In this work, we show that a counterfactual account of Lüders' rule—which we argue is naturally implied by the mathematical structure of the rule itself—rules out underlying-state models of quantum mechanics (a type of hidden-variable model, typically used in the contextuality and nonlocality literature, where quantum states are treated as probability measures over “better-defined states”). This incompatibility arises because the counterfactual update requires ontological models to update their states according to conditional probability, which in turn establishes an equivalence between compatibility and the existence of such models.

For arbitrary reciprocal single-mode structures, regardless of their geometric shapes or constituent materials, there must exist incident directions of plane waves for which they are optically achiral.

We report resonant inelastic x-ray scattering (RIXS) spectra of neon atoms interacting with intense x-ray pulses generated using an x-ray free-electron laser (XFEL). We find that an unexpected peak emerges near the $K\alpha$ line of Ne, which does not coincide with any physical resonances of neon ions. We perform theoretical calculations based on a quantum-state-resolved rate-equation approach with x-ray-induced processes including possible resonant excitations. Our dynamics simulations demonstrate that a sequence of multiple resonant photoabsorption events are involved and the interplay of those multiple resonances in combination with the relatively large spectral bandwidth of XFEL radiation leads to the emergent resonance-like structure at a position where no resonances exist. Our finding offers critical guidance for future applications of high-intensity RIXS at XFEL facilities.

Autonomous quantum thermal machines are particularly suited to understand how correlations between thermal baths, a load, and a thermal machine affect the overall thermodynamic functioning of the setup. Here, we show that by tuning the operating temperatures and the magnitude of the coupling between machine and load, the thermal machine can operate in four modes: engine, accelerator, heater, or refrigerator. In particular, we show that as we increase the coupling strength, the engine mode is suppressed and the refrigerator mode is no longer attainable, leaving the heater as the most pronounced functioning modality, followed by the accelerator. This regime switching can be amplified by quantum effects, such as the bosonic enhancement factor for a harmonic oscillator load, which effectively modifies the machine-load coupling, making the thermodynamic functioning sensitive to the initial preparation of the load.

Quantum logic operations between physically distinct qubits is an essential aspect of large-scale quantum information processing. We propose an approach to high-speed mixed-species entangling operations in trapped-ion quantum computers, based on mechanical excitation of spin-dependent ion motion by ultrafast pulsed lasers. We develop the theory and machine design of pulse sequences that realize megahertz-speed "fast gates" between a range of mixed-isotope and mixed-species ion pairings with experimentally realistic laser controls. We demonstrate the robustness of the gate mechanism against expected experimental errors, and identify errors in ultrafast single-qubit control as the primary technical limitation. The proposed mixed-species gate mechanism can be used for fast transfer of quantum information between specialized qubits and quantum memories, which we show enables the protection of matter-photon interfaces against rapid spin dephasing in optical networks of trapped-ion processors.

We investigate the magnetic-field dependence of the interaction between two Rydberg atoms, $|n{S}_{1/2},m_J\rangle$ and $|(n+1)S_{1/2},m_J\rangle$. In this setting, the effective spin-1/2 Hamiltonian takes the form of an XXZ model. We show that the anisotropy parameter of the XXZ model can be tuned by applying a magnetic field and, in particular, that it changes drastically near the Förster resonance points. Based on this result, we propose experimental realizations of spin-1/2 and spin-1 Heisenberg-type quantum spin models in Rydberg atom quantum simulators, without relying on Floquet engineering. Our results provide guidance for future experiments of Rydberg atom quantum simulators and offer insight into quantum many-body phenomena emerging in the Heisenberg model.

The Josephson effect characterizes superfluids and superconductors separated by a weak link, the so-called Josephson junction. A recent experiment has shown that Josephson oscillations can also be observed in a supersolid, where the weak link is not due to an external barrier, but is self-induced by interparticle interactions. Here we show theoretically that supersolids—despite their self-induced character—feature key properties of bosonic Josephson junction arrays, including macroscopic quantum self-trapping. We focus on the harmonically trapped dipolar supersolids of interest for current experiments and show that they can be described with a generalized Josephson model that takes into account spatial inhomogeneities. Our work sheds new light on the dynamics of supersolids and opens the way to the study of a novel class of Josephson junctions.

