We show that the annihilation dynamics of excited positronium (Ps) atoms can be controlled using parallel electric and magnetic fields. To achieve this, Ps atoms were optically excited to $n=2$ sublevels in fields that were adjusted to control the amount of short-lived and long-lived character of the resulting mixed states. Inclusion of the former offers a practical approach to detection via annihilation radiation, whereas the increased lifetimes due to the latter can be exploited to optimize resonance-enhanced two-photon excitation processes (e.g., $1{}^{3}S\to 2{}^{3}P\to nS/nD$), either by minimizing losses through intermediate state decay, or by making it possible to separate the excitation laser pulses in time. In addition, photoexcitation of mixed states with a $2{}^3{S}_1$ component represents an efficient route to producing long-lived pure $2{}^3{S}_1$ atoms via single-photon excitation.

We investigate the attractive Fermi polaron problem in two dimensions using nonperturbative Monte Carlo simulations. We introduce a new Monte Carlo algorithm called the impurity lattice Monte Carlo method. This algorithm samples the path integral in a computationally efficient manner and has only small sign oscillations for systems with a single impurity. As a benchmark of the method, we calculate the universal polaron energy in three dimensions in the scale-invariant unitarity limit and find agreement with published results. We then present the first fully nonperturbative calculations of the polaron energy in two dimensions and density correlations between the impurity and majority particles in the limit of zero-range interactions. We find evidence for a smooth crossover transition from fermionic quasiparticle to molecular state as a function of the interaction strength.

We theoretically analyze exciton-photon oscillatory dynamics within a homogenous polariton gas in the presence of energy detuning between the cavity and quantum well modes. Whereas pure Rabi oscillations consist of the particle exchange between the photon and exciton states in the polariton system without any oscillations of the phases of the two subcondensates, we demonstrate that any nonzero detuning results in oscillations of the relative phase of the photon and exciton macroscopic wave functions. Different initial conditions reveal a variety of behaviors of the relative phase between the two condensates, and a crossover from Rabi-like to Josephson-like oscillations is predicted.

In a kagome lattice, the time reversal symmetry can be broken by a staggered magnetic flux emerging from ferromagnetic ordering and intrinsic spin-orbit coupling, leading to several well-separated nontrivial Chern bands and intrinsic quantum anomalous Hall effect. Based on this idea and ab initio calculations, we propose the realization of the intrinsic quantum anomalous Hall effect in the single layer ${\mathrm{Cs}}_2{\mathrm{Mn}}_3{\mathrm{F}}_{12}$ kagome lattice and on the (001) surface of a ${\mathrm{Cs}}_2{\text{LiMn}}_3{\mathrm{F}}_{12}$ single crystal by modifying the carrier coverage on it, where the band gap is around 20 meV. Moreover, a simplified tight binding model based on the in-plane $dd\sigma$ antibonding states is constructed to understand the topological band structures of the system.

Nuclear spins in a spin-blocked quantum dot can be pumped and eventually polarized in either of two opposite directions that are selected by applying two different source-drain voltages. Applying a square pulse train as the source-drain voltage can continuously switch the pumping direction alternately. We propose and demonstrate a critical behavior in the polarization after alternate pumping, where the final polarization is sensitive to the initial polarization and pulse conditions. This sensitivity leads to stochastic behavior in the final polarization under nominally the same pumping conditions.

When liquid crystal elastomers are prepared without any alignment, disordered polydomain structures emerge as the materials are cooled into the nematic phase. These polydomain structures are often attributed to quenched disorder in the cross-linked polymer network. As an alternative explanation, we develop a theory for the dynamics of the isotropic-nematic transition in liquid crystal elastomers, and show that the dynamics can induce a polydomain structure with a characteristic length scale, through a mechanism analogous to the Cahn-Hilliard equation for phase separation.

We examine the fluid-mechanical interactions that occur between arrays of flapping wings when operating in close proximity at a moderate Reynolds number ($\mathrm{Re}\approx 100–1000$). Pairs of flapping wings are oscillated sinusoidally at frequency $f$, amplitude ${\theta }_M$, phase offset $\varphi$, and wing separation distance $D^{*}$, and outflow speed $v^{*}$ is measured. At a fixed separation distance, $v^{*}$ is sensitive to both $f$ and $\varphi$, and we observe both constructive and destructive interference in airspeed. $v^{*}$ is maximized at an optimum phase offset, ${\varphi }_{\max }$, which varies with wing separation distance, $D^{*}$. We propose a model of collective flow interactions between flapping wings based on vortex advection, which reproduces our experimental data.

Recent theoretical and experimental advances have clarified the major effects of knotting on the properties of stretched chains. Yet, how knotted chains respond to weak mechanical stretching and how this behavior differs from the unknotted case are still open questions and we address them here by profiling the complete stretching response of chains of hundreds of monomers and different topology. We find that the ratio of the knotted and unknotted chain extensions varies nonmonotonically with the applied force. This surprising feature is shown to be a signature of the crossover between the well-known high-force stretching regime and the previously uncharacterized low-force one. The observed differences of knotted and unknotted chain response increases with knot complexity and are sufficiently marked that they could be harnessed in single-molecule contexts to infer the presence and complexity of physical knots in micron-long biomolecules.

We consider interacting electrons in a one-dimensional lattice with an incommensurate Aubry-André potential in the regime when the single-particle eigenstates are localized. We rigorously establish the persistence of ground state localization in the presence of weak many-body interaction, for almost all the chemical potentials. The proof uses a quantum many-body extension of methods adopted for the stability of tori of nearly integrable Hamiltonian systems and relies on number-theoretic properties of the potential incommensurate frequency.

