Squeezed light can exponentially increase the signal-to-noise ratio (SNR) of dispersive qubit readout, especially using a combination of injected external squeezing (IES) and intracavity squeezing (ICS). We further investigate whether IES and ICS can also exponentially improve the measurement precision of temperature. In the case of fully thermalized qubits isolated from a thermal bath, the measurement precision of temperature can be improved exponentially when the temperature, the measurement time, or the input photon number approaches 0. In general, thermal fluctuations prevent the action of squeezed light. When multiple qubits maintain interacting with the thermal bath, the Heisenberg scaling can be achieved if the loss rate of the cavity is large and the coupling between the qubit and the optical cavity is weak enough. In the meantime, IES can also further promote the improvement of the measurement precision of the temperature exponentially.

We present an initialization method for variational quantum algorithms applicable to intermediate-scale quantum computers. The method explores the efficiently classically simulable Clifford points of a parameterized quantum circuit, using simulated annealing to find a low-energy initial state. We provide numerical evidence of the effectiveness of the technique for different choices of the Hamiltonian structure, number of qubits, and circuit depth. While a number of problems are considered, we note that the method is particularly useful for quantum chemistry problems, for which it is able to capture long-range correlation energy. The presented method could help achieve a quantum advantage in noisy or fault-tolerant intermediate-scale devices by preparing a high-quality initial state.

In the framework of the hybrid quantum-classical variational cluster approach to strongly correlated electron systems one of the goals of a quantum subroutine is to find single-particle correlation functions of lattice fermions in polynomial time. Previous works suggested to use variants of the Hadamard test for this purpose, which requires an implementation of controlled single-particle fermionic operators. However, for a number of locality-preserving mappings to encode fermions into qubits, a direct construction of such operators is not possible. In this work, we propose a quantum algorithm that uses an analog of the Kubo formula adapted to a quantum circuit simulating the Hubbard model. It allows to access the Green's function of a cluster directly using only bilinears of fermionic operators and circumvents the usage of the Hadamard test. We test our algorithm in practice by using open-access simulators of noisy IBM superconducting chips.

Quantum repeaters that incorporate quantum error correction codes have been shown to be a promising alternative compared with the original quantum repeaters that rely upon probabilistic quantum error detection depending on classical communication over remote repeater stations. A particularly efficient way of encoding qubits into an error correction code is through bosonic codes where even a single oscillator mode serves as a sufficiently large, physical system. Here we consider the bosonic Gottesman-Kitaev-Preskill (GKP) code as a natural choice for a loss-correction-based quantum repeater. However, unlike existing treatments, we focus on the excitation loss that occurs in the local, stationary memory qubits as represented by, for instance, collective atomic spin modes. We analyze and assess the performance of such a GKP-based quantum repeater where, apart from the initial state generations and distributions, all operations can be performed via deterministic linear mode transformations, as opposed to other existing memory-based quantum repeater schemes.

Assembly of ultracold polar molecules containing silver (Ag) from laser-cooled atoms requires knowledge of the dynamic polarizabilities of Ag at convenient laser wavelengths. We present calculations and analysis of the energies and electric-dipole dc and ac polarizabilities of the low-lying states of neutral Ag. Calculations of the properties of the $4d^{10}x$ states, where $x=5s,6s,7s,5p,6p,7p,5d,6d$, and $4f$, are performed using the linearized coupled cluster single-double method. The properties of the $4d^{9}5s^2{}^{2}D_{5/2,3/2}$ states are obtained within the framework of configuration interaction with 11 and 17 electrons in the valence field. We analyze the different contributions to the polarizabilities and estimate the uncertainties of our predictions.

