We develop a fast-forward scheme of the adiabatic spin dynamics of quantum entangled states. We settle the quasiadiabatic dynamics by adding the regularization terms to the original Hamiltonian and then accelerate it with the use of a large time-scaling factor. Assuming the experimentally realizable candidate Hamiltonian consisting of the exchange interactions and magnetic field, we solve the regularization terms. These terms, multiplied by the velocity function, give rise to the state-dependent counterdiabatic terms. The scheme needs neither knowledge of full spectral properties of the system nor solving the initial- and boundary-value problem. Our fast forward Hamiltonian generates a variety of state-dependent counterdiabatic terms for each of adiabatic states, which can include the state-independent one. We highlight this fact by using minimum (two-spin) models for a simple transverse Ising model, quantum annealing, and generation of entanglement.

When one tries to take into account the nontrivial vacuum structure of quantum field theory, the standard functional-integral tools, such as generating functionals or transitional amplitudes, are often quite inadequate for such purposes. Here we propose a generalized generating functional for Green's functions which allows one to easily distinguish among a continuous set of vacua that are mutually connected via unitary canonical transformations. In order to keep our discussion as simple as possible, we limit ourselves to quantum mechanics where the generating functional of Green's functions is constructed by means of phase-space path integrals. The quantum-mechanical setting allows us to accentuate the main logical steps involved without embarking on technical complications such as renormalization or inequivalent representations that should otherwise be addressed in the full-fledged quantum field theory. We illustrate the inner workings of the generating functional obtained by discussing Green's functions among vacua that are mutually connected via translations and dilatations. Salient issues, including connection with quantum field theory, vacuum-to-vacuum transition amplitudes, and perturbation expansion in the vacuum parameter, are also briefly discussed.

There is an ongoing search for a physical or operational definition for quantum mechanics. Several informational principles have been proposed which are satisfied by a theory less restrictive than quantum mechanics. Here, we introduce the principle of “many-box locality,” which is a refined version of the previously proposed “macroscopic locality.” These principles are based on coarse graining the statistics of several copies of a given box. The set of behaviors satisfying many-box locality for $N$ boxes is denoted $L_N^{MB}$. We study these sets in the bipartite scenario with two binary measurements, in relation with the sets $\mathcal{Q}$ and ${\mathcal{Q}}_{1+AB}$ of quantum and “almost quantum” correlations, respectively. We find that the $L_N^{MB}$ sets are, in general, not convex. For unbiased marginals, by working in the Fourier space we can prove analytically that $L_N^{MB}⊈\mathcal{Q}$ for any finite $N$, while $L_{\infty }^{MB}=\mathcal{Q}$. Then, with suitably developed numerical tools, we find an example of a point that belongs to $L_{16}^{MB}$ but not to ${\mathcal{Q}}_{1+AB}$. Among the problems that remain open is whether $\mathcal{Q}\subset L_{\infty }^{MB}$.

The relativistic semiclassical evolution of the position of an electron in the presence of an external electromagnetic field is studied in terms of a Newton equation that incorporates spin effects directly. This equation emerges from the Dirac equation and allows the identification of scenarios where spin effects are necessary to understand the main characteristics of the electron trajectories. It involves the eigenvalues of the non-Hermitian operator ${\mathrm{\Sigma }}_{\mu \nu }{F}^{\mu \nu }$, with ${\mathrm{\Sigma }}_{\mu \nu }$ and $F^{\mu \nu }$ as the spin and electromagnetic tensors. The formalism allows a deeper understanding on the physics behind known analytical solutions of the Dirac equation when translational dynamics seem to decouple from spin evolution. As an illustrative example, it is applied to an electron immersed in an electromagnetic field which exhibits chiral symmetry and optical vortices. It is shown that the polarization of intense structured light beams can be used to suppress or enhance spin effects on the electron semiclassical trajectory; the latter configuration yields a realization of a Stern-Gerlach apparatus for an electron.

We present an alternative form of master equation, applicable to the analysis of nonequilibrium dynamics of fermionic open quantum systems. The formalism considers a general scenario, composed by a multipartite quantum system in contact with several reservoirs, each one with a specific chemical potential and in thermal equilibrium. With the help of Jordan-Wigner transformation, we perform a fermion-to-qubit mapping to derive a set of Lindblad superoperators that can be straightforwardly used on a wide range of physical setups. To illustrate our approach, we explore the effect of a charge sensor, acting as a probe, over the dynamics of electrons on coupled quantum molecules. The probe consists of a quantum dot attached to source and drain leads, that allows a current flow. The dynamics of populations, entanglement degree, and purity show how the probe is behind the sudden deaths and rebirths of entanglement, at short times. Then, the evolution leads the system to an asymptotic state being a statistical mixture. Those are signatures that the probe induces dephasing, a process that destroys the coherence of the quantum system.

