We present phase coherence time measurements in quasi-one-dimensional mesoscopic wires made from high mobility two-dimensional electron gas. By implanting gallium ions into a $\mathrm{GaAs}/\mathrm{AlGaAs}$ heterojunction we are able to vary the diffusion coefficient over 2 orders of magnitude. We show that in the diffusive limit, the decoherence time follows a power law as a function of diffusion coefficient as expected by theory. When the disorder is low enough so that the samples are semiballistic, we observe a new and unexpected regime in which the phase coherence time is independent of disorder. In addition, for all samples the temperature dependence of the phase coherence time follows a power law down to the lowest temperatures without any sign of saturation and this strongly suggests that the frequently observed low temperature saturation is not intrinsic.

We show that qubits traveling along closed timelike curves are a resource that a party can exploit to distinguish perfectly any set of quantum states. As a result, an adversary with access to closed timelike curves can break any prepare-and-measure quantum key distribution protocol. Our result also implies that a party with access to closed timelike curves can violate the Holevo bound.

We experimentally demonstrate ultrafast electron diffraction from transiently aligned molecules in the absence of external (aligning) fields. A sample of aligned molecules is generated through photodissociation with femtosecond laser pulses, and the diffraction pattern is captured by probing the sample with picosecond electron pulses shortly after dissociation—before molecular rotation causes the alignment to vanish. In our experiments the alignment decays with a time constant of $2.6\pm 1.2\mathrm{ps}$.

We demonstrate that the third elemental group-IV semiconductor, germanium, exhibits superconductivity at ambient pressure. Using advanced doping and annealing techniques of state-of-the-art semiconductor processing, we have fabricated a highly Ga-doped Ge ($\mathrm{Ge}:\mathrm{Ga}$) layer in near-intrinsic Ge. Depending on the detailed annealing conditions, we demonstrate that superconductivity can be generated and tailored in the doped semiconducting Ge host at temperatures as high as 0.5 K. Critical-field measurements reveal the quasi-two-dimensional character of superconductivity in the $\sim 60\mathrm{nm}$ thick $\mathrm{Ge}:\mathrm{Ga}$ layer. The Cooper-pair density in $\mathrm{Ge}:\mathrm{Ga}$ appears to be exceptionally low.

Based on the observation that the thermodynamic equilibrium free energy of an open quantum system in contact with a thermal environment is the difference between the free energy of the total system and that of the bare environment, the validity of the Crooks theorem and of the Jarzynski equality is extended to open quantum systems. No restrictions on the nature of the environment or on the strength of the coupling between system and environment need to be imposed. This free energy entering the Crooks theorem and the Jarzynski equality is closely related to the Hamiltonian of mean force that generalizes the classical statistical mechanical concept of the potential of mean force.

We argue that in at least a portion of the history of the Universe the relic background neutrinos are spatially extended, coherent superpositions of mass states. We show that an appropriate quantum mechanical treatment affects the neutrino mass values derived from cosmological data. The coherence scale of these neutrino flavor wave packets can be an appreciable fraction of the causal horizon size, raising the possibility of spacetime curvature-induced decoherence.

A search for muon neutrinos from neutralino annihilations in the Sun has been performed with the IceCube 22-string neutrino detector using data collected in 104.3 days of live time in 2007. No excess over the expected atmospheric background has been observed. Upper limits have been obtained on the annihilation rate of captured neutralinos in the Sun and converted to limits on the weakly interacting massive particle (WIMP) proton cross sections for WIMP masses in the range 250–5000 GeV. These results are the most stringent limits to date on neutralino annihilation in the Sun.

A self-corrugated ${\mathrm{Cu}}_3\mathrm{N}\mathrm{\text{−}}\mathrm{Cu}(110)$ molecular network shows the potential to overcome the element dependence barrier as demonstrated by epitaxial growth of atomic nanowires ($\sim 1\mathrm{nm}$ in width) among various $3d$, $4d$, and $5d$ elements. Scanning tunneling microscopy shows that all of the investigated atomic nanowires share an identical structure, featuring uniform width, height, orientation and the same minimum separation distance. Ab initio study reveals that the formation mechanism of atomic nanowires can be directly attributed to a strain relief guided asymmetric occupation of atoms on the originally symmetric crest zone of the corrugated network.

We demonstrate a fiber-optical switch that is activated at tiny energies corresponding to a few hundred optical photons per pulse. This is achieved by simultaneously confining both photons and a small laser-cooled ensemble of atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber and using quantum optical techniques for generating slow light propagation and large nonlinear interaction between light beams.

