Indian Journal of Pure & Applied Physics Vol. 49, September 2011, pp. 590-595 Characterization of cold atomic cloud in a magneto-optical trap Poonam Arora, Swatilekha Chowdhury, Ashish Agarwal, Kavindra Pant & Amitava Sen Gupta Time & Frequency Division, National Physical Laboratory (CSIR), New Delhi 110 012, India E-mail: arorap@mail.nplindia.ernet.in Received 10 May 2011; revised 30 June 2011; accepted 20 July 2011 The results of characterization of cold cloud of cesium (Cs) atoms trapped in a magneto-optical trap (MOT) have been presented. The MOT is a part of the Physics package of the Cs Fountain Clock being developed at National Physical Laboratory (NPL), India. The number of atoms, size and temperature of the cloud have been measured and calculated. It is also been investigated how the number of trapped atoms changes with the trapping laser’s beam intensity, detuning and magnetic-field gradient of the trap. Keywords: Laser cooling and trapping, Frequency control, Magnetic field 1 Introduction Cold atoms produced using laser cooling and trapping techniques have been widely used for high- resolution spectroscopy 1,2 , precision measurements 3-6 , highly precise atomic clocks 7,8 etc. A magneto-optical trap (MOT) is, generally, used to create a cold, dense cloud of atoms 9,10 . Some of the crucial experiments performed with a MOT are cold collisions 11 , quantum degenerate gases experiments 12 and atomic standards of time and frequency. We have been developing a primary standard of time and frequency namely a Cs fountain clock at National Physical Laboratory (NPL) India and the MOT is a part of the Physics package of the clock. In a fountain clock, atoms are first cooled and trapped in a MOT and then launched up to about a meter high and fall under gravity. The cold atomic cloud interacts with microwaves on the way up and down and undergoes state interrogation in the detection zone. This way the frequency of an external oscillator is locked to the transition frequency of the atomic hyperfine states to form an extremely accurate clock. MOT is the first and most crucial part of the fountain clock. In our fountain clock, the MOT has been successfully demonstrated with a trapped Cs atomic cloud recently. Once the cloud is formed, measurements are done to characterize the cloud in terms of size of atomic cloud, number of atoms in the cloud, temperature of the cloud. Particularly for the fountain clock, parameters like temperature and number of atoms in the MOT are critical. The density/number of atoms are optimized in a way to have maximum S/N ratio with minimum frequency shifts. The temperature has to be low enough (<5 μK) (small velocity/kinetic energy) in order to keep all the trapped atoms together during the flight time. In order to optimize the performance of the fountain clock, measurements of these parameters become essential. This paper is a comprehensive report on details of measurement, calculations and results of characterization of cold atomic cloud trapped in a MOT. 2 Experimental Details The optical system as shown in Fig. 1 delivers three mutually orthogonal pairs of counter-propagating laser beams, which are well balanced with respect to their intensities (5 mW/cm 2 each) and have diameters of about 15 mm each for the horizontal beams and 10 mm each for the vertical beams. There are two vertical beams-upward and downward (z-axis) and four horizontal beams, counter-propagating along the x and the y-axis of a cartesian coordinate system, respectively. Besides the six cooling beams, the optical system delivers two beams for detection after the fountain operation cycle. The optical set-up provides the means for changing the laser intensity and frequency ν c in a precisely controlled way in order to properly cool, launch and detect the atoms. Generation of cooling, re-pump, launch and detection beams, and finally coupling them into eight single mode polarization maintaining (SM-PM) optical fibers is done on an environmentally controlled vibration free optical table of dimensions 1 m×1.9 m. An extended cavity diode laser (ECDL) in Littrow configuration is frequency locked to a Cs D 2 line