We investigate the degree of entanglement between an atom and a driven cavity mode in the presence of dissipation. Previous work has shown that in the limit of weak driving fields, the steady-state entanglement is proportional to the square of the driving intensity. This quadratic dependence is due to the generation of entanglement by the creation of pairs of photons or excitations. In this work we investigate the entanglement between an atom and a cavity in the presence of multiple photons. Nonlinearity of the atomic response is needed to generate entanglement, but as that nonlinearity saturates the entanglement vanishes. We posit that this is due to spontaneous emission, which puts the atom in the ground state and the atom-field state into a direct product state. An intermediate value of the driving field, near the field that saturates the atomic response, optimizes the atom-field entanglement. In a parameter regime for which multiphoton resonances occur, we find that entanglement recurs at those resonances. In this regime, we find that the entanglement decreases with increasing photon number. We also investigate, in the bimodal regime, the entanglement as a function of atom and/or cavity detuning. Here we find that there is evidence of a phase transition in the entanglement, which occurs at $2\epsilon /g\ge 1$.

The possible compatibility of density matrices for single-party subsystems is described by linear constraints on their respective spectra. Whenever some of those quantum marginal constraints are saturated, the total quantum state has a specific, simplified structure. We prove that these remarkable global implications of extremal local information are stable; i.e., they hold approximately for spectra close to the boundary of the allowed region. Application of this general result to fermionic quantum systems allows us to characterize natural extensions of the Hartree-Fock ansatz and to quantify their accuracy by resorting to one-particle information, only: The fraction of the correlation energy not recovered by such an ansatz can be estimated from above by a simple geometric quantity in the occupation number picture.

We study theoretically single-electron loss from the ground state of a heliumlike highly charged ion in fast collisions with an atomic particle (a nucleus or an atom), focusing on electron emission energies where the so-called excitation-autoionization channel of electron loss becomes of importance. The presence of this channel leads to the appearance of sharp structures in the energy distribution of the emitted electrons and may also noticeably influence the angular distributions of the emission in the vicinity of autoionization resonances. We performed calculations for electron loss from ${\mathrm{Ca}}^{18+}\left(1s^2\right)$ and ${\mathrm{Zn}}^{28+}\left(1s^2\right)$ in 100 MeV/u collisions with neon. It is shown that two qualitatively different subchannels (which involve either one or two interactions between the electrons of the ion and the incident atomic particle) substantially contribute to excitation-autoionization and take active part in the interference with the direct channel of electron loss; however, they practically do not interfere with each other. Our consideration also shows that the account of QED corrections is important for an accurate description of electron loss even from relatively light heliumlike HCIs.

We experimentally study the phase stabilization of a semiconductor double-quantum-dot (DQD) single-atom maser by injection locking. A voltage-biased DQD serves as an electrically tunable microwave frequency gain medium. The statistics of the maser output field demonstrate that the maser can be phase locked to an external cavity drive, with a resulting phase noise $\mathcal{L}=-99$ dBc/Hz at a frequency offset of 1.3 MHz. The injection locking range, and the phase of the maser output relative to the injection locking input tone are in good agreement with Adler's theory. Furthermore, the electrically tunable DQD energy level structure allows us to rapidly switch the gain medium on and off, resulting in an emission spectrum that resembles a frequency comb. The free running frequency comb linewidth is $\approx 8$ kHz and can be improved to less than 1 Hz by operating the comb in the injection locked regime.

Recent theoretical studies of a pair of atoms in a one-dimensional waveguide find that the system responds asymmetrically to incident fields from opposing directions at low powers. Since there are no explicit time-reversal symmetry-breaking elements in the device, this has caused some debate. Here we show that the asymmetry arises from the formation of a quasidark state of the two atoms, which saturates at extremely low power. In this case the nonlinear saturability explicitly breaks the assumptions of the Lorentz reciprocity theorem. Moreover, we show that the statistics of the output field from the driven system can be explained by a very simple stochastic mirror model and that at steady state, the two atoms and the local field are driven to an entangled, tripartite $\left|W\right.〉$ state. Because of this, we argue that the device is better understood as a saturable Yagi-Uda antenna, a distributed system of differentially tuned dipoles that couples asymmetrically to external fields.

To achieve a high degree of quantum noise squeezing, an optical cavity is often employed to enhance the interaction time between light and matter. Here, we propose to utilize the effect of coherent population trapping (CPT) to directly generate squeezed light without any optical cavity. Combined with the slow propagation speed of light in a CPT medium, a coherent state passing through an atomic ensemble with a high optical density (OD) can evolve into a highly squeezed state even in a single passage. Our study reveals that noise squeezing of more than 10 dB can be achieved with an OD of 1000, which is currently available in experiments. A larger OD can further increase the degree of squeezing. As the light intensity and two-photon detuning are key factors in the CPT interaction, we also demonstrate that the minimum variance at a given OD can be reached for a wide range of these two factors, showing the proposed scheme is flexible and robust. Furthermore, there is no need to consider the phase-matching condition in the CPT scheme. Our introduction of high OD in atomic media not only brings a long light-matter interaction time comparable to optical cavities, but also opens an alternative avenue in the generation of squeezed light for quantum interface.

