We experimentally study cosmological particle production in a two-dimensional Bose-Einstein condensate, whose density excitations map to an analog cosmology. The expansion of spacetime is realized with tunable interactions. The particle spectrum can be understood through an analogy to quantum mechanical scattering, in which the dynamics of the spacetime metric determine the shape of the scattering potential. Hallmark scattering phenomena such as resonant forward scattering and Bragg reflection are connected to their cosmological counterparts, namely linearly expanding space and bouncing universes. We compare our findings to a theoretical description that extends beyond the acoustic approximation, which enables us to apply the model to high-momentum excitations.

We introduce a new paradigm for the preparation of deeply entangled states useful for quantum metrology. We show that, when the quantum state is an eigenstate of an operator $A$, observables $G$ which are completely off diagonal with respect to $A$ have purely quantum fluctuations, as quantified by the quantum Fisher information, namely, $F_Q(G)=4⟨G^2⟩$. This property holds regardless of the purity of the quantum state, and it implies that off-diagonal fluctuations represent a metrological resource for phase estimation. In particular, for many-body systems such as quantum spin ensembles or bosonic gases, the presence of off-diagonal long-range order (for a spin observable or for bosonic operators) directly translates into a metrological resource, provided that the system remains in a well-defined symmetry sector. The latter is defined, e.g., by one component of the collective spin or by its parity in spin systems; and by the particle number for bosons. Our results establish the optimal use for metrology of arbitrarily non-Gaussian quantum correlations in a large variety of many-body systems.

We introduce and explore the notion of texture of an arbitrary quantum state, in a selected basis. In the first part of this Letter we develop a resource theory and show that state texture is adequately described by an easily computable monotone, which is also directly measurable. It is shown that textures are useful in the characterization of unknown quantum gates in universal circuit layers. By using randomized input states and recording the textures of the output qubits we are able to fully characterize the circuit layer, whenever it contains at least one CNOT gate. This can be done without the need of tomographic protocols and the use of ancillary systems.

Heavy neutral leptons (HNLs) are often among the hypothetical ingredients behind nonzero neutrino masses. If sufficiently light, they can be produced and detected in fixed-target-like experiments. We show that if the HNLs belong to a richer—but rather generic—dark sector, their production mechanism can deviate dramatically from expectations associated with the standard-model weak interactions. In more detail, we postulate that the dark sector contains an axionlike particle (ALP) that naturally decays into HNLs. Since ALPs mix with the pseudoscalar hadrons, the HNL flux might be predominantly associated with the production of neutral mesons (e.g., ${\pi }^0$, $\eta$) as opposed to charge hadrons (e.g., ${\pi }^{\pm }$, $K^{\pm }$). In this case, the physics responsible for HNL production and decay are not directly related and experiments like DUNE might be sensitive to HNLs that are too weakly coupled to the standard model to be produced via weak interactions, as is generically the case of HNLs that play a direct role in the type-I seesaw mechanism.

We report the first evidence for the $h_b(2\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S})\eta$ transition with a significance of 3.5 standard deviations. The decay branching fraction is measured to be $\mathcal{B}[h_b(2\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S})\eta ]=({7.1}_{-3.2}^{+3.7}\pm 0.8)\times {10}^{-3}$, which is noticeably smaller than expected. We also set upper limits on ${\pi }^0$ transitions of $\mathcal{B}[h_b(2\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S}){\pi }^0]<1.8\times {10}^{-3}$, and $\mathcal{B}[h_b(1\mathrm{P})\to \mathrm{\Upsilon }(1\mathrm{S}){\pi }^0]<1.8\times {10}^{-3}$, at the 90% confidence level. These results are obtained with a $131.4{\mathrm{fb}}^{-1}$ data sample collected near the $\mathrm{\Upsilon }(5\mathrm{S})$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.

