黑料传送门

Parahydrogen induced polarization (PHIP) achieves efficient hyperpolarisation of nuclear spins with the transfer of the singlet order of parahydrogen to target molecules through catalytic hydrogenation reactions and subsequent coherent control of the spin dynamics. However, in realistic conditions B₀/B₁ inhomogeneities lead to significant reduction in the polarisation transfer efficiency. Moreover, in high-concentration samples, dipolar fields arising from the magnetisation of the sample can degrade polarisation transfer efficiency significantly. In this work, we present a theoretical framework and a comprehensive analysis of both pulsed and continuous-wave (CW) control sequences designed to mitigate the detrimental effects of dipolar fields and B₀/B₁ inhomogeneities. By combining tools from average Hamiltonian theory with detailed numerical simulations, we introduce and characterise a wide range of transfer sequences, including dipolar-field adjusted and dipolar-field suppressing protocols. We identify conditions under which dipolar interactions either hinder or, perhaps surprisingly, stabilise polarisation transfer, depending on the sequence structure. Our results offer practical guidance for the selection and design of PHIP transfer sequences under realistic experimental constraints and open pathways towards robust hyperpolarisation in concentrated liquid-state NMR samples.

Quantum phase estimation is a core task in quantum technologies ranging from metrology to quantum computing, where it appears as a key subroutine in various algorithms. Here, we quantitatively connect the performance of phase estimation protocols with quantum coherence. To achieve this, we construct and characterize resource theories of quantum networks that cannot generate coherence. Given multiple copies of a unitary encoding an unknown phase and access to a fixed coherent state, we estimate the phase using such networks. For a unified and general approach, we assess the quality of the estimate using a generic cost function that penalizes deviations from the true value. We determine the minimal average cost that can be achieved in this manner and explicitly derive optimal protocols. From this, we construct a family of coherence measures that directly connect a state鈥檚 coherence with its value for phase estimation, demonstrating that every bit of coherence helps. This establishes coherence as a resource that quantifies the performance of phase estimation, and, thus, of any quantum technology relying on it as a subroutine.

Tensor network formalisms have emerged as powerful tools for simulating quantum state evolution. While widely applied in the study of optical quantum circuits, such as boson sampling, existing tensor network approaches fail to address the complexity mismatch between tensor contractions and the calculation of photon-counting probability amplitudes. Here, we present an alternative tensor network framework that exploits the input-output relations of quantum optical circuits encoded in the unitary interferometer matrix. Our approach bridges the complexity gap by enabling the computation of the permanent鈥攃entral to boson sampling鈥攚ith the same computational complexity as the best known classical algorithm based on a graphical representation of the operator-basis matrix product states that we introduce. Furthermore, we exploit the flexibility of tensor networks to extend our formalism to incorporate partial distinguishability and photon loss, two key imperfections in practical interferometry experiments. This work offers a significant step forward in the simulation of large-scale quantum optical systems and the understanding of their computational complexity.

Current proposals to probe the quantum nature of gravity in the low-energy regime predominantly focus on the Newtonian interaction term. In this work, we present a theoretical exploration of gravitationally mediated entanglement arising from a genuinely general relativistic effect: frame dragging. This interaction gives rise to an effective dipolar coupling between the angular momenta of two rotating, spherically symmetric masses, allowing entanglement generation between angular momentum degrees of freedom. We represent the quantum states by angular momentum eigenstates and show that, while the maximal entangling rate is achieved for highly delocalized initial states, non-negligible quantum correlations can still emerge even when the initial states are not prepared in superposition. We then analyze the robustness of the resulting entanglement in the presence of common noise sources, explicitly acknowledging the challenges associated with a potential implementation. We also note that, for spherically symmetric masses, angular momentum degrees of freedom are intrinsically insensitive to Casimir and Coulomb interactions, thereby mitigating key decoherence channels present in existing proposals. Finally, we discuss possible state preparation and detection strategies while framing our results within the broader landscape of gravitationally mediated entanglement schemes, emphasizing the role of this framework as a conceptual avenue for exploring genuinely relativistic quantum gravitational effects.

Metabolic monitoring and reaction rate estimation using hyperpolarized NMR technology requires accurate quantitative analysis of multidimensional data scenarios. Currently, this analysis is often performed in a two-stage procedure, which is prone to errors in uncertainty propagation and estimation. We propose an approach derived from a Bayesian hierarchical model that intrinsically propagates uncertainties and operates on the full data to maximize the precision at minimal uncertainty. In an analytic treatment, we reduce the estimation procedure to a least-squares optimization problem which can be understood as an extension of the Variable Projection (VarPro) approach for data scenarios with two predictors. We investigate the method鈥檚 efficacy in two experiments with hyperpolarized metabolites recorded with conventional high-field NMR devices and a micronscale NMR setup using Nitrogen-Vacancy centers in diamond for detection, respectively. In both examples, the new approach improves estimates compared to Fourier methods and proves operational advantages over a two-stage procedure employing VarPro. While the approach presented is motivated by NMR analysis, it is straightforwardly applicable to further estimation scenarios with similar data structure, such as time-resolved photospectroscopy.

