We study a degenerate Bose gas quenched to unitarity by solving a many-body model including three-body losses and correlations up to second order. As the gas evolves in this strongly interacting regime, the buildup of correlations leads to the formation of extended pairs bound purely by many-body effects, analogous to the phenomenon of Cooper pairing in the BCS regime of the Fermi gas. Through fast sweeps away from unitarity, we detail how the correlation growth and formation of bound pairs emerge in the fraction of unbound atoms remaining after the sweep, finding quantitative agreement with experiment. We comment on the possible role of higher-order effects in explaining the deviation of our theoretical results from experiment for slower sweeps and longer times spent in the unitary regime.

We present a simple model used for investigating the quantum reflection and quantum coherence in ultracold molecular collisions. Analytical expressions of reflection and transmission coefficients, short-range $S$ matrix, and transition matrix $T$ are derived for the hard-core plus inverse power potentials in the vicinity of the threshold. The effect of the quantum reflection and coherence on the loss rate depends on the scattering length and short-range loss parameter. The quantum coherence results from the phase difference $\theta$ between reflection paths. As an applied example, we calculate the loss rate of ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ molecules in the ultracold ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}-{}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ collision. We can well interpret the experimental results of the ultracold ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ molecular collision [Ye et al., Sci. Adv. 4, eaaq0083 (2018)]. It is found that the quantum coherence has a significant influence on the loss rate of ${}^{23}\mathrm{Na}{}^{87}\mathrm{Rb}$ molecules in ultracold molecule-molecule collisions.

There is a revival of the problem of how a relativistic particle with spin moves through an electromagnetic field. Our paper focuses on the spin-dependent dynamics of electrons in linearly polarized laser fields based on the Dirac equation and a classical spin model. It is proved that an electron at rest undergoes a three-dimensional motion moving out of the polarization-propagation plane when the initial spin is perpendicular to the magnetic field. We make a comprehensive investigation of the dynamics of quantum wave packets and classical electrons in both plane-wave and standing-wave fields. The wave packet travels in accordance with its center of mass in a plane-wave field but spreads heavily in a standing-wave field. The ensembles of classical electrons turn out to well simulate the quantum wave-packet dynamics in the standing-wave fields. We give the extended calculations of the electron dynamics with spin in the standing-wave field in the chaotic regime and find the quantum correspondence to the classical chaotic dynamics.

Computer simulations of many-body quantum dynamics of indistinguishable particles is a challenging task for computational physics. In this paper we demonstrate that the method of coupled coherent states (CCS) developed previously for multidimensional quantum dynamics of distinguishable particles can be used to study indistinguishable bosons in the second-quantization formalism. To prove its validity, the technique termed here coupled coherent states for indistinguishable bosons (CCSB) is tested on two model problems. The first is a system-bath problem consisting of a tunneling mode coupled to a harmonic bath, previously studied by CCS and other methods in distinguishable representation in 20 dimensions. The harmonic bath is comprised of identical oscillators, and may be second quantized for use with CCSB, so that this problem may be thought of as a bosonic bath with an impurity. The cross-correlation function for the dynamics of the system and Fourier transform spectrum compare extremely well with a benchmark calculation, which none of the prior methods of studying the problem achieved. The second model problem involves 100 bosons in a shifted harmonic trap. Breathing oscillations in the one-body density are calculated and shown to compare favorably to a multiconfigurational time-dependent Hartree for bosons calculation, demonstrating the applicability of the method as a new formally exact way to study the quantum dynamics of Bose-Einstein condensates.

