arXiv subjects

Wolfgang Ketterle

Publications and source records attributed to Wolfgang Ketterle.

108 records · Page 6Linked to original sources

arXiv2006

Atom trapping with a thin magnetic film

We have created a $^{87}$Rb Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to BEC on the surface with radiofrequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than 40 $μ$m from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.

cond-mat.other↗

arXiv2006

Imaging the Mott Insulator Shells using Atomic Clock Shifts

Microwave spectroscopy was used to probe the superfluid-Mott Insulator transition of a Bose-Einstein condensate in a 3D optical lattice. Using density dependent transition frequency shifts we were able to spectroscopically distinguish sites with different occupation numbers, and to directly image sites with occupation number n=1 to n=5 revealing the shell structure of the Mott Insulator phase. We use this spectroscopy to determine the onsite interaction and lifetime for individual shells.

cond-mat.other↗

arXiv2006

Continuous and Pulsed Quantum Zeno Effect

Continuous and pulsed quantum Zeno effects were observed using a $^{87}$Rb Bose-Einstein condensate(BEC). Oscillations between two ground hyperfine states of a magnetically trapped condensate, externally driven at a transition rate $ω_R$, were suppressed by destructively measuring the population in one of the states with resonant light. The suppression of the transition rate in the two level system was quantified for pulsed measurements with a time interval $δt$ between pulses and continuous measurements with a scattering rate $γ$. We observe that the continuous measurements exhibit the same suppression in the transition rate as the pulsed measurements when $γδt=3.60(0.43)$, in agreement with the predicted value of 4. Increasing the measurement rate suppressed the transition rate down to $0.005ω_R$.

cond-mat.other↗

arXiv2006

Fluctuations in Ideal and Interacting Bose-Einstein Condensates: From the laser phase transition analogy to squeezed states and Bogoliubov quasiparticles

We review the phenomenon of equilibrium fluctuations in the number of condensed atoms in a trap containing N atoms total. We start with a history of the Bose-Einstein distribution, the Einstein-Uhlenbeck debate concerning the rounding of the mean number of condensed atoms near a critical temperature, and a discussion of the relations between statistics of BEC fluctuations in the grand canonical, canonical, and microcanonical ensembles. Next we discuss different approaches capable of providing approximate analytical results and physical insight into the problem of fluctuations. In particular, we describe the master equation (similar to the quantum theory of the laser) and canonical-ensemble quasiparticle approaches which give the most accurate and physically transparent picture of the BEC fluctuations. In the last part we describe condensate fluctuations in the interacting Bose gas. In particular, we show that the canonical-ensemble quasiparticle approach works very well for the interacting gases and yields analytical formulas for the characteristic function and all moments of the condensate fluctuations. In most cases the ground-state occupation fluctuations are anomalously large and are not Gaussian even in the thermodynamic limit. We clarify a crossover between the ideal and weakly-interacting-gas statistics which is governed by a pair-correlation squeezing mechanism.

cond-mat.stat-mech↗

arXiv2006

Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases

Water freezes into ice, atomic spins spontaneously align in a magnet, liquid helium becomes superfluid: Phase transitions are dramatic phenomena. However, despite the drastic change in the system's behaviour, observing the transition can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and superfluidity in trapped, weakly interacting Bose gases is the sudden appearance of a dense central core inside a thermal cloud. In strongly interacting gases, such as the recently observed fermionic superfluids, this clear separation between the superfluid and the normal parts of the cloud is no longer given. Condensates of fermion pairs could be detected only using magnetic field sweeps into the weakly interacting regime. The quantitative description of these sweeps presents a major theoretical challenge. Here we demonstrate that the superfluid phase transition can be directly observed by sudden changes in the shape of the clouds, in complete analogy to the case of weakly interacting Bose gases. By preparing unequal mixtures of the two spin components involved in the pairing, we greatly enhance the contrast between the superfluid core and the normal component. Furthermore, the non-interacting wings of excess atoms serve as a direct and reliable thermometer. Even in the normal state, strong interactions significantly deform the density profile of the majority spin component. We show that it is these interactions which drive the normal-to-superfluid transition at the critical population imbalance of 70(5)%.

cond-mat.supr-con↗

arXiv2005

Fermionic Superfluidity with Imbalanced Spin Populations and the Quantum Phase Transition to the Normal State

Whether it occurs in superconductors, helium-3 or inside a neutron star, fermionic superfluidity requires pairing of fermions, particles with half-integer spin. For an equal mixture of two states of fermions ("spin up" and "spin down"), pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, not every particle can find a partner. Will the system nevertheless stay superfluid? Here we study this intriguing question in an unequal mixture of strongly interacting ultracold fermionic atoms. The superfluid region vs population imbalance is mapped out by employing two complementary indicators: The presence or absence of vortices in a rotating mixture, as well as the fraction of condensed fermion pairs in the gas. Due to the strong interactions near a Feshbach resonance, the superfluid state is remarkably stable in response to population imbalance. The final breakdown of superfluidity marks a new quantum phase transition, the Pauli limit of superfluidity.

cond-mat.supr-con↗

arXiv2005

Parametric Amplification of Atoms

We have observed parametric generation and amplification of ultracold atom pairs. A 87Rb Bose-Einstein condensate was loaded into a one-dimensional optical lattice with quasimomentum k0 and spontaneously scattered into two final states with quasimomenta k1 and k2 . Furthermore, when a seed of atoms was first created with quasimomentum k1 we observed parametric amplification of scattered atoms pairs in states k1 and k2 when the phase-matching condition was fulfilled. This process is analogous to optical parametric generation (OPG) and amplification (OPA) of photons and could be used to efficiently create entangled pairs of atoms. Furthermore, these results explain the dynamic instability of condensates in moving lattices observed in recent experiments.

