We show how the interference between spatially separated states of the center of mass (c.m.) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey interferometry). We propose to use an optically levitated diamond bead containing a nitrogen-vacancy center spin. The nanoscale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the c.m. state is required and thereby one dispenses with cavities, spatially resolved detection, and low-mass-dispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nanocrystal being in spatially distinct locations.

Noise is the main obstacle for the realization of fault-tolerant quantum information processing and secure communication over long distances. In this work, we propose a communication protocol relying on simple linear optics that optimally protects quantum states from non-Markovian or correlated noise. We implement the protocol experimentally and demonstrate the near-ideal protection of coherent and entangled states in an extremely noisy channel. Since all real-life channels are exhibiting pronounced non-Markovian behavior, the proposed protocol will have immediate implications in improving the performance of various quantum information protocols.

We establish a general mechanism for highly efficient quantum transport through finite, disordered 3D networks. It relies on the interplay of disorder with centrosymmetry and a dominant doublet spectral structure and can be controlled by the proper tuning of only coarse-grained quantities. Photosynthetic light harvesting complexes are discussed as potential biological incarnations of this design principle.

We numerically simulate planar shock wave collisions in anti–de Sitter space as a model for heavy ion collisions of large nuclei. We uncover a crossover between two different dynamical regimes as a function of the collision energy. At low energies the shocks first stop and then explode in a manner approximately described by hydrodynamics, in close similarity with the Landau model. At high energies the receding fragments move outwards at the speed of light, with a region of negative energy density and negative longitudinal pressure trailing behind them. The rapidity distribution of the energy density at late times around midrapidity is not approximately boost invariant but Gaussian, albeit with a width that increases with the collision energy.

We demonstrate the feasibility of levitating a small mirror using only radiation pressure. In our scheme, the mirror is supported by a tripod where each leg of the tripod is a Fabry-Perot cavity. The macroscopic state of the mirror is coherently coupled to the supporting cavity modes allowing coherent interrogation and manipulation of the mirror motion. The proposed scheme is an extreme example of the optical spring, where a mechanical oscillator is isolated from the environment and its mechanical frequency and macroscopic state can be manipulated solely through optical fields. We model the stability of the system and find a three-dimensional lattice of trapping points where cavity resonances allow for buildup of optical field sufficient to support the weight of the mirror. Our scheme offers a unique platform for studying quantum and classical optomechanics and can potentially be used for precision gravitational field sensing and quantum state generation.

We present investigations of the afterglow of oxygen-helium gas mixtures at cryogenic temperatures. The cooling of a helium jet containing trace amounts of oxygen after passing through a radio frequency discharge zone led to the observation of strong emissions from atomic oxygen. The effect results from the increasing efficiency of energy transfer from metastable helium atoms and molecules to oxygen impurities in the cold dense helium vapor. This effect might find an application for the detection of small quantities of the impurities in helium gas.

Collision-induced rotational angular momentum orientation is a fundamental property of molecular scattering, which is sensitive to the balance between attractive and repulsive forces at play during collision. Here, we quantify a new mechanism leading to orientation, which is purely quantum mechanical in origin. Although the new mechanism is quite general, and will operate more widely in atomic and molecular scattering, it is observed here for impulsive hard shell collisions, for which the orientation vanishes classically. The quantum mechanism can thus be studied in isolation from other processes. The orientation is proposed to originate from the nonlocal nature of the quantum mechanical collision encounter.

We introduce a simple protocol for adaptive quantum state tomography, which reduces the worst-case infidelity [$1-F(\widehat{\rho },\rho )$] between the estimate and the true state from $O(1/\sqrt{N})$ to $O(1/N)$. It uses a single adaptation step and just one extra measurement setting. In a linear optical qubit experiment, we demonstrate a full order of magnitude reduction in infidelity (from 0.1% to 0.01%) for a modest number of samples ($N\approx 3\times {10}^4$).

A major technological challenge in building a muon cooling channel is operating rf cavities in multitesla external magnetic fields. We report the first proof-of-principle experiment of a high pressure gas-filled rf cavity for use with intense ionizing beams and strong external magnetic fields. rf power consumption by beam-induced plasma is investigated with hydrogen and deuterium gases with pressures between 20 and 100 atm and peak rf gradients between 5 and $50\mathrm{MV}/\mathrm{m}$. The low pressure case agrees well with an analytical model based on electron and ion mobilities. Varying concentrations of oxygen gas are investigated to remove free electrons from the cavity and reduce the rf power consumption. Measurements of the electron attachment time to oxygen and rate of ion-ion recombination are also made. Additionally, we demonstrate the operation of the gas-filled rf cavity in a solenoidal field of up to 3 T, finding no major magnetic field dependence. All these results indicate that a high pressure gas-filled cavity is a viable technology for muon ionization cooling.

We study the elementary processes behind one of the pioneering works on scanning tunneling microscope controlled reactions of single molecules [Stipe et al., Phys. Rev. Lett. 81, 1263 (1998)]. Using the Keldysh-Green function approach for the vibrational generation rate in combination with density functional theory calculations to obtain realistic parameters we reproduce the experimental rotation rate of an acetylene molecule on a Cu(100) surface as a function of bias voltage and tunneling current. This combined approach allows us to identify the reaction coordinate mode of the acetylene rotation and its anharmonic coupling with the C-H stretch mode. We show that three different elementary processes, the excitation of C-H stretch, the overtone ladder climbing of the hindered rotational mode, and the combination band excitation together explain the rotation of the acetylene molecule on Cu(100).

We propose a nanoscale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators.

