arXiv subjects

Martin Weitz

Publications and source records attributed to Martin Weitz.

At least 73 records · Page 4Linked to original sources

arXiv2012

Collisional Redistribution Laser Cooling of a High Pressure Atomic Gas

We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained both via thermographic imaging and via thermographic deflection spectroscopy. The cooled gas has a density above 10$^{21}$ atoms/cm$^3$, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms.

physics.atom-ph↗

arXiv2012

Light Confinement by a Cylindric Metallic Waveguide in Dense Buffer Gas Environment

We report on the implementation of metallic microtubes in a system of rubidium vapour at 230\,bar of argon buffer gas. The high buffer gas pressure leads to a widely pressure broadened linewidth of several nanometers, interpolating between the sharp atomic physics spectra and the band structure of solid state systems. Tube-like metallic waveguide structures have been inserted in the high pressure buffer gas system, allowing for an enhancement of the atom-light interaction over an optical guiding length in the tube of up to 1\,mm. The system holds promise for nonlinear optics experiments and the study of atom-light polariton condensation.

physics.atm-clus↗

arXiv2011

Doppler-free frequency modulation spectroscopy of atomic erbium in a hollow cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose-Einstein condensate of atoms with a non-vanishing orbital angular momentum ($L \neq 0$) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 $nm$ laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.

physics.atom-ph↗

arXiv2011

Production of Sodium Bose--Einstein condensates in an optical dimple trap

We report on the realization of a sodium Bose--Einstein condensate (BEC) in a combined red-detuned optical dipole trap, formed by two beams crossing in a horizontal plane and a third, tightly focused dimple trap propagating vertically. We produce a BEC in three main steps: loading of the crossed dipole trap from laser-cooled atoms, an intermediate evaporative cooling stage which results in efficient loading of the auxiliary dimple trap, and a final evaporative cooling stage in the dimple trap. Our protocol is implemented in a compact setup and allows us to reach quantum degeneracy even with relatively modest initial atom numbers and available laser power.

physics.atom-ph↗

arXiv2011

Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices

We report on an experimental study of quantum transport of atoms in variable periodic optical potentials. The band structure of both ratchet-type asymmetric and symmetric lattice potentials is explored. The variable atom potential is realized by superimposing a conventional standing wave potential of λ/2 spatial periodicity with a fourth-order multiphoton potential of λ/4 periodicity. We find that the Landau-Zener tunneling rate between the first and the second excited Bloch band depends critically on the relative phase between the two spatial lattice harmonics.

cond-mat.other↗

arXiv2011

Laser cooling of a potassium-argon gas mixture using collisional redistribution of radiation

We study laser cooling of atomic gases by collisional redistribution, a technique applicable to ultradense atomic ensembles at a pressure of a few hundred bar. Frequent collisions of an optically active atom with a buffer gas shift atoms into resonance with a far red detuned laser beam, while spontaneous decay occurs close to the unperturbed resonance frequency. In such an excitation cycle, a kinetic energy of the order of the thermal energy kT is extracted from the sample. Here we report of recent experiments investigating the cooling of a potassium-argon gas mixture, which compared to an rubidium-argon mixture investigated in earlier experiments has a smaller fine structure of the optically active alkali atom. We observe a relative cooling of the potassium-argon gas mixture by 120 K.

cond-mat.quant-gas↗

arXiv2010

Bose-Einstein condensation of photons in an optical microcavity

Bose-Einstein condensation, the macroscopic ground state accumulation of particles with integer spin (bosons) at low temperature and high density, has been observed in several physical systems, including cold atomic gases and solid state physics quasiparticles. However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition, because the chemical potential of photons vanishes and, when the temperature is reduced, photons disappear in the cavity walls. Theoretical works have considered photon number conserving thermalization processes, a prerequisite for Bose-Einstein condensation, using Compton scattering with a gas of thermal electrons, or using photon-photon scattering in a nonlinear resonator configuration. In a recent experiment, we have observed number conserving thermalization of a two-dimensional photon gas in a dye-filled optical microcavity, acting as a 'white-wall' box for photons. Here we report on the observation of a Bose-Einstein condensation of photons in a dye-filled optical microcavity. The cavity mirrors provide both a confining potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped, massive bosons. By multiple scattering off the dye molecules, the photons thermalize to the temperature of the dye solution (room temperature). Upon increasing the photon density we observe the following signatures for a BEC of photons: Bose-Einstein distributed photon energies with a massively populated ground state mode on top of a broad thermal wing, the phase transition occurring both at the expected value and exhibiting the predicted cavity geometry dependence, and the ground state mode emerging even for a spatially displaced pump spot.

