Fermilab installs first component for new PIP-II accelerator

Fermilab has reached a significant milestone in its neutrino research program with the installation of the first beamline component for the new PIP-II linear accelerator. This accelerator, once completed, will be fundamental in powering future neutrino experiments, solidifying Fermilab's position as a leading center in particle physics. The PIP-II (Proton Improvement Plan-II) is designed to be a state-of-the-art particle accelerator, capable of producing a high-intensity and high-energy proton beam. This beam will be crucial for generating more powerful neutrino fluxes, allowing scientists to study the fundamental properties of these elusive particles with greater precision. The installation of this first component marks the beginning of the assembly phase in the tunnel that will house the accelerator.

Probability of Quantum Steering in Two-Qubit States

Researchers have quantified the probability of observing quantum steering in generic two-qubit states. Steering is a manifestation of entanglement where measurements performed by one party influence the conditional states of another, without the correlations being explainable by a local hidden state model. This study addresses the question of how common this behavior is in quantum systems, which is crucial for the development of quantum information technologies. The team derived analytical expressions for the steering probability ($\mathcal{P}_S$) in Werner states for two- and three-setting scenarios, restricting the latter case to coplanar projective measurements on the Bloch sphere. For a larger number of settings and various random state ensembles, numerical analyses showed that $\mathcal{P}_S$ systematically increases with the number of measurements. Furthermore, this probability substantially exceeds the probabilities associated with Bell nonlocality. The results indicate that random states with minimal environmental coupling exhibit a high probability of steering for a finite number of measurements ($m$), approaching genuine typicality, where $\mathcal{P}_S = 100\%$, as the number of settings increases. The study provides a detailed characterization of $\mathcal{P}_S$ across different state ensembles and specific families, such as Werner and Bell-diagonal states, identifying those with the greatest non-classical potential and highlighting their relevance for protocols where steering serves as a key resource in quantum communication and computation.

Plaquette: A Platform for Designing Fault-Tolerant Quantum Computers

Researchers have developed Plaquette, a theoretical and software platform designed to evaluate the logical performance of fault-tolerant quantum computing (FTQC) architectures based on the physical imperfections of devices. This tool addresses a critical need in the development of quantum computers, where error suppression is fundamental. Plaquette enables hardware teams to make informed decisions about which imperfections to mitigate, offering a precise view of how actual hardware noise affects the logical performance of an FTQC. Unlike stochastic Pauli models used by scalable stabilizer simulators, Plaquette considers a broader range of noise sources common in physical qubits. This includes leakage out of the computational subspace in superconducting qubits, scattering through intermediate states in neutral atoms, heating in trapped ions due to phonon absorption, and coherent errors from miscalibrated controls. The platform allows hardware error models to be specified using Kraus operators, Hamiltonian-Lindblad dynamics, or experimentally reconstructed quantum channels, automatically compiling them for different classes of samplers. Plaquette incorporates samplers such as stabilizer sampling for Pauli noise, the new XPauli sampler for leakage and environment sectors, near-Clifford samplers for coherent errors, and full-state simulation for exact reference calculations. Validation of the XPauli and near-Clifford samplers against full-state simulation has demonstrated their accuracy, matching within statistical uncertainty, while Pauli twirling can fall short depending on the error model. The tool has been demonstrated on three specific error models: leakage in superconducting qubits, intermediate-state scattering in neutral atoms, and heating in trapped ions. The discrepancy between Plaquette simulations and Clifford-only simulations varies with platform and noise process. This highlights the importance of using the most accurate simulation available to obtain reliable thresholds, error budgets, and overhead estimates. Plaquette provides a direct path from the open-system physics of a device to the evaluation of the logical performance of the FTQC built upon it, facilitating progress towards robust and functional quantum computers.

Robust Quantum Key Distribution Demonstrated with High-Dimensional Photons

Researchers have experimentally demonstrated a robust one-sided device-independent quantum key distribution (1sDI-QKD) protocol using photons entangled in their transverse-spatial degree-of-freedom. This advancement addresses practical limitations of QKD, including susceptibility to noise and losses in communication. The proposed protocol relies on high-dimensional entanglement and leverages quantum steering to certify security, enabling positive secret key rates even under adverse conditions. The study develops a systematic security analysis for high-dimensional 1sDI-QKD protocols, evaluating achievable secret key rates for different measurement configurations and system dimensions. Theoretical results indicate that increasing the dimension significantly enhances the protocol's robustness against both noise and loss. For the experimental implementation, the team developed a high-quality source of high-dimensional photonic entanglement and a fully programmable, multi-outcome measurement device capable of operating up to 11 dimensions. These components allowed for positive key rates across all investigated dimensions, with the highest rates achieved for dimension d=7, under the fair-sampling assumption. This work represents a crucial step towards the practical implementation of QKD with information-theoretic security. The ability to operate in high dimensions confers greater resilience to device imperfections and noisy or lossy communication channel conditions. Although the current demonstration relies on the fair-sampling assumption, the authors discuss the steps required for a loophole-free implementation in realistic regimes of loss and noise, paving the way for future applications in secure quantum communications.