The generation of the narrowband strong-correlated biphotons via spontaneous four-wave mixing can be effectively controlled and enhanced by an additional driving field which drives a transition with its upper level being a Rydberg state. We study the properties of the noise of the generated biphotons and show that in the region of weak pumping and low atomic density, a high degree of the photon correlation is maintained with the photon-pair generation rate significantly enhanced.

Thin-film nanostructures with embedded Mössbauer nuclei have been successfully used for x-ray quantum optical applications with hard x-rays coupling in grazing incidence. Here we address theoretically an alternative geometry, in which hard x rays are coupled in forward incidence (front coupling), setting the stage for waveguide QED with nuclear x-ray resonances. We present in a self-contained manner a general model based on the Green's function formalism of the field-nucleus interaction in one-dimensional waveguides and show that it combines aspects of both nuclear forward scattering, visible as dynamical beating in the spatiotemporal response, and the resonance structure from grazing incidence, visible in the spectrum of guided modes. The interference of multiple modes is shown to play an important role, resulting in beats with wavelengths on the order of tens of micrometers, on the scale of practical photolithography. This allows for the design of special sample geometries to explore the resonant response or microstriped waveguides, opening a toolbox of geometrical design for hard x-ray quantum optics.

In this work, we present a coherence measure based on the quantum optimal transport cost in terms of the convex roof extended method. We also obtain the analytical solutions of the quantifier for pure states. We also propose an operational interpretation of the coherence measure for pure states.

We investigate topology-dependent giant-atom–photon bound states in a giant-atom coupled topological waveguide system and find that the bound states can induce topology-dependent dipole-dipole interactions between giant atoms. For bridged giant atoms, when the waveguide is in the topological phase, there is no interaction between them and the atom-atom interaction can only survive in the trivial phase. Conversely, for separated giant atoms, they interact with each other only in the topological phase rather than in the trivial phase. The characteristics of these two cases provide us a way to model pure nearest-neighbor interactions and double chains, respectively. Our work may promote the application of topological physics in quantum information processes and quantum simulations.

Circuit cutting, the partitioning of quantum circuits into smaller independent fragments, has become a promising avenue for scaling up current quantum-computing experiments. Here, we introduce a scheme for joint cutting of two-qubit rotation gates based on a virtual gate-teleportation protocol. With that, we significantly lower the previous upper bounds on the sampling overhead and prove the optimality of the scheme. Furthermore, we show that no classical communication between the circuit partitions is required. For parallel two-qubit rotation gates we derive an optimal ancilla-free decomposition, which includes cnot gates as a special case.

NP-hard problems are not believed to be exactly solvable through general polynomial time algorithms. Hybrid quantum-classical algorithms to address such combinatorial problems have been of great interest in the past few years. Such algorithms are heuristic in nature and aim to obtain an approximate solution. Significant improvements in computational time and/or the ability to treat large problems are some of the principal promises of quantum computing in this regard. The hardware, however, is still in its infancy and the current noisy intermediate-scale quantum (NISQ) computers are not able to optimize industrially relevant problems. Moreover, the storage of qubits and introduction of entanglement require extreme physical conditions. An issue with quantum optimization algorithms such as the quantum approximate optimization algorithm is that they scale linearly with problem size. In this paper, we build upon a proprietary methodology which scales logarithmically with problem size—opening an avenue for treating optimization problems of unprecedented scale on gate-based quantum computers. To test the performance of the algorithm, we first find a way to apply it to a handful of NP-hard problems: Maximum Cut, Minimum Partition, Maximum Clique, Maximum Weighted Independent Set. Subsequently, these algorithms are tested on a quantum simulator with graph sizes of over a hundred nodes and on a real quantum computer up to graph sizes of 256. To our knowledge, these constitute the largest realistic combinatorial optimization problems ever run on a NISQ device, overcoming previous problem sizes by almost tenfold.

