We present a comprehensive study of co- and counterpropagating Stokes and anti-Stokes quantum fields in double Raman four-wave mixing parametric oscillators using full quantum Heisenberg-Langevin framework with noise operators. General analytical solutions of the fields operators at any point in the Raman medium are obtained for four cases: two possible copropagating (forward) and two counterpropagating (backward) Stokes and anti-Stokes. We analyze the symmetrical properties of the complex linear and nonlinear susceptibilities spectra of the quantum fields, nonclassicality of two-photon correlation functions, spatial variations of the quantum fields, and the two-mode relative intensity squeezing. We compare the results of forward and backward cases for several limiting double Raman schemes. We find interesting resonant effects of medium length, laser detunings and laser field strengths (Rabi frequencies) for backward-propagating geometries. Analysis of the solutions provide insights on the resonant conditions while computation over multiple variables enables us to identify the values of laser detuning, field strength, and propagation length that give enhanced nonclassical intensity squeezing and persistent correlations. The present work sets the crucial foundations for optimization of the nonclassicality of photons in double Raman systems and would be useful for quantum information storage, quantum nonlinear optics, and quantum spectroscopy.

We study nonreciprocal photon blockade in cavity optomagnonics, where a sphere of ferromagnetic crystal supports two optical whispering-gallery modes and one magnon mode. The interaction in cavity optomagnonics is subject to the selection rules involving conservation of angular momentum and energy, and the selection rules introduce a nonreciprocal coupling between linearly polarized photons and magnons. For given incident linearly polarized photons, the photon blockade occurs when the cavity is driven in one direction, but it disappears if the cavity is driven in the opposite direction. The results suggest that cavity optomagnonics could be a promising platform for studying chiral quantum optics and topological photonics.

We investigate the ultimate quantum limit of resolving the temperatures of two thermal sources affected by diffraction. More quantum Fisher information can be obtained with a priori information than without a priori information. We carefully consider two strategies: simultaneous estimation and individual estimation. We prove that the simultaneous estimation of two temperatures satisfies the saturation condition of the quantum Cramér-Rao bound and performs better than the individual estimation in the case of a small degree of diffraction given the same resources. However, in the case of a high degree of diffraction, the individual estimation performs better. In particular, at the maximum diffraction, the simultaneous estimation cannot get any information, which is supported by a practical measurement, while the individual estimation can still get the information. In addition, we find that for the individual estimation, a practical and feasible estimation strategy using the full Hermite-Gauss basis can saturate the quantum Cramér-Rao bound without being affected by the attenuation factor at the maximum diffraction.

We demonstrate interface states between two Floquet-driven photonic lattices having opposite chirality. Symmetrical and antisymmetrical localized states are found at an interface emerging in an otherwise static uniform lattice upon action of mirrored Floquet drives. These states locate near the edges of Floquet zones, highlighting the opening of band gaps initiated from lifted band crossings. They exist in a certain range of the driving frequency, which can be widened by increasing the driving strength. Chiral symmetry of the driven lattices is preferable to realize a large degree of localization of the interface states. We realize such states experimentally and observe different localization behaviors unpredicted in a tight-binding model, yet well understood via a continuum model that is more realistic for the experiment.

High-fidelity state preparation represents a fundamental challenge in the application of quantum technology. While the majority of optimal control approaches use feedback to improve the controller, the controller itself often does not incorporate explicit state dependence. Here, we present a general framework for training deep feedback networks for open quantum systems with continuous weak measurement that allows a variety of system and control structures that are prohibitive by many other techniques and can in effect react to unmodeled effects through nonlinear filtering. Our approach benefits from characteristics of both stochastic sampling and gradient-based optimization methods yet does not require differentiability as in backpropagation approaches. We demonstrate that this method is efficient due to inherent parallelizability, robust to open system interactions, and outperforms landmark state-dependent feedback control results in simulation.

Zero-noise extrapolation is a quantum error mitigation technique that has typically been studied under the ideal approximation that the noise acting on a quantum device is not time correlated. In this paper, we investigate the feasibility and performance of zero-noise extrapolation in the presence of time-correlated noise. We show that, in contrast to white noise, time-correlated noise is harder to mitigate via zero-noise extrapolation because it is difficult to scale the noise level without also modifying its spectral distribution. This limitation is particularly strong if “local” gate-level methods are applied for noise scaling. However, we find that “global” noise-scaling methods, e.g., global unitary folding, can be sufficiently reliable even in the presence of time-correlated noise.