We report on the optical polarizability of microwave-shielded ultracold NaCs molecules in an optical dipole trap. While dressing a pair of rotational states with a microwave field, we observe a marked dependence of the optical polarizability on the intensity and detuning of the dressing field. To precisely characterize differential energy shifts between dressed rotational states, we establish dressed-state spectroscopy. For strong dressing fields, we find that a magic rotational transition can be engineered and demonstrate its insensitivity to laser intensity fluctuations. The results of this Letter have direct relevance for evaporative cooling and the recent demonstration of molecular Bose-Einstein condensates [Bigagli et al., Nature (London) 631, 289 (2024)] and may open a door to precision microwave spectroscopy in interacting many-body systems of microwave-shielded molecules.

A combined experimental and theoretical study is carried out on the three-body recombination process in a gas of microwave-shielded polar molecules. For ground-state polar molecules dressed with a strong microwave field, field-linked bound states can appear in the intermolecular potential. We model three-body recombination into such bound states using classical trajectory calculations. Our results show that recombination can explain the enhanced loss rates observed at small microwave detunings in trapped samples of bosonic NaCs [Bigagli et al., Nat. Phys. 19, 1579–1584 (2023)]. Specifically, our calculations reproduce the experimentally measured three-body loss rates across a wide range of microwave Rabi couplings, detunings, and temperatures. This work suggests that for bosonic shielded molecular systems in which the two-body loss is sufficiently suppressed and a field-linked bound state is present, the dominant loss process will be three-body recombination.

Flat lines within a band structure represent constant frequency bands for all momentum values (i.e., they maintain zero group velocity for all wave numbers). These bands lead to energy localization near the source with very high intensity compared to regular pass bands. An all-flat photonic band structure was observed very recently. However, to date, due to the limitations on coupling stiffness ratios and the vectorial nature of phonon propagation, no existing metamaterial exhibits an all-flat band (AFB) for phonons. In this Letter, we utilize disks with magnetic couplings as a platform to realize an all-flat phononic band structure. We elucidate the required design methodology to obtain an AFB phononic band structure and report on the first direct observation of such metamaterials. Our design methodology can enable the realization of advanced materials with exotic wave control capabilities.

The exploration of quantum phases in moiré systems has drawn intense experimental and theoretical efforts. The realization of honeycomb symmetry has been a recent focus. The combination of strong interaction and honeycomb symmetry can lead to exotic electronic states such as fractional Chern insulator, unconventional superconductor, and quantum spin liquid. Accurate computations in such systems, with reliable treatment of long-ranged Coulomb interaction and approaching large system sizes to extract thermodynamic phases, are mostly missing. We study the two-dimensional electron gas on a honeycomb moiré lattice at quarter filling, using the fixed-phase diffusion Monte Carlo method. The ground state phases of this important model are determined in the parameter regime relevant to current experiments. With increasing moiré potential, the system transitions from a paramagnetic metal to an itinerant ferromagnetic semimetal then a charge-density-wave insulator.

Although quantum mechanics underpins the microscopic behavior of all materials, its effects are often obscured at the macroscopic level by thermal fluctuations. A notable exception is a zero-temperature phase transition, where scaling laws emerge entirely due to quantum correlations over a diverging length scale. The accurate description of such transitions is challenging for classical simulation methods of quantum systems, and is a natural application space for quantum simulation. These quantum simulations are, however, not without their own challenges—representing quantum critical states on a quantum computer requires encoding entanglement of a large number of degrees of freedom, placing strict demands on the coherence and fidelity of the computer’s operations. Using Quantinuum’s H1-1 quantum computer, we address these challenges by employing hierarchical quantum tensor-network techniques, creating the ground state of the critical transverse-field Ising chain on 128-sites with sufficient fidelity to extract accurate critical properties of the model. Our results suggest a viable path to quantum-assisted tensor-network contraction beyond the limits of classical methods.

The coupling between defects and extended critical degrees of freedom gives rise to the intriguing theory known as defect conformal field theory (CFT). In this work, we introduce a novel family of boundary and interface CFTs by coupling $N$ Majorana chains with ${\mathrm{SYK}}_q$ interactions at the defect. Our analysis reveals that the interaction with $q=2$ constitutes a new marginal defect. Employing a versatile saddle-point method, we compute unique entanglement characterizations, including the $g$ function and effective central charge, of the defect CFT. Furthermore, we analytically evaluate the transmission coefficient using CFT techniques. Surprisingly, the transmission coefficient deviates from the universal relation with the effective central charge across the defect at the large $N$ limit, suggesting that our defect CFT extends beyond all known examples of Gaussian defect CFT.

