We present a new method for determining pulse imperfections and improving the single-gate fidelity in a superconducting qubit. By applying consecutive positive and negative $\pi$ pulses, we amplify the qubit evolution due to microwave pulse distortions, which causes the qubit state to rotate around an axis perpendicular to the intended rotation axis. Measuring these rotations as a function of pulse period allows us to reconstruct the shape of the microwave pulse arriving at the sample. Using the extracted response to predistort the input signal, we are able to reduce the average error per gate by 37%, which enables us to reach an average single-qubit gate fidelity higher than 0.998.

We present a physical implementation of a Maxwell demon which consists of a conventional single electron transistor (SET) capacitively coupled to another quantum dot detecting its state. Altogether, the system is described by stochastic thermodynamics. We identify the regime where the energetics of the SET is not affected by the detection, but where its coarse-grained entropy production is shown to contain a new contribution compared to the isolated SET. This additional contribution can be identified as the information flow generated by the “Maxwell demon” feedback in an idealized limit.

The spectrum and morphology of gamma rays from the Galactic center and the spectrum of synchrotron emission observed from the Milky Way’s radio filaments have each been interpreted as possible signals of $\sim 7–10\mathrm{GeV}$ dark matter particles annihilating in the inner Galaxy. In dark matter models capable of producing these signals, the annihilations should also generate significant fluxes of $\sim 7–10\mathrm{GeV}$ positrons which can lead to a distinctive bumplike feature in a local cosmic ray positron spectrum. In this Letter, we show that while such a feature would be difficult to detect with PAMELA, it would likely be identifiable by the currently operating Alpha Magnetic Spectrometer experiment. As no known astrophysical (i.e., nondark matter) sources or mechanisms are likely to produce such a sharp feature, the observation of a positron bump at around 7–10 GeV would significantly strengthen the case for a dark matter interpretation of the reported gamma-ray and radio anomalies.

We present the complete calculation of the top-quark decay width at next-to-next-to-leading order in QCD, including next-to-leading electroweak corrections as well as finite bottom quark mass and $W$ boson width effects. In particular, we also show the first results of the fully differential decay rates for the top-quark semileptonic decay $t\to W^{+}(l^{+}\nu )b$ at next-to-next-to-leading order in QCD. Our method is based on the understanding of the invariant mass distribution of the final-state jet in the singular limit from effective field theory. Our result can be used to study arbitrary infrared-safe observables of top-quark decay with the highest perturbative accuracy.

Using spontaneous parametric down-conversion as a source of correlated photon pairs, correlations are measured between the orbital angular momentum (OAM) in a target beam (which contains an unknown object) and that in an empty reference beam. Unlike previous studies, the effects of the object on off-diagonal elements of the OAM correlation matrix are examined. Because of the presence of the object, terms appear in which the signal and idler OAM do not add up to that of the pump. Using these off-diagonal correlations, the potential for high-efficiency object identification by means of correlated OAM states is experimentally demonstrated for the first time. The higher-dimensional OAM Hilbert space enhances the information capacity of this approach, while the presence of the off-diagonal correlations allows for recognition of specific spatial signatures present in the object. In particular, this allows the detection of discrete rotational symmetries and the efficient evaluation of multiple azimuthal Fourier coefficients using fewer resources than in conventional pixel-by-pixel imaging. This represents a demonstration of sparse sensing using OAM states, as well as being the first correlated OAM experiment to measure properties of a real, stand-alone object, a necessary first step toward correlated OAM-based remote sensing.

Neutrons are unique particles to probe samples in many fields of research ranging from biology to material sciences to engineering and security applications. Access to bright, pulsed sources is currently limited to large accelerator facilities and there has been a growing need for compact sources over the recent years. Short pulse laser driven neutron sources could be a compact and relatively cheap way to produce neutrons with energies in excess of 10 MeV. For more than a decade experiments have tried to obtain neutron numbers sufficient for applications. Our recent experiments demonstrated an ion acceleration mechanism based on the concept of relativistic transparency. Using this new mechanism, we produced an intense beam of high energy (up to 170 MeV) deuterons directed into a Be converter to produce a forward peaked neutron flux with a record yield, on the order of ${10}^{10}\mathrm{n}/\mathrm{sr}$. We present results comparing the two acceleration mechanisms and the first short pulse laser generated neutron radiograph.