For paraxial light beams and electromagnetic fields, the Stokes vector and polarization matrix provide equivalent scalar measures of optical chirality, widely used in linear optics. However, growing interest in nonparaxial fields, with fully three-dimensional polarization components, necessitates an extended framework. Here, we develop a general theory for characterizing optical chirality in arbitrary electromagnetic fields, formulated through extensions of the polarization matrix approach. This framework applies to both near- and far-field optical helicity and chirality. As examples, we demonstrate its relevance to near-zone fields from chiral dipole emission and the focal plane of tightly focused beams.

Quantum machine learning is considered one of the current research fields with great potential. In recent years, Havlíček et al. [Nature (London) 567, 209 (2019)] have proposed a quantum machine learning algorithm with quantum-enhanced feature spaces, which effectively addressed a binary classification problem and offered a potential pathway to achieving quantum advantage. However, a straightforward binary classification algorithm falls short in solving multiclass classification problems. In this paper, we propose a quantum algorithm that rigorously demonstrates that quantum kernel methods enhance the efficiency of multiclass classification in real-world applications, providing quantum-enhanced performance. To demonstrate this, we design six distinct quantum kernels within the quantum algorithm to map input data into quantum state spaces and estimate the corresponding quantum kernel matrices. The results from quantum simulations reveal that the quantum algorithm outperforms its classical counterpart in handling six real-world multiclass classification problems. Furthermore, we leverage a variety of performance metrics to comprehensively evaluate the classification and generalization performance of the quantum algorithm. The results demonstrate that the quantum algorithm achieves superior classification and better generalization performance relative to classical counterparts.

In the process of interatomic Coulombic electron capture, an incident free electron is captured at an atomic center $A$ and the transition energy is transferred radiationlessly over a rather large distance to a neighboring atom $B$ of different species, upon which the latter is ionized. We consider two-step cascade processes where the electron emitted from atom $B$ is subsequently captured as well (either at center $A$ or yet another atomic center $C$), leading to the emission of a second electron from one of the centers. We derive formulas for the cross section of this cascading interatomic Coulombic electron capture, which can reach the order of kilobarn for suitable parameters and exhibits a very strong dependence on the interatomic distances. The relevance of this process, which leads to a substantial rearrangement of the electronic configuration, is discussed in various two- and three-center atomic systems.

We study intraband high-harmonic generation (HHG) in a crystal driven by quantum light. Previous theoretical studies have developed a framework based on coherent state expansions in terms of $P$ distributions to consider nonclassical driving fields for HHG in atoms. Here we adapt this framework to the context of solids and consider an intraband model of ZnO. We investigate the effect of the quantum optical nature of the driving field on the harmonic spectra including the cutoff and the intensity scaling of the harmonics with driving field intensity. Based on analytical calculations in the Floquet limit, we explain why driving with thermal light or bright-squeezed vacuum (BSV) produces a much higher cutoff than when driving with fields described by coherent or Fock states. Further, we derive an expression for the generated time-dependent electric field and its fluctuations and find that it inherits characteristics of the driving field. Finally, we discuss the limitations of an approximative positive $P$ representation, which is introduced to be able to reduce the numerical complexity for Fock and BSV driving fields.

In recent years, flat electronic bands in twisted bilayer graphene (TBG) have attracted significant attention due to their intriguing topological properties, extremely slow electron velocities, and enhanced density of states. Extending twisted bilayer systems to new configurations is highly desirable, as it offers promising opportunities to explore flat bands beyond TBG. Here, we study both topological and trivial flat bands in a twisted bilayer honeycomb lattice for ultracold atoms and present the evolution of the flat bands with different interlayer coupling strength (ICS). Our results demonstrate that an isolated topological flat band can emerge at the Dirac-point energy for a specific value of weak ICS, referred to as the “critical coupling.” This occurs over a wide range of twist angles, surpassing the limits of the magic angle in TBG systems. When the ICS is slightly increased beyond the critical coupling value, the topological flat band exhibits degenerate band crossings with both the upper and lower adjacent bands at the high-symmetry ${\mathrm{\Gamma }}_s$ point. As the ICS is further increased into the strong coupling regime, trivial flat bands arise around Dirac-point energy. Meanwhile, more trivial flat bands appear, extending from the lowest to higher energy bands, and remain flat as the ICS increases. The topological properties of the flat bands are studied through the winding pattern of the Wilson loop spectrum. Our research provides deeper insights into the formation of flat bands in ultracold atoms with highly controllable twisted bilayer optical lattices, and may contribute to the discovery of new strongly correlated states of matter.

