The radical pair mechanism is an important model that may provide a basis for biological magnetoreception. To account for the high orientation precision of the real avian compass, Hore et al. proposed an intriguing phenomenon called the quantum needle [Proc. Natl. Acad. Sci. USA 113, 4634 (2016)], where a spikelike feature emerges in the fractional yield signal. However, it is believed that the quantum needle requires the radical pair lifetime to be longer than a few microseconds and thus poses stern challenges in realistic biological systems. Here, we exploit the optimization techniques and find an alternative class of model system, which sustains much more prominent features of the quantum needle and significantly relaxes the requirement for radical pair lifetime. Even more surprisingly, we find that the characteristics of the quantum needle retain a narrow functional window around the geomagnetic field, which is absent in the previous model systems. Therefore, our work provides essential evidence for identifying the possible physical mechanism for the quantum needle in chemical magnetoreception.

Quantum signal processing combined with quantum eigenvalue transformation has recently emerged as a unifying framework for several quantum algorithms. In its standard form, it consists of two separate routines: block encoding, which encodes a Hamiltonian in a larger unitary, and signal processing, which achieves an almost arbitrary polynomial transformation of such a Hamiltonian using rotation gates. The bottleneck of the entire operation is typically constituted by block encoding, and in recent years, several problem-specific techniques have been introduced to overcome this problem. Within this framework, we present a procedure to block-encode a Hamiltonian based on its matrix product operator (MPO) representation. More specifically, we encode every MPO tensor in a larger unitary of dimension $D+2$, where $D=\lceil {log}_2\left(\chi \right)\rceil$ is the number of subsequently contracted qubits that scales logarithmically with the virtual bond dimension $\chi$. Given any system of size $L$, our method requires $L+D$ ancillary qubits in total, while the number of one- and two-qubit gates decomposing the block-encoding circuit scales as $O\left(L{\chi }^2\right)$. Moreover, block encodings generally require postselection. Our model exhibits a success probability decaying exponentially with $L$. Nevertheless, we present a way to mitigate this limitation for finite systems.

A common approach to deal with gate errors in modern quantum-computing hardware is zero-noise extrapolation. By artificially amplifying errors and extrapolating the expectation values obtained with different error strengths towards the zero-error (zero-noise) limit, the technique aims at rectifying errors in noisy quantum computing systems. For accurate extrapolation, it is essential to know the exact factors of the noise amplification. In this article, we propose a simple method for estimating the strength of errors occurring in a quantum circuit and demonstrate improved extrapolation results. The method determines the error strength for a circuit by appending to it the inverted circuit and measuring the probability of the initial state. The estimation of error strengths is easy to implement for arbitrary circuits and does not require previous characterization of noise properties. We compare this method with the conventional zero-noise extrapolation method and show that our method leads to a more accurate calculation of expectation values on current quantum hardware, showcasing its suitability for near-term quantum computing applications.

As measurement precision continues to advance, the accurate determination of multipolar polarizabilities and hyperpolarizabilities becomes increasingly crucial for precisely analyzing atomic light shifts in external fields, yet their investigation remains relatively limited. We employ the Dirac-Fock plus core polarization (DFCP) method to calculate the dynamic multipolar polarizabilities and hyperpolarizabilities for the lowest $s_{1/2}$ and $d_{5/2}$ states of alkaline-earth-metal ions. We take into account the contribution of negative-energy states to magnetic dipole polarizability, revealing its dominant impact at the $10{}^{-4}$ level. Furthermore, we conduct a comprehensive analysis of lattice light shift and find that ${\mathrm{Ca}}^{+}$, ${\mathrm{Sr}}^{+}$, and ${\mathrm{Ba}}^{+}$ ions exhibit promising candidates for developing all-optical-trapping ion clocks with a light shift uncertainty down to ${10}^{-18}$. Specifically, the ${\mathrm{Sr}}^{+}$ ion demonstrates potential for achieving an even lower uncertainty of ${10}^{-19}$ in light shift. Additionally, we predict the operational trap depths and laser detuning parameters necessary for conducting relevant experiments, thereby providing a solid theoretical framework to guide future endeavors in this promising research field.

A competition between electron detachment and radiative stabilization (fluorescence) of ${\mathrm{C}}_2{}^{-}$ was observed in the energy region slightly above the electron detachment threshold, where the dominant process is switched from fluorescence to electron detachment. To quantify the stabilization process, we measured the rotationally resolved excitation spectra of delayed detachment; these spectra were obtained from various rotational energy levels in the vibrationally excited $v^{\prime }=5$ state of the electronic excited $B$ state. The dependence of the electron detachment rates on the rotational energy was obtained by analyzing the intensities of the electron detachment signals. At low rotational levels ($N^{\prime }=7$), the electron detachment rate was one order of magnitude lower than the fluorescence rate, whereas at high rotational levels ($N^{\prime }=43$), the electron detachment rate was higher than the fluorescence rate. The rates exhibited a nonlinear increase against the rotational energy. This result is in contrast with the previously reported linear dependence of the detachment rates on the rotational energy for fast electron detachment at high vibrational states. The survival probability of ${\mathrm{C}}_2{}^{-}$ against electron detachment at 4500 ${\mathrm{cm}}^{-1}$ above the detachment threshold was 50%, indicating an equal rate of fluorescence and electron detachment.

