Advanced quantum technologies最新文献

An online estimated state feedback control for trajectory tracking in stochastic open quantum systems is proposed in this paper, which is based on the Lyapunov-based control method. By inducing the error between the controlled state, and the target state as the error state, the trajectory tracking problem of the quantum system is transformed into the error state transition control problem. The quantum state online estimation method QST-OADM is applied to estimate the state of the error state system online, and the tracking control laws are designed by using the quantum Lyapunov stability theorem for driving the stochastic open quantum system from an arbitrary initial state to an arbitrary trajectory. The numerical simulation experiments and results analyses are given.

A basic element of a quantum network based on two single-mode waveguides is proposed with different frequencies connected by a solid-state qubit. Using a simple example of a possible superconducting implementation, the usefulness of the simplifications used in the general theoretical consideration has been justified. The non-classical field in a single-mode with a frequency of $�{\omega }_1$ is fed to the input of a qubit controller and transformed into a non-classical field in an output single-mode with a frequency of $�{\omega }_2$. The interface can establish a quantum connection between solid-state and photonic flying qubits with adjustable pulse shapes and carrier frequencies. This allows quantum information to be transferred to other superconducting or atomic-based quantum registers or chips. The peculiarities of the wave-qubit interactions are described, showing how they help to control the quantum state of the non-classical field. On this basis, the operating principles of solid-state and flying qubits for the future quantum information platforms are considered.

Nondeterministic measurement-based techniques are efficient in reshaping the population distribution of a quantum system but suffer from a limited success probability of holding the system in the target state. To save the experimental cost, a two-step protocol is proposed to cool a resonator down to the ground state with a near-unit probability by exploiting the state-engineering mechanisms of both conditional and unconditional measurements on an ancillary qubit. In the first step, the unconditional measurements on the ancillary qubit are applied to reshape the target resonator from a thermal state to a reserved Fock state. The measurement sequence can be efficiently optimized by reinforcement learning for maximum fidelity. In the second step, the population on the reserved state can be faithfully transferred in a stepwise way to the resonator's ground state with a near-unit fidelity by the conditional measurements on the qubit. Properly designing the projection operator and the measurement interval enables the Kraus operator to act as a lowering operator for neighboring Fock states. Through dozens of measurements in all, the initial thermal average population of the resonator can be reduced by five orders in magnitude with a success probability of over 95%.

In recent years, mediated semi-quantum key distribution (MSQKD) has become a hot topic in quantum cryptography. In this study, the original MSQKD protocol is revisited and a new scheme for proving security based on information theory is developed. At first, a new bound on the key rate of the protocol is derived using an entropic uncertainty relation, thus proving the unconditional security of the protocol. In addition, in the asymptotic scenario, a higher noise tolerance that improves the previous results is found. The legitimate communicating parties have to abort the protocol when they observe the error rate is larger than the noise tolerance. Furthermore, the security of a single-state MSQKD protocol and a single-state semi-quantum key distribution (SQKD) protocol is proven using a similar scheme.

Phonon lasers have long been a subject of interest and possess broad application prospects. Much effort is devoted to lay the foundation of realizing phonon lasers using cavity magnomechanical systems, but up to now no related work is carried out to explore the quantum-squeezing-engineered phonon laser action in cavity magnomechanics. Here, the phonon laser action is investigated in a three-mode cavity magnomechanical system built based on a microwave resonator-yttrium iron garnet sphere composite device, focusing on the effect induced by the magnon-mode squeezing. It is found that the magnon squeezing can improve the effective magnon–photon and magnon–phonon coupling rates. It is demonstrated that the phonon laser action can be engineered and enhanced by changing the squeezing strength. This scheme provides a new mechanism to improve the effective magnon–photon and magnon–phonon couplings for various applications, and demonstrates the feasibility of realizing high-gain and low-threshold phonon lasers with cavity magnomechanical platforms.