We explore the phase diagram of the extended attractive $\mathrm{SU}\left(3\right)$ Hubbard chain with two-body hopping and nearest-neighbor attraction at half-filling. In the large on-site attraction limit, we identify three different phases: phase separation (PS); Tomonaga-Luttinger liquid (TLL); and charge density wave (CDW). Our analysis reveals that the $\eta$-clustering state, a three-component generalization of the $\eta$-pairing state, becomes the ground state at the boundary between the PS and TLL phases. On an open chain, this state exhibits an edge-edge correlation, which we call boundary off-diagonal long-range order (bODLRO). Using the density matrix renormalization group (DMRG) method, we numerically study the phase diagram of the model with large but finite on-site interactions and find that the numerical results align with those obtained in the strong coupling limit.

Terahertz nonlinear response has considerable potential for applications in fields such as high-speed communication and radar imaging. However, owing to the weakness of its nonlinear effects and the scarcity of high-power sources, its practical applications remain largely constrained. In this work, we present a way to manipulate third-harmonic generation (THG) at terahertz frequencies via the design and study of dual-resonance metagrating with a compound lattice. The compound lattice supports two photonic bands of the magnetic polariton mode and the spoof surface plasmon polariton (SSPP) mode, whose eigenfrequencies satisfy a threefold relationship. Due to the geometric symmetry of the structure, an accidental bound state in the continuum (BIC) emerges at the SSPP band. The combined contribution of the two distinct resonant modes can significantly enhance the THG process at a lower input power. Meanwhile, the quasi-BIC mode operating at the nonlinear frequency has the capacity to confine the generated nonlinear harmonics within the metagrating, thereby suppressing the radiation channel of the third-harmonic light. Specifically, based on the nonlinear efficiency and resonance mechanisms, four different THG regimes are identified, namely, region I (dual-resonance region), region II (annihilation region due to BIC), region III (transition region), and region IV (single-resonance region). Our findings offer an alternative paradigm to control nonlinear processes through metagrating to tailor the light-matter interactions.

In this article, we study bipartite quantum steering using a general class of measurement operators known as the generalized equiangular measurement. Our approach allows for the construction of steering inequalities that are applicable not only to informationally complete measurements but also to incomplete or lossy scenarios, where the resulting quantum assemblages may be subnormalized. We develop a method to analytically bound the value of steering functionals under local hidden state (LHS) models by employing a generalized inequality for the eigenvalues of positive semidefinite operators, extending classical results such as the Samuelson inequality. This yields universal upper bounds on the LHS value in terms of the parameters defining the frame. Furthermore, we analyze the asymptotic behavior of these bounds in high-dimensional Hilbert spaces and demonstrate the possibility of unbounded quantum violations of steering inequalities. Our formalism encompasses previously known results for mutually unbiased bases and mutually unbiased measurements as special cases, showing that these are particular instances within a unified and more general framework.

Many resource theories share an interesting property: An operation is free if and only if its renormalized Choi matrix is a free state. In this article, we refer to resource theories exhibiting this property as Choi-defined resource theories. We demonstrate how and under what conditions one can construct a Choi-defined resource theory, and we prove that when such a construction is possible, the free operations are all and only the completely resource-nongenerating operations. Moreover, we examine resource measures, a complete family of monotones, and conversion distances in such resource theories.

