Evaporative cooling by pulse width modulation (PWM) of optical dipole traps

TL;DR

This work addresses evaporative cooling in optical dipole traps without lowering trap laser power by employing pulse-width modulation (PWM) to create a time-averaged dipole potential. The trap depth is controlled via the duty cycle $D$ of high-frequency switching, with $\langle I(T)\rangle = D I_{\max}$ when $I_{\min}=0$, and the potential $U_{dipole}(\mathbf{r})$ scales accordingly; ramping $D$ reduces the effective trap depth to drive evaporation while keeping instantaneous power constant. The experiments demonstrate that, for modulation frequencies well above trap frequencies, cooling is achievable with modest atom loss, achieving $T$ down to $3\ \mu\text{K}$ from $120\ \mu\text{K}$ in 1 s and increasing phase-space density by about four orders of magnitude to $\sim 10^{-2}$ (optimal near $700\ \text{kHz}$). This PWM approach offers a simple, robust alternative to conventional evaporation, with potential benefits for quantum computation, precision spectroscopy, and microgravity experiments due to its constant trap power and digital control.

Abstract

We introduce a method for cooling atoms in an optical dipole trap using pulse-width modulation (PWM) technique, without reducing the laser power of the dipole trap. The PWM technique involves digital modulation of the trap at a fixed frequency. The effective time-averaged dipole potential is lowered by adjusting the duty cycle of the modulation, thereby implementing evaporative cooling. We show that, this technique effectively reduces temperature and enhances phase space density. A comparison with the standard method of evaporative cooling has also been made. Apart from the atom loss due to reduction of the effective trapping potential, we observe an additional loss channel originating from the lack of trapping potential during the trap off time. This atom loss is observed at different modulation frequencies which are an order of magnitude higher compared to trapping frequency of dipole trap. The PWM technique provides an alternative to traditional evaporative cooling in scenarios where it is preferred that the laser power of the trap should be constant.

Figures (7)

• Figure 1: Diagram illustrating the schematic arrangement of the PWM technique utilized for cooling atoms by time-averaged optical dipole trap.
• Figure 2: An optimized time-averaged potential (black solid line) created by numerical integration of PWM waveform. We experimentally generate this potential by changing the duty cycle ( blue markers) linearly in different time-segment.
• Figure 3: No. of atoms after 1 s hold time with different modulation frequency and fixed 50% duty cycle. Here, markers are experimental data and solid line is linear fit.
• Figure 4: Comparison of the loss of the atoms with and without modulation at fixed power. Here, markers are experimental data and solid lines are exponential fit.
• Figure 5: Evaporative cooling is shown with a decreasing duty cycle over time in blue, corresponding to a decrease in temperature in red. A time-of-flight image of cold atoms on the right, showing the state before evaporation and after 1 second of evaporation by PWM.