We use the strong intrinsic nonlinearity of a microwave superconducting qubit with a 4 GHz transition frequency to directly detect and control the energy of a micromechanical oscillator vibrating at 25 MHz. The qubit and the oscillator are coupled electrostatically at a rate of approximately $2\pi \times 22\mathrm{MHz}$. In this far off-resonant regime, the qubit frequency is shifted by 0.52 MHz per oscillator phonon, or about 14% of the 3.7 MHz qubit linewidth. The qubit behaves as a vibrational energy detector and from its line shape we extract the phonon number distribution of the oscillator. We manipulate this distribution by driving number state sensitive sideband transitions and creating profoundly nonthermal states. Finally, by driving the lower frequency sideband transition, we cool the oscillator and increase its ground state population up to $0.48\pm 0.13$, close to a factor of 8 above its value at thermal equilibrium. These results demonstrate a new class of electromechanics experiments that are a promising strategy for quantum nondemolition measurements and nonclassical state preparation.

Time crystals are a phase of matter, for which the discrete time symmetry of the driving Hamiltonian is spontaneously broken. The breaking of discrete time symmetry has been observed in several experiments in driven spin systems. Here, we show the observation of a space-time crystal using ultracold atoms, where the periodic structure in both space and time is directly visible in the experimental images. The underlying physics in our superfluid can be described ab initio and allows for a clear identification of the mechanism that causes the spontaneous symmetry breaking. Our results pave the way for the usage of space-time crystals for the discovery of novel nonequilibrium phases of matter.

In this work, we experimentally show that quantum entanglement can be symmetry protected in the interaction with a single subwavelength plasmonic nanoaperture, with a total volume of $V\sim 0.2{\lambda }^3$. In particular, we experimentally demonstrate that two-photon entanglement can be either completely preserved or completely lost after the interaction with the nanoaperture, solely depending on the relative phase between the quantum states. We achieve this effect by using specially engineered two-photon states to match the properties of the nanoaperture. In this way we can access a symmetry protected state, i.e., a state constrained by the geometry of the interaction to retain its entanglement. In spite of the small volume of interaction, we show that the symmetry protected entangled state retains its main properties. This connection between nanophotonics and quantum optics probes the fundamental limits of the phenomenon of quantum interference.

We study the elementary characteristics of turbulence in a quantum ferrofluid through the context of a dipolar Bose gas condensing from a highly nonequilibrium thermal state. Our simulations reveal that the dipolar interactions drive the emergence of polarized turbulence and density corrugations. The superfluid vortex lines and density fluctuations adopt a columnar or stratified configuration, depending on the sign of the dipolar interactions, with the vortices tending to form in the low-density regions to minimize kinetic energy. When the interactions are dominantly dipolar, the decay of the vortex line length is enhanced, closely following a $t^{-3/2}$ behavior. This system poses exciting prospects for realizing stratified quantum turbulence and new levels of generating and controlling turbulence using magnetic fields.

The dynamo effect is a class of macroscopic phenomena responsible for generating and maintaining magnetic fields in astrophysical bodies. It hinges on the hydrodynamic three-dimensional motion of conducting gases and plasmas that achieve high hydrodynamic and/or magnetic Reynolds numbers due to the large length scales involved. The existing laboratory experiments modeling dynamos are challenging and involve large apparatuses containing conducting fluids subject to fast helical flows. Here we propose that electronic solid-state materials—in particular, hydrodynamic metals—may serve as an alternative platform to observe some aspects of the dynamo effect. Motivated by recent experimental developments, this Letter focuses on hydrodynamic Weyl semimetals, where the dominant scattering mechanism is due to interactions. We derive Navier-Stokes equations along with equations of magnetohydrodynamics that describe the transport of a Weyl electron-hole plasma appropriate in this regime. We estimate the hydrodynamic and magnetic Reynolds numbers for this system. The latter is a key figure of merit of the dynamo mechanism. We show that it can be relatively large to enable observation of the dynamo-induced magnetic field bootstrap in an experiment. Finally, we generalize the simplest dynamo instability model—the Ponomarenko dynamo—to the case of a hydrodynamic Weyl semimetal and show that the chiral anomaly term reduces the threshold magnetic Reynolds number for the dynamo instability.