In this study, the interaction between a plasmonic nanoparticle and coupled quantum dots is investigated to identify how the coupled particles can manipulate the plasmonic nanoparticle decay rate. This subject is very important, because most applications of the plasmonic system are restricted due to the nanoparticle decay rate and the related losses. Therefore, in the present work, we try to find out how and by which method the plasmonic nanoparticle decay rate can be manipulated. For this purpose, a plasmonic system containing a nanoparticle coupled to some small quantum dots is designed. The system dynamics of motions are analyzed with Heisenberg-Langevin equations. These equations are analyzed to study the effect of the plasmonic nanoparticles on the quantum dots’ decay rate. In the following, as an interesting point, the quantum dot coupling influence on the nanoparticle's decay rate is theoretically analyzed in the transient and steady-state conditions. Additionally, a theoretical formula is derived by which one can explicitly find the dependency of the modified decay rate of the plasmonic nanoparticle on the number of the coupled quantum dots and the coupling strength. The simulation results show that it is possible to effectively control the nanoparticles’ decay rate with regard to the application for which they are utilized.

We show how to recover complete positivity (and hence positivity) of the Redfield equation via a coarse-grained averaging technique. We derive general bounds for the coarse graining timescale above which the positivity of the Redfield equation is guaranteed. It turns out that a coarse grain timescale has strong impact on the characteristics of the Lamb shift term and implies, in general, noncommutation between the dissipating and the Hamiltonian components of the generator of the dynamical semigroup. Finally, we specify the analysis to a two-level system or a quantum harmonic oscillator coupled to a fermionic or bosonic thermal environment via dipolelike interaction.

All known solutions of the Dirac equation describing states of electrons endowed with angular momentum are very far from our notion of the electron as a spinning charged bullet because they are not localized in the direction of propagation. We present here normalizable analytic exact solutions, eigenstates of the total angular momentum component $M_z$, that come very close to this notion. These solutions of the Dirac equation have also intricate topological properties similar to the hopfion solutions of the Maxwell equations.

One of the leading quantum computing architectures is based on the two-dimensional (2D) surface code. This code has many advantageous properties such as a high error threshold and a planar layout of physical qubits where each physical qubit need only interact with its nearest neighbors. However, the transversal logical gates available in 2D surface codes are limited. This means that an additional (resource-intensive) procedure known as magic state distillation is required to do universal quantum computing with 2D surface codes. Here, we examine three-dimensional (3D) surface codes in the context of quantum computation. We introduce a picture for visualizing 3D surface codes which is useful for analyzing stacks of three 3D surface codes. We use this picture to prove that the CZ and CCZ gates are transversal in 3D surface codes. We also generalize the techniques of 2D surface-code lattice surgery to 3D surface codes. We combine these results and propose two quantum computing architectures based on 3D surface codes. Magic state distillation is not required in either of our architectures. Finally, we show that a stack of three 3D surface codes can be transformed into a single 3D color code (another type of quantum error-correcting code) using code concatenation.

In a practical continuous-variable quantum key distribution (CVQKD) system, the optical attenuator can adjust the Gaussian-modulated coherent states and the local oscillator signal to an optimal value for guaranteeing the security of the system and optimizing the performance of the system. However, the performance of the optical attenuator may deteriorate due to the intentional and unintentional damage of the device. In this paper, we investigate the practical security of a CVQKD system with reduced optical attenuation. We find that the secret key rate of the system may be overestimated based on the investigation of parameter estimation under the effects of reduced optical attenuation. This opens a security loophole for Eve to successfully perform an intercept-resend attack in a practical CVQKD system. To close this loophole, we add an optical fuse at Alice's output port and design a scheme to monitor the level of optical attenuation in real time, which can make the secret key rate of the system evaluated precisely. The analysis shows that these countermeasures can effectively resist this potential attack.

The double-minimum 3 ${}^1\mathrm{\Sigma }_{\mathrm{u}}^{+}$ state in ${}^{85}\mathrm{Rb}_2$ and ${}^{85}\mathrm{Rb}{}^{87}\mathrm{Rb}$ was investigated by analyzing rotationally resolved spectra of the 3 ${}^1\mathrm{\Sigma }_{\mathrm{u}}^{+}⟵\mathrm{X}{}^1\mathrm{\Sigma }_{\mathrm{g}}^{+}$ band system simplified by polarization labeling. A total of 3665 transitions to rovibrational levels located in the inner well and above the internal barrier were identified and an accurate potential energy curve of the state reaching up to $R\approx 21$ Å was constructed using the inverted perturbation approach, making this state the best characterized double-minimum state in alkali dimers. The possibility of formation of ultracold rubidium dimers in the absolute ground state by photoassociation into levels of the 3 ${}^1\mathrm{\Sigma }_{\mathrm{u}}^{+}$ state above the internal barrier followed by spontaneous or stimulated emission is discussed.

