We present a first-principles analysis of the noise spectra of alkali-metal vapors in and out of the spin-exchange-relaxation-free (SERF) regime, and we predict nonintuitive features with a potential to further improve the sensitivity of SERF media, and which must be taken into account in their use in quantum optical applications. Studying the process of spin-noise spectroscopy (SNS), we derive analytic formulas for the observable noise spectra, and for the correlation functions among different hyperfine components, which give additional insight into the spin dynamics. The analytic results indicate a variety of distortions of the spin-noise spectrum relative to simpler models, including a broad spectral background that mimics optical shot noise, interference of noise contributions from the two ground-state hyperfine levels, noise reduction at the spin-precession frequency, and “hiding” of spin-noise power that can introduce a systematic error in noise-based calibrations, e.g., for spin-squeezing experiments.

We define the three-body scattering hypervolume $D_F$ for identical spin-polarized fermions in two dimensions by considering the wave function of three such fermions colliding at zero energy and zero orbital angular momentum. We derive the asymptotic expansions of such a wave function when three fermions are far apart or one pair and the third fermion are far apart, and $D_F$ appears in the coefficients of such expansions. For weak-interaction potentials, we derive an approximate formula of $D_F$ by using the Born expansion. We then study the shift of energy of three such fermions in a large periodic area due to $D_F$. This shift is proportional to $D_F$ times the square of the area of the triangle formed by the momenta of the fermions. We also calculate the shifts of energy and of pressure of spin-polarized two-dimensional Fermi gases due to a nonzero $D_F$ and the three-body recombination rate of spin-polarized ultracold atomic Fermi gases in two dimensions.

We analyze quantum effects involved in continuous variable synchronization between two self-sustaining resonators in the framework of Bohmian mechanics. Bohmian trajectories provide visual descriptions of significant nonclassical dynamics, which allow us to characterize the level of quantum synchronization more intuitively compared with only utilizing some designed synchronization measures. It is found that in the quantum limit, the Bohmian trajectories will deviate from their corresponding semiclassical limit cycle behavior after they are distorted by the quantum potential. We explore the roles of zero-point fluctuation, superposition, and nonlocal correlation in synchronization dynamics, corresponding to the oscillators in a coherent state, superposition state, and entanglement state, respectively. We also explore the influence of squeezed Hamiltonian on spontaneous synchronization.

We systematically studied the temporal profile of biphotons, pairs of time-correlated single photons, generated from a hot atomic vapor via the spontaneous four-wave mixing process. The measured temporal width of biphoton wave packet or two-photon correlation function against the coupling power was varied from about 70 to 580 ns. We derived an analytical expression of the biphoton's spectral profile in the Doppler-broadened medium. The analytical expression reveals that the spectral profile is mainly determined by the effect of electromagnetically induced transparency (EIT) and behaves like a Lorentzian function with a linewidth approximately equal to the EIT linewidth. Consequently, the biphoton's temporal profile influenced by the Doppler broadening is an exponential-decay function, which was consistent with the experimental data. Employing a weak input probe field of classical light, we further measured the EIT spectra under the same experimental conditions as those in the biphoton measurements. The theoretical predictions of the biphoton wave packets calculated with the parameters determined by the classical-light EIT spectra are consistent with the experimental data. The consistency demonstrates that in the Doppler-broadened medium, the classical-light EIT spectrum is a good indicator for the biphoton's temporal profile. The measured biphoton's temporal widths well approximated the predictions of the analytical formula based on the biphoton's EIT effect. This study provides an analytical way to quantitatively understand the biphoton's spectral and temporal profiles in the Doppler-broadened medium.

