For numerous applications of quantum theory it is desirable to be able to apply arbitrary unitary operations on a given quantum system. However, in particular situations only a subset of unitary operations is easily accessible. This raises the question of what additional unitary gates should be added to a given gate set in order to attain physical universality, i.e., to be able to perform arbitrary unitary transformation on the relevant Hilbert space. In this work, we study this problem for three paradigmatic cases of naturally occurring restricted gate sets: (A) particle-number preserving bosonic linear optics, (B) particle-number preserving fermionic linear optics, and (C) general (not necessarily particle-number preserving) fermionic linear optics. Using tools from group theory and control theory, we classify, in each of these scenarios, what sets of gates are generated, if an additional gate is added to the set of allowed transformations. This allows us to solve the universality problem completely for arbitrary number of particles and for arbitrary dimensions of the single-particle Hilbert space.

To investigate the performance of quantum information tasks on networks whose topology changes in time, we study the spatial search algorithm by continuous time quantum walk to find a marked node on a random temporal network. We consider a network of $n$ nodes constituted by a time-ordered sequence of Erdös-Rényi random graphs $G(n,p)$, where $p$ is the probability that any two given nodes are connected: After every time interval $\tau$, a new graph $G(n,p)$ replaces the previous one. We prove analytically that, for any given $p$, there is always a range of values of $\tau$ for which the running time of the algorithm is optimal, i.e., $\mathcal{O}(\sqrt{n})$, even when search on the individual static graphs constituting the temporal network is suboptimal. On the other hand, there are regimes of $\tau$ where the algorithm is suboptimal even when each of the underlying static graphs are sufficiently connected to perform optimal search on them. From this first study of quantum spatial search on a time-dependent network, it emerges that the nontrivial interplay between temporality and connectivity is key to the algorithmic performance. Moreover, our work can be extended to establish high-fidelity qubit transfer between any two nodes of the network. Overall, our findings show that one can exploit temporality to achieve optimal quantum information tasks on dynamical random networks.

We present a Monte Carlo method for computing the renormalized coupling constants and the critical exponents within renormalization theory. The scheme, which derives from a variational principle, overcomes critical slowing down, by means of a bias potential that renders the coarse grained variables uncorrelated. The two-dimensional Ising model is used to illustrate the method.

We study the statistical properties of an ensemble of weak gravitational waves interacting nonlinearly in a flat space-time. We show that the resonant three-wave interactions are absent and develop a theory for four-wave interactions in the reduced case of a $2.5+1$ diagonal metric tensor. In this limit, where only plus-polarized gravitational waves are present, we derive the interaction Hamiltonian and consider the asymptotic regime of weak gravitational wave turbulence. Both direct and inverse cascades are found for the energy and the wave action, respectively, and the corresponding wave spectra are derived. The inverse cascade is characterized by a finite-time propagation of the metric excitations—a process similar to an explosive nonequilibrium Bose–Einstein condensation, which provides an efficient mechanism to ironing out small-scale inhomogeneities. The direct cascade leads to an accumulation of the radiation energy in the system. These processes might be important for understanding the early Universe where a background of weak nonlinear gravitational waves is expected.

The decays ${\chi }_{c1}\to J/\psi {\mu }^{+}{\mu }^{-}$ and ${\chi }_{c2}\to J/\psi {\mu }^{+}{\mu }^{-}$ are observed and used to study the resonance parameters of the ${\chi }_{c1}$ and ${\chi }_{c2}$ mesons. The masses of these states are measured to be $m({\chi }_{c1})=3510.71\pm 0.04(\mathrm{stat})\pm 0.09(\mathrm{syst})\mathrm{MeV}$ and $m({\chi }_{c2})=3556.10\pm 0.06(\mathrm{stat})\pm 0.11(\mathrm{syst})\mathrm{MeV}$, where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference $m({\chi }_{c2})-m({\chi }_{c1})=45.39\pm 0.07(\mathrm{stat})\pm 0.03(\mathrm{syst})\mathrm{MeV}$. The natural width of the ${\chi }_{c2}$ meson is measured to be $\mathrm{\Gamma }({\chi }_{c2})=2.10\pm 0.20(\mathrm{stat})\pm 0.02(\mathrm{syst})\mathrm{MeV}$. These results are in good agreement with and have comparable precision to the current world averages.

