With high spatial resolution, polarimetric imaging of a supermassive black hole, like $\mathrm{M}{87}^{\star }$ or Sgr ${\mathrm{A}}^{\star }$, by the Event Horizon Telescope can be used to probe the existence of ultralight bosonic particles, such as axions. Such particles can accumulate around a rotating black hole through the superradiance mechanism, forming an axion cloud. When linearly polarized photons are emitted from an accretion disk near the horizon, their position angles oscillate due to the birefringent effect when traveling through the axion background. In particular, the observations of supermassive black holes $\mathrm{M}{87}^{\star }$ (Sgr ${\mathrm{A}}^{\star }$) can probe the dimensionless axion-photon coupling $c=2\pi g_{a\gamma }{f}_a$ for axions with mass around $O({10}^{-20})\mathrm{eV}$ [$O({10}^{-17})\mathrm{eV}$] and decay constant $f_a<O({10}^{16})\mathrm{GeV}$, which is complimentary to other axion measurements.

In 1935, Einstein, Podolsky, and Rosen (EPR) formulated an apparent paradox of quantum theory [Phys. Rev. 47, 777 (1935)]. They considered two quantum systems that were initially allowed to interact and were then later separated. A measurement of a physical observable performed on one system then had to have an immediate effect on the conjugate observable in the other system—even if the systems were causally disconnected. The authors viewed this as a clear indication of the inconsistency of quantum mechanics. In the parton model of the nucleon formulated by Bjorken, Feynman, and Gribov, the partons (quarks and gluons) are viewed by an external hard probe as independent. The standard argument is that, inside the nucleon boosted to an infinite-momentum frame, the parton probed by a virtual photon with virtuality $Q$ is causally disconnected from the rest of the nucleon during the hard interaction. Yet, the parton and the rest of the nucleon have to form a color-singlet state due to color confinement and so have to be in strongly correlated quantum states—we thus encounter the EPR paradox at the subnucleonic scale. In this Letter, we propose a resolution of this paradox based on the quantum entanglement of partons. We devise an experimental test of entanglement and carry it out using data on proton-proton collisions from the Large Hadron Collider. Our results provide a strong direct indication of quantum entanglement at subnucleonic scales.

Balancing nonlinear gain and loss automatically generates sub-Poissonian light, through negative feedback, when the gain is significantly reduced (increased) by the addition (subtraction) of a single photon. We show that micromaser trapping states can provide the necessary feedback in the presence of photon loss and, with the addition of external parametric control, realize a photon number on the order of 100 and a Mandel $Q$ parameter of $-0.998$, i.e., number squeezing of 27 dB.

The so-called stellar formalism allows us to represent the non-Gaussian properties of single-mode quantum states by the distribution of the zeros of their Husimi $Q$ function in phase space. We use this representation in order to derive an infinite hierarchy of single-mode states based on the number of zeros of the Husimi $Q$ function: the stellar hierarchy. We give an operational characterization of the states in this hierarchy with the minimal number of single-photon additions needed to engineer them, and derive equivalence classes under Gaussian unitary operations. We study in detail the topological properties of this hierarchy with respect to the trace norm, and discuss implications for non-Gaussian state engineering, and continuous variable quantum computing.

The first-order Fermi acceleration of electrons requires an injection of electrons into a mildly relativistic energy range. However, the mechanism of injection has remained a puzzle both in theory and observation. We present direct evidence for a novel stochastic shock drift acceleration theory for the injection obtained with Magnetospheric Multiscale observations at the Earth’s bow shock. The theoretical model can explain electron acceleration to mildly relativistic energies at high-speed astrophysical shocks, which may provide a solution to the long-standing issue of electron injection.

We investigate the microscopic mechanisms of ultralow lattice thermal conductivity (${\kappa }_l$) in ${\mathrm{Tl}}_3{\mathrm{VSe}}_4$ by combining a first principles density functional theory based framework of anharmonic lattice dynamics with the Peierls-Boltzmann transport equation for phonons. We include contributions of the three- and four-phonon scattering processes to the phonon lifetimes as well as the temperature dependent anharmonic renormalization of phonon energies arising from an unusually strong quartic anharmonicity in ${\mathrm{Tl}}_3{\mathrm{VSe}}_4$. In contrast to a recent report by Mukhopadhyay et al. [Science 360, 1455 (2018)] which suggested that a significant contribution to ${\kappa }_l$ arises from random walks among uncorrelated oscillators, we show that particlelike propagation of phonon excitations can successfully explain the experimentally observed ultralow ${\kappa }_l$. Our findings are further supported by explicit calculations of the off-diagonal terms of the heat current operator, which are found to be small and indicate that wavelike tunneling of heat carrying vibrations is of minor importance. Our results (i) resolve the discrepancy between the theoretical and experimental ${\kappa }_l$, (ii) offer new insights into the minimum ${\kappa }_l$ achievable in ${\mathrm{Tl}}_3{\mathrm{VSe}}_4$, and (iii) highlight the importance of high order anharmonicity in low-${\kappa }_l$ systems. The methodology demonstrated here may be used to resolve the discrepancies between the experimentally measured and the theoretically calculated ${\kappa }_l$ in skutterides and perovskites, as well as to understand the glasslike ${\kappa }_l$ in complex crystals with strong anharmonicity, leading towards the goal of rational design of new materials.

