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: (3)He (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.

Ultrafast double-pulse parametric amplification for precision Ramsey metrology.

We demonstrate phase stable, mJ-level parametric amplification of pulse pairs originating from a Ti:Sapphire frequency comb laser. The amplifier-induced phase shift between the pulses has been determined interferometrically with an accuracy of approximately 10 mrad. Typical phase shifts are on the order of 50-200 mrad, depending on the operating conditions. The measured phase-relation can be as stable as 20 mrad rms (1/300(th) of an optical cycle). This makes the system suitable for Ramsey spectroscopy at short wavelengths by employing harmonic upconversion of the double-pulses in nonlinear media.

A source of 2 terawatt, 2.7 cycle laser pulses based on noncollinear optical parametric chirped pulse amplification.

We demonstrate a noncollinear optical parametric chirped pulse amplifier system that produces 7.6 fs pulses with a peak power of 2 terawatt at 30 Hz repetition rate. Using an ultra-broadband Ti:Sapphire seed oscillator and grating-based stretching and compression combined with an LCD phase-shaper, we amplify a 310 nm wide spectrum with a total gain of 3x10(7), and compress it within 5% of its Fourier limit. The total integrated parametric fluorescence is kept below 0.2%, leading to a pre-pulse contrast of 2 x10(-8) on picosecond timescales.

Generation of few-cycle terawatt light pulses using optical parametric chirped pulse amplification.

We demonstrate the generation of 9.8+/-0.3 fs laser pulses with a peak power exceeding one terawatt at 30 Hz repetition rate, using optical parametric chirped pulse amplification. The amplifier is pumped by 140 mJ, 60 ps pulses at 532 nm, and amplifies seed pulses from a Ti:Sapphire oscillator to 23 mJ/pulse, resulting in 10.5 mJ/pulse after compression while amplified fluorescence is kept below 1%. We employ grating-based stretching and compression in combination with an LCD phase-shaper, allowing compression close to the Fourier limit of 9.3 fs.

Photoassociation spectroscopy of cold He(2 3S) atoms

We observe vibrational states by photoassociation spectroscopy of cold He(2 3S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3S1-2 3P2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.

Phase stability of terawatt-class ultrabroadband parametric amplification.

The phase stability of broadband (280 nm bandwidth) terawatt-class parametric amplification was measured, for the first time to our knowledge, with a combination of spatial and spectral interferometry. Measurements at four different wavelengths from 750 to 900 nm were performed in combination with numerical modeling. The phase stability is better than 1/23 rms of an optical cycle for all the measured wavelengths, depending on the phase-matching conditions in the amplifier.

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.

Local health systems in 21st century: who cares?-An exploratory study on health system governance in Amsterdam.

BACKGROUND: There is a growing awareness that there should be a public health perspective to health system governance. Its intrinsic population health orientation provides the ultimate ground for determining the health needs and governing collaborative care arrangements within which these needs can be met. Notwithstanding differences across countries, population health concerns are not central to European health reforms. Governments currently withdraw leaving governance roles to care providers and/or financiers. Thereby, incentives that trigger the uptake of a public health perspective are often ignored. METHODS: In this study we addressed this issue in the city of Amsterdam. Using a qualitative study design, we explored whether there is a public health perspective to the governance practices of the municipality and the major sickness fund in Amsterdam. And if so, what the scope of this perspective is. And if not, why not. RESULTS: Findings indicate that the municipality has a public health perspective to local health system governance, but its scope is limited. The municipality facilitates rather than governs health care provision in Amsterdam. Furthermore, the sickness fund runs major financial risks when adapting a public health perspective. It covers an insured population that partly overlaps the Amsterdam population. Returns on investments in population health are therefore uncertain, as competitors would also profit from the sickness fund's investments. CONCLUSION: The local health system in Amsterdam is not consistently aligned to the health needs of the Amsterdam population. The Amsterdam case is not unique and general consequences for local health system governance are discussed.

High-power parametric amplification of 11.8-fs laser pulses with carrier-envelope phase control.

Phase-stable parametric chirped-pulse amplification of ultrashort pulses from a carrier-envelope phase-stabilized mode-locked Ti:sapphire oscillator (11.0 fs) to 0.25 mJ/pulse at 1 kHz is demonstrated. Compression with a grating compressor and a LCD shaper yields near-Fourier-limited 11.8-fs pulses with an energy of 0.12 mJ. The amplifier is pumped by 532-nm pulses from a synchronized mode-locked laser, Nd:YAG amplifier system. This approach is shown to be promising for the next generation of ultrafast amplifiers aimed at producing terawatt-level phase-controlled few-cycle laser pulses.

Single-longitudinal-mode optical parametric oscillator for spectroscopic applications.

We have developed a tunable, narrow-bandwidth nanosecond optical parametric oscillator system and applied it to spectroscopic studies. The system consists of a narrow-bandwidth grazing-incidence oscillator and a seeded power oscillator, generating Fourier-transform-limited 1.5-ns pulses (bandwidth <500 MHz) in the wavelength range 435 to 2000 nm with energy of 3.5 mJ at a pump energy of 22 mJ. Continuous scanning over 30 to 100 GHz (depending on wavelength) is demonstrated by recording of the resonance line of the Hg atom at 253.7 nm and a vibrational transition of the CO (2) molecule at 1528 nm.

