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Journal of Physics Conference Series

Number Count Characteristics of Cold Atoms in a Magnetic Trap for Temperature Estimation

P Insang, Nuttida Kaewart, Nithiwadee Thaicharoen, N Chattrapiban

Published Jan 1, 2025• 0 citations
Publisher

Abstract

Abstract Temperature estimation of cold atoms in sub-Doppler to Doppler temperatures is crucial for determining various aspects such as diffusion, compressibility, trap stability, phase transition, and many-body collision dynamics. In this temperature regime, cold atoms are typically confined in a magnetic trap for further investigation. In some cases, temperature estimation from a typical time-of-flight method is not applicable due to the long switch-off time of the magnetic trap. In this work, we employ a non-invasive number counting method without having to turn off the magnetic field to estimate the temperature. We explore various number count conditions for a temperature range between 20 µK to 100 µK in such a way that the trace of number counts can be used as a reference to estimate the temperature of cold atoms in a magnetic trap.

DOI
10.1088/1742-6596/2934/1/012020

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Abstract Temperature estimation of cold atoms in sub-Doppler to Doppler temperatures is crucial for determining various aspects such as diffusion, compressibility, trap stability, phase transition, and many-body collision dynamics. In this temperature regime, cold atoms are typically confined in a magnetic trap for further investigation. In some cases, temperature estimation from a typical time-of-flight method is not applicable due to the long switch-off time of the magnetic trap. In this work, we employ a non-invasive number counting method without having to turn off the magnetic field to estimate the temperature. We explore various number count conditions for a temperature range between 20 µK to 100 µK in such a way that the trace of number counts can be used as a reference to estimate the temperature of cold atoms in a magnetic trap.
For the first time ultra-cold thulium atoms were trapped in a magnetic quadrupole trap with a small field gradient (20 Gs cm -1 ). The atoms were loaded from a cloud containing 4 × 10 5 atoms that were preliminarily cooled in a magneto-optical trap to the sub-Doppler temperature of 80 μK. As many as 4 × 10 4 atoms were trapped in the magnetic trap at the temperature of 40 μK. By the character of trap population decay the lifetime of atoms was determined (0.5 s) and an upper estimate was obtained for the rate constant of inelastic binary collisions for spin-polarised thulium atoms in the ground state (g in < 10 -11 cm 3 s -1 ).
Temperatures of rubidium atoms confined in a magneto-optical trap have been measured using time-of-flight. By varying the parameters of the trap, we find that lower temperatures are obtained with lower laser intensities, larger laser detunings, lower atomic densities, and with the 85 vs. the 87 isotope. Temperatures are below the Doppler limit (140 {mu}K for Rb) in most cases. By measuring the size of the trapped cloud as well as the temperature, we can infer the spring constant of the trap. For comparison purposes, we can measure temperatures in optical molasses by turning off the magnetic field of the trap.
We present a method for measuring the average temperature of a cloud of cold 85 Rb atoms in a magneto-optical trap using an optical nanofibre. A periodic spatial variation is applied to the magnetic fields generated by the trapping coils and this causes the trap centre to oscillate, which, in turn, causes the cloud of cold atoms to oscillate. The optical nanofibre is used to collect the fluorescence emitted by the cold atoms, and the frequency response between the motion of the centre of the oscillating trap and the cloud of atoms is determined. This allows us to make measurements of cloud temperature both above and below the Doppler limit, thereby paving the way for nanofibres to be integrated with ultracold atoms for hybrid quantum devices.
Based on the real sense of the time-of-flight , we demonstrate an alternative method of measuring the temperature of cold atoms in magneto-optical traps (MOTs) using a high-finesse optical microcavity, which acts as a pointlike single-atom counter. A cloud of atoms trapped in magneto-optical traps is positioned about 5 mm above the cavity, and the atoms fall freely down through the cavity. The temperature of the cold atoms in the MOT is determined by counting the exact arrival times of the single atoms. A theoretical model based on a ballistic expansion of a cloud of trapped atoms falling in the earth’s gravitational field is used to fit the probability distribution of atom arrivals, and the fittings agree very well with the experimental results. This method could be used for systems with little room, where an extra probe beam is hard to involve, or with fewer atoms initially.
Over the past decade there have been a lot of remarkable progress in cooling,trapping and manipulating atoms.The precise measurement of temperature and density of ultra-cold atoms becomes an important subject.In the measurement of the temperature of cold atoms,one needs to turn off the magnetic field and the light field in a very short time(less than several milliseconds).According to the requirements of the time-flying method in measuring the cold-atom temperature,one can design and make a pulse generator whose pulse-width and delay time are controllable.Then the pulses are applied to the switches of magneto-optical trap,and the magnetic and light field can be turned off in short time successfully.
We use a magneto-optical trap as an integrating counter for descending cold atoms. In the case of cold 87 Rb atoms with an initial mean temperature of 9 µK, 60% of the total number of 2.6 × 10 7 are captured after a 26-cm free-fall and trapped for a long time of 270 s. From the capture efficiency and the decay time, the minimum detectable flux intensity of a cold 87 Rb atomic beam is estimated to be 2 × 10 5 atom/cm 2 ·s. The atom-counting system is used for detection and evaluation of a cold atomic beam produced out of a near-field optical funnel.