Magnetic flux noise in dc SQUIDs: temperature and geometry dependence.

The spectral density S(Φ)(f) = A(2)/(f/1 Hz)(α) of magnetic flux noise in ten dc superconducting quantum interference devices (SQUIDs) with systematically varied geometries shows that α increases as the temperature is lowered; in so doing, each spectrum pivots about a nearly constant frequency. The mean-square flux noise, inferred by integrating the power spectra, grows rapidly with temperature and at a given temperature is approximately independent of the outer dimension of a given SQUID. These results are incompatible with a model based on the random reversal of independent, surface spins.

Broadband loop gap resonator for nitrogen vacancy centers in diamond.

We present an S-band tunable loop gap resonator (LGR), which provides strong, homogeneous, and directionally uniform broadband microwave (MW) drive for nitrogen-vacancy (NV) ensembles. With 42 dBm of input power, the composite device provides drive field amplitudes approaching 5 G over a circular area ≳50 mm2 or cylindrical volume ≳250 mm3. The wide 80 MHz device bandwidth allows driving all NV Zeeman resonances for bias magnetic fields below 20 G. The device realizes percent-scale MW drive inhomogeneity; we measure a fractional root-mean-square inhomogeneity σ rms = 1.6% and a peak-to-peak variation σ pp = 3% over a circular area of 11 mm2 and σ rms = 3.2% and σ pp = 10.5% over a larger 32 mm2 circular area. We demonstrate incident MW power coupling to the LGR using two methodologies: a printed circuit board-fabricated exciter antenna for deployed compact bulk sensors and an inductive coupling coil suitable for microscope-style imaging. The inductive coupling coil allows for approximately 2π steradian combined optical access above and below the device, ideal for envisioned and existing NV imaging and bulk sensing applications.

Generation of 20 GHz, sub-40 fs pulses at 960 nm via repetition-rate multiplication.

Optical filtering of a stabilized 1 GHz optical frequency comb produces a 20 GHz comb with approximately 40 nm bandwidth (FWHM) at 960 nm. Use of a low-finesse Fabry-Pérot cavity in a double-pass configuration provides a broad cavity coupling bandwidth (Deltalambda/lambda approximately 10%) and large suppression (50 dB) of unwanted modes. Pulse durations shorter than 40 fs with less than 2% residual amplitude modulation are achieved.