Efficient lambda-enhanced gray molasses using an EIT-based laser locking scheme

TL;DR

This work addresses the challenge of achieving sub-Doppler cooling in non-ideal beam geometries with a low-cost locking scheme. It implements lambda-enhanced gray molasses using two independently diode lasers stabilized via an EIT-based offset lock, enabling coherent dark-state cooling without GHz electronics. Experimental results include cooling $^{87}$Rb from $T\approx 43\,\mu\text{K}$ to $T=6.8\pm0.9\,\mu\text{K}$, supported by wave-function Monte Carlo modelling that reproduces the observed dark-state dynamics and momentum distributions. The combination of a simple locking scheme and adaptable beam geometry provides a scalable, cost-effective pathway to higher-fidelity neutral-atom devices for quantum computing and sensing, with strong agreement between theory and experiment.

Abstract

We present a novel implementation of lambda-enhanced gray molasses cooling in a non-standard beam geometry and with an inexpensive laser locking set-up. In contrast to the established use of resource-intensive phase locking methods, our laser system uses two independent lasers, frequency -locked to a spectral feature produced by an electromagnetically induced transparency (EIT) resonance. We show that this approach achieves sufficient coherence to enable effective gray molasses cooling without the need for costly GHz electronics, significantly reducing the complexity and cost of experimental setups and represents a step toward more accessible cold atom technologies. A wave-function Monte Carlo analysis supports the experimental findings, offering insight into the cooling dynamics of this unconventional scheme

Figures (8)

• Figure 1: The EIT offset locking scheme. The DL pro laser is stabilised to the $F=2\to F^\prime = 1,3$ saturated absorption crossover peak (the saturated absorption spectrometer is omitted for clarity). The DL 100 laser is phase modulated with a 10 MHz EOM, and then mixed on a polarizing beamsplitter cube with the stabilised DL pro laser. Both are then passed through a quarter waveplate (QWP) and the rubidium vapour cell, located in a $\upmu-$metal housing.
• Figure 2: The EIT signal (blue) and the derived error signal (black), and the energy levels and laser scheme. The power-broadened width of the EIT feature signal is adjustable down to about 750 kHz, likely limited by the linewidth of the free running DL 100, which is nominally $\sim 1\,\text{MHz}$. In this figure, the EIT signal is fit with a sum of two Lorentzians and here gives an EIT-linewidth of $2.08\pm 0.03\,\text{ MHz}$. The coupling laser is tuned blue of the $F=2\to F^\prime=2$ transition by $\Delta$. The Raman laser is tuned very near to Raman resonance.
• Figure 3: (a) The two gray molasses cooling beams are each down-shifted by $\sim 200\,\text{MHz}$ by an AOM, and then combined on a beam splitter. One of the output beams is selected for the experiment, and up-shifted by $\sim 160\,\text{MHz}$, so that the resulting cooling beams are shifted $\sim2\Gamma$ blue of the $F\to F^\prime = 2$ transitions. Finally, the beams are split on a polarizing beam splitter cube, with 1/3 of the incident power being sent to the vertical cooling beam and 2/3 sent to the horizontal cooling beams. In (b), the non-standard arrangement of the horizontal beams is depicted, showing the inclination of the beams from the horizontal $x-$axis and the location of the dipole beam shaping lenses symmetrically located on the $z$-axis.
• Figure 4: TOF analysis of the gray molasses cooled cloud. The temperature, extracted from the least-squares best-fit, is $T=6.8\pm0.9\,\upmu\text{K}$, and represents an improvement by a factor of 1/7 over our standard optical molasses cooling.
• Figure 5: F=2 state population as a function of $\langle\omega\rangle/ \langle\Omega\rangle$, where $\langle\Omega\rangle = 1.8\,\Gamma$. The experimental points (red dots) are measured using state selective fluorescence.