We propose a scheme for interrogating a warm rubidium vapor using two different clock lasers. Performance wise, this approach is distinctly different from the recently proposed two-color two-photon rubidium clocks as our scheme does not trade off the ac Stark suppression against an increased sensitivity to the cell temperature or pressure. Instead, our approach compensates both the ac Stark shift and the temperature and pressure-induced frequency shifts. The proposed scheme also makes use of the modulation transfer technique, which enables a two orders of magnitude increase in the signal-to-noise ratio compared to traditional clocks that rely on fluorescence measurements.

To investigate high-order harmonic generation in a monochromatic laser field, we derive an analytical expression for the return energy of an electron as a function of the time interval between ionization and return. We then expand the expression for kinetic energy to second order with respect to the Keldysh parameter $\gamma$. In this expansion, the zero-order term is the return energy in the simple man model and the second-order term corresponds to corrections to this model. The origin of this additional kinetic energy is frequently attributed to the nonzero exit of the initial tunneling step. Here, we show that this commonly used picture is incomplete. We present a framework to fully understand the additional kinetic energy as resulting from additive contributions of zero-order and second-order velocities. Our results show that the nonzero velocity of the initial tunneling step has a quantifiable effect on the cutoff energy measured in high harmonic generation (HHG). This opens the door to experimentally addressing the question of the initial electron velocity at the tunnel exit, with important implications for the correct calibration of the attoclock, as well as our interpretation of the strong field-ionization process more broadly.

We studied the tunneling ionization time in the elliptically polarized laser field with the recently proposed phase-of-the-phase spectroscopy, wherein a weak linearly polarized second-harmonic field was introduced to the attoclock frame. By monitoring the oscillation of the photoelectron yield with the relative phase between the two fields, the ionization time of the photoelectrons can be determined without any effort on handling the Coulomb interaction between the escaping electron and the parent ion. This advantage enables us to resolve the ionization time in the full photoelectron momentum distribution. By solving the time-dependent Schrödinger equation, we showed that the ionization time depends on the radial momentum of photoelectron. The photoelectron with larger radial momentum ionizes earlier at the emission angle before the most probable emission angle, while this radial-momentum dependence is reversed at the emission angle after the most probable emission angle. These results are confirmed by our experiments.

We address the effects of internal modes on the evolution dynamics of quantum droplets in symmetric binary Bose-Einstein condensates. Angle-dependent internal modes and the corresponding eigenvalues are solved numerically for both fundamental droplets and vortex droplets with different topological charges. When internal modes corresponding to the largest eigenvalues are excited, perturbed droplets are deformed and appear as regular polygons. While neutrally internal modes make the evolution of a droplet like a breather, the internal modes with higher angular indices result in a persistently periodic rotation of droplets.

Nonlinear optical frequency conversion, observed more than half a century ago, is a cornerstone in modern applications of nonlinear and quantum optics. It is well known that frequency conversion processes are constrained by conservation laws, such as momentum conservation that requires phase-matching conditions for efficient conversion. However, conservation laws alone could not fully capture the features of nonlinear frequency conversion. Here it is shown that topology can provide additional constraints in nonlinear multifrequency conversion processes. Unlike conservation laws, a topological constraint is concerned with the conserved properties under continuous deformation, and can be regarded as a new indispensable degree of freedom to describe multifrequency processes. We illustrate such a paradigm by considering sum frequency generation under a multifrequency pump wave, showing that, akin topological phases in topological insulators, topological phase transitions can be observed in the frequency conversion process both at the classical and quantum levels.

By choosing more orthogonality between preselection and postselection states, one can significantly improve the sensitivity in general optical quantum metrology based on the weak-value amplification (WVA) approach. However, increasing the orthogonality decreases the probability of detecting photons and makes the weak measurement difficult, especially when the weak measurement is disturbed by strong noise and the pointer is drowned in noise with a negative-decibel signal-to-noise ratio (SNR). In this article we introduce and numerically evaluate a modified weak-measurement protocol with a temporal pointer, namely, the autocorrelative weak-value amplification (AWVA) approach. Specifically, a small longitudinal time delay (tiny phase shift) $\tau$ of a Gaussian pulse is measured by implementing two simultaneous autocorrelative weak measurements under Gaussian white noise with different SNRs. The small quantities $\tau$ are obtained by measuring the autocorrelation coefficient of the pulses instead of fitting the shift of the mean value of the probe in the standard WVA technique. Simulation results show that the AWVA approach outperforms the standard WVA technique in the time domain with smaller statistical errors, remarkably increasing the precision of weak measurement under a strong noise background.

