We develop a scheme to prepare a macroscopic maximally entangled state between two atomic ensembles using adaptive quantum nondemolition (QND) measurements. The quantum state of the system is evolved using a sequence of QND measurements followed by adaptive unitaries such that the desired measurement outcome is obtained with asymptotically unit probability. This procedure is repeated in $z$- and $x$-spin bases alternately such that the state converges deterministically towards the maximally entangled state. Up to a local spin-basis rotation, the maximally entangled state has zero total spin angular momentum, i.e., it is a singlet state. Our protocol does not perform postselection and works beyond the Holstein-Primakoff regime for the atomic spin degrees of freedom, producing genuine macroscopic entanglement.

Spectroscopy and imaging in the mid-infrared ($2.5\mu \mathrm{m}\sim \lambda \sim 25\mu \mathrm{m}$) is bedeviled by the presence of a strong 300-K thermal background at room temperature that makes infrared (IR) detectors decades noisier than can be readily achieved in the visible. The technique of imaging with undetected photons (IUP) exploits the quantum correlations between entangled photon pairs to transfer image information from one spectral region to another, and here we show that it does so in a way that is immune to the thermal background. This means that IUP can be used to perform high-speed photon-counting measurements across the mid-IR spectrum, using uncooled visible detectors that are many times cheaper, faster, and more sensitive than their IR counterparts.

We study the dynamics of entanglement and atomic populations of ultracold dipolar bosons in an aligned three-well potential described by an extended Bose-Hubbard model. We focus on a sufficiently strong interacting regime where the couplings are tuned to obtain an integrable system, in which the time evolution exhibits a resonant behavior that can be exactly predicted. Within this framework, we propose a protocol that includes an integrability breaking step by tilting the edge wells for a short time through an external field, allowing the production of quantum states with a controllable degree of entanglement. We analyze this protocol for different initial states and show the formation of highly entangled states as well as NOON-like states. These results offer valuable insights into how entanglement can be controlled in ultracold atom systems that may be useful for the proposals of new quantum devices.

Orbital degree of freedom plays an important role for understanding quantum many-body phenomena. In this paper, we study an experimentally related setup with ultracold bosons loaded into hybridized bands of two-dimensional hexagonal optical lattices. We find that the system supports various quantum many-body phases at zero temperature, including chiral superfluid and chiral Mott insulating phases by breaking time-reversal symmetry, and the time-reversal-even insulating phase, based on dynamical mean-field theory. In the deep insulating regime, the time-reversal-even phase arises from the interplay of effective Dzyaloshinskii-Moriya and Heisenberg exchange interactions. To relate to experimental situations, we make band-structure calculations to obtain the Hubbard parameters, and show that these orbital ordering phases persist also in the presence of next-nearest-neighbor hopping.

We systematically investigate the excited-state quantum phase transition (ESQPT) in the anisotropic quantum Rabi model, which interpolates between the quantum Rabi model with ${\mathbb{Z}}_2$ symmetry and the Jaynes-Cummings model with $\mathbb{U}\left(1\right)$ symmetry. We calculate the model energy spectra and density of states (DOS) with the cumulants in both analytical and numerical ways to describe the ESQPT by the singularities. In the Jaynes-Cummings limit, its continuous $\mathbb{U}\left(1\right)$ symmetry presents different nonanalytic behaviors from the quantum Rabi model with the discrete ${\mathbb{Z}}_2$ symmetry, and there exists a finite discontinuous jump at the critical energy. For the general anisotropy case with ${\mathbb{Z}}_2$ symmetry, there are two types of ESQPTs characterized by the finite discontinuous jump and the logarithmic divergence in the DOS, respectively. Different from the ground-state quantum phase transition, the ESQPT of the anisotropic quantum Rabi model strongly depends on the anisotropy which leads to a discontinuity even though the ${\mathbb{Z}}_2$ symmetry is still preserved.

