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Journal of Physics B Atomic and Molecular Physics

Experimental study of quantum fluctuations of polarisation in superradiance

A. Crubellier, Sylvain Liberman, P. Pillet, Marie-Gabrielle Schweighofer

Published Mar 14, 1981• 21 citations
Publisher

Abstract

The superradiant light emitted on the transition 6p2P1/2 to 6s2S1/2 of Rb I has been found to be roughly linearly polarised with a polarisation direction changing randomly from pulse to pulse. The stochastic distribution of the polarisation which provides evidence of the quantum fluctuations of a two-level system has been investigated.
DOI
10.1088/0022-3700/14/5/004

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The authors report on an experimental study of intensity and delay fluctuations for a two-level system by analysing stochastic distribution of the polarisation of superradiant pulses emitted on the transition 6p 2 P 1 /2 to 6s 2 S 1/2 of rubidium atoms. They develop a theoretical interpretation of their results. Finally, use of these properties of polarisation fluctuations has permitted one to study the very beginning of the superradiant emission and to give a new estimate of the so-called 'tipping angle'.
Describes a theoretical study of the polarisation properties of the superradiant light emitted on a j to j' transition between degenerate levels. Two specific orthogonal polarisation vectors are associated with each set of initial conditions and the corresponding field components are shown to evolve independently during the whole linear-emission period. Due to a quenching effect, the polarisation of the emitted light is predicted to be identical to one of the two specific polarisations of the superradiant system if the intensities of the two field components are not equal during this period. If, on the other hand, the two intensities remain equal, a spontaneous symmetry breaking is expected to occur and consequently the polarisation of the superradiant light is expected to vary randomly from pulse to pulse.
Using laser excitation of the 6 2 P levels in the natural isotopic mixture of Rb atoms, modulation of superradiance intensity from some transitions to the lower levels was observed. On the transition 6 2 P 1/2 -6 2 S 1/2 this effect is interpreted as a quantum beat between the hyperfine-structure transitions of the Rb 85 and Rb 87 isotopes. A nearly 'single-pulse' superradiant signal was obtained on the 6 2 P-4 2 D transitions. When a magnetic field was applied transverse to the direction of the laser beam, the pulses obtained showed the properties forecast by two-level superradiance theory. Preliminary results are reported.
The anticorrelated fluctuation of the intensity of a driver beam pumping a \ensuremath{\Lambda}-like three-level system in the first transition from the initial to the intermediate level and that of the intensity of the radiation generated in the second transition from the intermediate to the final level initiated by noise is observed in an experimental arrangement designed for the investigation of far-infrared Dicke's superradiance. The fluctuation is interpreted as the result of a two-photon back-and-forth transition between the initial and the final level, emitting (absorbing) and absorbing (emitting) photons in the first and the second transition depending on the direction of the two-photon transition. The influence of this two-photon back-and-forth transition on the Dicke's superradiance is discussed.
We investigate the statistical properties of superradiant pulses emitted by a system of many atoms. By using a representation of the atomic density operator in terms of directed angular-momentum states (which are also known as atomic coherent states), we find that the statistical behavior of the atoms is formally equivalent to that of a harmonic oscillator subject to linear amplitude amplification and driven by Gaussian white noise. The emitted light pulses are then found to exhibit very large quantum fluctuations for atomic initial states corresponding to complete or nearly complete excitation. For all other atomic initial states the superradiant pulses show classical behavior.
Macroscopic fluctuation of superradiance most sensitively appears as the fluctuation of the polarized photon number. This fluctuation is not much affected by unexpected variation of the exciting pulse area. In the superradiance limit the relative fiuctuation of the polarized photon number is equal to 1/3\sqrt{1/3}1/3​, and it decreases rapidly as the atomic number density is reduced below a threshold density. Such a characteristic change is attributed to a transition from the superradiant emission to the amplified spontaneous emission.