In many condensed matter systems, long-range order emerges at low temperatures as thermal fluctuations subside. In the presence of competing interactions or quenched disorder, however, some systems can show unusual configurations that become more disordered at low temperature, a rare phenomenon known as “inverse melting.” Here, we discover an inverse melting of the polar order in a ferroelectric oxide with quenched chemical disorder (${\mathrm{BaTi}}_{1-x}{\text{Zr}}_x{\mathrm{O}}_3$) through direct atomic-scale visualization using in situ scanning transmission electron microscopy. In contrast to the clean ${\mathrm{BaTiO}}_3$ parent system in which long-range order tracks lower temperatures, we observe in the doped system ${\mathrm{BaTi}}_{1-x}{\text{Zr}}_x{\mathrm{O}}_3$ that thermally driven fluctuations at high temperature give way to a more ordered state and then to a reentrant disordered configuration at even lower temperature. Such an inverse melting of the polar order is likely linked to the random field generated by Zr dopants, which modulates the energy landscape arising from the competition between thermal fluctuations and random field pinning potential. These visualizations highlight a rich landscape of order and disorder in materials with quenched disorder, which may be key to understanding their advanced functionalities.

We report on the first observation of spin noise in a strongly birefringent semiconductor—halide perovskite single crystal ${\mathrm{MAPbI}}_3$. The observed spin noise resonance is attributed to resident free holes in the bulk of the crystal with one of the longest spin dephasing times $T_2=4\mathrm{ns}$. The spin dynamics is found to be affected by the residual light absorption of the crystal providing renormalization of the Larmor frequency. Extended spin noise spectroscopy with a rotating magnetic field allowed us not only to evaluate the $g$-factor anisotropy but also to distinguish two different spin subsystems tentatively associated with twinning of the crystal. This work opens the way to studies of spin physics of bulk anisotropic semiconductors in general and perovskites in particular.

This Letter makes progress on the issue of global vs local master equations. Global master equations like the Redfield master equation (following from standard Born and Markov approximation) require a full diagonalization of the system Hamiltonian. This is especially challenging for interacting quantum many-body systems. We discuss a short-bath-correlation-time expansion in reciprocal (energy) space, leading to a series expansion of the jump operator, which avoids a diagonalization of the Hamiltonian. For a bath that is coupled locally to one site, this typically leads to an expansion of the global Redfield jump operator in terms of local operators. We additionally map the local Redfield master equation to a novel local Lindblad form, giving an equation which has the same conceptual advantages of traditional local Lindblad approaches while being applicable in a much broader class of systems. Our ideas give rise to a nonheuristic foundation of local master equations, which can be combined with established many-body methods.

Non-Gaussian fluctuations are one of the essential properties of quantum systems. However, the direct detection of non-Gaussian fluctuations has not yet been achieved in experiments since the specific order of correlation has to be isolated from the complex dynamics of the whole system. In this Letter, we measure the second- and fourth-order correlations of ${}^{13}\mathrm{C}$ nuclear spins around a nitrogen vacancy center in diamond, using the recently developed quantum nonlinear spectroscopy. By adjusting the detection window and the spectral resolution, we observe the Gaussian to non-Gaussian transition of the multispin system. We find that the fourth-order correlation provides a fingerprint signal that can be used to identify individual spins with the same precession frequency, suggesting that Gaussianity is a potential quantum resource for quantum sensing. Our results shed light on the study of nonequilibrium quantum many-body dynamics and quantum materials at the nanoscale.

In light of recent experimental data indicating a substantial thermal Hall effect in square lattice antiferromagnetic Mott insulators, we investigate whether a simple Mott insulator can sustain a finite thermal Hall effect. We verify that the answer is “no” if one performs calculations within a spin-only low-energy effective spin model with noninteracting magnons. However, by performing determinant quantum Monte Carlo simulations, we show the single-band $t\text{−}{t}^{\prime }\text{−}U$ Hubbard model coupled to an orbital magnetic field does support a finite thermal Hall effect when $t^{\prime }\ne 0$ and $B\ne 0$ in the Mott insulating phase. We argue that the (carrier agnostic) necessary conditions for observing a finite thermal Hall effect are time-reversal and particle-hole symmetry breaking. By considering magnon-magnon scattering using a semiclassical Boltzmann analysis, we illustrate a physical mechanism by which finite transverse thermal conductivity may arise, consistent with our symmetry argument and numerical results. Our results indicate that square and triangular lattices with SU(2) symmetry can support a finite thermal Hall effect and call for a critical reexamination of thermal Hall effect data in insulating magnets, as the magnon contribution should not be excluded a priori.