Electromagnetically induced transparency (EIT) spectroscopy has been used for the development of sensitive sensors for quantum metrology applications. The sensitivity of a sensor is strongly dependent on the measurement precision of the EIT spectrum. In this work, we present a theoretical study of spectral lineshape measurement of three-level $\mathrm{\Lambda }$-type EIT media based on Fisher information (FI) analysis. Using two kinds of probing sources, the single-photon Fock state and the coherent state, we calculate the FI in an EIT medium and quantify the quantum advantage and limitations of the single-photon probe. The analysis of the FI structure also provides a clear picture for classifying the spectral lineshape into two different regimes, EIT and Aulter-Townes splitting (ATS). This work provides a systematic analysis of the single-photon EIT spectrum, which provides essential knowledge of quantum sensing based on EIT and deepens our understanding of the spectral characteristics of the $\mathrm{\Lambda }$-type media.

One possible realization of non-Hermitian systems is based on open quantum systems by omitting quantum jumping terms in the master equation. This is a good approximation at short times where the effects of quantum jumps can be ignored. However, the jumps can affect the long-time dynamics of the system, motivating us to take the jumps into account in these studies. In this paper, by treating the quantum jumps as perturbations, we examine the effect of the quantum jumps on the non-Hermitian system. For this purpose, we first derive an effective Hamiltonian to describe the dynamics of the open quantum system based on the master equation, then expand the eigenstates and eigenenergies up to the first and second order in the quantum jumps. Finally, we apply our theory to a dissipative two-level system and dissipative fermionic superfluids. The effect of the quantum jump on the dynamics and the nonequilibrium phase transition is demonstrated and discussed.

Solving a Poisson equation is generally reduced to solving a linear system with a coefficient matrix $A$ of entries $a_{ij}, i,j=1,2,...,n$, from the discretized Poisson equation. Although the variational quantum algorithms are promising algorithms to solve the discretized Poisson equation, they generally require that $A$ be decomposed into a sum of $O\left[\text{poly}\left({\text{log}}_{2}n\right)\right]$ simple operators in order to evaluate efficiently the loss function. A tensor product decomposition of $A$ with $2{\text{log}}_{2}n+1$ terms has been explored in previous works. In this paper, based on the decomposition of sparse Hamiltonians we greatly reduce the number of terms. We first write the loss function in terms of the operator ${\sigma }_x\otimes A$ with ${\sigma }_x$ denoting the standard Pauli operator. Then for the one-dimensional Poisson equations with different boundary conditions and for the $d$-dimensional Poisson equations with Dirichlet boundary conditions, we decompose ${\sigma }_x\otimes A$ into a sum of at most 7 and $(4d+1)$ Hermitian, one-sparse, and self-inverse operators, respectively. We design explicitly the quantum circuits to evaluate efficiently the loss function. The decomposition method and the design of quantum circuits can also be easily extended to linear systems with Hermitian and sparse coefficient matrices satisfying $a_{i,i+c}=a_c$ for $c=0,1,...,n-1$ and $i=0,\cdots ,n-1-c$.

A Clifford circuit is a pivotal tool in quantum computing and has extensive applications in quantum error correction codes and topological quantum computing. Hence, it is essential to benchmark and verify the effect of Clifford circuits against noise and errors. Standard quantum process tomography is a fundamental technique for fully characterizing quantum dynamics, but at the cost of exponential time, space, and computation with an increasing number of qubits. Here, we propose an efficient quantum process tomography method for Clifford circuits. Combining with the stabilizer formalism, we prove theoretically that, for an $n$-qubit Clifford circuit, our method merely needs $m$ ancillary qubits and $\lceil n/m\rceil$ input stabilizer states to obtain the quantum process. Numerical simulation results show that our method could perfectly rebuild the unknown Clifford quantum circuit with fidelity over $99.99\%$ to six qubit cases. Our work provides an efficient and practical approach to benchmark and verify Clifford circuits.

In this paper we propose the Quantum Data Center (QDC), an architecture combining Quantum Random Access Memory (QRAM) and quantum networks. We give a precise definition of QDC and discuss its possible realizations and extensions. We discuss applications of QDC in quantum computation, quantum communication, and quantum sensing, with a primary focus on QDC for $T$-gate resources, QDC for multiparty private quantum communication, and QDC for distributed sensing through data compression. We show that QDC will provide efficient, private, and fast services as a future version of data centers.

