Advanced quantum technologies最新文献

Depicted is a novel setup for realizing the photon number splitting (PNS) attack with current-day technology, namely, using the single-photon Raman interaction. In article number 2300437, Eliahu Cohen and co-workers analyze the amount of information which the eavesdropper (Eve) can obtain using this physical realization of PNS, concluding that while part of the secret key is at risk when weak coherent states are used, there is still a price for Eve to pay in terms of the induced noise. This stresses the importance of proper countermeasures.

Quantum coherence suffers from environmental influence but the topological feature may not. In article number 2400053, Zu-Jian Ying rigorously shows that the topological feature manifested by the fundamental Jaynes–Cummings model of light–matter interaction is robust against a general non-Hermiticity induced by dissipation and decay rates. The non-Hermiticity only tilts the spin winding plane, while the winding number is preserved. Several revealed properties may be useful for designing topological quantum devices.

This review article focuses on biphoton sources based on the double-$�\Lambda$ spontaneous four-wave mixing (SFWM) process in laser-cooled as well as room-temperature or hot atomic ensembles. These biphoton sources have the advantage of providing stable frequencies, ultranarrow linewidths, and a tunability of the temporal biphoton width of more than one order of magnitude for high-bandwidth applications. Therefore, the generated photons can be efficiently interfaced to, e.g., atomic quantum memories. In contrast, solid-state biphoton sources typically require assistance by an optical cavity to operate at narrow linewidth that limits the tunability of the temporal width of the biphotons. Present state-of-the-art double-$�\Lambda$ SFWM biphoton sources can achieve one of the following results: a spectral linewidth of 50 kHz (290 kHz) or a temporal width of 13 $��\mu �s�$ (580 ns) with cold (hot) atoms, a detection rate of about 7$��\times �{10}^3�$ cps, and a generation rate of $�{10}^7$ cps at a duty cycle of 0.4% or of $�{10}^5$ cps in the steady state. The theoretical background of these biphoton sources, experimental implementations with cold and hot atoms, and progress over the years, will be illustrated.

The discovery of the interface-enhanced superconductivity in the single-layer film of FeSe epitaxially grown on SrTiO₃ substrates has triggered a flurry of activity in the field of superconductivity. It raised the hope to find more conventional high transition temperature (T_c) superconductors that are purely driven by the electron-phonon interaction at ambient pressure. Here the epitaxial growth of the Pb nano-sized islands on SrTiO₃ (001) substrates with the island volumes ranging from 286 to 4945 nm³ is reported by molecular beam epitaxy, followed by systematic scanning tunneling microscopic/spectroscopic (STM/S) investigation. The observed STS gap for the nanoscale islands highly dependent on the volumes of nano-sized islands, can be divided into three regions. By performing a detailed spectroscopic investigation, it is founded that superconductivity in the volume above 3700 nm³ (Region I) has a zero temperature energy gap (Δ(0)) and T_c of 6.8 meV and 9.8 K obtained by BCS fitting, showing the largely-enhanced Δ and slightly-increased T_c by comparing to the bulk Pb (1.4 meV and 7.2 K). As the volume in the range from 1300 to 3700 nm³ (Region II), a large Coulomb gap induced by electron correlation emerged and shows a volume-dependent behavior, suggesting the reduced size can enhance electron correlation in Pb islands. As the volume decreases down to Region III, enhanced electron correlation and Coulomb gap become more dominant and superconductivity is totally suppressed. The experiment reveals that an electron-electron interaction in nano-sized Pb islands can be significantly enhanced by reducing the island sizes and suppresses the superconductivity, thus demonstrates a competition between superconductivity and electron correlation as the volume varies.

This work presents an all-fiber telecom-range optical gyroscope employing a spontaneous parametric down conversion crystal to produce ultra-low intensity thermal light by tracing-out one of the heralded photons. The prototype exhibits a detection limit on photon delay measurements of 249 zs over a 72 s averaging time and 26 zs in differential delay measurements at $��t�=�{10}^4�$ s averaging. The detection scheme proves to be the most resource-efficient possible, saturating $��>�99.5�\%�$ of the Cramér–Rao bound. These results are groundbreaking in the context of low-photon regime quantum metrology, paving the way to novel experimental configurations to bridge quantum optics with special or general relativity.

Classification is at the core of data-driven prediction and decision-making, representing a fundamental task in supervised machine learning. Recently, several quantum machine learning algorithms that use quantum kernels as a measure of similarities between data have emerged to perform binary classification on datasets encoded as quantum states. The potential advantages of quantum kernels arise from the ability of quantum computers to construct kernels that are more effective than their classical counterparts in capturing patterns in data or computing kernels more efficiently. However, existing quantum kernel-based classification algorithms do not harness the capability of having data samples in quantum superposition for additional enhancements. This work demonstrates how such capability can be leveraged in quantum kernelized binary classifiers (QKCs) through Quantum Amplitude Estimation (QAE) for quadratic speed-up. Additionally, new quantum circuits are proposed for the QKCs in which the number of qubits is reduced by one, and the circuit depth is reduced linearly with respect to the number of sample data. The quadratic speed-up over previous methods is verified through numerical simulations on the Iris dataset.

Quantum battery (QB) is an energy storage and extraction device conforming to the principles of quantum mechanics. In this study, the characteristics of QBs are considered for the Heisenberg spin chain models in the absence and presence of Dzyaloshinskii-Moriya (DM) interaction. The results show that the DM interaction can enhance the ergotropy and power of QBs, which shows the collective charging can outperform parallel charging regarding QB's performance. Besides, it turns out that first-order coherence is a crucial quantum resource during charging, while quantum steering between the cells is not conducive to the energy storage of QBs. The investigations offer insight into the properties of QBs with Heisenberg spin chain models with DM interaction and facilitate us to acquire the performance in the framework of realistic quantum batteries.

Cavity magnonics has become an intriguing field due to its potential to enable next-generation technologies centered around controlling information exchange in hybrid resonant systems. Investigating the tunability of magnon-photon coupling is key to advancing the field. Here, the observation of coupling between the first order magnon mode in a metallic thin film with a cavity photon mode is reported. An electromagnetic perturbation theory that takes account of perpendicular standing spin waves and their respective dissipation is utilized to estimate the coupling strength. The metallic thin film exhibits notably lower dissipation for the higher-order magnon mode, which is not observed in a thin film magnetic insulator. As such, and given that metallic Kittel magnons typically exhibit lower coherence times than their insulator counterparts, the excitation and coupling to specific higher order modes could lengthen these times compared to previous observations, which may be useful for future integration into quantum devices.