Search Results (235 total)

We propose a class of schemes for robust population transfer between quantum states that utilize trains of coherent pulses, thus forming a generalized adiabatic passage via a wave packet. We study piecewise stimulated Raman adiabatic passage with pulse-to-pulse amplitude variation, and piecewise chirped Raman passage with pulse-to-pulse phase variation, implemented with an optical frequency comb. In the context of production of ultracold ground-state molecules, we show that with almost no knowledge of the excited potential, robust high-efficiency transfer is possible.

We present experiments demonstrating high-resolution and wide-bandwidth coherent control of a four-level atomic system in a diamond configuration. A femtosecond frequency comb is used to excite a specific pair of two-photon transitions in cold ⁸⁷Rb. The optical-phase-sensitive response of the closed-loop diamond system is studied by controlling the phase of the comb modes with a pulse shaper. Finally, the pulse shape is optimized resulting in a 256% increase in the two-photon transition rate by forcing constructive interference between the mode pairs detuned from an intermediate resonance.

The ¹S₀-³P₀ clock transition frequency ν_Sr in neutral ⁸⁷Sr has been measured relative to the Cs standard by three independent laboratories in Boulder, Paris, and Tokyo over the last three years. The agreement on the 1×10^-15 level makes ν_Sr the best agreed-upon optical atomic frequency. We combine periodic variations in the ⁸⁷Sr clock frequency with ¹⁹⁹Hg⁺ and H-maser data to test local position invariance by obtaining the strongest limits to date on gravitational-coupling coefficients for the fine-structure constant α, electron-proton mass ratio μ, and light quark mass. Furthermore, after ¹⁹⁹Hg⁺, ¹⁷¹Yb⁺, and H, we add ⁸⁷Sr as the fourth optical atomic clock species to enhance constraints on yearly drifts of α and μ.

Using a Feshbach resonance, we create ultracold fermionic molecules starting from a Bose-Fermi atom gas mixture. The resulting mixture of atoms and weakly bound molecules provides a rich system for studying few-body collisions because of the variety of atomic collision partners for molecules; either bosonic, fermionic, or distinguishable atoms. Inelastic loss of the molecules near the Feshbach resonance is dramatically affected by the quantum statistics of the colliding particles and the scattering length. In particular, we observe a molecule lifetime as long as 100 ms near the Feshbach resonance.

The magnitude and spectral response of second-harmonic generation (SHG) from 0.4  nm single-walled carbon nanotubes (CNTs) in the channels of AlPO₄-5 zeolite is reported. The second harmonics (SH) was found to be polarized perpendicular to the tube axis and maximized by an excitation polarization at 45° to the tube axis. A SH resonance peak at 2  eV was observed, which corresponds to the lowest-energy excitonic state in chiral (4,2) CNTs. The second-order optical susceptibility χ⁽²⁾ of the system was determined to be 10⁻⁶  esu, which agrees with the large χ⁽²⁾ predicted for small diameter CNTs. These experimental results suggest that SHG can be used to characterize the symmetry and chirality of CNTs.

Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple-repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling of particles without closed transitions, identify conditions necessary to achieve efficient cooling, and suggest solutions given experimental constraints. Specifically, it is shown that cooperativities much greater than unity are required for cooling without loss, and that this could be achieved via the superradiant scattering associated with intracavity self-localization of the molecules. Particular emphasis is given to the polar hydroxyl radical (OH), cold samples of which are readily obtained from Stark deceleration.

We report on the coupled plasmon resonances in a monolayer consisting of metal or metallodielectric nanoparticles with the dipole and quadrupole single-particle resonances. The theoretical models included spherical gold and silver particles and also gold and silver nanoshells on silica and polystyrene cores forming two dimensional random clusters or square-lattice arrays on a dielectric substrate (glass in water). The parameters of the individual particles were chosen so that a quadrupole plasmon resonance could be observed along with the dipole-scattering band. By using an exact multipole cluster-on-a-substrate solution, we showed that particle-substrate coupling can be neglected in the calculation of the monolayer-extinction spectra, at least for the glass-in-water configuration. When the surface particle density in the monolayer was increased, the dipole resonance became suppressed and the spectrum for the cooperative system was determined only by the quadrupole plasmon. The dependence of this effect on the single-particle parameters and on the cluster structure was examined in detail. In particular, the selective suppression of the long-wavelength extinction band was shown to arise from the cooperative suppression of the dipole-scattering mode, whereas the short-wavelength absorption spectrum for the monolayer was shown to be little different from the single-particle spectrum. For experimental studies, the silica/gold-nanoshell monolayers were fabricated by the deposition of nanoshells on a glass substrate functionalized by silane-thiol cross-linkers. The measured single-particle and monolayer-extinction spectra are in reasonable agreement with simulations based on the nanoshell geometrical parameters (scanning electron microscopy data). Finally, we evaluated the sensitivity of the coupled quadrupole resonance to the dielectric environment to show a universal linear relation between the relative shift in the coupled-quadrupole-resonance wavelength and the relative increment in the environment refractive index.