Quantum private query (QPQ) faces many challenges in practical applications. At present, some scholars have made substantial work on overcoming channel loss, channel noise, and nonideal light sources, respectively. However, a protocol that would overcome all three of these problems has yet to materialize. We review a practical QPQ protocol that can actually work in noisy channels. This protocol has an upper bound on tolerable errors based on the required level of security and reliability. Then, we study the security of the above QPQ protocol under weak coherent pulses. The results indicate that the multiphoton pulses have induced significant vulnerabilities, which seriously threatens the privacy of users. Finally, we propose a decoy-state method to solve the serious threat to user security caused by multiphoton pulses. The analysis shows that the decoy-state method significantly improves the security of the QPQ protocol under weak coherent pulses. The improved protocol can not only tolerate transmission losses and channel noise, but also overcome the security vulnerability caused by a nonideal light source.

The spin-orbit coupling (SOC) affecting the center of mass of ultracold atoms can be simulated using a properly chosen periodic sequence of magnetic pulses. Yet such a method is generally accompanied by micromotion, which hinders precise control of atomic dynamics, thus complicating practical applications. Here we show how to bypass the micromotion emerging in the magnetically induced SOC by properly switching on and off the oscillating magnetic fields at the initial and final times. We consider the exact dynamics of the system and demonstrate that the overall dynamics can be immune to the micromotion. The exact dynamics is shown to agree well with the evolution of the system described by the slowly changing effective Floquet Hamiltonian including the SOC term. The agreement is shown to be best when the phase of the periodic driving takes a specific value for which the effect of the spin-orbit coupling is maximum.

Microchannel-plate (MCP) detectors, when used at pulsed-neutron-source instruments, offer the possibility of high spatial resolution and high contrast imaging with pixel-level spectroscopic information. Here we demonstrate the possibility of multimodal analysis including total neutron cross-section spectra measurements, quantitative material differentiation imaging, and texture-sensitive in-line phase imaging, from a single exposure using an MCP detector. This multimodal approach operates in full-field imaging mode, with the neutron transmission spectra acquired at each individual detector pixel. Due to the polychromatic nature of the beam and spectroscopic resolving capability of the detector, no energy scanning is required. Good agreement with the library reference data is demonstrated for neutron cross-section spectra measurements. Two different images corresponding to two selected energy bandwidths are used for elemental differentiation imaging. Moreover, the presence of changes in texture, i.e., preferred grain orientation, in the sample is identified from our phase-retrieval imaging results.

We report the experimental observation of the out-of-plane optical conductivity in bilayer graphene at a wavelength of 633 nm. While the in-plane optical constants vary by a few percent from the single-layer crystal up to the bulk, this physical quantity depends strongly on interlayer interactions, being at least three times larger in bilayer than in monolayer graphene.

For quantum error correction codes the required number of measurement rounds typically increases with the code distance when measurements are faulty. Single-shot error correction allows for an error threshold with only one round of noisy syndrome measurements regardless of the code size. Here we implement single-shot check operators for toric codes. The single-shot checks are constructed by Gaussian elimination following Campbell [Campbell, Quantum Sci. Technol. 4, 025006 (2019)]. The single-shot check operators result in a sustainable threshold at 5.62% for an error model with noisy measurements, outperforming the conventional toric code-check operators with multiple rounds of noisy measurement. The cost of the transformation is nonlocal high-weight stabilizer generators. We then consider a gate-based error model that leads to increased measurement error with stabilizer weight. Here we find no single-shot threshold behavior and instead find the code family will have an optimal code size for a fixed error rate. For this error model the conventional check operators with multiple measurements yields a lower logical error rate.