We report a spiking flip-flop memory mechanism that allows controllably switching between neural-like excitable spike-firing and quiescent dynamics in a resonant tunneling diode (RTD) neuron under low-amplitude ($<150\mathrm{mV}$ pulses) and high-speed (ns rate) inputs pulses. We also show that the timing of the set-reset input pulses is critical to elicit switching responses between spiking and quiescent regimes in the system. The demonstrated flip-flop spiking memory, in which spiking regimes can be controllably excited, stored, and inhibited in RTD neurons via specific low-amplitude, high-speed signals (delivered at proper time instants) offers high promise for RTD-based spiking neural networks, with the potential to be extended further to optoelectronic implementations where RTD neurons and RTD memory elements are deployed alongside for fast and efficient photonic-electronic neuromorphic computing and artificial intelligence hardware.

Athermal (i.e., zero-temperature) underconstrained systems are typically floppy, but they can be rigidified by the application of external strain, which is theoretically well understood. Here and in the companion paper [C. T. Lee and M. Merkel, Phys. Rev. E 110, 064147 (2024)], we extend this theory to finite temperatures for a very broad class of underconstrained systems. In the vicinity of the athermal transition point, we derive from first principles expressions for elastic properties such as isotropic tension $t$ and shear modulus $G$ on temperature $T$, isotropic strain $ϵ$, and shear strain $\gamma$, which we confirm numerically. These expressions contain only three parameters: entropic rigidity ${\kappa }_S$, energetic rigidity ${\kappa }_E$, and a parameter $b_{ϵ}$ describing the interaction between isotropic and shear strain, which can be determined from the microstructure of the system. Our results imply that in underconstrained systems, entropic and energetic rigidity interact like two springs in series. This also allows for a simple explanation of the previously numerically observed scaling relation $t\sim G\sim T^{1/2}$ at $ϵ=\gamma =0$. Our work unifies the physics of systems as diverse as polymer fibers and networks, membranes, and vertex models for biological tissues.

We uncover emergent universality arising in the equilibration dynamics of multimode continuous-variable systems. Specifically, we study the ensemble of pure states supported on a small subsystem of a few modes, generated by Gaussian measurements on the remaining modes of a globally pure bosonic Gaussian state. We find that beginning from highly entangled, complex global states, such as random Gaussian states and product squeezed states coupled via a deep array of linear optical elements, the induced ensemble attains a universal form, independent of the choice of measurement basis: it is composed of unsqueezed coherent states whose displacements are distributed normally and isotropically, with variance depending on only the particle-number density of the system. We further show that the emergence of such a universal form is consistent with a generalized maximum entropy principle, which endows the limiting ensemble, which we call the “Gaussian Scrooge distribution,” with a special quantum information-theoretic property of having minimal accessible information. Our results represent a conceptual generalization of the recently introduced notion of “deep thermalization” in discrete-variable quantum many-body systems—a novel form of equilibration going beyond thermalization of local observables—to the realm of continuous-variable quantum systems. Moreover, it demonstrates how quantum information-theoretic perspectives can unveil new physical phenomena and principles in quantum dynamics and statistical mechanics.

Diatomic molecular codes [V. V. Albert, J. P. Covey, and J. Preskill, Robust encoding of a qubit in a molecule, Phys. Rev. X 10, 031050 (2020).] are designed to encode quantum information in the orientation of a diatomic molecule, allowing error correction from small torques and changes in angular momentum. Here, we directly study noise native to atomic and molecular platforms—spontaneous emission, stray electromagnetic fields, and Raman scattering—and show that diatomic molecular codes fail against this noise. We derive simple conditions that are sufficient for codes to protect against such noise. We also identify existing and develop new absorption-emission (Æ) codes that are more practical than molecular codes, require lower average momentum, can directly protect against photonic processes up to arbitrary order, and are applicable to a broader set of atomic and molecular systems.

