Making an Ultracold Gas

In biology, the study of geomagnetic orientation has gained new momentum since the discovery of magnetic field detectors in aquatic organisms. Sharks and rays respond to dc and low frequency voltage gradients of 0.005 μV/cm. By moving through the earth's magnetic field, they induce electric fields well within the sensitivity range of their keen electric sense. As these fields depend on the direction in which the animal is heading, the induced voltage gradients may serve as the biophysical basis of an electromagnetic compass sense. The ability of sharks and rays to orient to the earth's magnetic field has been demonstrated in behavioral experiments. Also, various marine and freshwater mud bacteria are endowed with permanent magnetic dipole moments, directed parallel to the axis of motility. When separated from the sediments, these bacteria return to the mud by migrating downward along the earth's inclined magnetic field lines. Their orientation is largely determined by the principles of statistical mechanics and may be expressed in terms of the directive magnetic force, the randomizing effect of thermal agitation, and the cells' flagellar thrust. Observations on live bacteria yield individual dipole moments circa 15 × kT/G.

To explain the differences in protein-protein interactions (PPI) of concentrated versus dilute formulations of a model antibody. High frequency rheological measurements from pH 3.0 to 12.0 quantitated viscoelasticity and PPI at high concentrations. Dynamic light scattering (DLS) characterized PPI in dilute solutions. For concentrated solutions at low ionic strength, the storage modulus, a viscosity component and a measure of PPI, is highest at the isoelectric point (pH 9.0) and lowest at pH 5.4. This profile flattens at higher ionic strength but not completely, indicating PPI consist of long-range electrostatics and other short-range attractions. At low concentrations, PPI are near zero at pI but become repulsive as the pH is shifted. Higher salt concentrations completely flatten this profile to zero, indicating that these PPI are mainly electrostatic. This discrepancy occurs because long-range interactions are significant at low concentrations, whereas both long- and short-range interactions are significant at higher concentrations. Computer modeling was used to calculate antibody properties responsible for long- and short-range interactions, i.e. net charge and dipole moment. Charge-charge interactions are repulsive while dipole-dipole interactions are attractive. Their net effect correlated with the storage modulus profile. However, only charge-charge repulsions correlated with PPI determined by DLS.

We study the steady-state behavior of active, dipolar, Brownian spheroids in a planar channel subjected to an imposed Couette flow and an external transverse field, applied in the "downward" normal-to-flow direction. The field-induced torque on active spheroids (swimmers) is taken to be of magnetic form by assuming that they have a permanent magnetic dipole moment, pointing along their self-propulsion (swim) direction. Using a continuum approach, we show that a host of behaviors emerges over the parameter space spanned by the particle aspect ratio, self-propulsion and shear/field strengths, and the channel width. The cross-stream migration of the model swimmers is shown to involve a regime of linear response (quantified by a linear-response factor) in weak fields. For prolate swimmers, the weak-field behavior crosses over to a regime of full swimmer migration to the bottom half of the channel in strong fields. For oblate swimmers, a counterintuitive regime of reverse migration arises in intermediate fields, where a macroscopic fraction of swimmers reorient and swim to the top channel half at an acute "upward" angle relative to the field axis. The diverse behaviors reported here are analyzed based on the shear-induced population splitting (bimodality) of the swim orientation, giving two distinct, oppositely polarized, swimmer subpopulations (albeit very differently for prolate/oblate swimmers) in each channel half. In strong fields, swimmers of both types exhibit net upstream currents relative to the laboratory frame. The onsets of full migration and net upstream current depend on the aspect ratio, enabling efficient particle separation strategies in microfluidic setups.

The new derivation of the equation of the spin precession is given for a particle possessing electric and magnetic dipole moments. Contributions from classical electrodynamics and from the Thomas effect are explicitly separated. A fully covariant approach is used. The final equation is expressed in a very simple form in terms of the fields in the instantaneously accompanying frame. The Lorentz transformations of the electric and magnetic dipole moments and of the spin are derived from basic equations of classical electrodynamics. For this purpose, the Maxwell equations in matter are used and the result is confirmed by other methods. An antisymmetric four-tensor is correctly constructed from the electric and magnetic dipole moments.