The wave-particle duality, one of the expressions of Bohr complementarity, is usually quantified by path predictability and the visibility of interference fringes. With the development of quantum resource theory, quantitative analysis of wave-particle duality is increasing, most of which are expressed in the form of specific functions. In this paper, we obtain the path-information measure for pure states by converting the coherence measure for pure states into a symmetric concave function. Then we prove the function as a path-information measure is also valid for mixed states. Furthermore, we also establish a generalized wave-particle-mixedness triality. Although the mixedness proposed in the text is not a complete mixedness measure, it also satisfies some conditions of the mixedness measure. From the perspective of resource theory, the path information we establish can be used as the measure of the resource of predictability, and the triality relationship we establish reveals the relationship among coherence, predictability, purity, and mixedness degree to a certain extent. According to our method, given either the coherence measure or path information, a particular form of wave-particle-mixedness triality can be established. This will play an important role in establishing connections between wave, particle, and other physical quantifiers.

Restrictions imposed by existing infrastructure can make it hard to ensure an even spacing between the nodes of future fiber-based quantum networks. We investigate here the negative effects of asymmetric node placement by considering separately the placement of midpoint stations required for heralded entanglement generation, as well as of processing-node quantum repeaters in a chain. For midpoint stations, we describe the effect asymmetry has on the time required to perform one entangling attempt, the success probability of such an attempt, and the fidelity of the entangled states created. This includes accounting for the effects of chromatic dispersion on photon indistinguishability. For quantum-repeater chains, we numerically investigate how uneven spacing between repeater nodes leads to bottlenecks, thereby increasing both the waiting time and the time states are stored in noisy quantum memory. We find that while the time required to perform one entangling attempt may increase linearly with the midpoint's asymmetry, the success probability and fidelity of heralded entanglement generation and the distribution time and error rate for repeater chains all have vanishing first derivatives with respect to the amount of asymmetry. This suggests resilience of quantum-network performance against small amounts of asymmetry.

We investigate the impact of different connectivities on the decoherence time in quantum systems under quasistatic Heisenberg noise. We considered three types of elementary units, including node, stick and triangle and connect them into ring, chain, and tree configurations. We find that rings exhibit greater stability compared with chains, contrary to the expectation that higher average connectivity leads to decreased stability. Additionally, the stick configuration is more stable than the triangle configuration. We also observe similar trends in entanglement entropy and return probability, indicating their potential use in characterizing decoherence time. Our findings provide insights into the interplay between connectivity and stability in quantum systems, with implications for the design of robust quantum technologies and quantum error correction strategies.

The construction of quantum circuits to simulate Hamiltonian evolution is central to many quantum algorithms. State-of-the-art circuits are based on oracles whose implementation is often omitted, and the complexity of the algorithm is estimated by counting oracle queries. However, in practical applications, an oracle implementation contributes a large constant factor to the overall complexity of the algorithm. The key finding of this work is the efficient procedure for representation of a tridiagonal matrix in the Pauli basis, which allows one to construct a Hamiltonian evolution circuit without the use of oracles. The procedure represents a general tridiagonal matrix $2^n\times 2^n$ by systematically determining all Pauli strings present in the decomposition, dividing them into commuting subsets. The efficiency is in the number of commuting subsets $O\left(n\right)$. The method is demonstrated using the one-dimensional wave equation, verifying numerically that the gate complexity as a function of the number of qubits is lower than the oracle-based approach for $n<15$ and requires half the number of qubits. This method is applicable to other Hamiltonians based on the tridiagonal matrices.

Quantum-mechanical simulation of laser-induced alignment of asymmetric-top molecules at high temperatures has been a challenge due to the high computational cost required to model a large number of initial states of the molecular ensemble. Here, we report a method to reduce the computational time and memory requirements by using the symmetries of coefficients in the rotational wave packet. For the case of nonadiabatic alignment of asymmetric-top molecules with a linearly polarized laser pulse, we have theoretically derived the symmetries in the coefficients of the rotational wave packet produced by the induced-dipole interaction. The symmetries allow us to simulate rotational dynamics at higher rotational temperatures than previously possible by reducing both the memory requirements and the computation time by a factor of 8. We demonstrate simulated results of the rotational dynamics of 4-fluorobenzotrifluoride at multiple temperatures up to 300 K.