Bose-Einstein condensation in spin-polarized atomic hydrogen: A new superfluid

We find that in the spin-polarized hydrogen, Bose condensation occurs for certain quantized values of the magnetic field. Once the field is fixed, sweeping of the radio-frequency results in nuclear magnetic resonance so that condensation and NMR occur simultaneously. We have found that nuclear self-induced transparency occurs. A new excitation designated by the present author as superboojum, which is a discontinuity in the hydrodynamic equations in spin-polarized hydrogen having finite nuclear as well as electronic spin is discovered.     

Bose-Einstein condensation in dilute atomic gases

We summarize all experimental studies of Bose-Einstein condensation (BEC) in dilute atomic gases reported thus far and discuss the experimental techniques used to produce, manipulate and observe nanokelvin samples of atoms.     

Crossover temperature of Bose-Einstein condensation in an atomic Fermi gas

We show that in an atomic Fermi gas near a Feshbach resonance the crossover between a Bose-Einstein condensate of diatomic molecules and a Bose-Einstein condensate of Cooper pairs occurs at positive detuning, i.e., when the molecular energy level lies in the two-atom continuum. We determine the crossover temperature as a function of the applied magnetic field and find excellent agreement with the experiment of C. A. Regal et al. [Phys. Rev. Lett. 92, 040403 (2004)]] who has recently observed this crossover temperature.     

Creating maximally entangled atomic states in a Bose-Einstein condensate

We propose a protocol to create maximally entangled pairs, triplets, quartiles, and other clusters of Bose-condensed atoms starting from a condensate in the Mott insulator state. The essential element is to drive single atom Raman transitions using laser pulses. Our scheme is simple, efficient, and can be readily applied to the recent experimental system as reported by M. Greiner 413, 44 (2002)].     

Creating macroscopic atomic Einstein-Podolsky-Rosen states from Bose-Einstein condensates

We present a scheme for creating quantum entangled atomic states through the coherent spin-exchange collision of a spinor Bose-Einstein condensate. The state generated possesses macroscopic Einstein-Podolsky-Rosen correlation and the fluctuation in one of its quasispin components vanishes. We show that an elongated condensate with large aspect ratio is most suitable for creating such a state.     

On the soliton states of an atomic Bose-Einstein condensate

Numerical analysis of the one-, two-, and three-dimensional soliton solutions for the modified Gross-Pitaevski equation with regard to the nonlocality of interaction between atoms of a condensate is performed. In calculations of the parameter of interaction nonlocality, the Lennard-Jones potential for collisions between Li atoms in the triplet state was used. It is shown that the nonlocality stabilizes the three-dimensional solitons with sizes in a certain range. The lifetimes of solitons, which are determined by two-and three-particle collisions between atoms, are estimated.     

Bose-Einstein condensation in nonequilibrium systems

The properties of quantum statistically degenerate systems of Bosons which are created by an external pump field and decay within a finite lifetime are investigated by means of a Green's function treatment. These investigations help to understand the physical properties of such condensed Bose-systems as excitons, excitonic molecules and spin aligned hydrogen atoms. As an example, recent experiments by Hulin et al. on degenerate excitons in Cu₂O are analyzed and a condensate fraction of about 5% is obtained.     

Bose-Einstein condensation in a cavity

The effect of the physically correct boundary conditions and the nonvanishing ground state energy on Bose-Einstein condensation of quantum particles confined to a cubic volumeV=L ³ is evaluated. The transition point is shifted towards higher temperatures by the confinement, the specific heat below the onset of condensation is no longer proportional toT ^3/2, and the pressure does depend on the volume. Precise expressions for the modification of the ground state population and for the shift of the condensation temperature are derived, together with an expansion of the internal energy and of the specific heat. Numerical computations confirm the accuracy of our analytical approximations.Dedicated to Herbert Wagner, whose work on quantum Fermi liquids proved to be also very stimulating for quantum Bose liquids.     