We propose a quadrupolar exchange-only spin qubit that is highly robust against charge noise and nuclear spin dephasing, the dominant decoherence mechanisms in quantum dots. The qubit consists of four electrons trapped in three quantum dots, and operates in a decoherence-free subspace to mitigate dephasing due to nuclear spins. To reduce sensitivity to charge noise, the qubit can be completely operated at an extended charge noise sweet spot that is first-order insensitive to electrical fluctuations. Because of on-site exchange mediated by the Coulomb interaction, the qubit energy splitting is electrically controllable and can amount to several GHz even in the “off” configuration, making it compatible with conventional microwave cavities.

Characterizing quantum processes is a key task in the development of quantum technologies, especially at the noisy intermediate scale of today’s devices. One method for characterizing processes is randomized benchmarking, which is robust against state preparation and measurement errors and can be used to benchmark Clifford gates. Compressed sensing techniques achieve full tomography of quantum channels essentially at optimal resource efficiency. In this Letter, we show that the favorable features of both approaches can be combined. For characterizing multiqubit unitary gates, we provide a rigorously guaranteed and practical reconstruction method that works with an essentially optimal number of average gate fidelities measured with respect to random Clifford unitaries. Moreover, for general unital quantum channels, we provide an explicit expansion into a unitary 2-design, allowing for a practical and guaranteed reconstruction also in that case. As a side result, we obtain a new statistical interpretation of the unitarity—a figure of merit characterizing the coherence of a process.

The T2K experiment measures muon neutrino disappearance and electron neutrino appearance in accelerator-produced neutrino and antineutrino beams. With an exposure of $14.7(7.6)\times {10}^{20}$ protons on target in the neutrino (antineutrino) mode, 89 ${\nu }_e$ candidates and seven anti-${\nu }_e$ candidates are observed, while 67.5 and 9.0 are expected for ${\delta }_{CP}=0$ and normal mass ordering. The obtained $2\sigma$ confidence interval for the $CP$-violating phase, ${\delta }_{CP}$, does not include the $CP$-conserving cases (${\delta }_{CP}=0$, $\pi$). The best-fit values of other parameters are ${\sin }^2{\theta }_{23}=0.526_{-0.036}^{+0.032}$ and $\mathrm{\Delta }{m}_{32}^2={2.463}_{-0.070}^{+0.071}\times {10}^{-3}{\mathrm{eV}}^2/c^4$.

Kagome antiferromagnets are known to be highly frustrated and degenerate when they possess simple, isotropic interactions. We consider the entire class of these magnets when their interactions are spatially anisotropic. We do so by identifying a certain class of systems whose degenerate ground states can be mapped onto the folding motions of a generalized “spin origami” two-dimensional mechanical sheet. Some such anisotropic spin systems, including ${\mathrm{Cs}}_2{\mathrm{ZrCu}}_3{\mathrm{F}}_{12}$, map onto flat origami sheets, possessing extensive degeneracy similar to isotropic systems. Others, such as ${\mathrm{Cs}}_2{\mathrm{CeCu}}_3{\mathrm{F}}_{12}$, can be mapped onto sheets with nonzero Gaussian curvature, leading to more mechanically stable corrugated surfaces. Remarkably, even such distortions do not always lift the entire degeneracy, instead permitting a large but subextensive space of zero-energy modes. We show that for ${\mathrm{Cs}}_2{\mathrm{CeCu}}_3{\mathrm{F}}_{12}$, due to an additional point group symmetry associated with the structure, these modes are “Dirac” line nodes with a double degeneracy protected by a topological invariant. The existence of mechanical analogs thus serves to identify and explicate the robust degeneracy of the spin systems.