Tensor networks offer a novel and powerful tool for solving a variety of problems in mathematics, data science, and engineering. One such network is the multiscale entanglement renormalization ansatz (MERA). The MERA exhibits a hierarchical structure of layers, where each layer corresponds to a particular length (or energy) scale. The structure can be easily constructed using isometric and disentangler transformations. The following question arises: Is it possible to use the MERA to build hierarchical quantum secret sharing (HQSS)? The paper answers the question in the affirmative. In particular, it shows how a hierarchy of participant trust and authority relates to a MERA structure. The structure consists of binary and ternary MERA modules, which generate secret shares for participants. Because a binary MERA can be replaced by its ternary sibling and vice versa, our HQSS scheme is dynamic, allowing promotion and demotion of participants from the different layers but also enrollment of new ones and disenrollment of old ones from the same layer. The correctness and security of our dynamic hierarchical quantum secret sharing scheme are discussed.

In this research we analyze two models using the tensor network algorithm. The quantum correlations of a two-qubit system are first studied in different bosonic reservoirs. Both equilibrium and nonequilibrium scenarios are discussed. Non-Markovian effects can improve the survival time of the quantum correlations significantly and weaken the decoherence effect. Non-Markovian dynamics with existing memory can lead to entanglement rebirth in specific scenarios instead of the eventual entanglement decay or death seen in memoryless Markovian cases. The system reaches a steady state quickest in sub-Ohmic reservoirs and shows the most apparent non-Markovian behavior in super-Ohmic reservoirs. The Markovian approximation used in this paper is superior to that in the Bloch-Redfield master equation. The entanglement dynamics behaves similarly under different approaches when the system-bath coupling is weak and the memory effect is significant when the system-bath coupling is strong. We study not only the impact of the environment on quantum correlations, but also how to protect quantum correlations. Starting from a state in which the two ends are maximally entangled, a one-dimensional Aubry-André chain model is also studied. We identify distinct phases by monitoring the imbalance dynamics. When the chain is closed, the imbalance dynamics behaves differently in various phases and so does the entanglement evolution between the chain's ends. When the first site couples to a bath, we find that the imbalance dynamics can still be an effective indicator to differentiate various phases in an early evolution stage since the imbalance dynamics is only remarkably affected at relatively high temperatures. The distribution of the eigenenergy of the system can account for it. The entanglement of the chain ends decays rapidly in all phases due to one of the ends being coupled to the bath directly. However, the entanglement of the chain ends will persist for a perceptible amount of time in the localization phase if the bath is coupled to the middle site of the chain. Our research shows that one can utilize the disordered environment as a buffer to protect quantum correlations.

Frequency-multiplexed quantum communication usually requires a single-shot identification of the frequency mode of a single photon. In this paper, we propose a scheme that can identify the frequency mode with high resolution even for spontaneously emitted photons whose generation time is unknown, by combining the time-to-space and frequency-to-time mode mapping. We also demonstrate the mapping of the frequency mode (100 MHz intervals) to the temporal mode (435 ns intervals) for weak coherent pulses using atomic frequency combs. This frequency interval is close to the minimum frequency mode interval of the atomic frequency comb quantum memory with the ${\mathrm{Pr}}^{3+}$-ion-doped ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal, and the proposed scheme has the potential to maximize the frequency multiplexing of the quantum repeater scheme with the memory.

Recently, a $\mathcal{P},\mathcal{T}$-odd Faraday effect (PTFE), optical rotation in an external electric field parallel to the light propagation direction, was discussed as a promising tool for observation of the electron electric dipole moment in the intracavity absorption spectroscopy with atoms or molecules. The main mechanism leading to this effect, the linear Stark splitting of atomic levels (PTFE, LS) is well known. This mechanism is similar to the Zeeman splitting mechanism in case of the ordinary Faraday effect, i.e., optical rotation in an external magnetic field. In this paper we analyze the other possible mechanisms of PTFE, in particular, the atomic-level-mixing mechanism (PTFE, LM). In our analysis we perform considering PTFE (as well as the ordinary FE) as a kind of electro- (magneto-) optical circular birefringence. For the ordinary FE the level-mixing mechanism (FE, LM) is known apart from the main Zeeman level splitting mechanism (FE, LS). However, (PTFE, LM) mechanism for the PTFE was never considered in literature. Usually, the (FE, LS) mechanism dominates and the same was expected for (PTFE, LS). We demonstrate, however, that with heavy diatomic molecules, in particular in PbF, the contribution of (PTFE, LM) may essentially exceed the (PTFE, LS). This promises an additional enhancement of $\mathcal{P},\mathcal{T}$-odd effects in experiments with PbF molecule and makes this molecule one of the best candidates for performing such experiments.