Several recent studies have demonstrated the utility of the quantum switch as an important resource for enhancing the performance of various information processing tasks. In a quantum switch, the advantages appear significantly due to the coherent superposition of alternative configurations of the quantum components which are controlled by an additional control system. Here we explore the impact of increasing the Hilbert-space dimension of the control system on the performance of the quantum switch. In particular, we focus on a quantifier of the quantum switch through the emergence of non-Markovianity and explicitly study its behavior when we increase the Hilbert-space dimension of the control system. We observe that increasing the Hilbert-space dimension of the control system leads to the corresponding enhancement of the non-Markovian memory induced by it. Our study demonstrates how the dimension of the control system can be harnessed to improve the quantum-switch-based information processing or communication tasks.

Near-future experiments intend to detect strong-field Breit-Wheeler pair creation from the collision between bremsstrahlung bursts containing GeV-$\gamma$ quanta and high-intensity laser pulses. In this theoretical study, we investigate the influence of laser focusing, radiation reaction, and a broad bremsstrahlung $\gamma$ spectrum on the energy and angular distributions of created pairs. Understanding the role of these inherent reaction attributes provides relevant insights for experimental detection strategies and data interpretation. We show that the inclusion of radiation reaction leads to a narrowing of the particles' energy distribution, whose maximum is shifted to substantially lower energies as compared to the case in which radiative energy losses are ignored. The broad bremsstrahlung spectrum also has a distinct influence on the shape of the particle distributions, whereas the corresponding impact of laser focusing turns out to be rather moderate in the considered parameter regime.

Dynamic polarizabilities of cesium Rydberg states, explicitly $n{S}_{1/2}, n{P}_{1/2}, n{P}_{3/2}, n{D}_{3/2}$, and $n{D}_{5/2}$, where the principal quantum number $n$ is 40–70, are presented for linearly polarized light. The dynamic polarizability is calculated using the sum-over-state approach. We identify double magic wavelengths in the range of $1000–2000$ nm for simultaneous trapping of the ground state and a Rydberg state, which are red detuned and blue detuned, respectively, with respect to a low-lying excited auxiliary state. Based on calculations of the radiative lifetime, blackbody-radiation-induced transitions, and population transfer out of the Rydberg and auxiliary states (estimated within two-state and master-equation models), we conclude that magic wavelength trapping is particularly promising experimentally for the $n{D}_{J,|M_J|}$ Rydberg series with angular momentum $J=3/2$ and projection quantum numbers $M_J=\pm 1/2$ (auxiliary state $8P_{1/2}$) and $M_J=\pm 3/2$ (auxiliary state $8P_{3/2}$), using trap depths as large as $10\mu \mathrm{K}$. Moreover, by tuning the angle between the quantization axis and the polarization vector of the light, we identify triple and quadruple magic wavelengths, for which the polarizabilities of the ground state, a Rydberg state, and, respectively, one and two low-lying excited states are equal. Our comprehensive theoretical study provides much needed guidance for ongoing experimental efforts on cesium Rydberg-state-based quantum simulations that operate on timescales up to several microseconds.

In this work, quantum-optical radiative recombinations between electrons and ions are analyzed using a semiclassical approach without losing their specific quantum-optical nature. Under moderately strong driving field conditions, the results of the quantum-optical model are consistent with the semiclassically derived results in terms of both the spectral intensity and phase. Deviations between the two models, which are observed under extremely strong driving field conditions, are investigated in the time and frequency domains to clarify the underlying reasons. In addition, the radiative recombination dynamics between a free electron and a bare ion can be simulated using our quantum model. In recent decades, the semiclassical model combined with the classical equations of light has been used as an intuitive and powerful tool to study the macroscopic buildup of nonlinearly generated light. In this macroscopic approach, the phase information of individual atomic radiations plays an essential role. Since our model can provide spectral phases of quantum-optical lights generated from individual atoms, it is now feasible to pursue the macroscopic buildup of specific quantum-optical radiations and relevant phenomena by using the conventional approach used for the semiclassical model.

The equation of state (EOS) serves as a cornerstone in elucidating the properties of quantum many-body systems. A recent highlight along this research line consists of the derivation of the nonuniversal Lee-Huang-Yang (LHY) EOS for an ultracold quantum bosonic gas with finite-range interatomic interactions using one-loop effective path-integral field theory [Phys. Rev. Lett. 118, 130402 (2017)]. The purpose of this work is to extend Salasnich's pioneering work to uncover beyond-LHY corrections to the EOS by employing the Cornwall-Jackiw-Tomboulis (CJT) effective-field theory, leveraging its two-loop approximation. To this end, we expand Salasnich's remarkable findings of EOS to the next leading order characterized by ${\left(\rho a_{\text{s}}^3\right)}^2$, with $\rho$ and $a_{\text{s}}$ being the density and the $s$-wave scattering length. Notably, we derive analytical expressions for quantum depletion and chemical potential, representing the next-to-LHY corrections to nonuniversal EOS induced by finite-range effects. Moreover, we propose an experimental protocol of observing the nonuniversal next-to-LHY corrections to the EOS by calculating fractional frequency shifts in the breathing modes. The nonuniversal beyond-LHY EOS in this work paves the way of using LHY effects in quantum simulation experiments and for investigations beyond the LHY regime.