We consider the process of high-order harmonics generation (HHG) in the xenon atom enhanced by the intershell correlation between the valence $5p$ and inner $4d$ shells. We derive the HHG spectrum from a numerical solution of the one-electron time-dependent Schrödinger equation multiplied by the enhancement factor taken as the photoionization cross-sections ratio calculated with and without the intershell correlation. Such a simplified approach is adequate to describe the experimental HHG spectrum reported by Shiner et al. [Nat. Phys. 7, 464 (2011)] generated by a single color IR laser. Similarly, we find good agreement when applied to the two-color driven HHG spectra reported by Faccialá et al. [Phys. Rev. Lett. 117, 093902 (2016)].

Quantum systems exhibit recurrence phenomena after equilibration, but it is a difficult task to evaluate the recurrence time of a quantum system because it drastically increases as the system size increases (usually double exponential in the number of particles) and strongly depends on the initial state. Here, we analytically derive the recurrence times of the Lieb-Liniger model with relatively small particle numbers for the weak- and strong-coupling regimes. It turns out that these recurrence times are independent of the initial state and increase only polynomially in the system size.

It is shown that the maximal phase sensitivity of a two-path interferometer with high-intensity coherent light and squeezed vacuum in the input ports can be achieved by photon-number-resolving detection of only a small number of photons in a dark output port. It is then possible to achieve the quantum Cramér-Rao bound of the two-path interferometer using only the field displacement dependence of the photon number statistics in the single mode output of the dark port represented by a field-displaced squeezed vacuum state. We find that, at small field displacements, it is not sufficient to use the average photon number as the estimator, indicating that an optimal phase estimation depends critically on measurements of the precise photon number. We therefore analyze the effect of detection efficiency on the Fisher information and show that there is a transition from low robustness against photon losses associated with quantum interference effects at low field displacements to high robustness against photon losses at high field displacements. The transition between the two regimes occurs at field shifts proportional to the third power of the squeezing factor, indicating that squeezing greatly enhances the phase interval in which quantum effects are relevant in optimal phase estimations using photon resolving detectors. The case under study could thus be understood as a “missing link” between genuine multiphoton interference and the straightforward suppression of noise usually associated with squeezed light.

The effect of electromagnetically induced transparency (EIT) combined with Rydberg-state atoms provides high optical nonlinearity to efficiently mediate the photon-photon interaction. However, the decay rate of Rydberg coherence, i.e., the decoherence rate, plays an important role in optical nonlinear efficiency and can be largely influenced by laser frequency fluctuation. In this work, we carried out a systematic study of the effect of laser frequency fluctuation on the decoherence rate. We derive an analytical formula that quantitatively describes the relationship between the decoherence rate and laser frequency fluctuation. The formula is experimentally verified by using the $\mathrm{\Lambda }$-type EIT system of laser-cooled ${}^{87}\mathrm{Rb}$ atoms, in which one can either completely eliminate or controllably introduce the effect of laser frequency fluctuation. We also include the effect of Doppler shift caused by the atomic thermal motion in the formula, which can be negligible in the $\mathrm{\Lambda }$-type EIT experiment but significant in the Rydberg-EIT experiment. Utilizing the atoms of 350 $\mu \mathrm{K}$, we study the decoherence rate in the Rydberg-EIT system involving the state of $|32D_{5/2}\rangle$. The experimental data are consistent with the predictions from the formula. We are able to achieve a rather low decoherence rate of $2\pi \times 48$ kHz at a moderate coupling Rabi frequency of $2\pi \times 4.3$ MHz.