The electric dipole moment (EDM) of baryons provides a sensitive probe of CP-violating interactions beyond the standard model. Motivated by the recent BESIII measurement on the Λ hyperon EDM [Ablikim etal., arXiv:2506.19180.], we present the first perturbative QCD analysis of the Λ EDM form factor to elucidate its origin in CP-violating quark dipole interactions. In particular, we derive a QCD factorization formula that relates the Λ EDM form factor to quark EDMs and chromoelectric dipole moments through convolutions with light-cone distribution amplitudes of Λ. These connections allow us to extract constraints on CP-violating dipole couplings from current and future hyperon EDM measurements. Our numerical analysis demonstrates that the Λ EDM exhibits unique sensitivity to the strange-quark chromoelectric dipole moment, providing complementary information to that obtained from the neutron EDM.

Hadron electric dipole moments from CP-odd operators of dimension five via QCD sum rules: the vector meson

Many body localization (MBL) has emerged as a powerful paradigm for understanding non-equilibrium quantum dynamics. Folklore based on perturbative arguments holds that MBL only arises in systems with short range interactions. Here we advance non-perturbative arguments indicating that MBL can arise in systems with long range (Coulomb) interactions. In particular, we show using bosonization that MBL can arise in one dimensional systems with ~ r interactions, a problem that exhibits charge confinement. We also argue that (through the Anderson-Higgs mechanism) MBL can arise in two dimensional systems with log r interactions, and speculate that our arguments may even extend to three dimensional systems with 1/r interactions. Our arguments are `asymptotic' (i.e. valid up to rare region corrections), yet they open the door to investigation of MBL physics in a wide array of long range interacting systems where such physics was previously believed not to arise.

A rich internal structure and long-range interactions between them make molecules with non-vanishing dipole moments interesting for many applications. An experiment demonstrating the efficient transfer of loosely bound heteronuclear molecules into more deeply bound energy levels indicates a route towards producing dense ensembles of cold polar molecules. Polar molecules have bright prospects for novel quantum gases with long-range and anisotropic interactions1, and could find uses in quantum information science2 and in precision measurements3,4,5. However, high-density clouds of ultracold polar molecules have so far not been produced. Here, we report a key step towards this goal. We start from an ultracold dense gas of loosely bound 40K87Rb Feshbach molecules6,7 with typical binding energies of a few hundred kilohertz, and coherently transfer these molecules in a single transfer step into a vibrational level of the ground-state molecular potential bound by more than 10 GHz. Starting with a single initial state prepared with Feshbach association8, we achieve a transfer efficiency of 84%. Given favourable Franck–Condon factors9,10, the presented technique can be extended to access much more deeply bound vibrational levels and those exhibiting a significant dipole moment.

Constant temperature simulations of helix folding

We have studied the behavior of clouds of neutral atoms contained in a spontaneous force optical trap. Because of the low temperatures of the atoms (<1 mK) we have observed a variety of new phenomena which involve both short and long range interactions between the atoms. The short range interactions are manifested in two body collisions which are notable because of the very long collision times. This time scale causes the dynamics to be profoundly affected by absorption and spontaneous emission during the collision. An interesting assortment of collective behaviors are observed when the trapped atom cloud contains more than 105 atoms. These include the expansion of the cloud as the number is increased and dramatic changes in the distribution of the atoms at higher numbers. We can explain much of the collective behavior using a simple model that includes a 1/r2 force between the atoms arising from the multiple scattering of photons. Finally, we discuss the optical trapping of atoms directly from a low pressure vapor in a small glass cell. We have used these optically trapped atoms to load a magnetostatic trap in the same cell. This provided a high density sample of atoms with a temperature of less than 2 μK.

Dipole moments of aromatic heterocycles

The Stokes shift magnitudes for coumarin 153 (C153) in 13 organic solvents with various polarities have been determined by means of steady-state spectroscopy and reference interaction-site model-self-consistent-field (RISM-SCF) theory. RISM-SCF calculations have reproduced experimental results fairly well, including individual solvent characteristics. It is empirically known that in some solvents, larger Stokes shift magnitudes are detected than anticipated on the basis of the solvent relative permittivity, ɛr. In practice, 1,4-dioxane (ɛr = 2.21) provides almost identical Stokes shift magnitudes to that of tetrahydrofuran (THF, ɛr = 7.58), for C153 and other typical organic solutes. In this work, RISM-SCF theory has been used to estimate the energetics of C153-solvent systems involved in the absorption and fluorescence processes. The Stokes shift magnitudes estimated by RISM-SCF theory are ∼5 kJ mol(-1) (400 cm(-1)) less than those determined by spectroscopy; however, the results obtained are still adequate for dipole moment comparisons, in a qualitative sense. We have also calculated the solute-solvent site-site radial distributions by this theory. It is shown that solvation structures with respect to the C-O-C framework, which is common to dioxane and THF, in the near vicinity (∼0.4 nm) of specific solute sites can largely account for their similar Stokes shift magnitudes. In previous works, such solute-solvent short-range interactions have been explained in terms of the higher-order multipole moments of the solvents. Our present study shows that along with the short-range interactions that contribute most significantly to the energetics, long-range electrostatic interactions are also important. Such long-range interactions are effective up to 2 nm from the solute site, as in the case of a typical polar solvent, acetonitrile.