We propose physically realizable exceptional point degeneracies (EPDs) corresponding to synthetic reflectionless modes (SRMs). These are solutions of an auxiliary wave operator that is defined in synthetic frequency dimensions and describe incoming reflectionless waves onto a Floquet-driven cavity. The SRM-EPD emerges as a consequence of a local $\mathcal{PT}$ symmetry imposed on the auxiliary operator via appropriate Floquet driving. Its presence signifies the possibility to design wavefronts and time-modulated schemes with up or down targeted frequency conversion and flat transmission spectra. The theory is validated via simulations with driven RF resonators.

An accurate atomic mass of ${}^4\mathrm{He}$ (or the alpha particle) is required for deriving the atomic mass of the electron from proposed measurements of the ground-state $g$ factor of ${}^4\mathrm{He}^{+}$. However, there is a six-standard deviation discrepancy between a recent measurement [S. Sasidharan, O. Bezrodnova, S. Rau, W. Quint, S. Sturm, and K. Blaum, Phys. Rev. Lett. 131, 093201 (2023)] and the measurement by Van Dyck et al. [R. S. Van Dyck, Jr., D. B. Pinegar, S. Van Liew, and S. L. Zafonte, Int. J. Mass Spectrom. 251, 231 (2006)], on which the current values in the Atomic Mass Evaluation and CODATA tabulation of fundamental constants are based. By measuring the cyclotron frequency ratios of ${}^4\mathrm{He}^{+}$ to ${\mathrm{D}}_2^{+}$, and of ${}^4\mathrm{He}^{+}$ to ${\mathrm{H}}_2{\mathrm{D}}^{+}$, we find the mass of ${}^4\mathrm{He}$ to be $4.00260325467\left(8\right)\mathrm{u}$, favoring the more recent value.

We demonstrate a direct one-to-one correspondence between nonlinear optical fields in defocusing Kerr media and wave functions in weakly interacting Bose-Einstein condensates or quantum fluids. Based on this correspondence, we propose the existence of excitations in an optical vortex beam characterized by a helical deformation of its phase singularity core. These excitations are direct analogs of Kelvin waves known in quantum and classical fluid dynamics. We further show that the excitations exhibit two distinct branches, one of which includes a stationary solution. A feasible experimental scheme for generating these excitations is also discussed.

One core of quantum metrology is the utilization of entanglement to enhance measurement precision beyond the standard quantum limit. Here, we utilize the Floquet-engineered two-axis twisting (TAT) and turning dynamics to generate Greenberger-Horne-Zeilinger (GHZ)-like states for quantum metrology. Using both analytical semiclassical and quantum approaches, we find that the desired $N$-particle GHZ-like state can be produced in a remarkably short time, $t_{\mathrm{opt}}\propto lnN/N$, and its quantum Fisher information $F_{\mathrm{Q}}^{\mathrm{opt}}\propto N^2$ approaches the Heisenberg limit. Owing to the rapid state preparation, it shows outstanding robustness against decoherence. Moreover, using the Floquet-engineered anti-TAT-and-turn, one may implement an efficient interaction-based readout protocol to extract the signal encoded in this GHZ-like state. This Floquet-engineered anti-TAT-and-turn approach offers a viable method to achieve effective time-reversal dynamics to improve measurement precision and resilience against detection noise, all without the need to invert the sign of the nonlinear interaction. This study paves a way for achieving entanglement-enhanced quantum metrology via rapid generation of GHZ-like states at high particle numbers through continuous Floquet engineering.