We present a dynamic scaling theory to describe relaxation dynamics following a magnetic-field quench near an unconventional phase transition in the magnetic material spin ice. Starting from a microscopic model, we derive an effective description for the critical dynamics in terms of the seeding and growth of string excitations, and use this to find scaling forms in terms of time, reduced temperature, and monopole fugacity. We confirm the predictions of scaling theory using Monte Carlo simulations, which also show good quantitative agreement with analytical expressions valid in the limit of low monopole density. As well as being relevant for experiments in the spin ice materials, our results open the way for the study of dynamic critical properties in a family of unconventional classical phase transitions.

The properties of interfaces are key to understanding the physics of matter. However, the study of quantum interface dynamics has remained an outstanding challenge. Here, we use large-scale tree tensor network simulations to identify the dynamical signature of an interface roughening transition within the ferromagnetic phase of the 2D quantum Ising model. For initial domain wall profiles we find extended prethermal plateaus for smooth interfaces, whereas above the roughening transition the domain wall decays quickly. Our results can be readily explored experimentally in Rydberg atomic systems.

Finding a quantum battery model that displays a genuine quantum advantage, while being prone to experimental fabrication, is an extremely challenging task. In this Letter we propose a deceptively simple quantum battery model that displays a genuine quantum advantage, saturating the quantum speed limit. It consists of two harmonic oscillators (the charger and the battery), coupled during the charging dynamics by an anharmonic interaction. We first present the model, then certify the genuine quantum advantage, and finally briefly discuss how the battery can be fabricated through the use of superconducting circuits.

We study a stroboscopic quantum Ising model with Fibonacci dynamics. We use its boundary spin correlation functions in long but finite chains to identify regions in the phase diagram which exhibit Majorana zero modes (MZM) as well as Majorana golden-ratio modes (MGM). We find that these regions evolve in a self-similar manner with increasing simulation time and identify the self-similarity transform which governs this evolution of the phase diagram. Integrability-breaking perturbations lead to a temporal decay of the boundary spin correlations, ultimately limiting the self-similarity of the phase diagram. Our predictions are testable with current quantum information processors.

A distributed quantum computing network requires a quantum communication channel between spatially separated processing units. In superconducting circuits, such a channel can be implemented based on propagating microwave photons to encode and transfer quantum information between an emitter and a receiver. However, traveling microwave photons can be lost during the transmission, leading to the failure of information transfer. Heralding protocols can be used to detect such photon losses. In this Letter, we propose such a protocol and experimentally demonstrate a frequency-bin encoding method of microwave photonic modes using superconducting circuits. We deterministically encode the quantum information from a superconducting qubit by simultaneously emitting its information into two photonic modes at different frequencies, with a process fidelity of 94.9%. The frequency-bin-encoded photonic modes can be used, at the receiver processor, to detect the occurrence of photon loss. Our Letter thus provides a reliable method to implement high-fidelity quantum state transfer in a distributed quantum computing network, incorporating error detection to enhance performance and accuracy.