We advance all optical spin noise spectroscopy (SNS) in semiconductors to detection bandwidths of several hundred gigahertz by employing a sophisticated scheme of pulse trains from ultrafast laser oscillators as an optical probe. The ultrafast SNS technique avoids the need for optical pumping and enables nearly perturbation free measurements of extremely short spin dephasing times. We apply the technique to highly-$n$-doped bulk GaAs where magnetic field dependent measurements show unexpected large $g$-factor fluctuations. Calculations suggest that such large $g$-factor fluctuations do not necessarily result from extrinsic sample variations but are intrinsically present in every doped semiconductor due to the stochastic nature of the dopant distribution.

An analytical expression for the diffusion current in organic metal-insulator-metal diodes is derived. The derivation is based on the classical diffusion theory of Schottky, with adaptations to account for the absence of doping, a built-in voltage due to asymmetric contacts, and band bending at the Ohmic contact. The commonly observed deviation of the ideality factor from unity ($\sim 1.2$) is characteristic of diffusion-limited currents in undoped organic semiconductors. Summing with the classical space-charge limited current provides a full analytic description of the current as a function of voltage, temperature and layer thickness.

We consider dc-electronic transport through a nanowire suspended between two normal-metal leads in the presence of an external magnetic field. We show the very mechanism through which such a system, whose stationary current-voltage characteristic is essentially characterized by positive differential conductance, becomes unstable with respect to an onset of self-excited oscillations in electrical transport and mechanical vibrations. The self-excitation mechanism is based on the correlation between the occupancy of the quantized spin-split electronic energy levels inside the nanowire and the velocity of the nanowire with the crucial influence of strong enough retardation effects in magnetomotive coupling coming from mechanical vibrations.

Comparing high-resolution specific heat and thermal expansion measurements to exact finite-size diagonalization, we demonstrate that ${\mathrm{Cs}}_2{\mathrm{CoCl}}_4$ for a magnetic field along the crystallographic $b$ axis realizes the spin-$\frac{1}{2}$ $XXZ$ chain in a transverse field. Exploiting both thermal as well as virtual excitations of higher crystal-field states, we find that the spin chain is in the $XY$ limit with an anisotropy $J_z/J_{\perp }\approx 0.12$, substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of ${\mu }_0{H}_b^{\mathrm{cr}}\approx 2\mathrm{T}$ before around 3.5 T the description in terms of an effective spin-$\frac{1}{2}$ chain becomes inapplicable.

We report evidence of a photoinduced coupling between two spins provided by Mn dopants in their neutral acceptor state $A^0$ in a single $\mathrm{InAs}/\mathrm{GaAs}$ quantum dot. The coupling occurs due to simultaneous exchange interactions between each of the two dopant spins and a photocreated hole. Microphotoluminescence spectroscopy achieved both in longitudinal and perpendicular magnetic fields reveals the splitting of the four spin configurations $|J_1=\pm 1,J_2=\pm 1⟩$ due to the $2A^0$-hole exchange interaction. We obtain a comprehensive interpretation of the experimental data with a simplified spin Hamiltonian model, which more specifically shows that the hole-mediated coupling is similar to a ${\epsilon }_{12}\simeq -70\mu \mathrm{eV}$ exchange interaction between both $A^0$ spins.

Narrow bandwidth is a fundamental issue plaguing practical applications of metamaterial absorbers. In this Letter, we show that by deliberately controlling the dispersion and dissipation of a metamaterial, an ultrawideband perfect metamaterial absorber with complex-valued constitutive parameters strictly satisfying the modified model of a perfectly matched layer, can be achieved. The nearly perfect power absorption, better than 99%, was experimentally observed in an unprecedented bandwidth of 39%, approaching the theoretical Rozanov limit. We expect a wide range of applications to emerge from this general concept.

We report a new photoinduced transition from a nonordered phase to a transient ordered phase with symmetry breaking in an organic charge-transfer compound, dimethyltetrathiafulvalene (DMTTF)-dibromodichloro-$p$-benzoquinone ($2,6Q{\mathrm{Br}}_2{\mathrm{Cl}}_2$), which is a neutral compound located near the neutral-ionic phase boundary and shows quantum paraelectricity at low temperatures. By an irradiation of a femtosecond laser pulse, an ionic domain consisting of $\sim 40$ molecules is introduced into the neutral lattice per photon, giving rise to coherent molecular oscillations with fractional charge modulations over $\sim 400$ molecules. This response is due to the recovery of ferroelectric nature from the quantum paraelectricity by a photoinjection of an ionic domain with a large dipole moment.

Assembling a freestanding, sharp-edged slab of homogeneous material that is much denser than gas, but much more rarefied than a solid, is an outstanding technological challenge. The solution may lie in focusing a dense aerosol to assume this geometry. However, whereas the geometrical optics of dilute aerosols is a well-developed field, the dense aerosol limit is mostly unexplored. Yet controlling the geometrical optics of dense aerosols is necessary in preparing such a material slab. Focusing dense aerosols is shown here to be possible, but the finite particle density reduces the effective Stokes number of the flow, a critical result for controlled focusing.

Interactions among units in complex systems occur in a specific sequential order, thus affecting the flow of information, the propagation of diseases, and general dynamical processes. We investigate the Laplacian spectrum of temporal networks and compare it with that of the corresponding aggregate network. First, we show that the spectrum of the ensemble average of a temporal network has identical eigenmodes but smaller eigenvalues than the aggregate networks. In large networks without edge condensation, the expected temporal dynamics is a time-rescaled version of the aggregate dynamics. Even for single sequential realizations, diffusive dynamics is slower in temporal networks. These discrepancies are due to the noncommutability of interactions. We illustrate our analytical findings using a simple temporal motif, larger network models, and real temporal networks.