We construct an optical tweezer array for Rb atoms that provides the same trapping potentials for ground and Rydberg levels. By comparing the value of the trapping field detuning from the $40s\text{−}6p$ transition frequency required to create such an array with that previously measured in an optical lattice [J. Lampen et al., Phys. Rev. A 98, 033411 (2018)], we determine the modification of the polarizability of a Rydberg state atom confined in a spatially varying optical potential, as predicted by Topcu and Derevianko [Phys. Rev. A 88, 043407 (2013)]. A state-insensitive atom array should enable more complex quantum computation by allowing the tweezer potentials to remain on during Rydberg gate operations.

Photonics provides a viable path to a scalable fault-tolerant quantum computer. The natural framework for this platform is measurement-based quantum computation, where fault-tolerant graph states supersede traditional quantum error-correcting codes. However, the existing formalism for foliation—the construction of fault-tolerant graph states—does not reveal how certain properties, such as single-shot error correction, manifest in the measurement-based setting. We introduce the fault complex, a representation of dynamic quantum error-correction protocols particularly well suited to describe foliation. Our approach enables precise computation of fault tolerance properties of foliated codes and provides insights into circuit-based quantum computation. Analyzing the fault complex yields improved thresholds for three- and four-dimensional toric codes, a generalization of stability experiments, and the existence of single-shot lattice surgery with higher-dimensional topological codes.

We study classical and quantum echoes in a Kerr oscillator driven by a frequency-controlling pulsed perturbation. We consider the dynamical response to the perturbation for a single coherent state and for Schrödinger cat states constructed as both balanced and imbalanced superpositions of two coherent states. For individual coherent states, we demonstrate that a weak parametric drive yields a long-lived sequence of classical echoes. Cat states are found to exhibit distinct quantum echoes that are sensitive to the initial relative phase and weights of the coherent states in superposition. We examine the effect of dissipation on quantum echoes and quantum revivals of cat states. We demonstrate that, even when dissipation suppresses quantum revivals, quantum echoes can be recovered by properly tuning the timing and strength of the perturbation. These results may be useful for characterizing and mitigating errors of cat qubits.

The two-body contact is a fundamental quantity of a dilute Bose gas that relates the thermodynamics to the short-distance two-body correlations. For a Bose gas in an optical lattice, near the superfluid–Mott-insulator transition, we show that a “universal” contact $C_{\mathrm{univ}}$ can be defined from the singular part $P-P_{\mathrm{MI}}$ of the pressure ($P_{\mathrm{MI}}$ is the pressure of the Mott insulator). Its expression $C_{\mathrm{univ}}=C_{\mathrm{DBG}}\left(\right|n-n_{\mathrm{MI}}|,a^{*})$ coincides with that of a dilute Bose gas provided we consider the effective “scattering length” $a^{*}$ of the quasiparticles at the quantum critical point (QCP) rather than the scattering length in vacuum, and the excess density $|n-n_{\mathrm{MI}}|$ of particles (or holes) with respect to the Mott insulator. Close to the transition, we find that the singular part $n_{\mathbf{k}}^{\mathrm{sing}}=n_{\mathbf{k}}-n_{\mathbf{k}}^{\mathrm{MI}}$ of the momentum distribution exhibits a high-momentum tail of the form $Z_{\mathrm{QP}}{C}_{\mathrm{univ}}/{\left|\mathbf{k}\right|}^4$ over a broad region of the Brillouin zone, where $Z_{\mathrm{QP}}$ is the quasiparticle weight of the elementary excitations at the QCP. Our results demonstrate that the notion of contact extends to strongly correlated lattice bosons, and we argue that the contact $C_{\mathrm{univ}}$ can be measured in state-of-the-art experiments on Bose gases in optical lattices and magnetic insulators.

