We measure atom number statistics after splitting a gas of ultracold Rb87 atoms in a purely magnetic double-well potential created on an atom chip. Well below the critical temperature for Bose-Einstein condensation Tc, we observe reduced fluctuations down to -4.9\,\,dB below the atom shot noise level. Fluctuations rise to more than +3.8\,\,dB close to Tc, before reaching the shot noise level for higher temperatures. We use two-mode and classical field simulations to model these results. This allows us to confirm that the supershot noise fluctuations directly originate from quantum statistics.

The collision-induced Zeeman relaxation rate for collisions of CaF $X^2\Sigma (v^{\prime \prime }=0)$ with ${}^3\mathrm{He}$ is measured to be ${\Gamma }_{\mathrm{Z}}=(7.7+5.4/-2.5)\times {10}^{-15}{\mathrm{cm}}^3/\mathrm{s}$ at 2 K. This rate is a direct measurement of the influence of spin-rotation coupling on Zeeman relaxation in the first rotational level of CaF. The relationship of this rate to known molecular constants is consistent with recent theory of cold molecular collisions and outlines the ${}^2\Sigma$ molecules conducive to magnetic trapping.

The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for mode locking and, hence, pulse formation. Even so, the clear mode locking of terahertz (THz) QCLs has been demonstrated recently, but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using the phase-resolved detection of the electric field in QCLs embedded in metal–metal waveguides, we demonstrate that active mode locking requires the phase velocity of the microwave round-trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short-pulse generation. Mode locking was performed on QCLs employing phonon depopulation active regions, permitting the coherent detection of large gain bandwidths (500 GHz) and the generation of 11 ps pulses centered around 2.6 THz when the above “phase-matching” condition is satisfied. This work brings an enhanced understanding of QCL mode locking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.

Pulses from an actively modelocked terahertz quantum cascade laser are fully characterized using an optical sampling technique to detect the total instantaneous terahertz intensity. By triggering the quantum cascade laser electronics with a femtosecond laser, we are able to measure both the formation of modelocked pulses and the quasi-steady state. The dependence of the pulse width on the modulation power and drive current are investigated. At low drive currents, we measure transform-limited gaussian-shaped pulses with a FWHM of 19 ps.

A novel scheme to achieve mode-locking of a multimode laser is demonstrated. Traditional methods to produce ultrashort laser pulses are based on modulating the cavity gain or losses at the cavity roundtrip frequency, favoring the pulsed emission. Here, we rather directly act on the phases of the modes, resulting in constructive interference for the appropriated phase relationship. This was performed on a terahertz quantum cascade laser by multimode injection seeding with an external terahertz pulse, resulting in phase mode-locked terahertz laser pulses of 9ps duration, characterized unambiguously in the time domain.