Orbital degrees of freedom play an important role in understanding the emergence of unconventional quantum phases. Ultracold atomic gases in optical lattices provide a wonderful platform to simulate orbital physics. In this work, we consider spinless fermionic atoms loaded into $p$-orbital bands of a two-dimensional frustrated triangular lattice. The system can be described by an extended Fermi-Hubbard model, which is numerically solved by using the orbital version of real-space dynamical mean-field theory. Due to the interplay of anisotropic hoppings and geometrical frustration, a rich phase diagram is found, including stripe-orbital, paraorbital, and ferro-orbital phases, in contrast to its bosonic analog with orbital-skyrmion ordering in the identical lattice. In order to understand the underlying mechanics of competing orbital orders, we derive an effective orbital-exchange model, which yields consistent explanation with our main numerical results.

A multicavity quantization scheme is developed using quasinormal modes (QNMs) of optical cavities embedded in a homogeneous background medium for cases where retardation is significant in the intercavity coupling. Using quantities that can be calculated in computational optics with numerical Maxwell solvers, we extend previous QNM quantization schemes and define a quantitative measure to determine if a separate quantization of QNM cavities is justified or if a joint quantization of the system is necessary. We test this measure for the examples of two coupled one-dimensional dielectric slabs and a three-dimensional dimer consisting of metal nanorods acting as QNM cavities. For sufficiently large separations, the new scheme allows for an efficient treatment of multicavity phenomena using parameters defined for the individual cavities. Formulating the Hamiltonian in a familiar system-bath form, the scheme connects the rigorous QNM theory and widespread phenomenological models of open cavities coupled to a shared photonic bath with parameters obtained directly from Maxwell calculations.

Multipartite nonlocality is a measure of multipartite quantum correlations, and has been used to characterize quantum criticality in various low-dimensional quantum models. In this work, we propose the nonlocality spectrum $\left\{\right|{\lambda }_i\left|\right\}$ of the nonlocality transfer matrix as a tool to characterize multipartite nonlocality and quantum criticality in infinite-size translation-invariant quantum systems. The approach is used to analyze four typical quantum spin chain models, i.e., the transverse-field Ising model, an XXZ model, a three-body coupling cluster model and an XYZ model. In various types of quantum phase transitions in these models, it is found that at the critical points, the leading eigenvalue $|{\lambda }_1|$ presents identifiable signals, and the spectrum gap $\mathrm{\Delta }\left|\lambda \right|=|{\lambda }_1|-|{\lambda }_2|$ closes. Moreover, the nonlocality spectrum provides a unified mathematical framework to reveal the underlying mechanisms for various previously reported behaviors of multipartite nonlocality. Our results demonstrate the power of the nonlocality spectrum as a valuable tool for the comprehensive characterization of multipartite nonlocality and quantum criticality in one-dimensional quantum systems.

The Mpemba effect is a counterintuitive relaxation phenomenon that a system which begins hot can cool down faster than the identical system which is initially cold. Here, we investigate heat and work in a quantum open system with being alternatively driven by hot-cold heat reservoirs, which operates as an Otto cycle in finite time, either with or without the Mpemba effect. Based on the quantum master equation, the time duration of the cooling relaxation process is determined by exploring the distance function which can be used to observe the appearance of the Mpemba effect. We then numerically evaluate and compare the average and variance of the power of two scenarios with and without the Mpemba effect. We demonstrate that, given the efficiency, the Markovian Mpemba effect leads to improved performance by significantly increasing the power without sacrificing the stability.

The decomplexification procedure, which allows showing mathematically the isomorphism between classical and quantum dynamics of systems with a finite number of basis states, is exploited to propose resonant electric circuits with gyrator-based couplings and to experimentally study the quantum brachistochrone problem, particularly the passage time in Hermitian and parity-time-symmetric cases.

Coherence is the most fundamental quantum resource in quantum information processing. How fast a physical system gets coherence or decoherence is a critical ingredient. We present an attainable quantum speed limit based on the variation of quantum coherence subject to a dynamical process. It indicates that for a two-dimensional quantum state, one can always find corresponding dynamics driving it to evolve along the geodesic to another state with certain coherence variation. As applications, we study the coherence quantum speed limits of the dephasing and dissipative dynamics. It is shown that the dephasing dynamics can saturate our coherence quantum speed limit, and the decoherence of the state with identical populations will be faster than others. However, the dissipative dynamics have the opposite behavior. In addition, we illustrate a stronger tightness of our bound for the mentioned dynamics by comparison.