Bose-Einstein condensation in interacting gases

We study the occurrence of a Bose-Einstein transition in a dilute gas with repulsive interactions, starting from temperatures above the transition temperature. The formalism, based on the use of Ursell operators, allows us to evaluate the one-particle density operator with more flexibility than in mean-field theories, since it does not necessarily coincide with that of an ideal gas with adjustable parameters (chemical potential, etc.). In a first step, a simple approximation is used (Ursell-Dyson approximation), which allow us to recover results which are similar to those of the usual mean-field theories. In a second step, a more precise treatment of the correlations and velocity dependence of the populations in the system is elaborated. This introduces new physical effects, such as a change of the velocity profile just above the transition: the proportion of atoms with low velocities is higher than in an ideal gas. A consequence of this distortion is an increase of the critical temperature (at constant density) of the Bose gas, in agreement with those of recent path integral Monte-Carlo calculations for hard spheres. Received 13 November 1998     

Bose-Einstein condensation of chromium

We report on the generation of a Bose-Einstein condensate in a gas of chromium atoms, which have an exceptionally large magnetic dipole moment and therefore underlie anisotropic long-range interactions. The preparation of the chromium condensate requires novel cooling strategies that are adapted to its special electronic and magnetic properties. The final step to reach quantum degeneracy is forced evaporative cooling of 52Cr atoms within a crossed optical dipole trap. At a critical temperature of T(c) approximately 700 nK, we observe Bose-Einstein condensation by the appearance of a two-component velocity distribution. We are able to produce almost pure condensates with more than 50,000 condensed 52Cr atoms.     

Bose-Einstein condensation in one- and two-dimensional gases

We show that the one- and two-dimensional ideal Bose gases undergo a phase transition if the temperature is lowered at constant pressure. At the pressure-dependent transition temperature T_c (P) and in their thermodynamic limit the specific heat at constant pressure c_p and the particle densityn diverge, the entropyS and specific heat at constant volumec _v fall off sharply but continuously to zero, and the fraction of particles in the ground state N₀/N jumps discontinuously from zero to one. This Bose-Einstein condensation provides a remarkable example of a transition which has most of the properties of a second-order phase transition, except that the order parameter is discontinuous. The nature of the condensed state is described in the large but finiteN regime, and the width of the transition region is estimated. The effects of interactions in real one- and two-dimensional Bose systems and recent experiments on submonolayer helium films are discussed briefly.     

Bose-Einstein condensation in dense quark matter

We consider the problem of Bose condensation of charged pions in QCD at finite isospin chemical potential μ_I using the O(4)-symmetric linear sigma model as an effective field theory for two-flavor QCD. Using the 2PI 1/N-expansion, we determine the quasiparticle masses as well as the pion and chiral condensates as a function of the temperature and isospin chemical potential in the chiral limit and at the physical point. The calculations show that there is a competition between the condensates. At T=0, Bose condensation takes place for chemical potentials larger than μ_π. In the chiral limit, the chiral condensate vanishes for any finite value of μ_I.     

On Bose-Einstein condensation in any dimension

A general property of an ideal Bose gas as temperature tends to zero and when conditions of degeneracy are satisfied is to have an arbitrarily large population in the groud state (or in its neighborhood); thus, condensation occurs in any dimension D but for D ≤ 2 there is no critical temperature. Some astrophysical consequences, as well as the temperature-dependent mass case, are discussed.     

Bose-Einstein condensation and the glassy state

The distinction between a classical glass and a classical liquid is difficult, since both are disordered. The difference is in the fact that a glass is frozen while the liquid is not. In this Letter an equilibrium measure is suggested that distinguishes between a glass and a liquid. The choice of this measure is based on the idea that in a system which is not frozen symmetry under permutation of particles is physically relevant, because particles can be permuted by actual physical motion. This is not the case in a frozen system. In this Letter it is shown how to generalize naturally the quantum mechanical concept of Bose condensed fraction to classical systems in order to distinguish between the glass and the liquid. It is finite in the liquid and zero in the frozen state. The actual value of the condensed fraction in the liquid may serve also as a measure of the glassiness in the liquid.     

Bose-Einstein condensation in a circular waveguide

We have produced Bose-Einstein condensates in a ring-shaped magnetic waveguide. The few-millimeter diameter, nonzero-bias ring is formed from a time-averaged quadrupole ring. Condensates that propagate around the ring make several revolutions within the time it takes for them to expand to fill the ring. The ring shape is ideally suited for studies of vorticity in a multiply connected geometry and is promising as a rotation sensor.     

Bose-Einstein condensation in complex networks.

The evolution of many complex systems, including the World Wide Web, business, and citation networks, is encoded in the dynamic web describing the interactions between the system's constituents. Despite their irreversible and nonequilibrium nature these networks follow Bose statistics and can undergo Bose-Einstein condensation. Addressing the dynamical properties of these nonequilibrium systems within the framework of equilibrium quantum gases predicts that the "first-mover-advantage," "fit-get-rich," and "winner-takes-all" phenomena observed in competitive systems are thermodynamically distinct phases of the underlying evolving networks.