. Furthermore, by mapping the system to a periodically quenched lattice model, we obtain the topological edge states and unravel the bulk-edge correspondence of the non-Hermitian double-kicked rotor. These results reveal the richness of Floquet topological states in non-Hermitian dynamical kicking systems.

In this paper we introduce a pseudospectral shooting method to model intense, ultrashort optical pulses incident on remote targets. The method naturally extends the unidirectional pulse propagation equation to include backward propagating radiation due to either nonlinear coupling with the “forward” field or reflections generated at material boundaries. The general applicability of the method is highlighted with several examples, including a full simulation of a pulse propagating through air across an ultrathin gold film coated on fused silica.

At zero energy the difference in the number of spin-up and spin-down modes of the Dirac equation is determined by the topology of both space and the gauge field in which the system sits. Writing Maxwell's equations in a Dirac-like form, we identify cases where a combination of material parameters plays the role of “energy.” At zero energy we thus find electromagnetic modes that are indifferent to local changes in the material parameters, with a dispersion relation depending only on the asymptotic values of the material parameters at infinity. We give several examples and show that this theory has implications for non-Hermitian media, where it can be used to construct permittivity profiles that are either reflectionless or act as coherent perfect absorbers or lasers.

A two-mode optical parity-time ($\mathcal{PT}$) symmetric system, with gain and damping, described by a quantum quadratic Hamiltonian with additional small Kerr-like nonlinear terms, is analyzed from the point of view of nonclassical-light generation. Two kinds of stationary states with different types of (in)stability are revealed. Properties of one of these are related to the presence of semiclassical exceptional points, i.e., exotic degeneracies of the non-Hermitian Hamiltonian describing the studied system without quantum jumps. The evolution of the logarithmic negativity, principal squeezing variances, and sub-shot-noise photon-number correlations, considered as entanglement and nonclassicality quantifiers, is analyzed in the approximation of linear-operator corrections to the classical solution. Suitable conditions for nonclassical-light generation are identified in the oscillatory regime, especially at and around exceptional points that considerably enhance the nonlinear interaction and, thus, the nonclassicality of the generated light. The role of quantum fluctuations, inevitably accompanying attenuation and amplification in the evolution of quantum states, is elucidated. The evolution of the system is analyzed for different initial conditions.