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arXiv:1406.1322 · Physical Review A

Bose-Einstein condensate of metastable helium for quantum correlation experiments

Michael Keller, Mateusz Kotyrba, Florian Leupold, Mandip Singh, Maximilian Ebner, Anton Zeilinger

Published Dec 1, 2014• 36 citations
arXivHTMLPDF

Abstract

We report on the realization of Bose-Einstein condensation of metastable helium-4. After exciting helium to its metastable state in a novel pulse-tube cryostat source, the atomic beam is collimated and slowed. We then trap several ${10}^{8}$ atoms in a magneto-optical trap. For subsequent evaporative cooling, the atoms are transferred into a magnetic trap. Degeneracy is achieved with typically a few ${10}^{6}$ atoms. For detection of atomic correlations with high resolution, an ultrafast delay-line detector has been installed. Consisting of four quadrants with independent readout electronics that allow for true simultaneous detection of atoms, the detector is especially suited for quantum correlation experiments that require the detection of correlated subsystems. We expect our setup to allow for the direct demonstration of momentum entanglement in a scenario equivalent to the Einstein-Podolsky-Rosen gedanken experiment. This will pave the way to matter-wave experiments exploiting the peculiarities of quantum correlations.

DOI
10.1103/physreva.90.063607

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•Mar 17, 2021•0 citations
We report on the realization of Bose-Einstein condensation of metastable helium-4. After exciting helium to its metastable state in a novel pulse-tube cryostat source, the atomic beam is collimated and slowed. We then trap several 108{10}^{8}108 atoms in a magneto-optical trap. For subsequent evaporative cooling, the atoms are transferred into a magnetic trap. Degeneracy is achieved with typically a few 106{10}^{6}106 atoms. For detection of atomic correlations with high resolution, an ultrafast delay-line detector has been installed. Consisting of four quadrants with independent readout electronics that allow for true simultaneous detection of atoms, the detector is especially suited for quantum correlation experiments that require the detection of correlated subsystems. We expect our setup to allow for the direct demonstration of momentum entanglement in a scenario equivalent to the Einstein-Podolsky-Rosen gedanken experiment. This will pave the way to matter-wave experiments exploiting the peculiarities of quantum correlations.
We report the realization of Bose-Einstein condensation (BEC) of metastable helium atoms using an in-vacuum coil magnetic trap and a crossed-beam optical dipole trap. A quadrupole-Ioffe configuration magnetic trap made from in-vacuum hollow copper tubes provides fast switching times while generating traps with a 10-G bias, without compromising optical access. The bias enables in-trap one-dimensional Doppler cooling to be used, which is the only cooling stage between the magneto-optic trap and the optical dipole trap. This allows direct transfer to the dipole trap without the need for any additional evaporative cooling in the magnetic trap. The entire experimental sequence takes 3.3 s, with essentially pure BECs observed with $\ensuremath{\sim}{10}^{6}$ atoms after evaporative cooling in the dipole trap.
We report the realization of a Bose-Einstein condensate of metastable helium-444 atoms (4He^{4}\mathrm{He}4He∗{}^{*}∗) in an all-optical potential. Up to 1010105{}^{5}5 spin-polarized 4He^{4}\mathrm{He}4He∗{}^{*}∗ atoms are condensed in an optical dipole trap formed from a single, focused, vertically propagating far-off-resonance laser beam. The vertical trap geometry is chosen to best match the resolution characteristics of a delay-line anode microchannel plate detector capable of registering single He∗{}^{*}∗ atoms. We also confirm the instability of certain spin-state combinations of 4He^{4}\mathrm{He}4He∗{}^{*}∗ to two-body inelastic processes, which necessarily affects the scope of future experiments using optically trapped spin mixtures. In order to better quantify this constraint, we measure spin-state-resolved two-body inelastic loss rate coefficients in the optical trap.
We report on the Bose-Einstein condensation of metastable 4He^{4}\mathrm{He}4He atoms using a hybrid approach, consisting of a magnetic quadrupole and an optical dipole trap. In our setup we cross the phase transition with $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms, and we obtain pure condensates of $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms in the optical trap. This approach to cooling 4He^{4}\mathrm{He}4He provides enhanced cycle stability, large optical access to the atoms and results in the production of a condensate every 6 s---a factor 2 faster than the state of the art. This speed-up will significantly reduce the data acquisition time needed for the measurement of many particle correlations, made possible by the ability of metastable helium atoms to be detected individually.