Traditional classifications of crystalline phases focus on nuclear degrees of freedom. Through the examination of both electronic and nuclear structure, we introduce the concept of an electronic plastic crystal. Such a material is classified by crystalline nuclear structure, while localized electronic degrees of freedom—here lone pairs—exhibit orientational motion at finite temperatures. This orientational motion is an emergent phenomenon arising from the coupling between electronic structure and polarization fluctuations generated by collective motions, such as phonons. Using ab initio molecular dynamics simulations, we predict the existence of electronic plastic crystal motion in halogen crystals and halide perovskites, and suggest that such motion may be found in a broad range of solids with lone pair electrons. Such fluctuations in the charge density should be observable, in principle, via synchrotron scattering.

We study the temperature dependence of the magnetic penetration depth in a 3D topological superconductor (TSC), incorporating the paramagnetic current due to the surface states. A TSC is predicted to host a gapless 2D surface Majorana fluid. In addition to the bulk-dominated London response, we identify a $T^3$ power-law-in-temperature contribution from the surface, valid in the low-temperature limit. Our system is fully gapped in the bulk, and should be compared to bulk nodal superconductivity, which also exhibits power-law behavior. Power-law temperature dependence of the penetration depth can be one indicator of topological superconductivity.

Antiferromagnetic (AFM) spintronics exploits the Néel vector as a state variable for novel spintronic devices. Recent studies have shown that the fieldlike and antidamping spin-orbit torques (SOTs) can be used to switch the Néel vector in antiferromagnets with proper symmetries. However, the precise detection of the Néel vector remains a challenging problem. In this Letter, we predict that the nonlinear anomalous Hall effect (AHE) can be used to detect the Néel vector in most compensated antiferromagnets supporting the antidamping SOT. We show that the magnetic crystal group symmetry of these antiferromagnets combined with spin-orbit coupling produce a sizable Berry curvature dipole and hence the nonlinear AHE. As a specific example, we consider the half-Heusler alloy CuMnSb, in which the Néel vector can be switched by the antidamping SOT. Based on density-functional theory calculations, we show that the nonlinear AHE in CuMnSb results in a measurable Hall voltage under conventional experimental conditions. The strong dependence of the Berry curvature dipole on the Néel vector orientation provides a new detection scheme of the Néel vector based on the nonlinear AHE. Our predictions enrich the material platform for studying nontrivial phenomena associated with the Berry curvature and broaden the range of materials useful for AFM spintronics.

It is well established that the ground states of a two-dimensional electron gas with half-filled high ($N\ge 2$) Landau levels are compressible charge-ordered states, known as quantum Hall stripe (QHS) phases. The generic features of QHSs are a maximum (minimum) in a longitudinal resistance $R_{xx}$ ($R_{yy}$) and a nonquantized Hall resistance $R_H$. Here, we report on emergent minima (maxima) in $R_{xx}$ ($R_{yy}$) and plateaulike features in $R_H$ in half-filled $N\ge 3$ Landau levels. Remarkably, these unexpected features develop at temperatures considerably lower than the onset temperature of QHSs, suggestive of a new ground state.

Controlling magnetism of two-dimensional multiferroics by an external electric field provides special opportunities for both fundamental research and future development of low-cost electronic nanodevices. Here, we report a general scheme for realizing a magnetic phase transition in 2D type-I multiferroic systems through the reversal of ferroelectric polarization. Based on first-principles calculations, we demonstrate that a single-phase 2D multiferroic, namely, ${\mathrm{ReWCl}}_6$ monolayer, exhibits two different low-symmetric ($C_2$) phases with opposite in-plane electric polarization and different magnetic order. As a result, an antiferromagnetic-to-ferromagnetic phase transition can be realized by reversing the in-plane electric polarization through the application of an external electric field. These findings not only enrich the 2D multiferroic family, but also uncover a unique and general mechanism to control magnetism by electric field, thus stimulating experimental interest.

Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500–800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in $1\mu \mathrm{s}$, well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed.