The atomic numbers and the masses of fragments formed in quasifission reactions are simultaneously measured at scission in ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ reactions at a laboratory energy of 286 MeV. The atomic numbers are determined from measured characteristic fluorescence x rays, whereas the masses are obtained from the emission angles and times of flight of the two emerging fragments. For the first time, thanks to this full identification of the quasifission fragments on a broad angular range, the important role of the proton shell closure at $Z=82$ is evidenced by the associated maximum production yield, a maximum predicted by time-dependent Hartree-Fock calculations. This new experimental approach gives now access to precise studies of the time dependence of the $N/Z$ (neutron over proton ratios of the fragments) evolution in quasifission reactions.

The dissociation of acids in aqueous environments at low temperatures in the presence of a limited amount of water is underlying a wealth of processes from atmospheric to interstellar science. For the paradigmatic case of $\mathrm{HCl}({\mathrm{H}}_2\mathrm{O}{)}_n$ clusters, our extensive ab initio path integral simulations quantify in terms of free energy differences and barriers that $n=4$ water molecules are indeed required to dissociate HCl at low temperatures. Increasing the temperature, however, reverses the process and thus counteracts dissociation by fluctuation-driven recombination. The size of the electric dipole moment is shown to not correlate with the acid being in its dissociated or molecular state, thus rendering its measurement as a function of $n$ unable to detect the dissociation transition.

We theoretically investigate charge transport through electronic bands of a mesoscopic one-dimensional system, where interband transitions are coupled to a confined cavity mode, initially prepared close to its vacuum. This coupling leads to light-matter hybridization where the dressed fermionic bands interact via absorption and emission of dressed cavity photons. Using a self-consistent nonequilibrium Green’s function method, we compute electronic transmissions and cavity photon spectra and demonstrate how light-matter coupling can lead to an enhancement of charge conductivity in the steady state. We find that depending on cavity loss rate, electronic bandwidth, and coupling strength, the dynamics involves either an individual or a collective response of Bloch states, and we explain how this affects the current enhancement. We show that the charge conductivity enhancement can reach orders of magnitudes under experimentally relevant conditions.

A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. We show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in ${\mathrm{Fe}}_{1.08}\mathrm{Te}$. Around 2 GPa, the orthorhombic and incommensurate antiferromagnetic ground state of ${\mathrm{Fe}}_{1.08}\mathrm{Te}$ is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a “type-0” transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of ${\mathrm{Fe}}_{1.08}\mathrm{Te}$ is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.

We report measurements of optical absorption in the zigzag antiferromagnet $\alpha \text{−}{\mathrm{RuCl}}_3$ as a function of temperature $T$, magnetic field $B$, and photon energy $\hslash \omega$ in the range $\sim 0.3$–8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at $B=0$ that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of $B$, the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

We report on terahertz spectroscopy of quantum spin dynamics in $\alpha \text{−}{\mathrm{RuCl}}_3$, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at $T_{s2}=62\mathrm{K}$. With the onset of a long-range magnetic order at $T_N=6.5\mathrm{K}$, spectral weight is transferred to a well-defined magnetic excitation at $\hslash {\omega }_1=2.48\mathrm{meV}$, which is accompanied by a higher-energy band at $\hslash {\omega }_2=6.48\mathrm{meV}$. Both excitations soften in a magnetic field, signaling a quantum phase transition close to $B_c=7\mathrm{T}$, where a broad continuum dominates the dynamical response. Above $B_c$, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

The surface relaxation dynamics of supported star-shaped polymer thin films are shown to be slower than the bulk, persisting up to temperatures at least 50 K above the bulk glass transition temperature $T_g^{\mathrm{bulk}}$. This behavior, exhibited by star-shaped polystyrenes with functionality $f=8$ arms and molecular weights per arm $M_{\mathrm{arm}}<M_e$ ($M_e$ is the entanglement molecular weight), is shown by molecular dynamics simulations to be associated with a preferential localization of these macromolecules at the free surface. This new phenomenon is in notable contrast to that of linear-chain polymer thin film systems, where the surface relaxations are enhanced in relation to the bulk; this enhancement persists only for a limited temperature range above the bulk $T_g^{\mathrm{bulk}}$. Evidence of the slow surface dynamics, compared to the bulk, for temperatures well above $T_g$ and at length and time scales not associated with the glass transition has not previously been reported for polymers.

Fission-fragment mass distributions were measured for ${}^{237–240}\mathrm{U}$, ${}^{239–242}\mathrm{Np}$, and ${}^{241–244}\mathrm{Pu}$ populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction ${}^{18}\mathrm{O}+{}^{238}\mathrm{U}$ at the Japan Atomic Energy Agency tandem facility. Among them, the data for ${}^{240}\mathrm{U}$ and ${}^{240,241,242}\mathrm{Np}$ were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission.