Research Work

Present Status

Currently we are trying to build an atomic interferometer for detecting magnetic fields with high precision. Moreover, we are planning to manipulate the atom-atom interaction to investigate whether the measurement sensitivity gets enhanced.

Storing Optical Information in Atom

In presence of control field when the probe field passes through atomic medium, the medium apparently becomes transparent to the probe. This phenomenon is termed electromagnetically induced transparency [EIT] where the two transition probability-amplitude interferes destructively. Such ground-state coherences without an effective excited state are called dark state polaritons (DSPs). When the control field is adiabatically turned off, the probe pulse gets stored in those ground state coherences. We turn the control again to map the stored atomic excitation to optical field. In our experiment, the storage time is of the order of 1 μs.

Experimental Setup and Sequence

Trapped cold atomic ensemble of Rb-87 atoms is first pumped to the F=1 ground state manifold. Then they are further pumped to create the initial state as a mixed state of |F=1, m=-1⟩ and |F=1, m=1⟩ [ or state |F=1, m=0⟩ depending on the requirement ]. After initial state preparation, we coherently couple F=1 manifold with |F'=0, m=0⟩ by applying probe and control fields that are on resonance with the corresponding transition. Probe pulse is stored in the ground state coherence by adiabatically turning off the control field. We retrieve the stored pulse by turning on the control pulse again. The entire experimental sequence from laser cooling to storage and retrieval is done in a computer-controlled 13 ms cycle! The data is taken by repeating this cycle multiple times.

Laser cooling and magneto-optical trapping of Rb-87 atoms

We use Doppler cooling method to cool the cloud of millions of Rb-87 atoms. Laser beams (MOT & repumper) are sent through the atomic ensemble from six directions. As the laser frequencies are kept slightly less than the atomic transition frequencies, the high-velocity atoms will absorb less energy due to the Doppler effect. Therefore, they lose more energy while radiating and slow down. This method helps us bring the atomic cloud's temperature down to the order of milliKelvin. The trapping potential is generated by anti-Helmholtz coils.

Laser Frequency Locking

Laser frequency is locked using the beat-note technique. We use a master laser as a reference, whose frequency is fixed by saturation absorption spectroscopy. All the other lasers are made to beat with first the master laser and then the voltage-controlled oscillator before their frequency is locked. We make the locking circuits in our Lab.

Temperature and Current Controllers

We build electronic circuits for controlling the temperature and current of the laser. These temperature and current controllers bring the laser frequencies to the desired level.

External Cavity Diode Lasers [ECDL]

We assemble Lasers in our Lab. The components like laser diode [780 nm], thermistor, TEC, piezoelectric chips, reflective gratings, etc are bought from Thorlabs. The frame is manufactured by Physics Workshop of our institute. Electronic signal from the piezo controls the angle of the reflective grating. The desired frequency from the first order of diffraction grating is fed back to the laser diode for amplification.