The quantum chromodynamics (QCD) axion arises as the pseudo-Goldstone mode of a spontaneously broken Abelian Peccei-Quinn (PQ) symmetry. If the scale of PQ symmetry breaking occurs below the inflationary reheat temperature and the domain wall number is unity, then there is a unique axion mass that gives the observed dark matter (DM) abundance. Computing this mass has been the subject of intensive numerical simulations for decades since the mass prediction informs laboratory experiments. Axion strings develop below the PQ symmetry-breaking temperature, and as the string network evolves, it emits axions that go on to become the DM. A key ingredient in the axion mass prediction is the spectral index of axion radiation emitted by the axion strings. We compute this index in this Letter using the most precise and accurate large-scale simulations to date of the axion-string network leveraging adaptive mesh refinement to achieve the precision that would, otherwise, require a static lattice with 262,${144}^3$ lattice sites. We find a scale-invariant axion radiation spectrum to within 1% precision and find no evidence that the spectral index of radiation evolves with time. Accounting for axion production from strings prior to the QCD phase transition leads us to predict that the axion mass should be approximately $m_a\in (45,65)\mathrm{\mu }\mathrm{eV}$. However, we provide preliminary evidence that axions are produced in greater quantities from the string-domain-wall network collapse during the QCD phase transition, potentially increasing the mass prediction to as much as $300\mathrm{\mu }\mathrm{eV}$.

We present a minimal cosmological solution to the hierarchy problem. Our model consists of a light pseudoscalar and an extra Higgs doublet in addition to the standard model field content. We consider a landscape of vacua with varying values of the electroweak vacuum expectation value (VEV). The vacuum energy in our model peaks in a region of the landscape where the electroweak VEV is nonzero and much smaller than the cutoff. During inflation, due to exponential expansion, such regions of the landscape with maximal vacuum energy dominate the universe in volume, thus explaining the observed smallness of the electroweak scale with respect to the cutoff. The pseudoscalar potential in our model is that of a completely generic pseudogoldstone boson—not requiring the clockwork mechanism—and its field value never exceeds its decay constant or the Planck scale. Our mechanism is robust to the variation of other model parameters in the landscape as the electroweak VEV is varied. It also predicts a precise and falsifiable relationship between the masses and couplings of the different Higgs boson mass eigenstates. Moreover, the pseudoscalar in our model can account for the observed dark matter relic density.

The neutrinoless double-beta decay ($0\nu \beta \beta$) of two neutrons $nn\to ppee$ is the elementary subprocess of $0\nu \beta \beta$ decay in nuclei. Accurate knowledge of the $nn\to ppee$ amplitude is required to pin down the short-range contributions in the nuclear matrix elements of the candidate nuclei for large-scale $0\nu \beta \beta$ searches. In this Letter, we report the first next-to-leading-order prediction of the $nn\to ppee$ amplitude, with Bayesian uncertainty quantification. This is made possible by the development of the relativistic chiral effective field theory, in which no unknown contact term is required up to next-to-leading order. The theory is validated by reproducing in a parameter-free way the available data on the charge independence and charge symmetry breaking contributions in the two-nucleon scattering. The present work makes an essential step toward addressing the uncertainty in the theoretical calculations of the nuclear matrix elements relevant for $0\nu \beta \beta$ searches.

High-frequency phonons hold great promise as carriers of quantum information on chip and as quantum memories. Because of their coherent interaction with several systems, their compact mode volume, and slow group velocity, multiple experiments have recently demonstrated coherent transport of information on chip using phonon modes, interconnecting distinct quantum devices. Strongly confined phonons in waveguidelike geometries are particularly interesting because of their long lifetime. However, spectral diffusion has been observed to substantially limit their coherence times [S. M. Meenehan et al., Silicon optomechanical crystal resonator at millikelvin temperatures, Phys. Rev. A 90, 011803(R) (2014), A. Wallucks et al., A quantum memory at telecom wavelengths, Nat. Phys. 16, 772 (2020), and G. S. MacCabe et al., Nano-acoustic resonator with ultralong phonon lifetime, Science 370, 840 (2020)]. Coupling to two-level systems is suspected to be a major contributor to the diffusion; however, to date, the origin and underlying mechanisms are still not fully understood. Here, we perform a time-domain study on two adjacent mechanical modes (separated by around 5 MHz) and show that the frequency positions of the two modes are not correlated in time, in agreement with our theoretical model and Monte Carlo simulations. This result is an important step in fully understanding the microscopic mechanisms of dephasing in mechanical quantum buses and memories.