W. Vassen

Comparison of the Hanbury Brown-Twiss effect for bosons and fermions.

Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source, highlighting the importance of two-photon correlations and stimulating the development of modern quantum optics. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms. By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: 3He (a fermion) and 4He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom–atom correlation measurements can be used to reveal details in the spatial density or momentum correlations in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations.

Cold and trapped metastable noble gases

Experimental work on cold, trapped metastable noble gases is reviewed. The aspects which distinguish work with these atoms from the large body of work on cold, trapped atoms in general is emphasized. These aspects include detection techniques and collision processes unique to metastable atoms. Several experiments exploiting these unique features in fields including atom optics and statistical physics are described. Precision measurements on these atoms including fine structure splittings, isotope shifts, and atomic lifetimes are also discussed.

Frequency metrology in quantum degenerate helium: direct measurement of the 2 3S1 --> 2 1S0 transition.

Measurement of an extremely weak spectroscopic transition in helium hones fundamental atomic theories. Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 3S1 and first excited singlet state 2 1S0), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic 3He and bosonic 4He atoms are used to obtain a precision of 8 × 10−12, providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.

Degenerate Bose-Fermi mixture of metastable atoms

We report the observation of simultaneous quantum degeneracy in a dilute gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3 (fermion) by helium-4 (boson), both in the lowest triplet state, allows us to produce ensembles containing more than 10(6) atoms of each isotope at temperatures below 1 microK, and achieve a fermionic degeneracy parameter of T/TF = 0.45. Because of their high internal energy, the detection of individual metastable atoms with subnanosecond time resolution is possible, permitting the study of bosonic and fermionic quantum gases with unprecedented precision. This may lead to metastable helium becoming the mainstay of quantum atom optics.

An intense collimated beam of metastable helium atoms by two-dimensional laser cooling

A compact apparatus has been developed to generate an intense beam of helium atoms in the metastable 2 3S state. A liquid nitrogen cooled DC discharge with a 0.25 mm diameter nozzle produces a beam with an intensity of 2 × 1014 metastable atoms per second per sterad, an average velocity ν = 1030 m/s and a velocity spread of ΔνFWHMν=0.3. Transverse two-dimensional laser cooling using curved wavefronts has been applied to capture atoms within a 30 mrad cone, to collimate the atomic beam and to enhance its intensity by a factor 50. An atomic beam with a cross section of 3 × 3 mm2 and an intensity of 1 × 1010 metastable triplet atoms per second per mm2 has been realised using a laser cooling section with an interaction length of 18 cm.

Universal three-body parameter in ultracold 4He*

We have analyzed our recently measured three-body loss rate coefficient for a Bose-Einstein condensate of spin-polarized metastable triplet 4He atoms in terms of Efimov physics. The large value of the scattering length for these atoms, which provides access to the Efimov regime, arises from a nearby potential resonance. We find the loss coefficient to be consistent with the three-body parameter (3BP) found in alkali-metal experiments, where Feshbach resonances are used to tune the interaction. This provides evidence for a universal 3BP outside the group of alkali-metal elements. In addition, we give examples of other atomic systems without Feshbach resonances but with a large scattering length that would be interesting to analyze once precise measurements of three-body loss are available.

Metastable helium Bose-Einstein condensate with a large number of atoms

We have produced a Bose-Einstein condensate of metastable helium (4He*) containing over 1.5x10^7 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*), 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.

Quasi-Landau Structure of Diamagnetic Helium Rydberg Atoms

Diamagnetism in helium Rydberg atoms is studied near the ionisation threshold using constant scaled-energy laser spectroscopy. Quasi-Landau resonances in the Fourier transform of the energy spectrum are explained using the classical periodic-orbit theory. Long laser scans combined with a high-frequency resolution allow for a detailed comparison between experiment and theory. Resonances with a scaled action /π up to 20 are identified and reproduced with an accuracy of 0.015. It is demonstrated that interference effects between resonances should be taken into account.

High-power frequency-stabilized laser for laser cooling of metastable helium at 389 nm

A high-power, frequency-stabilized laser for cooling of metastable helium atoms using the 2S13→3P23 transition at 389 nm has been developed. The 389 nm light is generated by frequency doubling of a titanium:sapphire laser in an external enhancement cavity containing a lithium–triborate nonlinear crystal. With a maximum conversion efficiency of 75%, 1 W of useful 389 nm power is produced out of 2 W at 778 nm. While being stabilized to the 2S13→3P23 transition, the 389 nm frequency is tunable over ±150MHz with respect to the field-free atomic resonance frequency. This is accomplished by Zeeman tuning of the absorption lines used in the frequency-stabilization scheme. The setup for saturated absorption spectroscopy in an rf discharge cell, used to stabilize the 389 nm laser to the atomic transition, is described in detail.

Magic wavelengths for the 2 3 S → 2 1 S transition in helium

We have calculated ac polarizabilities of the 2 3 S and 2 1 S states of both He 4 and He 3 in the range 318 nm to 2.5 µm and determined the magic wavelengths at which these polarizabilities are equal for either isotope. The calculations, only based on available ab initio tables of level energies and Einstein A coefficients, do not require advanced theoretical techniques. The polarizability contribution of the continuum is calculated using a simple extrapolation beyond the ionization limit, yet the results agree to better than 1% with such advanced techniques. Several promising magic wavelengths are identified around 320 nm with sufficient accuracy to design an appropriate laser system. The extension of the calculations to He 3 is complicated due to the additional hyperfine structure, but we show that the magic wavelength candidates around 320 nm are predominantly shifted by the isotope shift.