A quantum channel describes the general evolution of quantum systems, which is therefore a critical issue for quantum simulation and a fundamental element of quantum information. To realize quantum channels, an efficient method is to randomly implement Kraus operators, which does not require any ancillary quantum system. For open systems, it is natural to extend the method from implementation of unitary Kraus operators to nonunitary ones. However, when some Kraus operators are not proportional to unitary ones, this method becomes probabilistic even if the channel is trace preserving. In this paper, to overcome this drawback, we propose an algorithm to deterministically realize arbitrary trace-preserving channels with only one ancillary qubit and finite iterations of evolutions of the combined states of the system and the ancilla. Moreover, to show the validity of the method, we experimentally realize conventional and modified Landau-Streater channels based on the algorithm. Our results shed light on quantum simulation of quantum channels for open systems.

While most approaches to geometric quantum computation are based on geometric phases in cyclic evolution, noncyclic geometric gates have been proposed to increase further the flexibility. While these gates remove the dynamical phase of the computational basis, they do not, in general, remove it from the eigenstates of the time evolution operator, which makes the geometric nature of the gates ambiguous. Here, we resolve this ambiguity by proposing a scheme for genuinely noncyclic geometric gates. These gates are obtained by evolving the computational basis along open paths consisting of geodesic segments, and simultaneously assuring that no dynamical phase is acquired by the eigenstates of the time evolution operator. While we illustrate the scheme for the simplest nontrivial case of two geodesic segments starting at each computational basis state of a single qubit, the scheme can be straightforwardly extended to more elaborate paths, more qubits, or even qudits.

We report the investigation of the influence of atomic motion on the fluorescence dynamics of a dilute atomic ensemble driven by resonance pulse radiation. We show that even for sub-Doppler temperatures, the motion of atoms can significantly affect the nature of both superradiation and subradiation. We also demonstrate that, in the case of an ensemble of moving scatterers, it is possible to observe the nonmonotonic time dependence of the fluorescence rate. This leads to the fact that, in certain time intervals, increasing temperature causes not an decrease but an increase of the fluorescence intensity in the cone of coherent scattering. We analyze the role of the frequency diffusion of secondary radiation as a result of multiple light scattering in an optically dense medium. It is shown that spectrum broadening is the main factor which determines radiation trapping upon resonant excitation. Along with broadening, we found a shift and distortion of the shape of the spectrum over time. At later time, after the trapping stage, the dynamics is dominated by close pairs of atoms (dimers). The dynamics of the excited states of these dimers has been studied in detail. It is shown that the change in the lifetime of the given adiabatic term of the diatomic quasimolecule induced by the change in the interatomic distance as well as possible nonadiabatic transitions between sub- and superradiant states caused by atomic motion can lead not to the anticipated weakening of subradiation effect, but to its enhancement.

We study hidden nonlocality in a linear network with independent sources. In the usual paradigm of Bell nonlocality, there are certain states which exhibit nonlocality only after the application of suitable local filtering operations, which, in turn, are some special stochastic local operations assisted with classical communication (SLOCC). In the present work, we introduce the notion of hidden non $n$-locality. The notion is detailed using a bilocal network. We provide instances of hidden nonbilocality and nontrilocality, where we notice quite intriguingly that nonbilocality is observed even when one of the sources distributes a mixed two-qubit separable state. Furthermore, a characterization of hidden nonbilocality is also provided in terms of the Bloch-Fano decomposition, wherein we conjecture that, to witness hidden nonbilocality, one of the two states (used by the sources) must have nonnull local Bloch vectors. Noise is inevitable in practical scenarios, which makes it imperative to study any possible method to enhance the possibility of detecting nonclassicality in the presence of noise in the network. We find that local filtering enhances the robustness to noise, which we demonstrate using bit-flip and amplitude-damping channels.