We realize a nondegenerate two-mode Dicke model with competing interactions in a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization modes of a single optical cavity. The BEC is coupled to the cavity modes via the scalar and vectorial part of the atomic polarizability. We can independently change these couplings and determine their effect on a self-organization phase transition. Measuring the phases of the system, we characterize a crossover from a single-mode to a two-mode Dicke model. This work provides perspectives for the realization of coupled phases of spin and density.

Nonsequential two-photon ionization of inner-shell $np$ subshell of neutral atoms by circularly polarized light is investigated. Detection of subsequent fluorescence as a signature of the process is proposed and the dependence of fluorescence degree of polarization on incident photon beam energy is studied. It is generally expected that the degree of polarization remains approximately constant, except when the beam energy is tuned to an intermediate $n^{\prime }s$ resonance. However, strong unexpected change in the polarization degree is discovered for nonsequential two-photon ionization at specific incident beam energy due to a zero contribution of the otherwise dominant ionization channel. Polarization degree of the fluorescence depends less on the beam parameters, and its measurements at this specific beam energy, whose position is very sensitive to the details of the employed theory, are highly desirable for evaluation of theoretical calculations of nonlinear ionization at hitherto unreachable accuracy.

The methods for controlling spin states of negatively charged nitrogen-vacancy (NV) centers using microwave (MW) or radio frequency (rf) excitation fields for electron spin and nuclear spin transitions are effective in strong magnetic fields where a level anticrossing (LAC) occurs. A LAC can also occur at zero field in the presence of transverse strain or electric fields in the diamond crystal, leading to mixing of the spin states. In this Rapid Communication, we investigate zero-field LAC of NV centers using dual-frequency excitation spectroscopy. Under rf modulation of the spin states, we observe sideband transitions and Autler-Townes splitting in the optically detected magnetic resonance (ODMR) spectra. Numerical simulations show that the splitting originates from a Landau-Zener transition between electron spin $|\pm 1\rangle$ states, which potentially provides a way of manipulating NV center spin states in zero or weak magnetic field.

Quantum walks (QWs) describe the evolution of quantum systems on graphs. An intrinsic degree of freedom—called the coin and represented by a finite-dimensional Hilbert space—is associated with each node. Scalar quantum walks are QWs with a one-dimensional coin. We propose a general strategy allowing one to construct scalar QWs on a broad variety of graphs, which admit embedding in Eulidean spaces, thus having a direct geometric interpretation. After reviewing the technique that allows one to regroup cells of nodes into new nodes, transforming finite spatial blocks into internal degrees of freedom, we prove that no QW with a two-dimensional coin can be derived from an isotropic scalar QW in this way. Finally, we show that the Weyl and Dirac QWs can be derived from scalar QWs in spaces of dimension up to three, via our construction.

We aim to construct tests for evaluating whether policies for adaptive quantum-enhanced metrology are robust against unknown phase noise and are tractable to construct and to execute. Specifically, one of our tests determines scaling of phase-estimate precision with respect to photon number; the other two tests concern resource complexity with respect to the training time required to construct the policy and the execution time for the policy so constructed. The robustness test is performed on quantum-enhanced adaptive phase estimation by simulating the scheme under four phase-noise models corresponding to normal-distribution noise, random-telegraph noise, skew-normal-distribution noise, and log-normal-distribution noise. Control policies are devised either by an evolutionary algorithm under the same noisy conditions, albeit ignorant of its properties, or a Bayesian-based feedback method that assumes no noise. We have introduced an approach to evaluating quantum-control policies for metrology that relies on testing against unusual phase-noise models and accepting that the policies are robust only if scaling of phase-estimate precision beats the standard quantum limit and policy-design resource complexity, in the presence of phase noise, is polynomial in the number of photons.

We analyze the fidelity of the generalized Pauli channels governed by memory kernel master equations. It is shown that by appropriate engineering of parameters of the corresponding memory kernel, the quantum evolution with nonlocal noise can have higher fidelity than the corresponding purely Markovian evolution governed by the Markovian semigroup. Similar engineering can substantially influence the evolution of quantum entanglement, entropy, and quantum coherence. Interestingly, by adding a nonlocal noise, one can protect the system against a sudden death of entanglement.