Single-domain Bose condensate magnetometer achieves energy resolution per bandwidth below ℏ.

We present a magnetic sensor with energy resolution per bandwidth [Formula: see text] We show how a Rb single-domain spinor Bose-Einstein condensate, detected by nondestructive Faraday rotation probing, achieves single-shot low-frequency magnetic sensitivity of 72(8) fT measuring a volume [Formula: see text] for 3.5 s, and thus, [Formula: see text] We measure experimentally the condensate volume, spin coherence time, and readout noise and use phase space methods, backed by three-dimensional mean-field simulations, to compute the spin noise. Contributions to the spin noise include one-body and three-body losses and shearing of the projection noise distribution, due to competition of ferromagnetic contact interactions and quadratic Zeeman shifts.

Absolute frequency references at 1529 and 1560 nm using modulation transfer spectroscopy.

We demonstrate a double optical frequency reference (1529 and 1560 nm) for the telecom C-band using 87Rb modulation transfer spectroscopy. The two reference frequencies are defined by the 5S(1/2)F=2→5P(3/2)F'=3 two-level and 5S(1/2)F=2→5P(3/2)F'=3→4D(5/2)F''=4 ladder transitions. We examine the sensitivity of the frequency stabilization to probe power and magnetic field fluctuations, calculate its frequency shift due to residual amplitude modulation, and estimate its shift due to gas collisions.