Color centers in diamond provide a possible hardware for quantum computation, where the most basic quantum information processing unit are nitrogen-vacancy (NV) centers, each in contact with adjacent carbon nuclear spins. With specifically tailored dynamical decoupling sequences, it is possible to execute selective, high-fidelity two-body gates between the electron spin of the NV center and a targeted nuclear spin. In this work, we present a method to determine the optimal execution time that balances the trade-off between fidelity and execution speed for gates generated by adaptive XY sequences. With these optimized gates, we use the nuclear spin environment as a code space for quantum error correction within a color center register.

In this paper, we revisit the interpretation of the circular Unruh effect. To this aim, we rely on the principle of general covariance applied to the decay properties of noninertial particles. Specifically, we show how the tree-level decay rate of an inverse-尾 process involving scalar fields does not require the introduction of a thermal (or nonthermal) bath in the comoving frame to be a scalar under general coordinate transformations. Instead, we interpret any decay process as an emission of negative-energy quanta, whose existence is motivated by the absence of a global vacuum state for uniformly rotating observers. This implies that, in principle, no uniformly rotating particle can be regarded as stable.

Quantum sensors hold considerable promise for precision measurement, yet their capabilities are inherently constrained by environmental noise. A fundamental task in quantum sensing is determining the precision limit of noisy sensor devices. For continuously monitored quantum sensors, characterizing the optimal precision in the presence of environments other than the measurement channel is an outstanding open theoretical challenge, due to the infinite-dimensional nature of the sensor output field and the complex temporal correlation of the photons therein. Here, we establish a numerically efficient method to determine the quantum Cram茅r-Rao bound for continuously monitored quantum sensors subject to general environmental noise鈥擬arkovian or non-Markovian, and showcase its application with paradigmatic models of continuously monitored quantum sensors. Applicable to both constant-parameter and waveform estimation, our method provides a rigorous and practical framework for assessing and enhancing the sensor performance in realistic settings, with broad applications across experimental quantum physics.

Hybrid quantum architectures that integrate matter and photonic degrees of freedom present a promising pathway toward scalable fault-tolerant quantum computing. This approach needs to combine well-established entangling operations between distant registers using photonic degrees of freedom with direct interactions between matter qubits within a solid-state register. The high-fidelity control of such a register, however, poses significant challenges. In this work, we address these challenges with pulsed control sequences that modulate all interspin interactions to preserve the nearest-neighbor couplings while eliminating unwanted long-range interactions. We derive pulse sequences, including broadband and selective gates, using composite-pulse and shaped-pulse techniques as well as optimal-control methods. This ensures a general pulse sequence in the presence of spin-position bias, robustness against static offset detunings, and Rabi-frequency fluctuations of the control fields. The control techniques developed here apply well beyond the present setting to a broad range of physical platforms. We demonstrate the efficacy of our methods for the resource-state generation for fusion-based quantum computing in four- and six-spin systems encoded in the electronic ground states of nitrogen-vacancy centers or other molecular solid-state qubits. We also outline other elements of the proposed architecture, highlighting its potential for advancing quantum computing technology.

Traditionally, the characterization of quantum resources has focused on individual quantum states. Recent literature, however, has increasingly explored the characterization of resources in multistates鈥攐rdered collections of states indexed by a varying parameter. In this work, we provide a unitary-invariant framework to pinpoint imaginarity and coherence in sets of qubit states: We prove that Bloch vectors must be coplanar to be imaginarity free and collinear to be incoherent, yielding exact rank-based tests of coherence and imaginarity and closed-form bounds for existing robustness quantifiers, all based on two-state overlaps only. We also show that the set of imaginarity-free multistates is not convex, and that third-order invariants completely characterize multistate imaginarity of single-qubits but not of higher-dimensional systems. As our main technical result, we show that every Bargmann invariant of single-qubit states is determined (up to conjugation) by two-state overlaps. Beyond qubits, we give purity and system-agnostic coherence witnesses from equality constraints on higher-order invariants and connect our results to practical protocols鈥攃haracterization of partial distinguishability, spin-chirality detection, and subchannel discrimination.