Achieving the ultimate quantum precision in the estimation of multiple physical parameters simultaneously is a challenge in quantum metrology due to fundamental limitations and experimental challenges in harnessing the necessary quantum resources. We propose an experimentally feasible scheme to reach Heisenberg-scaling precision in the simultaneous estimation of two unknown phase parameters in a Mach-Zehnder interferometer by using squeezed and coherent states of light as input and homodyne detections at the outputs. Our results open a new research paradigm in harnessing the full quantum advantage of multiparameter estimation in distributed quantum sensing and quantum information processing, as enabled by multiphoton quantum interference with scalable experimental resources.

Collective measurements on identical and independent quantum systems can offer advantages in information extraction compared with individual measurements. However, little is known about the distinction between restricted collective measurements and genuine collective measurements in the multipartite setting. In this work we establish a rigorous performance gap based on a simple and old estimation problem, the estimation of a random spin state given three parallel spins. Notably, we derive an analytical formula for the maximum estimation fidelity of biseparable measurements and clarify its fidelity gap from genuine collective measurements. Moreover, we clarify the structure of optimal biseparable measurements. It turns out the maximum estimation fidelity can be achieved by two- and one-copy measurements assisted by one-way communication in one direction, but not the other way. Our work reveals a rich landscape of multipartite nonclassicality in quantum measurements instead of quantum states and is expected to trigger further studies.

Estimates of noise channels for quantum gates are required for most error-mitigation techniques and are desirable for informing quantum error-correction decoders. These estimates can be obtained by resource-intensive offline characterization techniques, but can become stale due to device drift and fluctuations. We propose a method to address this issue by performing real-time adaptation of noise channel estimates during the execution of a quantum algorithmic circuit using extended flag gadgets, midcircuit measurements, and Bayesian inference. We carry out analytical calculations and numerical simulations employing a Dirichlet prior distribution for the error rates in a Pauli channel to demonstrate and evaluate the technique, which can be seen as a protocol for real-time calibration of high-level gate error information.

We report on the experimental characterization of various types of spin-exchange interactions between two individual atoms, where pseudospin degrees of freedom are encoded in different Rydberg states. For the case of the direct dipole-dipole interaction between states of opposite parity, such as between $|nS\rangle$ and $|nP\rangle$, we investigate the effects of positional disorder arising from the residual atomic motion on the coherence of spin-exchange oscillations. We then characterize an indirect dipolar spin exchange, i.e., the off-diagonal part of the van der Waals effective Hamiltonian that couples the states $|nS\rangle$ and $|(n+1)S\rangle$. Finally, we report on the observation of a new type of dipolar coupling, made resonant using addressable light shifts and involving four different Rydberg levels: this exchange process is akin to electrically induced Förster resonance, but featuring local control. It exhibits an angular dependence distinct from the usual $1-3{cos}^2\left(\theta \right)$ form of the resonant dipolar spin-exchange.

We present the triple-differential cross sections (TDCSs) for electron-impact single ionization of a potential radiosensitizer molecule, 1-methyl,5-nitroimidazole (${\mathrm{C}}_4{\mathrm{H}}_5{\mathrm{N}}_3{\mathrm{O}}_2$), at an impact energy of 97 eV. The experiment was done using a momentum spectrometer, a so-called reaction microscope, employing the $(e,2e+\text{ion})$ coincidence technique. Modifications of the existing experimental setup to incorporate a vapor target produced from a solid sample are described. Experimental results are compared with the multicenter three-distorted-wave model. The post-collision interaction (PCI) effects are included using the Ward-Macek approximation in the multicenter three-distorted-wave Ward-Macek model. Both experimental and theoretical TDCSs include contributions from the first five valence orbitals which lead to the formation of the parent ${\mathrm{C}}_4{\mathrm{H}}_5{\mathrm{N}}_3{\mathrm{O}}_2{}^{+}$ ion. Qualitative agreement between the theory and the experiment is obtained. The prominent $\pi$ character of the ionized orbitals is evident in both results, along with indications of PCI effects and multiple scattering at the ionic potential, manifested as significant out-of-plane electron emissions.