cond-mat.quant-gas↗

arXiv2010

Atomic Bloch-Zener Oscillations and Stückelberg Interferometry in Optical Lattices

We report on experiments investigating quantum transport and band interferometry of an atomic Bose-Einstein condensate in an optical lattice with a two-band miniband structure, realized with a Fourier-synthesized optical lattice potential. Bloch-Zener oscillations, the coherent superposition of Bloch oscillations and Landau-Zener tunneling between the two bands are observed. When the relative phase between paths in different bands is varied, an interference signal is observed, demonstrating the coherence of the dynamics in the miniband system. Measured fringe patterns of this Stückelberg interferometer allow to interferometrically map out the band structure of the optical lattice over the full Brillouin zone.

cond-mat.quant-gas↗

arXiv2010

Thermalisation of a two-dimensional photonic gas in a 'white-wall' photon box

Bose-Einstein condensation, the macroscopic accumulation of bosonic particles in the energetic ground state below a critical temperature, has been demonstrated in several physical systems. The perhaps best known example of a bosonic gas, blackbody radiation, however exhibits no Bose-Einstein condensation at low temperatures. Instead of collectively occupying the lowest energy mode, the photons disappear in the cavity walls when the temperature is lowered - corresponding to a vanishing chemical potential. Here we report on evidence for a thermalised two-dimensional photon gas with freely adjustable chemical potential. Our experiment is based on a dye filled optical microresonator, acting as a 'white-wall' box for photons. Thermalisation is achieved in a photon number-conserving way by photon scattering off the dye-molecules, and the cavity mirrors both provide an effective photon mass and a confining potential - key prerequisites for the Bose-Einstein condensation of photons. As a striking example for the unusual system properties, we demonstrate a yet unobserved light concentration effect into the centre of the confining potential, an effect with prospects for increasing the efficiency of diffuse solar light collection.

cond-mat.quant-gas↗

arXiv2010

Non-dispersive optics using storage of light

We demonstrate the non-dispersive deflection of an optical beam in a Stern-Gerlach magnetic field. An optical pulse is initially stored as a spin-wave coherence in thermal rubidium vapour. An inhomogeneous magnetic field imprints a phase gradient onto the spin wave, which upon reacceleration of the optical pulse leads to an angular deflection of the retrieved beam. We show that the obtained beam deflection is non-dispersive, i.e. its magnitude is independent of the incident optical frequency. Compared to a Stern-Gerlach experiment carried out with propagating light under the conditions of electromagnetically induced transparency, the estimated suppression of the chromatic aberration reaches 10 orders of magnitude.

physics.atom-ph↗

arXiv2009

Directed Transport of Atoms in a Hamiltonian Quantum Ratchet

We demonstrate the operation of a quantum ratchet in the absence of dissipative processes within the observation time (Hamiltonian regime). An atomic rubidium Bose-Einstein condensate is exposed to a sawtooth-like optical lattice potential, whose amplitude is periodically modulated in time. The ratchet transport arises from broken spatiotemporal symmetries of the driven potential, resulting in a desymmetrisation of transporting Eigenstates (Floquet states). The measured atomic current oscillates around a non-zero stationary value at longer observation times, shows resonances at positions determined by the photon recoil and depends on the initial phase of the drive, providing different lines of evidence for the full quantum character of the ratchet transport. The results provide a proof of principle demonstration of a quantum motor.

cond-mat.quant-gas↗

arXiv2009

Frequency matching in light storage spectroscopy of atomic Raman transitions

We investigate the storage of light in an atomic sample with a lambda-type coupling scheme driven by optical fields at variable two-photon detuning. In the presence of electromagnetically induced transparency (EIT), light is stored and retrieved from the sample by dynamically varying the group velocity. It is found that for any two-photon detuning of the input light pulse within the EIT transparency window, the carrier frequency of the retrieved light pulse matches the two-photon resonance frequency with the atomic ground state transition and the control field. This effect which is not based on spectral filtering is investigated both theoretically and experimentally. It can be used for high-speed precision measurements of the two-photon resonance as employed e.g. in optical magnetometry.