We report the sudden and spontaneous evolution of an initially correlated gas of repulsively interacting Rydberg atoms to an ultracold plasma. Under continuous laser coupling we create a Rydberg ensemble in the strong blockade regime, which at longer times undergoes an ionization avalanche. By combining optical imaging and ion detection, we access the full information on the dynamical evolution of the system, including the rapid increase in the number of ions and a sudden depletion of the Rydberg and ground state densities. Rydberg-Rydberg interactions are observed to strongly affect the dynamics of plasma formation. Using a coupled rate-equation model to describe our data, we extract the average energy of electrons trapped in the plasma, and an effective cross section for ionizing collisions between Rydberg atoms and atoms in low-lying states. Our results suggest that the initial correlations of the Rydberg ensemble should persist through the avalanche. This would provide the means to overcome disorder-induced heating, and offer a route to enter new strongly coupled regimes.

The superfluid phases in resonant dipolar Fermi gases are investigated by the standard mean-field theory. In contrast to the crossover from Bose-Einstein condensation (BEC) to Bardeen-Cooper-Schrieffer superfluid in Fermi gases with isotropic interactions, resonant dipolar interaction leads to two completely different BEC phases of the tight-binding Fermi molecules on both sides of the resonance, which are characterized by two order parameters with distinct internal symmetries. We point out that, near the resonances, the two competitive phases can coexist, and an emergent relative phase between the two order parameters spontaneously breaks time-reversal symmetry, which could be observed in momentum resolved rf spectroscopy.

It is known from the solution of the two-body problem that an anisotropic dipolar interaction can give rise to $s$-wave scattering resonances, which are named dipolar interaction induced resonances (DIIR). In this Letter, we study the zero-temperature many-body physics of a two-component Fermi gas across a DIIR. In the low-density regime, it is very striking that the resulting pairing order parameter is a nearly isotropic singlet pairing and the physics can be well described by an $s$-wave resonant interaction potential with finite range conditions, despite the anisotropic nature of the dipolar interaction. The pairing energy is as strong as a unitary Fermi gas near a magnetic Feshbach resonance. In the high-density regime, the anisotropic effect plays an important role. We find phase transitions from singlet pairing to a state with mixed singlet and triplet pairing and then from mixed pairing to pure triplet pairing. The state with mixed pairing spontaneously breaks the time-reversal symmetry.

We consider a phase transition from an antiferromagnetic to a phase separated ground state in a spin-1 Bose-Einstein condensate of ultracold atoms. We demonstrate the occurrence of two scaling laws, for the number of spin domain seeds just after the phase transition, and for the number of spin domains in the final, stable configuration. Only the first scaling can be explained by the standard Kibble-Żurek mechanism. We explain the occurrence of two scaling laws by a model including postselection of spin domains due to the conservation of condensate magnetization.

We report on first-principles calculations of spin-dependent properties in graphene induced by its interaction with a nearby magnetic insulator (europium oxide, EuO). The magnetic proximity effect results in spin polarization of graphene $\pi$ orbitals by up to 24%, together with a large exchange-splitting band gap of about 36 meV. The position of the Dirac cone is further shown to depend strongly on the graphene-EuO interlayer. These findings point toward the possible engineering of spin gating by the proximity effect at a relatively high temperature, which stands as a hallmark for future all-spin information processing technologies.

We report measurements of the Kondo effect in a double quantum dot, where the orbital states act as pseudospin states whose degeneracy contributes to Kondo screening. Standard transport spectroscopy as a function of the bias voltage on both dots shows a zero-bias peak in conductance, analogous to that observed for spin Kondo in single dots. Breaking the orbital degeneracy splits the Kondo resonance in the tunneling density of states above and below the Fermi energy of the leads, with the resonances having different pseudospin character. Using pseudospin-resolved spectroscopy, we demonstrate the pseudospin character by observing a Kondo peak at only one sign of the bias voltage. We show that even when the pseudospin states have very different tunnel rates to the leads, a Kondo temperature can be consistently defined for the double quantum dot system.

We experimentally investigate the charge (isospin) frustration induced by a geometrical symmetry in a triangular triple quantum dot. We observe the ground-state charge configurations of sixfold degeneracy, the manifestation of the frustration. The frustration results in omnidirectional charge transport, and it is accompanied by nearby nontrivial triple degenerate states in the charge stability diagram. The findings agree with a capacitive interaction model. We also observe unusual transport by the frustration, which might be related to elastic cotunneling and the interference of trajectories through the dot. This work demonstrates a unique way of studying geometrical frustration in a controllable way.

We present density functional and many body perturbation theory calculations of the electronic, optical, and impact ionization properties of Si nanoparticles (NPs) with core structures based on high-pressure bulk Si phases. Si particles with a BC8 core structure exhibit significantly lower optical gaps and multiple exciton generation (MEG) thresholds, and an order of magnitude higher MEG rate than diamondlike ones of the same size. Several mechanisms are discussed to further reduce the gap, including surface reconstruction and chemistry, excitonic effects, and embedding pressure. Experiments reported the formation of BC8 NPs embedded in amorphous Si and in amorphous regions of femtosecond-laser doped “black silicon.” For all these reasons, BC8 nanoparticles may be promising candidates for MEG-based solar energy conversion.