cond-mat.other↗

arXiv2005

Large atom number Bose-Einstein Condensate machines

We describe experimental setups for producing large Bose-Einstein condensates of 23Na and 87Rb. In both, a high flux thermal atomic beam is decelerated by a Zeeman slower and is captured and cooled in a magneto-optical trap. The atoms are then transfered into a cloverleaf style Ioffe-Pritchard magnetic trap and cooled to quantum degeneracy with radio frequency induced forced evaporation. Typical condensates contain 20 million atoms. The design includes a second vacuum chamber into which ultracold atoms can be transported with an optical tweezers. This allows the flexibility to rapidly prepare and perform a wide variety of experiments.

cond-mat.other↗

arXiv2005

Photon recoil momentum in dispersive media

A systematic shift of the photon recoil due to the index of refraction of a dilute gas of atoms has been observed. The recoil frequency was determined with a two-pulse light grating interferometer using near-resonant laser light. The results show that the recoil momentum of atoms caused by the absorption of a photon is n\hbar k, where n is the index of refraction of the gas and k is the vacuum wavevector of the photon. This systematic effect must be accounted for in high-precision atom interferometry with light gratings.

cond-mat.other↗

arXiv2004

Quantum reflection of atoms from a solid surface at normal incidence

We observed quantum reflection of ultracold atoms from the attractive potential of a solid surface. Extremely dilute Bose-Einstein condensates of ^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak gravito-magnetic trap were normally incident on a silicon surface. Reflection probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s. The velocity dependence agrees qualitatively with the prediction for quantum reflection from the attractive Casimir-Polder potential. Atoms confined in a harmonic trap divided in half by a solid surface exhibited extended lifetime due to quantum reflection from the surface, implying a reflection probability above 50 %.

cond-mat.other↗

arXiv2003

Raman Amplification of Matter Waves

We demonstrate a Raman amplifier for matter-waves, where the amplified atoms and the gain medium are in two different hyperfine states. This amplifier is based on a novel form of superradiance that arises from self-stimulated Raman scattering in a Bose-Einstein condensate.

cond-mat↗

arXiv2003

Spectroscopic insensitivity to cold collisions in a two-state mixture of fermions

We have experimentally demonstrated the absence of spectroscopic resonance shifts in a mixture of two interacting Fermi gases. This result is linked to observations in an ultracold gas of thermal bosons. There, the measured resonance shift due to interstate collisions is independent of the coherence in the system, and twice that expected from the equilibrium energy splitting between the two states in a fully decohered cloud. We give a simple theoretical explanation of these observations, which elucidates the effect of coherent radiation on an incoherent mixture of atoms.

cond-mat.soft↗

arXiv2003

Collisions in zero temperature Fermi gases

We examine the collisional behavior of two-component Fermi gases released at zero temperature from a harmonic trap. Using a phase-space formalism to calculate the collision rate during expansion, we find that Pauli blocking plays only a minor role for momentum changing collisions. As a result, for a large scattering cross-section, Pauli blocking will not prevent the gas from entering the collisionally hydrodynamic regime. In contrast to the bosonic case, hydrodynamic expansion at very low temperatures is therefore not evidence for fermionic superfluidity.

cond-mat.soft↗

arXiv2001

Collective enhancement and suppression in dilute Bose-Einstein condensates

The coherent and collective nature of a Bose-Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to matter wave amplification, and the suppression of dissipation leads to superfluidity. In this summer school notes we present several experiments where enhancement and suppression have been observed and discuss the common roots of and differences between these phenomena.

cond-mat.soft↗

arXiv2000

Collective enhancement and suppression in Bose-Einstein condensates

The coherent and collective nature of Bose-Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to atom amplification, and the suppression of dissipation leads to superfluidity. In this paper, we review several experiments where suppression and enhancement have been observed and discuss the common roots of and differences between these phenomena.

cond-mat.soft↗

arXiv2000

Does matter wave amplification work for fermions?

We discuss the relationship between bosonic stimulation, density fluctuations, and matter wave gratings. It is shown that enhanced stimulated scattering, matter wave amplification and atomic four-wave mixing are in principle possible for fermionic or non-degenerate samples if they are prepared in a cooperative state. In practice, there are limitations by short coherence times.

cond-mat.stat-mech↗

arXiv2000

Comment on the paper: Quantum backaction of optical observations on Bose-Einstein condensates by U. Leonhardt, T. Kiss, and P. Piwnicki, Eur. Phys. J. D7, 413 (1999)

A recent paper, Quantum backaction of optical observations on Bose-Einstein condensates by U. Leonhardt, T. Kiss, and P. Piwnicki, Eur. Phys. J. D7, 413 (1999), emphasized that the limit of dispersive imaging of Bose-Einstein condensates with off-resonant light is not residual absorption, but a different form of quantum backaction of the probe light. This note points out that these conclusions are incorrect, and that Rayleigh scattering is the dominant quantum backaction of dispersive imaging.

cond-mat.soft↗

arXiv2000

Spinor condensates and light scattering from Bose-Einstein condensates

These notes discuss two aspects of the physics of atomic Bose-Einstein condensates: optical properties and spinor condensates. The first topic includes light scattering experiments which probe the excitations of a condensate in both the free-particle and phonon regime. At higher light intensity, a new form of superradiance and phase-coherent matter wave amplification were observed. We also discuss properties of spinor condensates and describe studies of ground--state spin domain structures and dynamical studies which revealed metastable excited states and quantum tunneling.

cond-mat.soft↗