arXiv subjects

H. R. Sadeghpour

Publications and source records attributed to H. R. Sadeghpour.

At least 91 records · Page 5Linked to original sources

arXiv2012

Influence of monolayer contamination on electric-field-noise heating in ion traps

Electric field noise is a hinderance to the assembly of large scale quantum computers based on entangled trapped ions. Apart from ubiquitous technical noise sources, experimental studies of trapped ion heating have revealed additional limiting contributions to this noise, originating from atomic processes on the electrode surfaces. In a recent work [A. Safavi-Naini et al., Phys. Rev. A 84, 023412 (2011)] we described a microscopic model for this excess electric field noise, which points a way towards a more systematic understanding of surface adsorbates as progenitors of electric field jitter noise. Here, we address the impact of surface monolayer contamination on adsorbate induced noise processes. By using exact numerical calculations for H and N atomic monolayers on an Au(111) surface representing opposite extremes of physisorption and chemisorption, we show that an additional monolayer can significantly affect the noise power spectrum and either enhance or suppress the resulting heating rates.

quant-ph↗

arXiv2012

Quantum Phases of Dipolar Bosons in Bilayer Geometry

We investigate the quantum phases of hard-core dipolar bosons confined to a square lattice in a bilayer geometry. Using exact theoretical techniques, we discuss the many-body effects resulting from pairing of particles across layers at finite density, including a novel pair supersolid phase, superfluid and solid phases. These results are of direct relevance to experiments with polar molecules and atoms with large magnetic dipole moments trapped in optical lattices.

cond-mat.quant-gas↗

arXiv2012

Observation of blue-shifted ultralong-range Cs$_{2}$ Rydberg molecules

We observe ultralong-range blue-shifted Cs$_{2}$ molecular states near $ns_{1/2}$ Rydberg states in an optical dipole trap, where $31\leq n\leq34$. The accidental near degeneracy of $(n-4)l$ and $ns$ Rydberg states for $l>2$ in Cs, due to the small fractional $ns$ quantum defect, leads to non-adiabatic coupling among these states, producing potential wells above the $ns$ thresholds. Two important consequences of admixing high angular momentum states with $ns$ states are the formation of large permanent dipole moments, $\sim 15-100\,$Debye, and accessibility of these states via two-photon association. The observed states are in excellent agreement with theory. Both projections of the total angular momentum on the internuclear axis are visible in the experiment.

physics.atom-ph↗

arXiv2012

Angular momentum changing transitions in proton-Rydberg atom collisions

Collisions between electrically charged particles and neutral atoms are central for understanding the dynamics of neutral gases and plasmas in a variety of physical situaziones of terrestrial and astronomical interest. Specifically, redistribution of angular momentum states within the degenerate shell of highly excited Rydberg atoms occurs efficiently in distant collisions with ions. This process is crucial in establishing the validity of the local thermal equilibrium assumption and may also play a role in determining a precise ionization fraction in primordial recombination. We provide an accurate expression for the non-perturbative rate coefficient of collsions between protons and H(n_l) ending in a final state H(n_l'), with n being the principal quantum number and l,l' the initial and final angular momentum quantum numbers, respectively. The validity of this result is confirmed by results of classical trajectory Monte Carlo simulations. Previous results, obtained by Pengelly and Seaton only for dipole-allowed transitions, l--->l+-1, overestimate the l-changing collisional rate approximately by a factor of six, and the physical origin of this overestimation is discussed.

astro-ph.IM↗

arXiv2012

Electric field control in ultralong-range triatomic polar Rydberg molecules

We explore the external electric field control of a species of ultralong-range molecules that emerge from the interaction of a ground state polar molecule with a Rydberg atom. The external field mixes the Rydberg electronic states and therefore strongly alters the electric field seen by the polar diatomic molecule due to the Rydberg electron. As a consequence, the adiabatic potential energy curves responsible for the molecular binding can be tuned in such a way that an intersection with neighboring curves occurs. The latter leads to admixture of s-wave character in the Rydberg wave function and will substantially facilitate the experimental preparation and realization of this particular class of Rydberg molecule species.