The quantum circuit model is the most widely used theoretical model for quantum computing. Therefore, determining whether two quantum circuits whose internal structures cannot be seen have the same functionality will be a fundamental problem in future quantum industries, which however turns out to be QMA-hard. Here, based on a photonic system we experimentally implement the equivalence checking of two unknown quantum circuits with real unitary matrix representations, where quantum nonlocality plays a key role and allows us to measure an “average-case” distance between the two quantum circuits very efficiently. Particularly, we also experimentally apply such protocols to verify the correctness of quantum circuit optimizations, which is another crucial topic in future quantum industries. Our work constitutes a proof of concept for the equivalence checking of quantum circuits without relying on their detailed structures.

The Mu2e and COMET experiments are expected to improve existing limits on charged lepton flavor violation (CLFV) by roughly 4 orders of magnitude. $\mu \to e$ conversion experiments are typically optimized for electrons produced without nuclear excitation, as this maximizes the electron energy and minimizes backgrounds from the free decay of the muon. Here we argue that Mu2e and COMET will be able to extract additional constraints on CLFV from inelastic $\mu \to e$ conversion, given the ${}^{27}\mathrm{Al}$ target they have chosen and backgrounds they anticipate. We describe CLFV scenarios in which inelastic CLFV can induce measurable distortions in the near-endpoint spectrum of conversion electrons, including cases where certain contributing operators cannot be probed in elastic $\mu \to e$ conversion. We extend the nonrelativistic EFT treatment of elastic $\mu \to e$ conversion to include the new nuclear operators needed for the inelastic process, evaluate the associated nuclear response functions, and describe several new-physics scenarios where the inelastic process can provide additional information on CLFV.

The spin tensor is fundamental to relativistic spin hydrodynamics, but its definition is ambiguous due to the pseudogauge symmetry. We show that this ambiguity can be solved in interacting field theories. We prove that the mean-field limit of a modified Nambu-Jona-Lasinio model with spin-spin interactions is equivalent to nondissipative spin hydrodynamics with a canonical spin tensor.

Time-dependent density functional theory (TDDFT) is widely used for understanding and predicting properties and behaviors of matter. As one of the fundamental theorems in TDDFT, Van Leeuwen theorem [Phys. Rev. Lett. 82, 3863 (1999)] guarantees how to construct a unique potential with the same one-body density evolution. Here we extend Van Leeuwen theorem by exploring truncation criteria in Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy. Our generalized theorem demonstrates the existence of a unique nonlocal potential to accurately reconstruct the multibody density evolution in binary interacting systems. Under nonstringent conditions, truncation of the BBGKY-hierarchy equations aligns with the behavior of multibody density evolution and maintains consistency in the reduced equations. Based on this theorem, we further construct a static many-body potential and extend the well-known Casida equation for multiple-excitation energy. The extended TDDFT provides an accurate and first-principles framework capable of describing the kinetic processes of correlated systems, including strongly coupled particle transport, multiple-excitation, and ionization processes.

A superresolution (SR) method for the reconstruction of Navier-Stokes (NS) flows from noisy observations is presented. In the SR method, first the observation data are averaged over a coarse grid to reduce the noise at the expense of losing resolution and, then, a dynamic observer is employed to reconstruct the flow field by reversing back the lost information. We provide a theoretical analysis, which indicates a chaos synchronization of the SR observer with the reference NS flow. It is shown that, even with noisy observations, the SR observer converges toward the reference NS flow exponentially fast, and the deviation of the observer from the reference system is bounded. Counterintuitively, our theoretical analysis shows that the deviation can be reduced by increasing the length scale of the spatial average, i.e., making the resolution coarser. The theoretical analysis is confirmed by numerical experiments of two-dimensional NS flows. The numerical experiments suggest that there is a critical length scale for the spatial average, below which making the resolution coarser improves the reconstruction.