We propose a precision measurement of time variations of the proton-electron mass ratio using ultracold molecules in an optical lattice. Vibrational energy intervals are sensitive to changes of the mass ratio. In contrast to measurements that use hyperfine-interval-based atomic clocks, the scheme discussed here is model independent and does not require separation of time variations of different physical constants. The possibility of applying the zero-differential–Stark-shift optical lattice technique is explored to measure vibrational transitions at high accuracy.

Starting from the Ginzburg-Landau free energy describing the normal state to Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state transition, we evaluate the free energy of seven most common lattice structures such as stripe, square, triangular, simple cubic, face centered cubic, body centered cubic, and quasicrystal. We find that the stripe phase, which is the original LO state, is the most stable phase. This result may be relevant to the detection of FFLO state in some heavy fermion compounds and the pairing lattice structure of fermions with unequal populations in the BCS side of Feshbach resonance in ultracold atoms.

The phase coherence of an ultrastable optical frequency reference is fully maintained over actively stabilized fiber networks of lengths exceeding 30 km. For a 7-km link installed in an urban environment, the transfer instability is 6×10^-18 at 1 s. The excess phase noise of 0.15 rad, integrated from 8 mHz to 25 MHz, yields a total timing jitter of 0.085 fs. A 32-km link achieves similar performance. Using frequency combs at each end of the coherent-transfer fiber link, a heterodyne beat between two independent ultrastable lasers, separated by 3.5 km and 163 THz, achieves a 1-Hz linewidth.

The photocatalysts Ba₃NiNb₂O₉ and Ba₃NiTa₂O₉ with the triple perovskite crystal structure were synthesized by using solid-state-reaction method. They were characterized by x-ray diffraction and UV-visible reflectance spectroscopy. Both of the photocatalysts absorb UV and visible light photons obviously, which can be attributed to their special crystal and electronic structures. The valence band of Ba₃NiNb₂O₉ was determined by ultraviolet photoelectron spectroscopy. Capacitance-voltage (C-V) measurements of Ba₃NiNb₂O₉ film as deposited by using a pulsed laser deposition technique suggested that the lower conduction band should be composed of the Ni  e_g states. The band structures were proposed through the theoretical calculation of the electronic structures by using density functional theory with the local spin-density approximation. The hybridized O  2p and Ni  t_2g states in the photocatalysts Ba₃NiNb₂O₉ and Ba₃NiTa₂O₉ act as the new valence band instead of the O  2p orbital, and the Ni  e_g states act as the lower conduction band in Ba₃NiNb₂O₉ and Ba₃NiTa₂O₉.

We present a detailed experimental and theoretical study of the effect of nuclear spin on the performance of optical lattice clocks. With a state-mixing theory including spin-orbit and hyperfine interactions, we describe the origin of the ¹S₀-³P₀ clock transition and the differential g factor between the two clock states for alkaline-earth-metal(-like) atoms, using ⁸⁷Sr as an example. Clock frequency shifts due to magnetic and optical fields are discussed with an emphasis on those relating to nuclear structure. An experimental determination of the differential g factor in ⁸⁷Sr is performed and is in good agreement with theory. The magnitude of the tensor light shift on the clock states is also explored experimentally. State specific measurements with controlled nuclear spin polarization are discussed as a method to reduce the nuclear spin-related systematic effects to below 10⁻¹⁷ in lattice clocks.

We report magnetic confinement of neutral, ground state OH at a density of ∼3×10³  cm^-3 and temperature of ∼30  mK. An adjustable electric field sufficiently large to polarize the OH is superimposed on the trap in various geometries, making an overall potential arising from both Zeeman and Stark effects. An effective molecular Hamiltonian is constructed, with Monte Carlo simulations accurately modeling the observed single-molecule dynamics in various trap configurations. Magnetic trapping of cold polar molecules under adjustable electric fields may enable study of low energy dipolar interactions.

