We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822+/-3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.

The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40 μs at its flux-insensitive point. Qubit relaxation times T1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f-flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T2≈85 μs, approximately the 2T1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary qubits based on transverse qubit–resonator interaction.

We investigate the recovery of superconducting NbN-nanowire photon counters after detection of an optical pulse at a wavelength of 1550 nm, and present a model that quantitatively accounts for our observations. The reset time is found to be limited by the large kinetic inductance of these nanowires, which forces a tradeoff between counting rate and either detection efficiency or active area. Devices of usable size and high detection efficiency are found to have reset times orders of magnitude longer than their intrinsic photoresponse time. . Of particular interest would be a detector that combines ultrafast count rates (≥ GHz) with high single-photon detection efficiency at near-infrared wavelengths; however, current near-infrared photon-counting technologies such as avalanche photodiodes [6] and photomultiplier tubes [7] are limited to much lower count rates by long reset times.A promising detector technology was reported recently, in which ultrathin superconducting NbN wires are biased with a DC current I bias slightly below the critical value I C [8]. An incident photon of sufficient energy can produce a resistive "hotspot" which in turn disrupts the superconductivity across the wire, resulting in a voltage pulse. Observations of this photoresponse showed promise for high counting rates, with measured intrinsic response times as low as ∼30 ps [9], and counting rates in the GHz regime [10,11]. In this Letter, we present our own investigation into the counting-rate limitation of these devices, in which we directly observe the recovery of the detection efficiency as the device resets (after a detection event), and develop a quantitative model of this process. We find that detectors having both high detection efficiency and usable active area are limited to much lower count rates than studies of their intrinsic response time had suggested [9].We fabricated our nanowires using a newly developed process [12], on ultrathin (3 − 5 nm) NbN films [13]. We used several geometries, including straight nanowires having widths from 20−400 nm and lengths from 0.5−50 µm, as well as large-area "meander" structures [8,10] (e.g., Fig. 1(b)) having active-area aspect ratios from 1 − 50, fill factors from 25 − 50%, and sizes up to 10-µm square. The devices had critical temperatures T C ∼ 9 − 10 K, and critical current densities J C ∼ 2 − 5 × 10 10

We have fabricated and tested superconducting single-photon detectors and demonstrated detection efficiencies of 57% at 1550-nm wavelength and 67% at 1064 nm. In addition to the peak detection efficiency, a median detection efficiency of 47.7% was measured over 132 devices at 1550 nm. These measurements were made at 1.8K, with each device biased to 97.5% of its critical current. The high detection efficiencies resulted from the addition of an optical cavity and anti-reflection coating to a nanowire photodetector, creating an integrated nanoelectrophotonic device with enhanced performance relative to the original device. Here, the testing apparatus and the fabrication process are presented. The detection efficiency of devices before and after the addition of optical elements is also reported.

We have produced ultracold, polar RbCs * molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the RbCs * molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound X 1 Σ + ground state RbCs vibrational levels as high as 5×10 5 s −1 per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.PACS numbers: 32.80. Pj, 33.80.Ps, 34.50.Gb, 34.50.Rk Ultracold polar molecules, due to their strong, longrange, anisotropic dipole-dipole interactions, may provide access to qualitatively new regimes previously inaccessible to ultracold atomic and molecular systems. For example, they might be used as the qubits of a scalable quantum computer [1]. New types of highly-correlated many-body quantum states could become accessible such as BCS-like superfluids [2], supersolid and checkerboard states [3], or "electronic" liquid crystal phases [4]. Ultracold chemical reactions between polar molecules have been discussed [5], and might be controlled using electric fields [6]. Finally, the sensitivity of current moleculebased searches for violations of fundamental symmetries [7] might be increased to unprecedented levels.Cold, trapped polar molecules have so far only been produced using either buffer-gas cooling [8] or Starkslowing [9], at temperatures of ∼10-100 mK [8,9]. This is much higher than the ∼1-100 µK accessible with atoms, and attempts to bridge this gap with evaporative cooling may run afoul of predicted molecular Feshbach resonances [10] or inelastic losses [11].Another approach is to extend well-known techniques for producing ultracold (non-polar) homonuclear diatomic molecules in binary collisions of ultracold atoms, either through photoassociation [12,13,14,15,16], or Feshbach resonance [17,18]. In these methods, the translational and rotational temperatures of the molecules are limited only by the initial atomic sample, possibly providing access all the way to the quantum-degenerate regime [15,18]. An important limitation, however, is that the molecules are typically formed in weakly bound vibrational levels near dissociation, which may have vanishing electric dipole moments [19], and are unstable with respect to inelastic collisions [10,11,15]; therefore, a method for transferring them to the vibrational ground state is desirable [14].Several authors have discussed the extension of these methods to the formation of (heteronuclear) polar molecules in collisions between different atomic species [20,21,22,23]. In recent experiments NaCs + and RbCs + ions formed in the presence of near-resonant light have indeed been observed in small numbers [21]; however, these observations did not permit an analysis of their formation mechanism, nor demonstrate a method for producing neutral, ultracold polar molecules.In this Letter, we ...

We demonstrate a simple, general purpose method to cool neutral atoms. A sample containing 3x10(8) cesium atoms prepared in a magneto-optical trap is cooled and simultaneously spin polarized in 10 ms at a density of 1.1x10(11) cm (-3) to a phase space density nlambda(3)(dB) = 1/500, which is almost 3 orders of magnitude higher than attainable in free space with optical molasses. The technique is based on 3D degenerate Raman sideband cooling in optical lattices and remains efficient even at densities where the mean lattice site occupation is close to unity.

We have observed and located more than 60 magnetic field-induced Feshbach resonances in ultracold collisions of ground-state 133 Cs atoms. Multiple extremely weak Feshbach resonances associated with g-wave molecular states are detected through variations in the radiative collision cross sections. The Feshbach spectroscopy allows us to determine the interactions between ultracold cesium atoms and the molecular energy structure near the dissociation continuum with unprecedented precision. Our work not only represents a very successful collaboration of experimental and theoretical efforts, but also provides essential information for cesium Bose-Einstein condensation, Cs 2 molecules, and atomic clock experiments.

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