We report an Fe $K\beta$ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide ${\mathrm{Ca}}_{1-x}{\mathrm{RE}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($\mathrm{RE}=\mathrm{La}$, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from $\sim 0.9{\mu }_B$ at $T=300\mathrm{K}$ to $\sim 0.45{\mu }_B$ at $T=70\mathrm{K}$. In the collapsed tetragonal phase of Nd- and Pr-doped samples ($T<70\mathrm{K}$) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that ${\mathrm{Ca}}_{1-x}{\mathrm{RE}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($\mathrm{RE}=\mathrm{Pr}$ and Nd) exhibits a spin-state transition and provide direct evidence for a nonmagnetic ${\mathrm{Fe}}^{2+}$ ion in the collapsed tetragonal phase; spin state as predicted by Yildirim. We argue that the gradual change of the spin state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe $3d$ orbitals and the Hund’s rule coupling.

We demonstrate that the exact nonequilibrium steady state of the one-dimensional Heisenberg $XXZ$ spin chain driven by boundary Lindblad operators can be constructed explicitly with a matrix product ansatz for the nonequilibrium density matrix where the matrices satisfy a quadratic algebra. This algebra turns out to be related to the quantum algebra $U_q[\mathrm{SU}(2)]$. Coherent state techniques are introduced for the exact solution of the isotropic Heisenberg chain with and without quantum boundary fields and Lindblad terms that correspond to two different completely polarized boundary states. We show that this boundary twist leads to nonvanishing stationary currents of all spin components. Our results suggest that the matrix product ansatz can be extended to more general quantum systems kept far from equilibrium by Lindblad boundary terms.

Illumination with laser sources leads to the creation of excited electronic states of particular symmetries, which can drive isosymmetric vibrations. Here, we use a combination of ultrafast stimulated and cw spontaneous Raman scattering to determine the lifetime of $A_{1g}$ and $E_g$ electronic coherences in Bi and Sb. Our results both shed new light on the mechanisms of coherent phonon generation and represent a novel way to probe extremely fast electron decoherence rates. The $E_g$ state, resulting from an unequal distribution of carriers in three equivalent band regions, is extremely short lived. Consistent with theory, the lifetime of its associated driving force reaches values as small as 2 (6) fs for Bi (Sb) at 300 K.

The remnant polarization of weakly coupled ferroelectric-dielectric superlattices is distributed unequally between the component layers, and as a result the components respond differently to applied electric fields. The difference is apparent in both the nanometer-scale structure of striped polarization domains and in the development of piezoelectric strain and field-induced polarization. Both effects are probed with in situ time-resolved synchrotron x-ray diffraction in a ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ superlattice in fields up to $2.38\mathrm{MV}/\mathrm{cm}$. Domains are initially distorted to increase the polarization in the ${\mathrm{SrTiO}}_3$ layer while retaining the striped motif. The subsequent transformation to a uniform polarization state at a later time leads to piezoelectric expansion dominated by the field-induced polarization of the ${\mathrm{SrTiO}}_3$ layers. The results are consistent with theoretical predictions of the field dependence of the domain structure and electrical polarization.

A two-dimensional (2D) solid lacks long-range positional order and is diffusive by means of the cooperative motion of particles. We find from molecular dynamics simulations of hard discs that 2D colloids in solid and hexatic phases show seemingly Fickian but strongly heterogeneous dynamics. Beyond translational relaxation time, the mean-square displacement is linear with time, $t$, implying that discs would undergo Brownian diffusion and the self-part of the van Hove correlation function [$G_s(r,t)$] might be Gaussian. But dynamics is still heterogeneous and $G_s(r,t)$ is exponential at large $r$ and oscillatory with multiple peaks at intermediate length. We attribute the existence of several such peaks to the observation that there are several clusters of discs with discretized mobility. The cluster of marginally mobile discs grows with time and begins to percolate around translational relaxation time while clusters of fast discs emerge in the middle of the marginally mobile cluster.

Biorecognition leads to the formation of a specific complex between a couple of biological partners to accomplish a functional task. Its occurrence can be inferred a posteriori by analyzing the unbinding force curves in a dynamic force spectroscopy experiment. Because of nonspecific interactions, the method is not, however, exempt from ambiguities and subjectivity. A fingerprint of the partner recruitment in the complex has been disclosed in the fluctuations of the atomic force microscopy cantilever. We demonstrate that the formation of the biotin-avidin specific complex strongly correlates with a $1/f^{\alpha }$ noise in the force curve fluctuations.