Target recognition technique plays an important role in many fields. The conventional methods perform recognition from images of scenes. However, noise or insufficient obtainable information would cause difficulty in image reconstruction, highly impeding the recognition. Ghost imaging (GI) can accomplish a recognition far below Nyquist limit under which a feasible image would not be reconstructed. However, recent GI recognition methods worked with targets of only $32\times 32$ in resolution, and recognition for high-resolution targets with an extremely low sampling rate is still challenging. We here propose a target recognition method combining GI and generative adversarial network (GAN), which implements a recognition for a target of a $1024\times 1024$ resolution at a sampling rate of 0.07%. GI samples spatial frequencies by illuminating a target with a set of random speckle patterns. The detected bucket signals (the intensities of the echo light from the target) are input into the GAN. The generator of the GAN is exploited to redraw the input bucket signals (detected in the high-resolution setup) to a low-dimension data (as if detected in a low-resolution setup). The discriminator can quickly classify the dimension-reduced data at a very low sampling rate. The method not only brings a way to high-resolution target recognition before image reconstruction, but also proposes an application of GI to circumvent its major challenge: a slow imaging speed due to the requirement of a large number of illumination patterns.

When a quantum particle moves in a curved space, a geometric potential can arise. In spite of a long history of extensive theoretical studies, to experimentally observe the geometric potential remains a challenge. What are the physically observable consequences of such a geometric potential? Solving the Schrödinger equation on a truncated conic surface, we uncover a class of quantum scattering states that bear a strong resemblance to the quasiresonant states associated with atomic collapse about a Coulomb impurity, a remarkable quantum phenomenon in which an infinite number of quasiresonant states emerge. A characteristic defining feature of such collapse states is the infinite oscillations of the local density of states (LDOS) about the zero energy point separating the scattering from the bound states. The emergence of such states in the curved (Riemannian) space requires neither a relativistic quantum mechanism nor any Coulomb impurity: they have zero angular momentum and their origin is purely geometrical, hence the term “geometry-induced wave-function collapse.” We establish the collapsing nature of these states through a detailed comparative analysis of the behavior of the LDOS for both the zero and finite angular momentum states as well as the corresponding classical picture. Potential experimental schemes to realize the geometry-induced collapse states are articulated. Not only does our paper uncover an intrinsic connection between the geometric potential and atomic collapse, it also provides a method to experimentally observe and characterize geometric potentials arising from different subfields of physics. For example, in nanoscience and nanotechnology, curved geometry has become increasingly common. Our finding suggests that wave-function collapse should be an important factor of consideration in designing and developing nanodevices.

We give a detailed theoretical derivation of the master equation for the coherent Ising machine. This is a quantum computational network with feedback, that approximately solves NP-hard combinatoric problems, including the traveling salesman problem and various extensions and analogs. There are two possible types of master equation, either conditional on the feedback current or unconditional. We show that both types can be accurately simulated in a scalable way using stochastic equations in the positive-P phase-space representation. This depends on the nonlinearity present, and we use parameter values that are typical of current experiments. While the two approaches are in excellent agreement, they are not equivalent with regard to efficiency. We find that unconditional simulation has much greater efficiency, and is more scalable to large sizes. This is a case where too much knowledge is a dangerous thing. Conditioning the simulations on the feedback current is not essential to determining the success probability, but it greatly increases the computational complexity. To illustrate the speed improvements obtained with the unconditional approach, we carry out full quantum simulations of the master equation with up to 1000 nodes.

Theoretical and experimental studies have suggested the relevance of quantum coherence to the performance of photovoltaic and light-harvesting complex molecular systems. However, there are ambiguities regarding the validity of statements we can make about the coherence in such systems. Here, we analyze the general procedure for coherence detection in quantum systems and show the counterintuitive phenomenon of detecting a quantum system's initial coherence when both the input and output states of the probe interacting with the system are locally completely incoherent. Our analysis yields the necessary and sufficient conditions for valid claims regarding the coherence of directly inaccessible systems. We further provide a proof-of-principle protocol that uses entangled probes to detect quantum coherence satisfying these conditions and discuss its potency for detecting coherence.