In this work, a scalable quantum gate-based algorithm for accelerating causal inference is introduced. Specifically, the formalism of causal hypothesis testing presented in [Nat Commun 10, 1472 (2019)] is considered. Through the algorithm, the existing definition of error probability is generalized, which is a metric to distinguish between two competing causal hypotheses, to a practical scenario. The results on the Qiskit validate the predicted speedup and show that in the realistic scenario, the error probability depends on the distance between the competing hypotheses. To achieve this, the causal hypotheses are embedded as a circuit construction of the oracle. Furthermore, by assessing the complexity involved in implementing the algorithm's subcomponents, a numerical estimation of the resources required for the algorithm is offered. Finally, applications of this framework for causal inference use cases in bioinformatics and artificial general intelligence are discussed.

This work presents a pioneering photonic crystal (PC) heterostructure design exploiting tailored topological corner states and cavities to unleash a fascinating topological rainbow effect. This effect arises from the strategic integration of a nontrivial topological PC with sharp corners within a trivial PC matrix, resulting in a heterostructure rich in corner states and cavities. The critical innovation lies in manipulating the sector angle of circular columns, granting dynamic control over the rainbow effect and light localization. This manipulation induces distinct group velocities for different light frequencies, leading to their separation and localization at specific corner states. This remarkable “rainbow trapping” phenomenon manifests as highly confined light exhibiting exceptional resilience against disorder. These findings illuminate a pathway toward crafting next-generation photonic devices boasting unparalleled functionalities. The reconfigurable rainbow trapping holds immense potential for applications in wavelength division multiplexing, optical sensing, and even venturing into quantum information processing.

The observation of superconductivity with a transition temperature (T_c) up to 8.7 K in KTaO₃ single crystals annealed with CaH₂ at 900–1000 °C is reported. The superconductivity is confirmed by both resistance and magnetization measurements and is 3D in nature. Characterizations of X-ray photoelectron spectroscopy, X-ray diffraction, and scanning transmission electron microscopy reveal that it locates in a 1-µm-order-thick polycrystalline surface layer that shows a rock-salt type structure, with a lattice constant of 0.454 nm, and can be chemically identified as K_xTaO_y (0.04 ≤ x ≤ 0.08, 1.24 ≤ y ≤ 1.35), depending on annealing conditions. Within the experimental ranges, the T_c is peaked at x ≈0.05, and increases with decreasing y, and the highest T_c is observed in K_0.05TaO_1.24. The T_c observed here is much higher than that of KTaO₃, Ta, and pure TaO, and is also one of the highest among of all the known oxide superconductors with the same rock-salt structure. The rather high T_c and its close connection with KTaO₃ and Ta, both of which are promising materials for quantum computing, make K_xTaO_y potentially interesting as a building block in constructing future superconducting quantum devices.

Recently, topological insulators (TIs) have regained extensive attention in spintronics due to their potential applications in new-generation spintronic devices, following the discovery of the quantum Hall effect and quantum anomalous Hall effect, which introduce the topological concept. In this review, the exotic spin transport phenomena are explored in TIs. The review offers a concise overview of the fundamental principles of TIs, followed by an exploration of diverse fabrication methods for TI materials. Characterization techniques of the topological surface states are also presented. The review delves into the intriguing spin current transport phenomena, focusing on spin-to-charge and charge-to-spin conversions in TI/ferromagnet bilayers, respectively. The review culminates summarizing key insights and project future directions for research on spin transport phenomena in TIs, emphasizing practical implications.

Einstein–Podolsky–Rosen (EPR) steering is the key resources in quantum information processing. EPR steering light sources typically obtained through spontaneous parametric down conversion below a threshold are relatively weak. Strong bright EPR steering light source can be obtained by operating the resonant cavity above the threshold. Here, the quantum steering correlations of the system above the threshold are analyzed using numerical simulations based on the quantum random trajectory method in Wigner representation. By employing the genuine multipartite EPR steering criterion, it is demonstrated that genuine bright tripartite EPR steering can be generated above the threshold, and investigate the influences of the nonlinear parameters on the genuine tripartite EPR steering. The scheme of bright EPR steering light source generated by cascaded nonlinear process can be used as a quantum repeater in quantum communication.