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 fermion and the boson HBT effects realised in the same apparatus with two different isotopes of helium, 3 He (a fermion) and 4 He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible, and the contrasting bunching and antibunching behaviours can be fully attributed to the different quantum statistics. Our result shows how atom-atom correlation measurements can be used not only for revealing details in the spatial density 7,8 or momentum correlations 9 in an atomic ensemble, but also to directly observe phase 2 effects linked to the quantum statistics in a many body system. It may thus find applications to study more exotic situations 10 .Two-particle correlation analysis is an increasingly important method for studying complex quantum phases of ultracold atoms 7,8,9,10,11,12,13 . It goes back to the discovery by Hanbury Brown and Twiss 1 , that photons emitted by a chaotic (incoherent) light source tend to be bunched: the joint detection probability is enhanced, compared to that of statistically independent particles, when the two detectors are close together.Although the effect is easily understood in the context of classical wave optics 14 , it took some time to find a clear quantum interpretation 3,15 . The explanation relies upon interference between the quantum amplitude for two particles, emitted from two source points S 1 and S 2 , to be detected at two detection points D 1 and D 2 (see fig. 1). For bosons, the two amplitudes D S D S must be added, which yields a factor of 2 excess in the joint detection probability, if the two amplitudes have the same phase. The sum over all pairs (S 1 ,S 2 ) of source points washes out the interference, unless the distance between the detectors is small enough that the phase difference between the amplitudes is less than one radian, or equivalently if the two detectors are separated by a distance less than the coherence length. Study of the joint detection rates vs. detector separation along the i-direction then reveals a bump whose width l i is the coherence length along that axis 1,5,16,17,18,19 . For a source size s i along i (standard half width at e -1/2 of a Gaussian density profile), one has a half width at 1/e of l i = ht / 2πms i , where m is the mass of the particle, t the time of flight from the source to the detector, and h Planck's constant. This formula is the analogue of the formula l i = Lλ / 2πs i for photons if one identifies λ = h / mv with the de Broglie wavelength for particles travelling at velocity v = L / t from the source to the detector.For indistinguishable fermions, the two-body wave function is antisymmetric, and the two amplitudes must be subtracted, yielding a null probability for joint detection in the same coherence volume. In the language of particles, it means th...

With a phase-modulated extreme ultraviolet pulsed laser source the frequency of the 1 1 S-2 1 P transition of helium at 58 nm has been measured. The phase modulation scheme enabled measurement and reduction of frequency chirp, usually limiting pulsed precision spectroscopy. From the measured transition frequency of 5 130 495 083͑45͒ MHz, a fourfold improved value of the ground state Lamb shift of 41 224͑45͒ MHz is deduced, in good agreement with a theoretical value of 41 233͑35͒ MHz based on QED calculations up to order ␣ 5 Z 6 . From these measurements, the well-known binding energy of the 2 1 P state and the previously determined 4 He-3 He isotope shift, accurate values for the ionization energies of the helium atom follow: 198 310.6672 (15) cm Ϫ1 for 4 He and 198 301.8808(15) cm Ϫ1 for 3 He. ͓S1050-2947͑97͒05403-6͔ PACS number͑s͒: 32.30.Jc, 12.20.Fv, 42.65.Ky

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Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 Â 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude.In recent years, the invention of the femtosecond frequency comb laser (1-3) has brought about a revolution in metrology. A frequency comb acts as a bridge between the radio frequency (RF) domain (typically tens of MHz) and the optical frequency domain (typically hundreds of THz). Thus, in precision spectroscopy, the optical cycles of a continuous wave (CW) ultrastable laser can be phase-locked and counted directly with respect to an absolute frequency standard such as an atomic clock (4, 5). The resultant frequency measurements approach a precision of 1 part in 10 15 in certain cases, potentially enabling the detection of possible drift in the fundamental constants (6, 7), among other quantum mechanical applications.Here, we perform precision metrology without the use of a CW laser. Instead, an atomic transition is excited directly with amplified and frequency-converted pulses from a femtosecond frequency comb laser. As a result of quantum interference effects in the atomic excitation process, we can achieve an accuracy that is about six orders of magnitude higher than the optical bandwidth of the individual laser pulses.The method used is related to Ramsey_s principle of separated oscillatory fields (8), which probes the phase evolution of an atom in spatially separated interaction zones. This technique is widely used in the RF domain for atomic fountain clocks (9). By extension, in the optical domain, excitation can be performed by pulses separated in time (rather than in space) to maintain phase coherence between the excitation contributions. Several experiments have been performed to investigate Ramsey-type quantum interference fringes in the optical domain (10 -14) and phase-stable amplification of single pulses (15). Actual quantitative spectroscopy with phase-coherent oscillator pulses has been limited to a few relative frequency measurements on fine and hyperfine structure of atoms (13,14,16) and relative and absolute measurements on rubidium (17); absolute frequency measurements with amplified pulses have been frustrated by an unknown phase difference between the pulses or by limited resolution.We generate powerful laser pulses with a precise phase relationship by amplifyin...

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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.

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We report loading of 1.5ϫ10 9 metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams. We fully characterized this trap by measuring trap losses and absorption of a probe beam. From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: 5.3(9)ϫ10 Ϫ9 cm 3 /s at Ϫ35 MHz and 3.7(6)ϫ10 Ϫ9 cm 3 /s at Ϫ44 MHz. Also, we find that the loss rate is maximum at Ϫ5 MHz detuning, where the rate is 1.3(3)ϫ10 Ϫ8 cm 3 /s, much larger than recent theoretical and experimental values. In the absence of light the S-S ionization rate constant is measured to be 1.3(2)ϫ10 Ϫ10 cm 3 /s.