The electronic energy loss of Li ion in diamond under a channeling condition is studied using real-time time-dependent density functional theory. Simulations with and without inner-shell electrons explicitly considered in describing the electron configuration of projectile Li ions are performed to understand their contribution to the dissipation mechanism. It is found that the explicit involvement of inner-shell electrons is particularly important in describing the charge state of the projectile during collision, especially in the high velocity regime where the inner-shell orbital is indeed far from fully occupied during collision. Due to the overestimate of the electronic screening, Li ions with inner-shell electrons that are frozen show only a reduced and hydrogenlike capability in perturbing the host electronic system.

We present an approach to mitigating coating thermal noise in optical cavities by using multiple transverse electromagnetic (TEM) spatial modes to read out and stabilize laser frequency. With optimal weightings we synthesize a wider sampling of mirror surfaces, improving the averaging of Brownian thermal fluctuations. We show a thermal noise improvement factor of 1.57, comparable to a mesa beam of a nominal 12 m prototype cavity, and a factor 1.61 improvement over the ${\text{TEM}}_{00}$ using three modes in a 0.1 m cavity for a practical laboratory experiment, equivalent to cooling mirrors to 120 K from room temperature.

Ultrafast spin-phonon entanglement based on spin-dependent momentum kicks (SDKs) provides an approach to realize fast entangling gates with intrinsic robustness and scalability for trapped ion quantum computing. Such SDKs so far have been implemented on a nanosecond timescale by off-resonant Raman transitions where each laser pulse is split into a sequence of perturbation pulses with carefully designed temporal patterns. Here we report an experimental realization of ultrafast qubit manipulation and spin-phonon entanglement in picoseconds using SDKs from single resonant laser pulses on the magnetic-field-insensitive hyperfine qubit states. This experiment demonstrates a convenient approach to ultrafast SDKs on noise-insensitive ion-spin qubits, with improvement in its speed by more than an order of magnitude. It removes the need to engineer the pattern of a sequence of perturbation pulses and is less vulnerable to noise, simplifying the approach to large-scale trapped-ion quantum computing based on fast quantum gates with SDKs.

Fast electron impact was used to study the generalized oscillator strengths of the inner-shell excitations in atoms and molecules previously. In this work, a nonresonant inelastic x-ray scattering technique is extended to determine the generalized oscillator strengths of $(1{\sigma }_g\to 1{\pi }_g)+(1{\sigma }_u\to 1{\pi }_g)$ inner-shell excitations of molecular nitrogen in the squared momentum transfer range of 0.86 to 14.40 a.u. at a photon energy of about 10 keV and an energy resolution of about 1.3 eV. The present generalized oscillator strength strictly follows the first Born approximation and thus provides a rigorous test to the previously calculated results and the experimental ones measured by the electron energy loss spectroscopy. The presently extrapolated optical oscillator strength at zero momentum transfer is in accord with most experimental results and the theoretical data with the correlation effect considered carefully. This work indicates that the nonresonant inelastic x-ray scattering technique is a powerful tool to study the core excitation mechanism in atoms and molecules.

In this paper we propose an experimentally viable scheme to enhance the sensitivity of force detection in a hybrid optomechanical setup assisted by squeezed vacuum injection, beyond the standard quantum limit (SQL). The scheme is based on a combination of the coherent quantum noise cancellation (CQNC) strategy with a variational homodyne detection of the cavity output spectrum in which the phase of the local oscillator is optimized. In CQNC, realizing a negative-mass oscillator in the system leads to exact cancellation of the backaction noise from the mechanics due to destructive quantum interference. Squeezed vacuum injection enhances this cancellation and allows sub-SQL sensitivity to be reached in a wide frequency band and at much lower input laser powers. We show here that the adoption of variational homodyne readout enables us to enhance this noise cancellation up to $40\mathrm{dB}$ compared to the standard case of detection of the optical output phase quadrature, leading to a remarkable force sensitivity of the order of ${10}^{-19}\mathrm{N}/\sqrt{\mathrm{Hz}}$, about $70\%$ enhancement compared to the standard case. Moreover, we show that at nonzero cavity detuning, the signal response can be amplified at a level three to five times larger than that in the standard case without variational homodyne readout, improving the signal-to-noise ratio. Finally, the variational readout CQNC developed in this paper may be applied to other optomechanical-like platforms such as levitated systems and multimode optomechanical arrays or crystals as well as Josephson-based optomechanical systems.

