Cold Atoms and Quantum Gases
Atoms Molecules and Lasers group webpage
The laser cooling force between an atom and near-resonant laser radiation can be used to decelerate and cool atoms down to low temperatures. Using cold helium-4 atoms in a metastable 2 3S state (He*, 20 eV internal energy) Bose-Einstein condensation has been observed on January 27, 2005. Fermionic helium-3 was brought to quantum degeneracy at the end of 2005. Measuring the arrival time and position of both isotopes on a microchannel plate detector we observed bunching for bosons and antibunching for fermions in 2006. In 2010 we excited the 1557-nm transition between the two metastable states of helium. In a project, finished in 2004, metastable helium atoms are also explored for the purpose of fabricating small, nanoscale structures (nanolithography). In an already in 2000 terminated project the level structure of helium Rydberg atoms in the presence of a varying dc electric- or magnetic field (scaled energy spectroscopy) was investigated.
Our experiments are described in more detail below.
When a gas of bosons is cooled to temperatures close to absolute zero (T~1 microK) at densities in the order of 1013cm-3 Bose-Einstein Condensation (BEC) can be observed. This effect was observed for the first time in 1995 with Rb-87 atoms in the group of Cornell and Wieman at JILA. The experimental setup is schematically shown in the figure below.
A beam of He* atoms (from a DC discharge source) is collimated and deflected by transverse laser beams in two dimensions. Only the He* atoms then enter the Zeeman slower where they are decelerated by an opposing laser beam. Up to 2x109 atoms are trapped in a magneto optical trap close to the end of the Zeeman slower. The temperature of the atoms is then 1 mK. The laser cooling transition used is 23S-23P at 1083 nm. The cold atoms are subsequently spin-polarized and trapped in a magnetic trap. We can trap up to 1x109 atoms at a temperature of 0.1 mK exploiting 1D optical molasses for 2 s inside the magnetic trap. After 1D laser cooling we compress the cloud by increasing the current of the magnetic trap coils and cool the atoms further by rf evaporative cooling to BEC at about 1 microK. In our second-generation experiment we operate at a vacuum pressure of 10-11 mbar resulting in a lifetime up to 3 minutes. For our most recent experiments we transfer the atoms from the magnetic trap to a crossed dipole trap. For this purpose the light of a 1557-nm laser is focussed on the atoms in two dimensions as shown in the figure. The same laser is used for spectroscopy on the transition to the metastable singlet state.
In a 15 s exponential ramp we have increased the phase space density 5 orders of magnitude and observe BEC applying two different techniques. In the traditional way we detect the transition by absorption imaging at 1083 nm. The figure below shows a condensate after 10 ms expansion time. The cloud is clearly nonisotropic. This absorption image is taken with an InGaAs camera. From the optical density of the cloud we deduce that the condensate contains ~106 atoms. Actually the image is taken after transferring the ultracold cloud from the magnetic trap to the optical dipole trap.
We also can detect BEC on a microchannel plate detector mounted 17 cm below the trap centre. This MCP detector can be translated in the horizontal plane allowing both detection of condensate by absorption imaging and, in a separate realization, a possibly small fraction on the MCP. As a third detection method a second MCP detector is available to measure the ion production inside the He* cloud.
Since our first demonstration of BEC in metastable helium we have improved our BEC production.
We have demonstrated that we can condense at least 107 atoms. Only for Rubidium, Sodium and Hydrogen
condensates containing so many atoms have been realized. We also observe the growth of the condensate online
on the second MCP detector.
Helium has the advantage that both a bosonic (helium-4) and a fermionic (helium-3) isotope are available. We also cool and trap helium-3. In contrast to helium-4 helium-3 has hyperfine structure and the 23S metastable state is split in an F=1/2 and F=3/2 state (see figure). The F=1/2 - F=3/2 hyperfine interval is 6.7 GHz. As helium-3 can not be cooled via evaporative cooling due to the absence of elastic collisions for identical fermions evaporative cooling of helium-4 brings helium-3 towards quantum degeneracy. In 2004 we for the first time confined both isotopes in a MOT. The trapped cloud contained up to 1.5x108 atoms of each isotope. We investigated optical pumping of helium-3 atoms to the non-trapped hyperfine state and found that large numbers can be confined without additional repumpers.
By loading a MOT with a mixture of both helium isotopes we can, after transfer to the magnetic trap, sympathetically cool helium-3 to quantum degeneracy. We have realized a degenerate Fermi gas of more than 1x106 atoms (Fig.A) as well as a mixture of degenerate gases of helium-3 and helium-4 atoms (Fig.B).
Fifty years ago Robert Hanbury Brown and Richard Twiss (HBT) discovered photon bunching in light emitted by a chaotic source and used this effect to determine stellar diameters. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is linked to the Bose nature of photons. The bunching effect can also be observed with bosonic atoms, as demonstrated by the group of Chris Westbrook and Alain Aspect in Orsay for metastable Helium-4. In Amsterdam we have, in a collaboration with the Orsay group, made a comparison of the HBT effect for bosonic Helium-4 and fermionic Helium-3. Our experimental setup allows cooling and trapping of both helium isotopes to temperatures around one microKelvin above absolute zero. The experimental setup is shown in the figure below.