In some models of physics beyond the Standard Model (SM), one of the leading low energy consequences of the model appears in the form of the chromo-electric dipole moments (CEDMs) of the gluons and light quarks. We examine if these CEDMs can be distinguished from the QCD θ-term through the experimentally measurable nuclear and atomic electric dipole moments (EDMs) in both cases with and without the Peccei-Quinn (PQ) mechanism solving the strong CP problem. We find that the nucleon EDMs show a distinctive pattern when the EDMs are dominantly induced by the light quark CEDMs without the PQ mechanism. In the presence of the PQ mechanism, the QCD θ-parameter corresponds to the vacuum value of the axion field, which might be induced either by CEDMs or by UV-originated PQ breaking other than the QCD anomaly, for instance the PQ breaking by quantum gravity effects. We find that in case with the PQ mechanism the nucleon EDMs have a similar pattern regardless of what is the dominant source of EDMs among the CEDMs and θ-term, unless there is a significant cancellation between the contributions from different sources. In contrast, some nuclei or atomic EDMs can have characteristic patterns significantly depending on the dominant source of EDMs, which may allow identifying the dominant source among the CEDMs and θ-term. Yet, discriminating the gluon CEDM from the QCD θ-parameter necessitates additional knowledge of low energy parameters induced by the gluon CEDM, which is not available at the moment. Our results imply that EDMs can reveal unambiguous sign of CEDMs while identifying the origin of the axion vacuum value, however it requires further knowledge of low energy parameters induced by the gluon CEDM.

An asymptotic expression for the van-der-Waals constant C6(n) ≈ –0.03n12K p (x) is derived for the long-range interaction between two highly excited hydrogen atoms A and B in their extreme Stark states of equal principal quantum numbers n A = n B = n ≫ 1 and parabolic quantum numbers n1(2) = n - 1, n2(1) = m = 0 in the case of collinear orientation of the Stark-state dipolar electric moments and the interatomic axis. The cubic polynomial K3(x) in powers of reciprocal values of the principal quantum number x = 1/n and quadratic polynomial K2(y) in powers of reciprocal values of the principal quantum number squared y = 1/n2 were determined on the basis of the standard curve fitting polynomial procedure from the calculated data for C6(n). The transformation of attractive van-der-Waals force (C6 > 0) for low-energy states n < 23 into repulsive force (C6 < 0) for all higher-energy states of n ≥ 23, is observed from the results of numerical calculations based on the second-order perturbation theory for the operator of the long-range interaction between neutral atoms. This transformation is taken into account in the asymptotic formulas (in both cases of p = 2, 3) by polynomials K p tending to unity at n → ∞ (K p (0) = 1). The transformation from low-n attractive van-der-Waals force into high-n repulsive force demonstrates the gradual increase of the negative contribution to C6(n) from the lower-energy two-atomic states, of the A(B)-atom principal quantum numbers n′A(B) = n-Δn (where Δn = 1, 2, … is significantly smaller than n for the terms providing major contribution to the second-order series), which together with the states of n″B(A) = n+Δn make the joint contribution proportional to n12. So, the hydrogen-like manifold structure of the energy spectrum is responsible for the transformation of the power-11 asymptotic dependence C6(n) ∝ n11of the low-angular-momenta Rydberg states in many-electron atoms into the power-12 dependence C6(n) ∝ n12 for the dipolar states of the Rydberg manifold.

The magnetic dipole moment (MDM) and the electric dipole moment (EDM) of leptons are calculated under the assumption of lepton flavor violation (LFV) induced by spin-1 unparticles with both vector and axial-vector couplings to leptons,including a CP-violating phase. The experimental limits on the muon MDM and LFV process such as the decay l_i->i_jl_kl_k are then used to constrain the LFV couplings for particular values of the unparticle operator dimension d_U and the unparticle scale \Lambda_U, assuming that LFV transitions between the tau and muon leptons are dominant. It is found that the current experimental constraints favor a scenario with dominance of the vector couplings over the axial-vector couplings. We also obtain estimates for the EDMs of the electron and the muon, which are well below the current experimental limits.