We study the extended Bose-Hubbard model on a two-dimensional honeycomb lattice by using large-scale quantum Monte Carlo simulations. We present the ground-state phase diagrams for both the hard-core and the soft-core bosons. For the hard-core case, the transition between the ρ=1∕2 solid and the superfluid is first order, and the supersolid state is unstable toward phase separation. For the soft-core case, due to the presence of multiple occupation, a stable particle-induced supersolid (SS-p) phase emerges when 1∕2<ρ<1. The transition from the solid at ρ=1∕2 to the SS-p phase is second order with the superfluid density scaling as ρ_s∼ρ−1∕2. The SS-p phase has the same diagonal order as the solid at ρ=1∕2. As the chemical potential increases further, the SS-p phase turns into a solid where two bosons occupy each site of one sublattice through a first-order transition. We also calculate the critical exponents of the transition between the ρ=1∕2 solid and superfluid at the Heisenberg point for the hard-core case. We find the dynamical critical exponent z=0.15, which is smaller than results obtained on smaller lattices. This indicates that z approaches zero in the thermodynamic limit, and thus the transition is also first order even at the Heisenberg point.

We construct a quantum Ginsburg-Landau theory to study the quantum phase transition from the excitonic superfluid to a possible pseudospin density wave (PSDW) at some intermediate distances driven by the magnetoroton minimum collapsing at a finite wave vector. We explicitly show that the PSDW takes a square lattice structure. We suggest the existence of zero-point quantum fluctuation generated vacancies in the PSDW and that correlated hopping of vacancies in the active and passive layers in the PSDW state leads to very large and temperature dependent drag consistent with the experimental data. Comparisons with previous numerical calculations are made. Further experimental implications are given.

We present a general and highly efficient scheme for performing narrow-band Raman transitions between molecular vibrational levels using a coherent train of weak pump-dump pairs of shaped ultrashort pulses. The use of weak pulses permits an analytic description within the framework of coherent control in the perturbative regime, while coherent accumulation of many pulse pairs enables near unity transfer efficiency with a high spectral selectivity, thus forming a powerful combination of pump-dump control schemes and the precision of the frequency comb. Simulations verify the feasibility and robustness of this concept, with the aim to form deeply bound, ultracold molecules.

Aided by ultrahigh resolution spectroscopy, the overall systematic uncertainty of the ¹S₀-³P₀ clock resonance for lattice-confined ⁸⁷Sr has been characterized to 9×10^-16. This uncertainty is at a level similar to the Cs-fountain primary standard, while the potential stability for the lattice clocks exceeds that of Cs. The absolute frequency of the clock transition has been measured to be 429 228 004 229 874.0(1.1) Hz, where the 2.5×10^-15 fractional uncertainty represents the most accurate measurement of a neutral-atom-based optical transition frequency to date.

We theoretically explore photoassociation by adiabatic passage of two colliding cold ⁸⁵Rb atoms in an atomic trap to form an ultracold Rb₂ molecule. We consider the incoherent thermal nature of the scattering process in a trap and show that coherent manipulations of the atomic ensemble, such as adiabatic passage, are feasible if performed within the coherence time window dictated by the temperature, which is relatively long for cold atoms. We show that a sequence of ∼2×10⁷ pulses of moderate intensities, each lasting ∼750  ns, can photoassociate a large fraction of the atomic ensemble at temperature of 100  μK and density of 10¹¹  atoms∕cm³. Use of multiple pulse sequences makes it possible to populate the ground vibrational state. Employing spontaneous decay from a selected excited state, one can accumulate the molecules in a narrow distribution of vibrational states in the ground electronic potential. Alternatively, by removing the created molecules from the beam path between pulse sets, one can create a low-density ensemble of molecules in their ground rovibrational state.

We perform precision microwave spectroscopy—aided by Stark deceleration—to reveal the low-magnetic-field behavior of OH in its ²Π_3∕2 rovibronic ground state, identifying two field-insensitive hyperfine transitions suitable as qubits and determining a differential Landé g factor of 1.267(5)×10⁻³ between opposite-parity components of the Λ doublet. The data are successfully modeled with an effective hyperfine Zeeman Hamiltonian, which we use to make a tenfold improvement of the magnetically sensitive, astrophysically important ΔF=±1 satellite-line frequencies, yielding 1  720  529  887(10)  Hz and 1  612  230  825(15)  Hz.