Stable and rigid carbon atomic chains were experimentally realized by removing carbon atoms row by row from graphene through the controlled energetic electron irradiation inside a transmission electron microscope. The observed structural dynamics of carbon atomic chains such as formation, migration, and breakage were well explained by density-functional theory calculations. The method we reported here is promising to investigate all-carbon-based devices with the carbon atomic chains as the conducting channel, which can be regarded as the ultimate basic component of molecular devices.

We present the first experimental demonstration of focusing ultrasound waves through a flat acoustic metamaterial lens composed of a planar network of subwavelength Helmholtz resonators. We observed a tight focus of half-wavelength in width at 60.5 kHz by imaging a point source. This result is in excellent agreement with the numerical simulation by transmission line model in which we derived the effective mass density and compressibility. This metamaterial lens also displays variable focal length at different frequencies. Our experiment shows the promise of designing compact and lightweight ultrasound imaging elements.

Recent experiments have reported that ferroelectric nanoparticles have drastic effects on nematic liquid crystals—increasing the isotropic-nematic transition temperature by about 5 K, and greatly increasing the sensitivity to applied electric fields. To understand these effects, we develop a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals. This theory predicts the enhancements of liquid-crystal properties, in good agreement with experiments. These predictions apply even when electrostatic interactions are partially screened by moderate concentrations of ions.

The interaction of a $3\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ laser pulse with a metallic wire has been investigated using proton radiography. The pulse is observed to drive the propagation of a highly transient field along the wire at the speed of light. Within a temporal window of 20 ps, the current driven by this field rises to its peak magnitude $\sim {10}^4\mathrm{A}$ before decaying to below measurable levels. Supported by particle-in-cell simulation results and simple theoretical reasoning, the transient field measured is interpreted as a charge-neutralizing disturbance propagated away from the interaction region as a result of the permanent loss of a small fraction of the laser-accelerated hot electron population to vacuum.

Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of the neutron star crust. With multimillion ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Because of the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gravitational wave radiation could limit the spin periods of some stars and might be detectable in large-scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in magnetar giant flares and microflares.

Divalent Eu ($4f^7$, $J=7/2$) possesses a strong local magnetic moment which suppresses superconductivity. Under sufficient pressure it is anticipated that Eu will become trivalent ($4f^6$, $J=0$) and a weak Van Vleck paramagnet, thus opening the door for a possible superconducting state, in analogy with Am metal ($5f^6$, $J=0$) which superconducts at 0.79 K. We present ac susceptibility and electrical resistivity measurements on Eu metal for temperatures 1.5–297 K to pressures as high as 142 GPa. At approximately 80 GPa Eu becomes superconducting at $T_c\simeq 1.8\mathrm{K}$; $T_c$ increases linearly with pressure to 2.75 K at 142 GPa. Eu metal thus becomes the 53rd known elemental superconductor in the periodic table.

It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing, the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation, in that high values of the geometric measure of entanglement preclude states from offering a universal quantum computational speedup. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful $n$-qubit pure states is less than $\exp (-n^2)$. This work highlights a new aspect of the role entanglement plays for quantum computational speedups.

We show the following: a randomly chosen pure state as a resource for measurement-based quantum computation is—with overwhelming probability—of no greater help to a polynomially bounded classical control computer, than a string of random bits. Thus, unlike the familiar “cluster states,” the computing power of a classical control device is not increased from P to BQP (bounded-error, quantum polynomial time), but only to BPP (bounded-error, probabilistic polynomial time). The same holds if the task is to sample from a distribution rather than to perform a bounded-error computation. Furthermore, we show that our results can be extended to states with significantly less entanglement than random states.

We introduce a class of states, called minimally entangled typical thermal states, designed to resemble a typical state of a quantum system at finite temperature with a bias towards classical (minimally entangled) properties. These states reveal in an intuitive way properties such as short-range order which may often be hidden. A finite-$T$ density matrix renormalization group algorithm is presented which is only modestly slower than the $T=0$ density matrix renormalization group.

An optical analogue of a quantum particle bouncing on a hard surface under the influence of gravity (a quantum bouncer) is experimentally demonstrated using a circularly curved optical waveguide. Spatially resolved tunneling optical microscopy measurements of multiple beam reflections at the waveguide edge clearly show the appearance of wave packet collapses and revivals (either integer and fractional), corresponding to the full quantum regime of the quantum bouncer.

We predict a huge interference effect contributing to the conductance through large ultraclean quantum dots of chaotic shape. When a double-dot structure is made such that the dots are the mirror image of each other, constructive interference can make a tunnel barrier located on the symmetry axis effectively transparent. We show (via theoretical analysis and numerical simulation) that this effect can be orders of magnitude larger than the well-known universal conductance fluctuations and weak localization (both less than a conductance quantum). A small magnetic field destroys the effect, massively reducing the double-dot conductance; thus a magnetic field detector is obtained, with a similar sensitivity to a SQUID, but requiring no superconductors.