A Dense Grid of Reference Iodine Lines for Optical Frequency Calibration in the Range 571-596 nm

A high precision dense grid of reference lines in the hyperfine structure of the B-X system of molecular iodine (127I2) is presented. A simple parametrization has been derived, predicting the hyperfine line positions of the "t" components for vibrational bands (13-1) up to (18-1) and rotational quantum numbers J = (9-140). The analysis in this paper is based on Doppler-free saturation spectroscopy spectra used for calibration of more than 100 new components in the hyperfine structure of 127I2. The data presented contains a prediction (with 2 MHz accuracy) for the positions of 1584 "t" components in the rotational structure of iodine, covering the wavelength interval 571-596 nm. Copyright 1998 Academic Press. Copyright 1998Academic Press

A Dense Grid of Reference Iodine Lines for Optical Frequency Calibration in the Range 595-655 nm.

A dense grid of reference lines in the hyperfine structure of the B-X transitions of molecular iodine ((127)I(2)) is presented. Frequencies of 481 "t"-hyperfine components in 15 vibrational bands covering the wavelength range 595-655 nm were determined with an absolute accuracy of 1.0 MHz (1varsigma uncertainty). Spectra were obtained by Doppler-free saturation spectroscopy using a cw ring-dye laser of 1-MHz bandwidth. Center-of-gravity frequencies of rovibronic lines, determined by correcting for the calculated hyperfine shift, are included separately in a least-squares parametrization for each band. From the obtained molecular constants a prediction is made for all t-hyperfine components of the 15 bands covering 595-655 nm. Hence a dense grid of reference lines of 1 MHz (1varsigma) absolute accuracy is created, with a line in each interval of 1 cm(-1) in the entire range. This spectral atlas of about 3400 lines may provide a useful calibration tool in many laboratories. Copyright 2000 Academic Press.

Double-well states of ungerade symmetry in H2: first observation and comparison with Ab initio calculations.

The observation of a new class of long-lived outer well states of ungerade symmetry (B"B1Sigma+u) in molecular hydrogen, lying above the ionization threshold, is reported. Rovibrational levels within a potential extended over internuclear separations of R = 7-25 a.u. are experimentally investigated in a triple resonance scheme. Good agreement ( <0.5 cm(-1)) with updated ab initio calculations is found for vibrational levels up to v = 26, demonstrating that such calculations can now be extended to this energetic range above ionization, as long as interaction with the Rydberg manifolds is shielded by a barrier. The dynamical behavior (predissociation and autoionization) of this class of " u" symmetry states is remarkably different from similar outer well states of " g" symmetry; this phenomenon can be understood from the structure of doubly excited electronic states.

The air-abrasion technique versus the conventional acid-etching technique: A quantification of surface enamel loss and a comparison of shear bond strength.

The purpose of this study was to quantify the surface enamel loss that results when an air-abrasive technique is used and to compare the shear bond strength of different prebonding and bonding methods. Enamel loss was determined for 2 enamel-conditioning methods: acid etching with 37% phosphoric acid; and sandblasting with 50 microm aluminum oxide particles under different conditions. A profilometer was used to determine the surface enamel loss. Forty-two bovine teeth were divided into 7 groups (N = 6). The statistical comparison of the different groups was carried out by analysis of variance. The results showed that under certain conditions the enamel loss associated with sandblasting is equal to or smaller than that resulting from acid etching. In addition, the effectiveness of different prebonding and bonding techniques used in the bonding of orthodontic brackets was evaluated by means of shear bond strength measurements. For bonding, 1 resin and 1 glass ionomer cement were evaluated; for prebonding, a sandblaster, 2 different polyacrylic acids and phosphoric acid were tested. Seventy bovine teeth were divided into 7 groups (N = 10) and then stored in water for 24 hours. An Instrom testing machine was used to determine bond strengths, and the 7 groups were compared by means of analysis of variance. The significance of post hoc tests was predetermined at P <.003. The results showed that the bond strength of the sandblasted groups was significantly lower than that of the etching groups. This indicates that sandblasting is not an alternative for the acid-etching technique currently used in orthodontic practice.

Quantitative analysis of decay transients applied to a multimode pulsed cavity ringdown experiment.

The intensity and noise properties of decay transients obtained in a generic pulsed cavity ringdown experiment are analyzed experimentally and theoretically. A weighted nonlinear least-squares analysis of digitized decay transients is shown that avoids baseline offset effects that induce systematic deviations in the estimation of decay rates. As follows from simulations not only is it a method that provides correct estimates for the values of the fit parameters, but moreover it also yields a correct estimate of the precision of the fit parameters. It is shown experimentally that a properly aligned stable optical resonator can effectively yield monoexponential decays under multimode excitation. An on-line method has been developed, based on a statistical analysis of the noise properties of the decay transients, to align a stable resonator toward this monoexponential decay.

Simultaneous magneto-optical trapping of a boson-fermion mixture of metastable helium atoms.

We simultaneously confine fermionic metastable 3He atoms and bosonic metastable 4He atoms in a magneto-optical trap. The trapped clouds, containing up to 1.5 x 10(8) atoms of each isotope, are characterized by measuring ions and metastable helium atoms escaping from the trap. Optical pumping of 3He atoms to a nontrapped hyperfine state is investigated and it is shown that large atom numbers can be confined without additional repumping lasers. Unique possibilities for quantum degeneracy experiments with mixtures of spin-polarized metastable 3He and 4He atoms are indicated.

Photoassociation spectroscopy of cold he(2

We observe vibrational states by photoassociation spectroscopy of cold He(2 ^{3}S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 ^{3}S_{1}-2 ^{3}P_{2} transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.