We have produced a Bose-Einstein condensate of metastable helium (4He∗)(^{4}\mathrm{He}^{*})(4He∗) containing over $1.5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms, which is a factor of 25 higher than previously achieved. The improved starting conditions for evaporative cooling are obtained by applying one-dimensional Doppler cooling inside a magnetic trap. The same technique is successfully used to cool the spin-polarized fermionic isotope (3He∗)(^{3}\mathrm{He}^{*})(3He∗), for which thermalizing collisions are highly suppressed. Our detection techniques include absorption imaging, time-of-flight measurements on a microchannel plate detector, and ion counting to monitor the formation and decay of the condensate.
The main subject of this thesis is to demonstrate that helium is a promising candidate for matter wave interferometry, with the goal of providing an improved value for the fine-structure constant, α. In particular, fast and efficient momentum transfer via Bloch oscillations in an optical lattice is demonstrated, together with the enhanced sensitivity offered by an ion-detector based detection scheme that is unique to metastable helium (He*). The experiments are performed in a setup that was built to produce, slow down and trap ultracold (~0.2 μK) He-4 atoms. The approach to realising an ultracold sample of He* atoms consists of a sequence of trapping and cooling stages. The experiment starts at the plasma discharge source, where helium atoms are excited to the so-called metastable 2-3S1 state, with a lifetime of ~7800 seconds, 20 eV above the atomic ground state. The atomic beam exiting from the source is 2-D collimated by 1083 nm laser light, resonant to the closed 2-3S1 - 2-3P2 cooling transition. Next, the atoms are slowed down from about 1000- to 50 m/s in a two meter long Zeeman slower through continuous absorption of resonant 1083 nm light. The slowed atoms are captured in a Magneto-Optical Trap (MOT), that holds up to 5x10^8 atoms at a temperature of 1 mK. The MOT cloud is compressed, spin-polarised and transferred to a cloverleaf Magnetic Trap (MT), which provides both axial and radial trapping through a set of coils outside the vacuum. In the MT, 1D-Doppler cooling is applied to the atomic sample to reduce the temperature to about 150 μK. Following this step, the temperature is reduced further by forced evaporative radio-frequency cooling, resulting in the onset of a Bose-Einstein Condensate (BEC). The atoms are transferred from the MT to a crossed horizontal optical dipole trap (ODT), in which the sample is further evaporatively cooled to produce a BEC of 5x10^6 atoms. The BEC is then transferred from the ODT to either a horizontal or vertical optical lattice, consisting of two counter-propagating laser beams at 1557 nm. By detuning the frequency of one of the lattice beams, the standing wave can be accelerated. The trapped helium atoms can follow the accelerating lattice through a process called Bloch oscillations (BO's), which describes the behaviour of particles in a periodic potential that are subjected to a force. Specifically, in this scenario, particles' velocities increase in increments of two recoil velocities, instead of continuously. Bloch oscillations of cold atoms in periodic potentials formed by optical lattices were first studied in 1996. Importantly, BO’s provide a way to efficiently accelerate atoms by coherently transferring a well controlled, large number of photon momenta, which has been used to increase the resolution of recoil measurements with caesium and rubidium atoms. In this thesis, Bloch oscillations with a metastable helium BEC are demonstrated. Due to its light mass, helium can be efficiently accelerated to high velocities which makes it a candidate for experiments that benefit from splitting the atomic wave-function over large distances. Additionally, Rabi-oscillations in an optical lattice are shown, which serve to probe the depth of the lattice, and a measurement of gravity through Bloch oscillations in a static vertical optical lattice is demonstrated, with a statistical uncertainty of 4x10^-5. Finally, the first proof of principle demonstration of a pulsed light He* atom interferometer is shown. The measurements illustrate the high signal-to-noise that can be obtained using an MCP detector. Next to that, we have demonstrated a method for determining the polarizability of atoms at specific wavelengths, by observing the induced change of the characteristic Bloch oscillation frequency by introducing that wavelength to the system.