physics.atom-ph↗

arXiv2009

Laser Cooling by Collisional Redistribution of Radiation

The general idea that optical radiation may cool matter was put forward by Pringsheim already in 1929. Doppler cooling of dilute atomic gases is an extremely successful application of this concept, and more recently anti-Stokes cooling in multilevel systems has been explored, culminating in the optical refrigeration of solids. Collisional redistribution of radiation is a proposed different cooling mechanism that involves atomic two-level systems, though experimental investigations in gases with moderate density have so far not reached the cooling regime. Here we experimentally demonstrate laser cooling of an atomic gas based on collisional redistribution of radiation, using rubidium atoms subject to 230 bar of argon buffer gas pressure. The frequent collisions in the ultradense gas transiently shift a far red detuned laser beam into resonance, while spontaneous decay occurs close to the unperturbed atomic resonance frequency. During each excitation cycle, a kinetic energy of order of the thermal energy k_B T is extracted from the dense atomic sample. In a proof of principle experiment with a thermally non-isolated sample, we experimentally demonstrate relative cooling by 66 K. The cooled gas has a density of more than 10 orders of magnitude above the typical values in Doppler cooling experiments, and the cooling power reaches 87 mW. Future prospects of the demonstrated effect include studies of supercooling beyond the homogeneous nucleation temperature and optical chillers.

cond-mat.quant-gas↗

arXiv2009

Bloch oscillations of atoms in an optical multiphoton potential

We report on experiments studying transport properties of an atomic Bose-Einstein condensate in an optical lattice of spatial period $λ/2n$, where $n$ is an integer, realized with the dispersion of multiphoton Raman transitions. We observe Bloch oscillations, as a clear effect of quantum transport, in the sub-wavelength scale periodicity lattice. The unusually strong localisation of atoms is evident from the measured effective mass. Future prospects of the novel lattice structures are expected in the search for new quantum phases in tailored lattice structures up to quantum computing in optical nanopotentials.

cond-mat.other↗

arXiv2008

Resonance beating of light stored using atomic spinor polaritons

We investigate the storage of light in atomic rubidium vapor using a multilevel-tripod scheme. In the system, two collective dark polariton modes exist, forming an effective spinor quasiparticle. Storage of light is performed by dynamically reducing the optical group velocity to zero. After releasing the stored pulse, a beating of the two reaccelerated optical modes is monitored. The observed beating signal oscillates at an atomic transition frequency, opening the way to novel quantum limited measurements of atomic resonance frequencies and quantum switches.

quant-ph↗

arXiv2007

Fourier synthesis of optical potentials for atomic quantum gases

We demonstrate a scheme for the Fourier synthesis of periodic optical potentials with asymmetric unit cells for atoms. In a proof of principle experiment, an atomic Bose-Einstein condensate is exposed to either symmetric or sawtooth-like asymmetric potentials by superimposing a conventional standing wave potential of $λ/2$ spatial periodicity with a fourth-order lattice potential of $λ/4$ periodicity. The high periodicity lattice is realized using dispersive properties of multiphoton Raman transitions. Future applications of the demonstrated scheme could range from the search for novel quantum phases in unconventionally shaped lattice potentials up to dissipationless atomic quantum ratchets.

cond-mat.other↗

arXiv2007

A Stern-Gerlach experiment for slow light

Electromagnetically induced transparency allows for light transmission through dense atomic media by means of quantum interference. Media exhibiting electromagnetically induced transparency have very interesting properties, such as extremely slow group velocities. Associated with the slow light propagation are quasiparticles, the so-called dark polaritons, which are mixtures of a photonic and an atomic contribution. We here demonstrate that these excitations behave as particles with a nonzero magnetic moment, which is in clear contrast to the properties of a free photon. It is found that light passing through a rubidium gas cell under the conditions of electromagnetically induced transparency is deflected by a small magnetic field gradient. The deflection angle is proportional to the optical propagation time through the cell. The observed beam deflection can be understood by assuming that dark state polaritons have an effective magnetic moment. Our experiment can be described in terms of a Stern-Gerlach experiment for the polaritons.

quant-ph↗

arXiv2005

Interference of an array of atom lasers

We report on the observation of interference of a series of atom lasers. A comb-like array of coherent atomic beams is generated by outcoupling atoms from distinct Bose-Einstein condensates confined in the independent sites of a mesoscopic optical lattice. The observed interference signal arises from the spatial beating of the overlapped atom laser beams, which is sampled over a vertical region corresponding to 2 ms of free fall time. The average relative de Broglie frequency of the atom lasers was measured.

cond-mat.other↗