Thursday, January 22, 2009

Noise-supressed large area atom interferometers

A paper-review

Atom interferometers can use just cold atoms or a BEC. The difference is that a BEC is a coherent source of cold atoms like a LASER unlike a beam of cold atoms which is at temperatures much above the BEC temperature. The researchers all around the world are working to realize more sensitive and more efficient atom interferometers by using either of these sources. One reason for that is - an atom interferometer is much more sensitive than an optical interferometer in the sense that the signal-to-noise ratio of an atom interferometer is greater than 10^11, compared to an optical interferometer of comparable area and the particle flux. Therefore, atom interferometers can prove to be more effective in precision measurements.

Atom interferometers are of trapped-atom-type or free-space-type. Recently, Herrmann et. al. have shown that a pair of simultaneous conjugate Ramsey-Borde atom interferometers can be operated at large momentum transfer to cold atoms from the optical pulses to supress the vibrational noise and to enhance the enclosed space-time area by a factor of 2500 of the area of the existing atom interferometers. They have used this interferometer to measure the fine structure constant more precisely.
A cold atom beam of Cs-133 shot vertically upward in a space of about a meter high is split by using a pi/2 laser pulse. It is split again after an interval of time by using another pi/2 pulse and two simultaneous interferometers are formed by using further two consecutive pi/2 pulses before they are finally recombined. Each atom can be given a momentum as large as that of the momenta of twenty optical photons (20*hbar*k). The total phase gained by the atoms during the interferometric cycle time has three contributions - the phase due to recoil velocity of the atoms, the phase due to free-evolution of the wave packet between the beam splitters and the phase due to the interaction of the wave packet with beam-splitting pulses. By operating both interferometers simultaneously, the phase due to free evolution and the effects of noise/vibrations can be subtracted off, leaving the contributions of the recoil velocity and the splitting pulses only. This is how the simultaneous conjugate Ramsey-Borde atom interferometers can be operated to get a large area without losing contrast.

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