physics.atom-ph↗

arXiv2011

A microscopic model of electronic field noise heating in ion traps

Motional heating of ions in micro-fabricated traps is a challenge hindering experimental realization of large-scale quantum processing devices. Recently a series of measurements of the heating rates in surface-electrode ion traps characterized their frequency, distance, and temperature dependencies, but our understanding of the microscopic origin of this noise is still vague. In this work we develop a theoretical model for the electric field noise which is associated with a random distribution of adsorbed atoms on the trap electrode surface. By using first principle calculations of the fluctuating dipole moments of the adsorbed atoms we evaluate the distance, frequency and temperature dependence of the resulting electric field fluctuation spectrum.Our theory calculates the noise spectrum beyond the standard scenario of two-level fluctuators, by incorporating all the relevant vibrational states. The $1/f$ noise is shown to commence at roughly the frequency of the fundamental phonon transition rate and the $d^{-4}$ dependence with distance of the ion from the electrode surface is established.

quant-ph↗

arXiv2011

Formation of van der Waals molecules in buffer gas cooled magnetic traps

We show that a large class of helium-containing cold polar molecules form readily in a cryogenic buffer gas, achieving densities as high as 10^12 cm^-3. We explore the spin relaxation of these molecules in buffer gas loaded magnetic traps, and identify a loss mechanism based on Landau-Zener transitions arising from the anisotropic hyperfine interaction. Our results show that the recently observed strong T^6 thermal dependence of spin change in buffer gas trapped silver (Ag) is accounted for by the formation and spin change of AgHe, thus providing evidence for molecular formation in a buffer gas trap.

physics.atom-ph↗

arXiv2011

Formation and dynamics of van der Waals molecules in buffer-gas traps

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxations are too slow to affect trap lifetimes. However, helium-3-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag3He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps.

physics.atm-clus↗

arXiv2011

Ultralong-range polyatomic Rydberg molecules formed by a polar perturber

The internal electric field of a Rydberg atom electron can bind a polar molecule to form a giant ultralong-range stable polyatomic molecule. Such molecules not only share their properties with Rydberg atoms, they possess huge permanent electric dipole moments and in addition allow for coherent control of the polar molecule orientation. In this work, we include additional Rydberg manifolds which couple to the nearly degenerate set of Rydberg states employed in [S. T. Rittenhouse and H. R. Sadeghpour, Phys. Rev. Lett. 104, 243002 (2010)]. The coupling of a set of $(n+3)s$ Rydberg states with the $n(l>2)$ nearly degenerate Rydberg manifolds in alkali metal atoms leads to pronounced avoided crossings in the Born-Oppenheimer potentials. Ultimately, these avoided crossings enable the formation of the giant polyatomic Rydberg molecules with standard two-photon laser photoassociation techniques.

physics.atom-ph↗

arXiv2010

A fundamental limit to the efficiency of spin-exchange optical pumping of 3He nuclei

We establish the existence of a fundamental limit to the efficiency of spin-exchange optical pumping of 3He nuclei by collisions with spin-polarized alkali-metal atoms. Using accurate ab initio calculations of molecular interactions and scattering properties, we show that the maximum 3He spin polarization that can be achieved in spin-exchange collisions with potassium (39K) and silver (107Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions, suggesting the possibility of using Ag in spin-exchange optical pumping experiments to increase the production rate of hyperpolarized 3He. Our analysis indicates that measurements of trap loss rates of 2S atoms in the presence of cold 3He gas may be used to probe anisotropic spin-exchange interactions in atom-He collisions.

physics.atom-ph↗

arXiv2010

CrRb: a molecule with large magnetic and electric dipole moments

We report calculations of Born-Oppenheimer potential energy curves of the chromium-rubidium heteronuclear molecule 52Cr-87Rb, and the long-range dispersion coefficient for the interaction between ground state Cr and Rb atoms. Our calculated van der Waals coefficient (C6=1770 a.u.) has an expected error of 3%. The ground state 6Sigma+ molecule at its equilibrium separation has a permanent electric dipole moment of de (Re=3.34 A) = 2.90 D. We investigate the hyperfine and dipolar collisions between trapped Cr and Rb atoms, finding elastic to inelastic cross section ratio of 100-1000.

physics.atom-ph↗

arXiv2010

Ultracold giant polyatomic Rydberg molecules: coherent control of molecular orientation

We predict the existence of a class of ultracold giant molecules formed from trapped ultracold Rydberg atoms and polar molecules. The interaction which leads to the formation of such molecules is the anisotropic charge-dipole interaction ($a/R^2$). We show that prominent candidate molecules such as KRb and deuterated hydroxyl (OD) should bind to Rydberg rubidium atoms, with energies $E_b\simeq 5-25$ GHz at distances $R\simeq 0.1-1 \ μ$m. These molecules form in double wells, mimicking chiral molecules, with each well containing a particular dipole orientation. We prepare a set of correlated dressed electron-dipole eigenstates which are used in a resonant Raman scheme to coherently control the dipole orientation and to create cat-like entangled states of the polar molecule.