We have experimentally tested whether spin-transport and charge-transport in pristine $\pi$-conjugated polymer films at room temperature occur via the same electronic processes. We have obtained the spin diffusion coefficient of several $\pi$-conjugated polymer films from the spin diffusion length measured by the technique of inverse spin Hall effect and the spin relaxation time measured by pulsed electrically detected magnetic resonance spectroscopy. The charge diffusion coefficient was obtained from the time-of-flight mobility measurements on the same films. We found that the spin diffusion coefficient is larger than the charge diffusion coefficient by about 1–2 orders of magnitude and conclude that spin and charge transports in disordered polymer films occur through different electronic processes.

Antiparallel spins are superior in orienteering to parallel spins. This intriguing phenomenon is tied to entanglement associated with quantum measurements rather than quantum states. Using photonic systems, we experimentally realize the optimal orienteering protocols based on parallel spins and antiparallel spins, respectively. The optimal entangling measurements for decoding the direction information from parallel spins and antiparallel spins are realized using photonic quantum walks, which is a useful idea that is of wide interest in quantum information processing and foundational studies. Our experiments clearly demonstrate the advantage of antiparallel spins over parallel spins in orienteering. In addition, entangling measurements can extract more information than local measurements even if no entanglement is present in the quantum states.

What is the fastest way to heat a system which is coupled to a temperature controlled oven? The intuitive answer is to use only the hottest temperature available. However, we show that often it is possible to achieve an exponentially faster heating protocol. Surprisingly, this protocol can have a precooling stage—cooling the system before heating it shortens the heating time significantly. To demonstrate such improvements in many-body systems, we developed a projection-based method with which such protocols can be found in large systems, as we demonstrate on the 2D antiferromagnet Ising model.

Cosmological models with a dynamical dark energy field typically lead to a modified propagation of gravitational waves via an effectively time-varying gravitational coupling $G(t)$. The local variation of this coupling between the time of emission and detection can be probed with standard sirens. Here we discuss the role that lunar laser ranging (LLR) and binary pulsar constraints play in the prospects of constraining $G(t)$ with standard sirens. In particular, we argue that LLR constrains the matter-matter gravitational coupling $G_N(t)$, whereas binary pulsars and standard sirens constrain the quadratic kinetic gravity self-interaction $G_{gw}(t)$. Generically, these two couplings could be different in alternative cosmological models, in which case LLR constraints are irrelevant for standard sirens. We use the Hulse-Taylor pulsar data and show that observations are highly insensitive to time variations of $G_{gw}(t)$ yet highly sensitive to $G_N(t)$. We thus conclude that future gravitational waves data will become the best probe to test $G_{gw}(t)$, and will hence provide novel constraints on dynamical dark energy models.

We study entanglement entropy after a double local quench in two-dimensional conformal field theories (CFTs). In the holographic CFT, such a state with double excitation is dual to an anti–de Sitter space with two massive particles injected from the boundary. We show that the growth after the double local excitations in pure CFT is universal and is given by the sum of two local quenches with an additional negative term. This negative contribution can be interpreted naturally in holography as being due to the attractive force of gravity. On the CFT side, this evaluation of the entanglement entropy is accomplished by a special limit of six-point functions, where we employ the fusion matrix approach for multipoint conformal blocks developed by the authors [J. High Energy Phys. 08 (2019) 063].

The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical $r$ process in producing nuclei heavier than $A\sim 190$. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region, neutron excitations in ${}^{207}\mathrm{Hg}$ have been probed using the neutron-adding ($d$,$p$) reaction in inverse kinematics. The radioactive beam of ${}^{206}\mathrm{Hg}$ was delivered to the new ISOLDE Solenoidal Spectrometer at an energy above the Coulomb barrier. The spectroscopy of ${}^{207}\mathrm{Hg}$ marks a first step in improving our understanding of the relevant structural properties of nuclei involved in a key part of the path of the $r$ process.

We report the precision measurement of the absolute frequencies, hyperfine splitting, and $2P$ fine structure splitting in cold atoms of ${}^6\mathrm{Li}$. Using the stabilized optical frequency comb and developed heterodyne detection technique, the photon shot-noise limited optical spectroscopy is achieved. The measurement of absolute frequencies of $D_1$ lines is reached with an uncertainty of about 1 kHz, which is 1 order of magnitude more accurate than previous measurements. The hyperfine splitting of the $D_1$ line and $2P$ fine structure splitting of ${}^6\mathrm{Li}$ are 26.103 1 (14) and 10 052.780 4 (18) MHz, respectively, in agreement with recent theoretical calculations. Our results could provide a benchmark to test the theory at the higher precision and help to resolve large discrepancies among previous experiments.