Based upon an over-one-month frequency comparison of two ${}^{40}{\mathrm{Ca}}^{+}$ optical clocks, the frequency difference between the two clocks is measured to be $3.2\times {10}^{-17}$ with a measurement uncertainty of $5.5\times {10}^{-17}$, considering both the statistic ($1.9\times {10}^{-17}$) and the systematic ($5.1\times {10}^{-17}$) uncertainties. This is the first performance of a ${}^{40}{\mathrm{Ca}}^{+}$ clock better than that of Cs fountains. A fractional stability of $7\times {10}^{-17}$ in 20 000 s of averaging time is achieved. The evaluation of the two clocks shows that the shift caused by the micromotion in one of the two clocks limits the uncertainty of the comparison. By carefully compensating the micromotion, the absolute frequency of the clock transition is measured to be 411 042 129 776 401.7(1.1) Hz.

The change in the speed of light as it propagates through a moving material has been a subject of study for almost two centuries. This phenomenon, known as the Fresnel light-drag effect, is quite small and usually requires a large interaction path length and/or a large velocity of the moving medium to be observed. Here, we show experimentally that the observed drag effect can be enhanced by over 2 orders of magnitude when the light beam propagates through a moving slow-light medium. Our results are in good agreement with the theoretical prediction, which indicates that, in the limit of large group indices, the strength of the light-drag effect is proportional to the group index of the moving medium.

We measure the fundamental noise processes associated with a continuous linear position measurement of a micromechanical membrane incorporated in a microwave cavity optomechanical circuit. We observe the trade-off between the two fundamental sources of noise that enforce the standard quantum limit: the measurement imprecision and radiation pressure backaction from photon shot noise. We demonstrate that the quantum backaction of the measurement can overwhelm the intrinsic thermal motion by 24 dB, entering a new regime for cavity optomechanical systems.

We report two theoretical discoveries for ${\mathbb{Z}}_2$ topological metals and semimetals. It is shown first that any dimensional ${\mathbb{Z}}_2$ Fermi surface is topologically equivalent to a Fermi point. Then the famous conventional no-go theorem, which was merely proven before for $\mathbb{Z}$ Fermi points in a periodic system without any discrete symmetry, is generalized so that the total topological charge is zero for all cases. Most remarkably, we find and prove an unconventional strong no-go theorem: all ${\mathbb{Z}}_2$ Fermi points have the same topological charge ${\nu }_{{\mathbb{Z}}_2}=1$ or 0 for periodic systems. Moreover, we also establish all six topological types of ${\mathbb{Z}}_2$ models for realistic physical dimensions.

Modulated phases, commensurate or incommensurate with the host crystal lattice, are ubiquitous in solids. The transition between such phases involves formation and rearrangement of domain walls and is generally slow. Using ultrafast electron diffraction, we directly record the photoinduced transformation between a nearly commensurate and an incommensurate charge-density-wave phase in $1T\text{−}{\mathrm{TaS}}_2$. The transformation takes place on the picosecond time scale, orders of magnitude faster than previously observed for commensurate-to-incommensurate transitions. The transition speed and mechanism can be linked to the peculiar nanoscale structure of the photoexcited nearly commensurate phase.

Spherical shock wave implosion in argon is studied both theoretically and experimentally. It is shown that as the strength of the converging shock increases the nonideal gas effects become dominant and govern the evolution of thermal and transport gas properties limiting the shock acceleration, lowering the gas adiabatic index and the achievable energy density at the focus. Accounting for multiple-level ionization, excitation, Coulomb interaction and radiation effects, the limiting equilibrium temperatures to be achieved during the shock implosion are estimated. Focal temperatures of the order of 30 000 K are measured in experiments where converging spherical shock waves are created using a conventional gas-dynamic shock tube facility.

We show by atomistic simulations that, in the thermodynamic limit, the in-plane elastic moduli of graphene at finite temperature vanish with system size $L$ as a power law $L^{-{\eta }_u}$ with ${\eta }_u\simeq 0.325$, in agreement with the membrane theory. We provide explicit expressions for the size and strain dependence of graphene’s elastic moduli, allowing comparison to experimental data. Our results explain the recently experimentally observed increase of the Young modulus by more than a factor of 2 for a tensile strain of only a few per mill. The difference of a factor of 2 between the measured asymptotic value of the Young modulus for tensilely strained systems and the value from ab initio calculations remains, however, unsolved. We also discuss the asymptotic behavior of the Poisson ratio, for which our simulations disagree with the predictions of the self-consistent screening approximation.

