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Nonlinear Dynamics in Optical Systems

Laser Cooling with Squeezed Light.

Yaakov Shevy

Published Jan 1, 1990• 0 citations
Publisher

Abstract

The process of laser cooling in a quasi resonant standing laser wave is one of the principal technics of laser cooling of atoms. Recent advances in this field include breaking both the "Doppler and the recoil limits". On another front modification of the quantum statistical properties of the "vacuum" fluctuations by squeezing has been achieved in the laboratory. It is, therefore, interesting to find what is the nature of the modification of the laser cooling process when the atoms are also embedded in squeezed vacuum. Such a state can be produced in principle by a degenerate parametric amplifier whose input are vacuum fluctuations. It is shown that due to the modified decay rates of an atom which is embedded in such a state, profound modification of the cooling processes in a standing wave is achieved . Among the important results obtained is the change of sign of the stimulated force from a heating force in normal vacuum to a cooling force in squeezed vacuum. This phenomenon can result in cooling forces more than two orders of magnitude larger than the maximum force in normal vacuum. In this talk the method for calculating the force in squeezed vacuum will be given. The modified force will be compared to the force in normal vacuum and finally the practical implications of these results for laser cooling will be discussed.

DOI
10.1364/nldos.1990.stsa581

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The process of laser cooling in a quasi resonant standing laser wave is one of the principal technics of laser cooling of atoms. Recent advances in this field include breaking both the "Doppler and the recoil limits". On another front modification of the quantum statistical properties of the "vacuum" fluctuations by squeezing has been achieved in the laboratory. It is, therefore, interesting to find what is the nature of the modification of the laser cooling process when the atoms are also embedded in squeezed vacuum. Such a state can be produced in principle by a degenerate parametric amplifier whose input are vacuum fluctuations. It is shown that due to the modified decay rates of an atom which is embedded in such a state, profound modification of the cooling processes in a standing wave is achieved . Among the important results obtained is the change of sign of the stimulated force from a heating force in normal vacuum to a cooling force in squeezed vacuum. This phenomenon can result in cooling forces more than two orders of magnitude larger than the maximum force in normal vacuum. In this talk the method for calculating the force in squeezed vacuum will be given. The modified force will be compared to the force in normal vacuum and finally the practical implications of these results for laser cooling will be discussed.
The process of laser cooling in a quasi resonant standing laser wave is one of the principal techniques of laser cooling of free atoms. It will be shown that the nonlinear interaction between the counter propagating waves in a standing wave have a profound affect on the mechanical manifestations of the light on the atoms. This can be understood simply by noting that in the Two Wave Mixing (TWM) process, a momentum kick of 2ℏk is transferred to the atom, as a photon is absorbed from one wave and emitted into the counter propagating wave. This process is in fact responsible for the so called "Dipole force" Or "Stimulated Molasses" and has been verified experimentally!. However, since the TWM process requires high laser intensity under normal conditions it does not usually lead to lower equilibrium temperatures. Nevertheless, It has been shown that the TWM process can be modified when the normal decay rates of the atom are altered. This modification can give rise to the appearance of the TWM at lower intensity and can even change its lineshape. This in turn have an important implications on the laser cooling process in a standing wave. Among the important effects: (i) The appearance of the stimulated force at lower intensity when the dipole decay rate is increased by dephasing events.(ii) The appearance of a narrow resonance characterized by the ground state decay in an open system. (iii) The modification of the stimulated force when the quantum fluctuations of the vacuum field are modified by quadrature squeezing3. Processes (ii) and (iii) can change the sign of the stimulated force from a heating force to a cooling force at the red side of the atomic resonance and give additional cooling force much larger than the radiation pressure force.
In this paper we report that the noisy quadrature, a undesirable feature of the squeezed light, could be effectively used in realizing higher cooling force in the Cs fountain clock. We know that the laser cooling is basically the result of the combination of undesirable phenomena like light shift, Doppler shift and polarization gradient. We establish in this paper that the noisy quadrature, a unwelcome feature of the squeezed light is useful in achieving larger cooling force. When the atoms interact with the squeezed vacuum, they decay into two quadratures which respectively have quantum noise below and above the normal vacuum fluctuations. The noisy quadrature corresponds to the faster decay rate. On subjecting the Cs atoms to the noisy quadrature of the squeezed vacuum, superimposed with the squeezed coherent light, the cooling force is increased. The use of the noisy quadrature of the squeezed vacuum for obtaining larger cooling force and the temperatures much below the Doppler limit, without resorting to large detuning of the driving field, is a novel idea discussed in this paper. We report that the S/N of the clock signal may also be improved with the squeezed light.