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