Disorder plays a crucial role in spin dynamics in solids and condensed matter systems. We demonstrate that for a spin-orbit coupled Bose-Einstein condensate in a random potential two mechanisms of spin evolution that can be characterized as “precessional” and “anomalous” are at work simultaneously. The precessional mechanism, typical for solids, is due to the condensate displacement. The unconventional anomalous mechanism is due to the spin-dependent velocity producing the distribution of the condensate spin polarization. The condensate expansion is accompanied by a random displacement and fragmentation, where it becomes sparse, as clearly revealed in the spin dynamics. Thus, different stages of the evolution can be characterized by looking at the condensate spin.

Quantum states can be stabilized in the presence of intrinsic and environmental losses by either applying an active feedback condition on an ancillary system or through reservoir engineering. Reservoir engineering maintains a desired quantum state through a combination of drives and designed entropy evacuation. We propose and implement a quantum-reservoir engineering protocol that stabilizes Fock states in a microwave cavity. This protocol is realized with a circuit quantum electrodynamics platform where a Josephson junction provides direct, nonlinear coupling between two superconducting waveguide cavities. The nonlinear coupling results in a single-photon-resolved cross-Kerr effect between the two cavities enabling a photon-number-dependent coupling to a lossy environment. The quantum state of the microwave cavity is discussed in terms of a net polarization and is analyzed by a measurement of its steady state Wigner function.

We have constrained possible new interactions which produce nonrelativistic potentials between polarized neutrons and unpolarized matter proportional to $\alpha \overrightarrow{\sigma }·\overrightarrow{v}$ where $\overrightarrow{\sigma }$ is the neutron spin and $\overrightarrow{v}$ is the relative velocity. We use existing data from laboratory measurements on the very long $T_1$ and $T_2$ spin relaxation times of polarized ${}^3\mathrm{He}$ gas in glass cells. Using the best available measured $T_2$ of polarized ${}^3\mathrm{He}$ gas atoms as the polarized source and the Earth as an unpolarized source, we obtain constraints on two new interactions. We present a new experimental upper bound on possible vector–axial-vector ($V_{\mathrm{VA}}$) type interactions for ranges between 1 and ${10}^8\mathrm{m}$. In combination with previous results, we set the most stringent experiment limits on $g_V{g}_A$ ranging from $\sim \mu \mathrm{m}$ to $\sim {10}^8\mathrm{m}$. We also report what is to our knowledge the first experimental upper limit on the possible torsion fields induced by the Earth on its surface. Dedicated experiments could further improve these bounds by a factor of $\sim 100$. Our method of analysis also makes it possible to probe many velocity dependent interactions which depend on the spins of both neutrons and other particles which have never been searched for before experimentally.

Glassy materials display numerous important properties which relate to the presence and intensity of the secondary ($\beta$) relaxations that dominate the dynamics below the glass transition temperature. However, experimental protocols such as annealing allow little control over the $\beta$ relaxation for most glasses. Here we report on the $\beta$ relaxation of toluene in highly stable glasses prepared by physical vapor deposition. At conditions that generate the highest kinetic stability, about 70% of the $\beta$ relaxation intensity is suppressed, indicating the proximity of this state to the long-sought “ideal glass.” While preparing such a state via deposition takes less than an hour, it would require $\sim 3500$ years of annealing an ordinary glass to obtain similarly suppressed dynamics.

An expanding polariton condensate is investigated under pulsed nonresonant excitation with a small laser pump spot. Far above the condensation threshold we observe a pronounced increase in the dispersion curvature, with a subsequent linearization of the spectrum and strong luminescence from a ghost branch orthogonally polarized with respect to the linearly polarized condensate emission. Polarization of both branches is understood in terms of spin-dependent polariton-polariton scattering. The presence of the ghost branch has been confirmed in time-resolved measurements. The effects of disorder and dissipation in the photoluminescence of polariton condensates and their excitations are discussed.

We performed calculations of electronic, optical, and transport properties of graphene on hexagonal boron nitride with realistic moiré patterns. The latter are produced by structural relaxation using a fully atomistic model. This relaxation turns out to be crucially important for electronic properties. We describe experimentally observed features such as additional Dirac points and the “Hofstadter butterfly” structure of energy levels in a magnetic field. We find that the electronic structure is sensitive to many-body renormalization of the local energy gap.

We argue that, as a consequence of the graviton’s spin-2, its bremsstrahlung in trans-Planckian-energy ($E\gg M_P$) gravitational scattering at small deflection angle can be nicely expressed in terms of helicity-transformation phases and their transfer within the scattering process. The resulting spectrum exhibits deeply sub-Planckian characteristic energies of order $M_P^2/E\ll M_P$ (reminiscent of Hawking radiation), a suppressed fragmentation region, and a reduced rapidity plateau, in broad agreement with recent classical estimates.

We suggest that at least some of the strongly coupled $\mathcal{N}=2$ quantum field theories in 4D can have a nonconformal $\mathcal{N}=1$ Lagrangian description flowing to them at low energies. In particular, we construct such a description for the $\mathcal{N}=2$ rank one superconformal field theory with $E_6$ flavor symmetry, for which a Lagrangian description was previously unavailable. We utilize this description to compute several supersymmetric partition functions.