Crystal growth often proceeds by atomic step flow. When the surface area available for growth is limited, the nucleation and progression of the steps can be affected. This issue is particularly relevant to the formation of nanocrystals. We examine the case of Au-catalyzed GaAs nanowires, which we grow in a transmission electron microscope. Our in situ observations show that atomic layers nucleate at the periphery of the interface between the nanowire and the catalyst droplet. From this starting location, the atomic step flows within a restricted area of hexagonal shape. At specific partial coverages, the monolayer configuration changes abruptly. A simple model based on the geometry of the system and its edge energies explains these observations. In particular, we observe an inversion of the step curvature which reveals that the effective energy per unit length of monolayer edge is much lower at the interface periphery than inside the catalyst droplet.

We report on an upward traveling, radio-detected cosmic-ray-like impulsive event with characteristics closely matching an extensive air shower. This event, observed in the third flight of the Antarctic Impulsive Transient Antenna (ANITA), a NASA-sponsored long-duration balloon payload, is consistent with a similar event reported in a previous flight. These events could be produced by the atmospheric decay of an upward-propagating $\tau$ lepton produced by a ${\nu }_{\tau }$ interaction, although their relatively steep arrival angles create tension with the standard model neutrino cross section. Each of the two events have a posteriori background estimates of $\lesssim {10}^{-2}$ events. If these are generated by $\tau$-lepton decay, then either the charged-current ${\nu }_{\tau }$ cross section is suppressed at EeV energies, or the events arise at moments when the peak flux of a transient neutrino source was much larger than the typical expected cosmogenic background neutrinos.

The Clausius inequality has deep implications for reversibility and the arrow of time. Quantum theory is able to extend this result for closed systems by inspecting the trajectory of the density matrix on its manifold. Here we show that this approach can provide an upper and lower bound to the irreversible entropy production for open quantum systems as well. These provide insights on how the information on the initial state is forgotten through a thermalization process. Limits of the applicability of our bounds are discussed and demonstrated in a quantum photonic simulator.

By making use of a recently proposed framework for the inference of thermodynamic irreversibility in bosonic quantum systems, we experimentally measure and characterize the entropy production rates in the nonequilibrium steady state of two different physical systems—a micromechanical resonator and a Bose-Einstein condensate—each coupled to a high finesse cavity and hence also subject to optical loss. Key features of our setups, such as the cooling of the mechanical resonator and signatures of a structural quantum phase transition in the condensate, are reflected in the entropy production rates. Our work demonstrates the possibility to explore irreversibility in driven mesoscopic quantum systems and paves the way to a systematic experimental assessment of entropy production beyond the microscopic limit.

Coherent control of reactive atomic and molecular collision processes remains elusive experimentally due to quantum interference-based requirements. Here, with insights from symmetry conditions, a viable method for controlling Penning and associative ionization in atomic collisions is proposed. Computational applications to ${\mathrm{He}}^{*}({}^{3}S)\text{−}\mathrm{Li}({}^{2}S)$ and ${\mathrm{Ne}}^{*}({}^3{P}_2)\text{−}\mathrm{Ar}({}^1{S}_0)$ show extensive control over the ionization processes under experimentally feasible conditions.

On 17 August 2017, the LIGO and Virgo observatories made the first direct detection of gravitational waves from the coalescence of a neutron star binary system. The detection of this gravitational-wave signal, GW170817, offers a novel opportunity to directly probe the properties of matter at the extreme conditions found in the interior of these stars. The initial, minimal-assumption analysis of the LIGO and Virgo data placed constraints on the tidal effects of the coalescing bodies, which were then translated to constraints on neutron star radii. Here, we expand upon previous analyses by working under the hypothesis that both bodies were neutron stars that are described by the same equation of state and have spins within the range observed in Galactic binary neutron stars. Our analysis employs two methods: the use of equation-of-state-insensitive relations between various macroscopic properties of the neutron stars and the use of an efficient parametrization of the defining function $p(\rho )$ of the equation of state itself. From the LIGO and Virgo data alone and the first method, we measure the two neutron star radii as $R_1={10.8}_{-1.7}^{+2.0}\mathrm{km}$ for the heavier star and $R_2={10.7}_{-1.5}^{+2.1}\mathrm{km}$ for the lighter star at the 90% credible level. If we additionally require that the equation of state supports neutron stars with masses larger than $1.97{\mathrm{M}}_{\odot }$ as required from electromagnetic observations and employ the equation-of-state parametrization, we further constrain $R_1={11.9}_{-1.4}^{+1.4}\mathrm{km}$ and $R_2={11.9}_{-1.4}^{+1.4}\mathrm{km}$ at the 90% credible level. Finally, we obtain constraints on $p(\rho )$ at supranuclear densities, with pressure at twice nuclear saturation density measured at ${3.5}_{-1.7}^{+2.7}\times {10}^{34}\text{dyn}{\mathrm{cm}}^{-2}$ at the 90% level.

