专题 · 量子纠缠

Deterministic remote entanglement of superconducting qutrits via time-bin microwave photons surpasses the negativity bound of two-qubit states.

Spectral crowding of collective motional modes limits the fidelity of entangling interactions in trapped-ion quantum processors by inducing off-resonant coupling to spectator modes. We introduce a geometric-phase entangling interaction driven by a transverse, time-dependent structured-light force. By applying the force in a plane orthogonal to the optical propagation direction, we reduce the effects of spectral crowding while preserving single-ion addressing. The scheme is compatible with arbitrary qubit encodings, provided that the qubit states experience a differential AC Stark shift. We experimentally realize high-fidelity two-qubit gates with error rates below 5 × 10 − 3 in ion crystals containing up to 12 ions confined within a single potential well. These results establish gradient-field light-shift gates as a scalable approach to high-fidelity entanglement generation in spectrally crowded trapped-ion systems.

We show that a bound impurity in a Bose-Einstein condensate can be directly mapped to an Unruh-DeWitt detector interacting with a relativistic quantum field. We provide explicit experimental parameters for an implementation using K 41 impurities coupled to a Rb 87 condensate via finite-time Feshbach tuning. As an application, we study the extraction of vacuum entanglement from distant regions of the condensate and find viable parameters for the implementation of entanglement harvesting.

Modern precision experiments often probe unknown classical fields with bosonic sensors in quantum-noise-limited regimes where vacuum fluctuations limit conventional readout. We introduce quantum signal learning, a sensing framework that extends metrology to a broader property-learning setting, and propose a quantum-enhanced protocol that simultaneously estimates many properties of a classical signal with shot noise suppressed below the vacuum level. Our scheme requires only two-mode squeezing, passive optics, and static homodyne measurements, and enables post hoc classical estimation of many properties from the same experimental dataset. We prove that our protocol enables a quantum speedup for common classical sensing tasks, including measuring electromagnetic correlations, real-time feedback control of interferometric cavities, and Fourier-domain matched filtering. To establish these separations, we introduce an optimal-transport conditioning method and show both worst-case exponential separations from all entanglement-free strategies and practical speedups over homodyne and heterodyne baselines. We further show that when squeezing is treated as a resource, a protocol with squeezed light can sense a structured classical background exponentially faster than any coherent classical probe.

Quantum nonlocality, typically revealed through entanglement distribution across quantum networks, is a cornerstone of quantum information science. Long-distance distribution of entanglement requires the information carrier, i.e., flying photons, to operate in the minimum-loss telecom band of optical fiber. While extensive efforts have been devoted to the direct generation of entanglement between C-band telecom photons and various stationary spins, the verification of quantum nonlocality remains an outstanding challenge. Here, utilizing a dipole transition in rubidium atoms with a wavelength of 1530 nm and a cavity-assisted protocol, we achieve resonant excitation and direct emission of C-band telecom photons from a single atom, generating spin–photon entanglement with a measured Bell state fidelity exceeding 91.4%. We then verify Bell nonlocality by observing a Bell inequality violation of 2.455(77) > 2 using this high-quality entangled pair. These results extend the wavelength of a single-atom quantum emitter to the telecom C-band, achieving sufficiently high-fidelity spin–photon entanglement to finally verify Bell nonlocality. The authors demonstrate the direct generation of telecom C-band spin-photon entanglement from a single neutral atom, achieving >91.4% Bell-state fidelity and a CHSH violation of 2.455(77), thereby establishing Bell nonlocality in a fiber-compatible quantum networking platform

Frequency combs are multimode photonic systems that underlie countless precision sensing and metrology applications. Since their invention over two decades ago, numerous efforts have pushed frequency combs to broader bandwidths and more stable operation. More recently, quantum squeezing and entanglement have been explored in single-comb systems for continuous-variable quantum computing and quantum-enhanced spectroscopy. However, the production of bright quantum light across multiple nondegenerate pulses or frequency combs, a key technology for multispectral applications, remains largely unexplored. In this work, we explore a mechanism that generates a series of nonlinearly coupled bright frequency combs through cascaded three-wave upconversion and downconversion processes mediated by a single idler comb. We consider both discrete combs and broadband ultrafast pulse systems, showing how these systems generate inter- and intracomb two-mode squeezing and entanglement directly in the frequency basis, spanning a large spectral range. Furthermore, we demonstrate how these systems can be engineered to produce target cluster states and concentrated two-mode squeezing through covariance matrix optimization. Our findings uncover a new multimode quantum regime enabled by hierarchical nonlinear interactions in two frequency dimensions, unlocking applications such as entanglement-assisted spectroscopy, squeezing-enhanced pump-probe measurements, quantum spectral multiplexing, and spectrally addressable cluster state generation for quantum information science.

Entanglement is a central resource in quantum technologies, and the realization of photonic entanglement necessarily relies on interaction with matter. Resonance fluorescence, originating from the coherent interaction between a driving field and a two-level system, plays a pivotal role in quantum optics. Here, we experimentally demonstrate high-quality generation of post-selected temporal-mode entanglement using vacuum–one-photon number superposition states from a single quantum dot, an approach conceptually aligned with early proposals based on identical single photons, yet whose high-quality experimental realization has remained absent. By delocalizing this superposition via a beam splitter, we realize post-selected temporal-mode entanglement and observe a clear violation of the Clauser–Horn–Shimony–Holt Bell inequality using Franson-type interferometry. Our scheme removes the need for multiphoton generation, simplifies the experimental requirements, and establishes a scalable pathway toward solid-state entangled photon sources.