We simulate the dissipative dynamics of a mesoscopic system of long-range interacting particles which can be mapped into non-Hermitian spin models with a $\mathcal{PT}$ symmetry. We find rich $\mathcal{PT}$ phase diagrams with $\mathcal{PT}$-symmetric and $\mathcal{PT}$-broken phases. The dynamical regimes can be further enriched by modulating tunable parameters of the system. We outline how the $\mathcal{PT}$ symmetries of such systems may be probed by studying their dynamics. We note that systems of Rydberg atoms and systems of Rydberg ions with strong dipolar interactions are particularly well suited for such studies. We present a viable proposal for implementing non-Hermitian physics with $\mathcal{PT}$ symmetry in Rydberg systems. We show that for realistic parameters, long-range interactions allow the emergence of new $\mathcal{PT}$-symmetric regions, generating new $\mathcal{PT}$ phase transitions. In addition, such $\mathcal{PT}$-symmetry phase transitions are found by changing the configurations of the Rydberg atoms. We propose a postselection scheme on an ensemble of Rydberg ions described by an effective three-level system. Detecting the population dynamics, the system shows an oscillatory behavior in the $\mathcal{PT}$-unbroken phase and a stationary population for long times in the $\mathcal{PT}$-broken phase.

Photonic structures offer a flexible platform for studying and demonstrating parity-time ($\mathcal{PT}$) symmetry phenomena. In these platforms, electric dipoles are often used as accurate models for electromagnetic sources, and elliptical dipoles were shown to provide for directional mode excitation. Here we introduce tailored dipole sources for directional excitation in a $\mathcal{PT}$-symmetric structure made of two coupled waveguides. By eliminating one mode in the device, a qualitatively different wave propagation on the two sides of the dipole is achieved. Interestingly, before the exceptional point, a linear dipole suffices to have mode beatings on only one side. Furthermore, beyond the exceptional point, gain can be created on one side only. Finally, at the exceptional point, a near-complete directionality can be achieved due to the mode merging. We explain these effects via a detailed analysis of the modes, and the subsequent mode excitation of the dipole is analytically described. In the end, these various types of contrasting phenomena offer possibilities for integrated photonics applications, routing setups, and lasing behavior.

High-order harmonic generation of the long-chain molecule 1-nonene (${\mathrm{C}}_9{\mathrm{H}}_{18}$) under a few-cycle linearly polarized laser field was studied using time-dependent density-functional theory. The efficiency and the cutoff frequency of the ${\mathrm{C}}_9{\mathrm{H}}_{18}$ harmonic spectrum are significantly higher than those of the BF molecule irradiated by the same laser pulse. By analyzing the time-frequency behavior of the harmonic and the classical simulation of electron, it is found that the ${\mathrm{C}}_9{\mathrm{H}}_{18}$ harmonics near the cutoff energy are caused by the recombination of electrons from one atom to others of the parent molecule. In addition, the high-order harmonic generation of the long-chain molecule 1,3-Butadiene (${\mathrm{C}}_4{\mathrm{H}}_6$) is also investigated. The harmonics generated by this mechanism can be applied to produce high-intensity isolated attosecond pulses.

Reliable tomography of a quantum state of atoms in room-temperature vapors offers interesting applications in quantum-information science. To step toward the applications, here, we theoretical investigate a technique of reconstruction of a collective density matrix of atoms in a state of a total angular momentum equal to unity. By studying transformation of density matrix due to magnetic-field pulses and deriving explicit relations between optical signals and specific density-matrix elements, we demonstrate full reconstruction of quantum states. Numerical simulations allow us to demonstrate that the reconstruction fidelity of pure and mixed states may exceed 0.95 and that the technique is robust against noise and uncertainties.