Quantum computers hold great promise for exact simulations of nuclear dynamical processes (e.g., scattering and reactions), which are paramount to the study of nuclear matter at the limit of stability and in the formation of chemical elements in stars. However, quantum simulations of the unitary (real) time dynamics of fermionic many-body systems require a currently prohibitive number of reliable and long-lived qubits. We propose a co-processing algorithm for the simulation of real-time dynamics in which the time evolution of the spatial coordinates is carried out on a classical processor, while the evolution of the spin degrees of freedom is carried out on quantum hardware. We demonstrate this hybrid scheme with the simulation of two neutrons scattering at the Lawrence Berkeley National Laboratory's Advanced Quantum Testbed. After implementing error mitigation strategies to improve the accuracy of the algorithm in addition to a combination of circuit compression techniques and tomography as methods to elucidate the onset of decoherence, our results validate the principle of the proposed co-processing scheme. A generalization of this present scheme will open the way for (real-time) path integral simulations of nuclear scattering.

We study the problem of measuring errors in non-trace-preserving quantum operations, with a focus on their impact on quantum computing. We propose an error metric that efficiently provides an upper bound on the trace distance between the normalized output states from imperfect and ideal operations, while remaining compatible with the diamond distance. As a demonstration of its application, we apply our metric in the analysis of a lossy beam splitter and a nondeterministic conditional sign-flip gate, two primary non-trace-preserving operations in the Knill-Laflamme-Milburn protocol. We then turn to the leakage errors of neutral-atom quantum computers, finding that these errors scale worse than previously anticipated, implying a more stringent fault-tolerant threshold. We also assess the quantum Zeno gate's error using our metric. In a broader context, we discuss the potential of our metric to analyze general postselected protocols, where it can be employed to study error propagation and estimate thresholds in fault-tolerant quantum computing. The results highlight the critical role of our proposed error metric in understanding and addressing challenges in practical quantum information processing.

Second-harmonic generation (SHG) has emerged as a promising tool for detecting electronic and magnetic structures in noncentrosymmetric materials, but $100\%$ spin-polarized SHG has not been reported. In this work, we demonstrate nearly $100\%$ spin-polarized SHG from the half-metallic ferrimagnet ${\mathrm{Mn}}_2\mathrm{RuGa}$. A band gap in the spin-down channel suppresses SHG, so the spin-up channel contributes nearly all the signal, as large as 3614 pm/V about ten times larger than that of GaAs. In the spin-up channel, ${\chi }_{xyz}^{\left(2\right)}$ is dominated by the large intraband current in three highly dispersed bands near the Fermi level. With the spin-orbit coupling, the reduced magnetic point group allows additional SHG components where the interband contribution is enhanced. Our finding is important as it predicts a large and complete spin-polarized SHG in an all-optical spin-switching ferrimagnet. This opens the door for future applications.

We propose a scheme to realize nonreciprocal mechanical squeezing in a spinning optomechanical system, where a spinning resonator is driven by a periodically modulated laser field via a tapered optical fiber. The spinning resonator supports two counterpropagating optical modes and a mechanical breathing mode induced by the radiation pressure. According to the optical Sagnac effect, the two counterpropagating optical modes obtain opposite frequency shifts. By utilizing the modulated pump field with a given frequency, mechanical squeezing can only be achieved by driving the resonator from one direction but not from the other. We analyze the impact of backscattering losses on mechanical squeezing and find that this negative impact can be almost completely avoided by increasing the angular velocity of the resonator. We also show that the presented scheme is robust to mechanical thermal noise and can be realized under the current experimental conditions. Therefore, this work may be meaningful for the study of quantum nonreciprocity and quantum precision measurement.

We study the concepts of complete positivity and positivity in a two-level open quantum system whose dynamics are governed by a time-local quantum master equation. We establish necessary and sufficient conditions on the time-dependent relaxation rates to ensure complete positivity and positivity of the dynamical map. We discuss their relations with the non-Markovian behavior of the open system. We also analyze the long-time asymptotic behavior of the dynamics as a function of the rates. We show under which conditions on the rates the system tends to an equilibrium state. Different examples illustrate this general study.