The bilayer quantum Hall system (BLQH) differs from its single layer counterparts (SLQH) by its symmetry breaking ground state and associated neutral gapless mode in the pseudospin sector. Due to the gapless mode, qualitatively good ground-state and low energy excited-state wave functions at any finite distance are still unknown. We investigate this important open problem by the composite boson (CB) theory developed by one of the authors to study BLQH systematically. We derive the ground state, quasihole, and a pair of quasihole wave functions from the CB theory and its dual action. We find that the ground-state wave function is the product of two parts, one in the charge sector which is the well known Halperin (111) wave function and the other in the spin sector which is nontrivial at any finite d due to the gapless mode. So the total ground-state wave function differs from the well known (111) wave function at any finite d. In addition to commonly known multiplicative factors, the quasihole and a pair of quasihole wave functions also contain nontrivial normalization factors multiplying the correct ground state wave function. We expect that the quasihole and pair wave function not only has logarithmically divergent energy and well localized charge distribution, but also correct interlayer correlations. All the distance dependencies in all the wave functions are encoded in the spin part of the ground-state wave function. The instability encoded in the spin part of the ground-state wave function leads to the pseudo-spin-density wave proposed by one of the authors previously. Some subtleties related to the Lowest Landau Level (LLL) projection and shortcomings of the CB theory are also noted.

We propose a combination of electromagnetically induced transparency–Raman and pulsed spectroscopy techniques to accurately cancel frequency shifts arising from electromagnetically induced transparency fields in forbidden optical clock transitions of alkaline earth atoms. At appropriate detunings, time-separated laser pulses are designed to trap atoms in coherent superpositions while eliminating off-resonance ac Stark contributions, achieving efficient population transfer up to 60% with inaccuracy <10^-17. Results from the wave-function formalism are confirmed by the density matrix approach.

We extend the composite boson theory to study slightly imbalanced bilayer quantum Hall systems. In the global U(1) symmetry breaking excitonic superfluid side, as the imbalance increases, the system supports continuously changing fractional charges. In the translational symmetry breaking pseudospin density wave (PSDW) side, there are two quantum phase transitions from the commensurate PSDW to an incommensurate PSDW and then to the excitonic superfluid state. We compare our theory with experimental data and also the previous microscopic calculations.

We perform a detailed numerical analysis of Fabry-Perot cavities used for state-of-the-art laser stabilization. Elastic deformation of Fabry-Perot cavities with various shapes and mounting methods is quantitatively analyzed using finite-element analysis. We show that with a suitable choice of mounting schemes it is feasible to minimize the susceptibility of the resonator length to vibrational perturbations. This investigation offers detailed information on stable optical cavities that may benefit the development of ultrastable optical local oscillators in optical atomic clocks and precision measurements probing the fundamental laws of physics.

We develop a simple Ginsburg-Landau theory to study all the possible phases and phase transitions in ⁴He, analyze the condition for the existence of the supersolid (SS) and map out its global phase diagram from a unified framework. If the condition favors the existence of the SS, we use the GL theory to address several experimental facts and also make some predictions that are amenable to experimental tests. A key prediction is that the x-ray scattering intensity from the SS ought to have an additional modulation over that of the normal solid. The modulation amplitude is proportional to the nonclassical rotational-inertial observed in the torsional oscillator experiments.

Atomistic kinetic Monte Carlo simulations are employed to analyze the dynamical stabilization of nanoscale patterning of L1₀ chemical order in a model binary alloy subjected to sustained irradiation. The effect of irradiation-induced displacement cascades on the chemical order is modeled by the introduction at a controlled rate of nearly fully disordered spherical zones, which compete with the reordering promoted by the thermally activated migration of vacancies. When the size of the disordered zones is small, the alloy reaches a steady state that is either long-range ordered at low irradiation-induced ballistic jump frequency, Γ_b, or disordered at high Γ_b, with a first-order dynamical transition between these two steady states at Γ_b=Γ_b^c. Furthermore, in the disordered steady state, the intensity of order fluctuations scales with the reduced variable Γ_b∕Γ_b^c, a scaling that is consistent with an effective temperature approach. For larger cascade sizes, however, an additional steady state is stabilized at intermediate ballistic jump frequency, with a microstructure comprised of well-ordered nanoscale domains. In this patterning-of-order steady state, the above rescaling breaks down but we show that, after deconvolution of the structure factor into Gaussian and Lorentzian components, scaling of the Gaussian component is recovered by introducing a new reduced variable, Γ_b∕Γ_b^p, where 1∕Γ_b^p is interpreted as the characteristic time for new domains to form in a disordered zone. This new scaling relationship provides a rigorous definition of the regime of patterning of order. This regime corresponds to the steady states stabilized by cascade sizes and ballistic jump frequencies satisfying Γ_b^c≤Γ_b≤Γ_b^p. A dynamical phase diagram based on this new criterion is constructed and it agrees well with direct visualization of atomic configurations. Extensions to nonstoichiometric compositions are investigated. Consequences for the direct synthesis of functional nanocomposite structures comprised of chemically ordered phases are discussed.