A cold cloud of metastable helium atoms is released at the switch-off of a magnetic trap. The cloud falls under the influence of gravity onto a time-resolved and position-sensitive detector and detects single atoms. The inset shows conceptually the two 2-particle amplitudes (in black or grey) that interfere to give either bunching or antibunching.
bunching and antibunching
To observe this effect helium atoms have to be cooled to temperatures close to absolute zero. Only then can the atoms be confined to a small enough volume in space with velocities that are low enough to see the effect. How this is done can be found here. In the plot below the bunching effect for helium-4 (in red) and the antibunching effect for helium-3 (in blue) are shown in a plot of the normalized cor relation function, that describes the number of pairs that are observed at one detector point for different time differences. These time differences are translat ed to vertical pair separations. Each point represents the normalized number of pairs observed in a certain time difference bin.
Nature 445 (2007) 402-405
a forbidden transition
The 1557-nm orthohelium - parahelium transition between the two metastable states, both in helium-4 and helium-3, has been observed. This transition is an excellent testing ground for fundamental theory of atomic structure. The present accuracy in the QED calculations is 2 MHz. The natural linewidth of the transition is 8 Hz, determined by the 20-ms lifetime of the metastable singletstate.
Either one, or both, of the two isotopes are transferred from the magnetic trap into a crossed-beam optical dipole trap, two focused 1557-nm laser beams, intersecting at their foci (see figure). Up to 106 atoms are transferred to the dipole trap. A heterodyne signal is set up between the 1557- nm laser and a mode of a femtosecond frequency comb laser to deduce the absolute frequency of the spectroscopy laser.
Because the excited state is anti-trapped, the trap is depleted when the spectroscopy beam is resonant with the atomic transition. By measuring the remaining number of metastable triplet atoms for various laser frequencies, the atomic resonance frequency is determined from a Gaussian fit to the data (see figure).
Several systematic shifts in the transition frequency are taken into account. The largest shift is due to the Zeeman effect. The second largest due to the AC Stark shift associated with the intense 1557-nm light of the dipole trap. The resonance frequency for a range of applied laser powers and extrapolate to zero laser intensity. The absolute frequency for Helium-4 was measured to be 192 510 702 145.6(1.8) kHz and for Helium-3 (F =3/2 - F =1/2) 192 504 914 426.4(1.5) kHz. For both isotopes, the results agree with QED calculations. The experimental error is three orders of magnitude smaller than estimates of non-evaluated higher-order terms in state-of-the-art QED calculations and presents a significant challenge for groups involved in atomic structure theory.
the size of the nucleus
Isotope shift measurements, combined with high-precision QED theory, provide a method to isolate contributions due to finite nuclear size effects. The difference in nuclear charge radii between Helium-3 and Helium-4 is determined by comparing experiment and theory. Presently, the accuracy to which the Helium-4 charge radius is known sets a lower limit on the uncertainty of the Helium-3 charge radius determined from helium spectroscopy. The measurement presents a value for the Helium-3 nuclear charge radius of 1.961(4) fm.
He* atoms have an internal energy of 20 eV. This internal energy can be selectively deposited on a self-assembling monolayer, attached to a substrate, to locally damage the surface. Using masks and laser-cooled atomic beams this process is studied and 50 nm resolution can be obtained in this way. Applying an intense, far blue-detuned standing wave in front of the substrate, 100 nm width lines have been written, in good agreement with model calculations of the dipole force. 2D-structures are investigated as well. The figure below shows a 1D pattern written in a gold substrate with a 375 MHz blue detuned standing wave.
CW lasers in the ultraviolet (wavelengths 312 nm and 260 nm) have been used in a crossed laser-atomic beam experiment to excite helium-4 atoms to Rydberg states in the presence of a varying electric- or magnetic field. The external field is so strong that many Rydberg states 1snl mix and a very complicated spectrum is recorded. The complex spectrum is due to the fact that for the highly excited electron (n~100) the Coulomb attraction to the He+ core is of comparable strength as the external force. Semi-classical closed-orbit theory, describing the motion of the electron around the He+ core, is applied to interpret the spectra. Electron orbits, closed at the core, provide sinusoidal modulations in the excitation spectrum that can be uncovered by Fourier transformation of the experimental spectrum. Comparison to calculations provides accurate tests of closed-orbit theory. An example, showing an almost perfect agreement between experiment and theory, is shown in the figure below for the electric field case.
In the magnetic field case classical dynamics is chaotic in a high field. Experimentally this was demonstrated by measuring the nearest neighbour statistics. For that purpose the atomic beam was collimated and intensified using two-dimensional transverse laser cooling. In a regular regime Poisson statistics should be observed and in a fully chaotic regime a Wigner distribution. In the figure below these two limiting situations are shown as well as experiments where the relative strength of the magnetic field was varied (epsilon = - 0.7, in the regular regime; epsilon = - 0.3, in the mixed regular-chaotic regime).