We analyze the scaling of quantum Fisher information with the number of system particles in the limit of large number of particles, as a function of the number of parties interacting with each other, for encoding Hamiltonians having $k$-body interactions, where $k$ is arbitrary. We find that estimation of coupling strength of such arbitrary-body encoding Hamiltonians provides a super-Heisenberg scaling that increases monotonically with an increase in the number of interacting particles, in the limit of large number of system particles. Moreover, we ask if genuine multiparty entanglement is indispensable in attaining the best metrological precision if we employ nonlocal terms in the Hamiltonian. We identify a dichotomy in the answer. Specifically, we find that Hamiltonians having odd-body interactions necessarily require genuine multipartite entanglement in probes to attain the best metrological precision, but the situation is opposite in the case of Hamiltonians with even-body interactions. The optimal probes corresponding to Hamiltonians that contain even-body interaction terms may be entangled, but certainly not so in all bipartitions, and particularly it is possible to attain optimal precision using asymmetric probes. Asymmetry, which therefore is a resource in this scenario rather than genuine multiparty entanglement, refers to the disparity between states of local parts of the global system. Thereby we find a complementarity in the requirement of asymmetry and genuine entanglement in optimal probes for estimating strength of odd- and even-body interactions respectively. Additionally, we provide an upper bound on the number of parties up to which one can always obtain an asymmetric product state that gives the best metrological precision for even-body interactions. En route, we find the quantum Fisher information in closed form for two- and three-body interactions for arbitrary number of parties. Additionally, we analyze the finite-size scaling of the QFI and examine the impact of local dephasing noise during the encoding process. We also provide an analysis of the case when the Hamiltonian contains local fields and up to $k$-body interaction terms, where the strength of interaction gradually decreases with an increase in the number of parties interacting with each other. Interestingly, we find a similar dichotomy in the nature of the optimal probe in this case as well, i.e., for encoding Hamiltonians with up to even- and odd-body interactions. Further, we identify conditions on the local component of the Hamiltonian for which this dichotomy is still shown to exist for two- and three-body encoding Hamiltonians with arbitrary local dimensions.

Smoothing is a technique for estimating the state of an imperfectly monitored open system by combining both prior and posterior measurement information. In the quantum regime, current approaches to smoothing either give unphysical outcomes, due to the noncommutativity of the measurements at different times, or require assumptions about how the environment is measuring the system, which with current technology is unverifiable. We propose a novel definition of the smoothed quantum state based on quantum Bayesian retrodiction, which mirrors the classical retrodictive approach to smoothing. This approach always yields physical results and does not require any assumption on the environment. We show that this smoothed state has, on average, greater purity than the state reconstructed using just the prior information. Finally, we make a connection with the smoothing theory of Guevara and Wiseman in a well-studied regime, and describe from a purely quantum perspective how it conditions on the posterior information.

Nonlocal correlations created in networks with multiple independent sources enable surprising phenomena in quantum information and quantum foundations. The presence of independent sources, however, makes the analysis of network nonlocality challenging, and even in the simplest nontrivial scenarios a complete characterization is lacking. In this Letter we study one of the simplest of these scenarios, namely that of distributions invariant under permutations of parties in the minimal triangle network, which features no inputs and binary outcomes. We perform an exhaustive search for triangle-local models, and from it we infer analytic expressions for the boundaries of the set of distributions that admit such models, which we conjecture to be all the tight Bell inequalities for the scenario. Armed with them and with improved outer approximations of the set, we provide insights on the existence of a classical-quantum gap in the triangle network with binary outcomes.

The ${\mathrm{RaOCH}}_3$ molecule is one of the most sensitive systems for electron electric dipole moment ($e\mathrm{EDM}$) searches. Its hyperfine and $K\text{−}\mathrm{doubling}$ structure in the external electric field is of key importance for preparing and interpreting the experiment. We proposed the method for calculation of the symmetric top molecules and applied it for ${\mathrm{RaOCH}}_3$. Hyperfine structure, $K$ doubling, and sensitivity of the molecule to $e\mathrm{EDM}$ in the external electric field were calculated.

We present a method for target finding in LIDAR applications using high-dimensional Bell states. Combined with a sequential decision rule, this scheme asymptotically achieves zero error probability with finite average energy expenditure. This result stems from the synergy of high-dimensional entanglement suppressing false positives and the sequential decision rule optimizing energy usage. This analysis provides a theoretical foundation to further explore quantum sensing protocols that may significantly outperform classical methods in noisy environments.