We theoretically investigate mechanisms of higher-order harmonic generation in solid-state materials under a high-intensity ac electric field. A new theoretical framework presented in this Letter holds the legitimacy of Bloch’s theorem even under the influence of the high-intensity electric field and provides an exact treatment of the diabatic processes of Bloch electrons. Utilizing this framework, we first discovered that the diabatic processes, namely, ac Zener tunneling and semimetallization of semiconductors, are key factors for nonperturbative mechanisms of HHG. These mechanisms are classified by the field intensity and could be understood by an extended simple man model based on an analogy between tunnel ionization in gaseous media and Zener tunneling in semiconductors. These conclusions would stimulate the universal understanding of HHG mechanisms in both atomic and solid cases.

We investigate the heat conductivity $\kappa$ of the Heisenberg spin-$1/2$ ladder at finite temperature covering the entire range of interchain coupling $J_{\perp }$, by using several numerical methods and perturbation theory within the framework of linear response. We unveil that a perturbative prediction $\kappa \propto J_{\perp }^{-2}$, based on simple golden-rule arguments and valid in the strict limit $J_{\perp }\to 0$, applies to a remarkably wide range of $J_{\perp }$, qualitatively and quantitatively. In the large $J_{\perp }$ limit, we show power-law scaling of opposite nature, namely, $\kappa \propto J_{\perp }^2$. Moreover, we demonstrate the weak and strong coupling regimes to be connected by a broad minimum, slightly below the isotropic point at $J_{\perp }=J_{\parallel }$. Reducing temperature $T$, starting from $T=\infty$, this minimum scales as $\kappa \propto T^{-2}$ down to $T$ on the order of the exchange coupling constant. These results provide for a comprehensive picture of $\kappa (J_{\perp },T)$ of spin ladders.

The emission times of laser-triggered electrons from a sharp tungsten tip are directly characterized under ultrafast, near-infrared laser excitation at Keldysh parameters of $6.6<\gamma <19.1$. Emission delays up to 10 fs are observed, which are inferred from the energy gain of photoelectrons emitted into a synchronously driven microwave cavity. Few femtosecond timing resolution is achieved in a configuration capable of measuring timing shifts up to 55 ps. The technique can also be used to measure the microwave phase inside the cavity with a precision below 70 fs upon the energy resolved detection of a single electron.

Reducing work fluctuation and dissipation in heat engines or, more generally, information heat engines that perform feedback control, is vital to maximize their efficiency. The same problem arises when we attempt to maximize the efficiency of a given thermodynamic task that undergoes nonequilibrium processes for arbitrary initial and final states. We find that the most general trade-off relation between work fluctuation and dissipation applicable to arbitrary nonequilibrium processes is bounded from below by the information distance characterizing how far the system is from thermal equilibrium. The minimum amount of dissipation is found to be given in terms of the relative entropy and the Renyi divergence, both of which quantify the information distance between the state of the system and the canonical distribution. We give an explicit protocol that achieves the fundamental lower bound of the trade-off relation.

Biological and engineered systems operate by coupling function to the transfer of heat and/or particles down a thermal or chemical gradient. In idealized deterministically driven systems, thermodynamic control can be exerted reversibly, with no entropy production, as long as the rate of the protocol is made slow compared to the equilibration time of the system. Here we consider fully realizable, entropically driven systems where the control parameters themselves obey rules that are reversible and that acquire directionality in time solely through dissipation. We show that when such a system moves in a directed way through thermodynamic space, it must produce entropy that is on average larger than its generalized displacement as measured by the Fisher information metric. This distance measure is subextensive but cannot be made small by slowing the rate of the protocol.

We demonstrate an algorithm for the retrieval of a qubit, encoded in spin angular momentum, that has been dropped into a no-firewall black hole. Retrieval is achieved analogously to quantum teleportation by collecting Hawking radiation and performing measurements on the black hole. Importantly, these methods require only the ability to perform measurements from outside the event horizon.

With an integrated luminosity of $2.47{\mathrm{fb}}^{-1}$ recorded by the ATLAS experiment at the LHC, the exclusive decays $B_s^0\to J/\psi \varphi$ and $B_d^0\to J/\psi K^{*0}$ of $B$ mesons produced in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ are used to determine the ratio of fragmentation fractions $f_s/f_d$. From the observed $B_s^0\to J/\psi \varphi$ and $B_d^0\to J/\psi K^{*0}$ yields, the quantity $(f_s/f_d)[\mathcal{B}(B_s^0\to J/\psi \varphi )/\mathcal{B}(B_d^0\to J/\psi K^{*0})]$ is measured to be $0.199\pm 0.004(\text{stat})\pm 0.008(\text{syst})$. Using a recent theory prediction for $[\mathcal{B}(B_s^0\to J/\psi \varphi )/\mathcal{B}(B_d^0\to J/\psi K^{*0})]$ yields $(f_s/f_d)=0.240\pm 0.004(\text{stat})\pm 0.010(\text{syst})\pm 0.017(\mathrm{th})$. This result is based on a new approach that provides a significant improvement of the world average.