physics.atom-ph↗

arXiv2009

Cold and Ultracold Rydberg Atoms in Strong Magnetic Fields

Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms such as giant dipole states are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for trapping and cooling of such atoms are investigated.

physics.atom-ph↗

arXiv2007

Accurate long-range coefficients for two excited like isotope He atoms: He($2 ^1P$)--He($2 ^1P$), He($2 ^1P$)--He($2 ^3P$), and He($2 ^3P$)--He($2 ^3P$)

A general formalism is used to express the long-range potential energies in inverse powers of the separation distance between two like atomic or molecular systems with $P$ symmetries. The long-range molecular interaction coefficients are calculated for the molecular symmetries $Δ$, $Π$, and $Σ$, arising from the following interactions: He($2 ^1P$)--He($2 ^1P$), He($2 ^1P$)--He($2 ^3P$), and He($2 ^3P$)--He($2 ^3P$). The electric quadrupole-quadrupole term, $C_{5}$, the van der Waals (dispersion) term $C_{6}$, and higher-order terms, $C_{8}$, and $C_{10}$, are calculated \textit{ab initio} using accurate variational wave functions in Hylleraas coordinates with finite nuclear mass effects. A comparison is made with previously published results where available.

physics.atom-ph↗

arXiv2006

Long-range interactions for He($n S$)--He$(n' S)$ and He($n S$)--He$(n' P)$

The energetically lowest five states of a helium atom are: He($1^1S$), He($2^3S$), He($2^1S$), He($2^3P$), and He($2^1P$). Long-range interaction coefficients $C_3$, $C_6$, $C_8$, $C_9$, and $C_{10}$ for all $S-S$ and $S-P$ pairs of these states are calculated precisely using correlated wave functions in Hylleraas coordinates. Finite nuclear isotope mass effects are included.

physics.atom-ph↗

arXiv2005

Long-range interactions between a He($2 ^3S$) atom and a He($2 ^3P$) atom for like isotopes

For the interactions between a He($2 ^3S$) atom and a He($2 ^3P$) atom for like isotopes, we report perturbation theoretic calculations using accurate variational wave functions in Hylleraas coordinates of the coefficients determining the potential energies at large internuclear separations. We evaluate the coefficient $C_{3}$ of the first order resonant dipole-dipole energy and the van der Waals coefficients $C_{6}$, $C_{8}$, and $C_{10}$ for the second order energies arising from the mutual perturbations of instantaneous electric dipole, quadrupole, and octupole interactions. We also evaluate the coefficient $C_{9}$ of the leading contribution to the third order energy. We establish definitive values including treatment of the finite nuclear mass for the ${}^3$He($2 ^3S$)--${}^3$He($2 ^3P$) and ${}^4$He($2 ^3S$)--${}^4$He($2 ^3P$) interactions.

physics.atom-ph↗

arXiv2003

Collisional properties of trapped cold chromium atoms

We report on calculations of the elastic cross section and thermalization rate for collision between two maximally spin-polarized chromium atoms in the cold and ultracold regimes, relevant to buffer-gas and magneto-optical cooling of chromium atoms. We calculate ab initio potential energy curves for Cr2 and the van der Waals coefficient C6, and construct interaction potentials between two colliding Cr atoms. We explore the effect of shape resonances on elastic cross section, and find that they dramatically affect the thermalization rate. Our calculated value for the s-wave scattering length is compared in magnitude with a recent measurement at ultracold temperatures.

physics.atom-ph↗

arXiv2002

Controlling two-species Mott-insulator phses in an optical lattice to form an array of dipolar molecules

We consider the transfer of a two-species Bose-Einstein condensate into an optical lattice with a density such that that a Mott-insulator state with one atom per species per lattice site is obtained in the deep lattice regime. Depending on collision parameters the result could be either a `mixed' or a `separated' Mott-insulator phase. Such a `mixed' two-species insulator could then be photo-associated into an array of dipolar molecules suitable for quantum computation or the formation of a dipolar molecular condensate. For the case of a $^{87}$Rb-$^{41}$K two-species BEC, however, the large inter-species scattering length makes obtaining the desired `mixed' Mott insulator phase difficult. To overcome this difficulty we investigate the effect of varying the lattice frequency on the mean-field interaction and find a favorable parameter regime under which a lattice of dipolar molecules could be generated.

cond-mat↗