Using a noncontact atomic force microscope, we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds on the hydrogen terminated silicon surface. An attractive tip surface interaction mechanically manipulates the equilibrium position of a surface silicon atom, causing rehybridization that stabilizes a negative charge at the dangling bond. This is applied to controllably switch the charge state of individual dangling bonds. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged dangling bonds, we observe charge configurations of dangling bond structures that remain stable for many seconds at 4.5 K. Subsequently, we use charge manipulation to directly prepare the ground state and metastable charge configurations of dangling bond structures composed of up to six atoms.

We report on quantum capacitance measurements of high quality, graphite and hexagonal boron nitride encapsulated Bernal stacked trilayer graphene devices. At zero applied magnetic field, we observe a number of electron density- and electrical displacement-tuned features in the electronic compressibility associated with changes in Fermi surface topology. At a high displacement field and low density, strong trigonal warping gives rise to three new emergent Dirac cones in each valley, which we term “gullies.” The gullies are centered around the corners of a hexagonal Brillouin zone and related by threefold rotation symmetry. At low magnetic fields of $B=1.25\mathrm{T}$, the gullies manifest as a change in the degeneracy of the Landau levels from two to three. Weak incompressible states are also observed at integer filling within these triplet Landau levels, which a Hartree-Fock analysis indicates are associated with Coulomb-driven nematic phases that spontaneously break rotation symmetry.

We propose a scenario to retrodict the top and bottom mass and the Abelian gauge coupling from first principles in a microscopic model including quantum gravity. In our approximation, antiscreening quantum-gravity fluctuations induce an asymptotically safe fixed point for the Abelian hypercharge leading to a uniquely fixed infrared value that is observationally viable for a particular choice of microscopic gravitational parameters. The unequal quantum numbers of the top and bottom quark lead to different fixed-point values for the top and bottom Yukawa couplings under the impact of gauge and gravity fluctuations. This results in a dynamically generated mass difference between the two quarks. To work quantitatively, the preferred ratio of electric charges of bottom and top in our approximation lies in close vicinity to the standard-model value of $Q_b/Q_t=-1/2$.

Many neurodegenerative diseases are related to the propagation and accumulation of toxic proteins throughout the brain. The lesions created by aggregates of these toxic proteins further lead to cell death and accelerated tissue atrophy. A striking feature of some of these diseases is their characteristic pattern and evolution, leading to well-codified disease stages visible to neuropathology and associated with various cognitive deficits and pathologies. Here, we simulate the anisotropic propagation and accumulation of toxic proteins in full brain geometry. We show that the same model with different initial seeding zones reproduces the characteristic evolution of different prionlike diseases. We also recover the expected evolution of the total toxic protein load. Finally, we couple our transport model to a mechanical atrophy model to obtain the typical degeneration patterns found in neurodegenerative diseases.

We perform the first global QCD analysis of parton distribution functions (PDFs) in the pion, combining $\pi A$ Drell-Yan data with leading neutron electroproduction from HERA within a Monte Carlo approach based on nested sampling. Inclusion of the HERA data allows the pion PDFs to be determined down to much lower values of $x$, with relatively weak model dependence from uncertainties in the chiral splitting function. The combined analysis reveals that gluons carry a significantly higher pion momentum fraction, $\sim 30\%$, than that inferred from Drell-Yan data alone, with sea quarks carrying a somewhat smaller fraction, $\sim 15\%$, at the input scale. Within the same effective theory framework, the chiral splitting function and pion PDFs can be used to describe the $\overline{d}-\overline{u}$ asymmetry in the proton.