We derived quantum trajectories for a system interacting with the environment prepared in a continuous mode single-photon state as the limit of a discrete filtering model with an environment defined as a series of independent qubits prepared initially in the entangled state being an analog of a continuous mode state. The environment qubits interact with the quantum system and they are subsequently measured. The initial correlation between the bath qubits is the source of the non-Markovianity. The conditional evolutions of the quantum system for the limit of the continuous in time observations together with the formulas for the photon counting probabilities are given.

Atom interferometers employing optical cavities to enhance the beam splitter pulses promise significant advances in science and technology, notably for future gravitational wave detectors. Long cavities, on the scale of hundreds of meters, have been proposed in experiments aiming to observe gravitational waves with frequencies below 1 Hz, where laser interferometers, such as LIGO, have poor sensitivity. Alternatively, short cavities have also been proposed for enhancing the sensitivity of more portable atom interferometers. We explore the fundamental limitations of two-mirror cavities for atomic beam splitting, and establish upper bounds on the temperature of the atomic ensemble as a function of cavity length and three design parameters: the cavity $g$ factor, the bandwidth, and the optical suppression factor of the first and second order spatial modes. A lower bound to the cavity bandwidth is found which avoids elongation of the interaction time and maximizes power enhancement. An upper limit to cavity length is found for symmetric two-mirror cavities, restricting the practicality of long baseline detectors. For shorter cavities, an upper limit on the beam size was derived from the geometrical stability of the cavity. These findings aim to aid the design of current and future cavity-assisted atom interferometers.

We study the Fano resonance and slow light in a hybrid optomechanical system, where a mechanical resonator is coupled to a cavity via radiation pressure and to a two-level system (qubit) via the Jaynes-Cummings coupling. The absorption spectra of the weak probe field can exhibit a series of asymmetric Fano line shapes when the frequency of the strong control field varies. The positions of the double Fano resonances can be determined by the cavity–control-field detuning, the qubit-resonator coupling strength, and the transition frequency of the qubit. Furthermore, the transparency windows in the probe transmission spectrum are accompanied by the rapid normal phase dispersion, which indicates the slow light effect. We find that the group delay of the transmitted probe field can be tuned by the frequency and amplitude of the control field and is inversely proportional to the decay rate of the qubit.

We study continuous-variable entanglement of bright quantum states in a pair of evanescently coupled nonlinear ${\chi }^{\left(2\right)}$ waveguides operating in the regime of degenerate down conversion. We consider the case where only the energy of the nonlinearly generated fields is exchanged between the waveguides while the pump fields stay independently guided in each original waveguide. We show that this device, when operated in the depletion regime, entangles the two noninteracting bright pump modes due to a nonlinear cascade effect. It is also shown that two-color quadripartite entanglement can be produced when certain system parameters are appropriately set. This device works in the traveling-wave configuration, such that the generated quantum light shows a broad spectrum. The proposed device can be easily realized with current technology and therefore stands as a good candidate for a source of bipartite or multipartite entangled states for the emerging field of optical continuous-variable quantum information processing.

We propose an effective method to achieve extremely slow light by using both the mechanism of electromagnetically induced transparency (EIT) and the localization of a coupled cavity waveguide (CCW). Based on quantum mechanics theory and the dispersion relation of a CCW, we derive a group-velocity formula that reveals both the effects of the EIT and CCW. Results show that ultralow light velocity at the order of several meters per second or even static light, could be obtained feasibly. In comparison with the EIT mechanism in a background of vacuum, this proposed method is more effective and realistic to achieve extremely slow light. And it exhibits potential values in the field of light storage.

We pump low-pressure nitrogen gas with ionizing femtosecond laser pulses at 1.5 $\mu \mathrm{m}$ wavelength. The resulting rotationally excited ${\mathrm{N}}_2^{+}$ molecular ions generate directional, forward-propagating stimulated and isotropic spontaneous emissions at 428 nm wavelength. Through high-resolution spectroscopy of these emissions, we quantify rotational population distributions in the upper and lower emission levels. We show that these distributions are shifted with respect to each other, which has a strong influence on the transient optical gain in this system. Although we find that electronic population inversion exists in our particular experiment, we show that sufficient dissimilarity of rotational distributions in the upper and lower emission levels could, in principle, lead to gain without net electronic population inversion.

The Gross-Pitaevskii or nonlinear Schrödinger equation relevant to ultracold atomic gaseous Bose-Einstein condensates possesses a modified Schrödinger symmetry in two spatial dimensions, in the presence of a harmonic trapping potential, an (artificial) constant magnetic field (or rotation), and an (artificial) electric field of a quadratic electrostatic potential. We find that a variance and a center of a trapped gas with or without a vorticity can be regarded as massive Nambu-Goldstone (NG) modes associated with spontaneous breaking of the modified Schrödinger symmetry. We show that the Noether theorem for the modified Schrödinger symmetry gives universal equations of motion which describe exact time evolutions of the trapped gases such as a harmonic oscillation, a cyclotron motion, and a breathing oscillation with frequencies determined by the symmetry independent of the details of the system. We further construct an exact effective action for all the NG modes.