Observation of quantum noise in the polarization of laser light in a rubidium-vapor cell

Abstract

We present experimental study of polarization quantum noise of laser radiation passed through optically think vapor of Rb 87 . We observe a step-like noise spectrum. We discuss various factor which may result in such noise spectrum and prevent observation of squeezing of quantum fluctuations predicted in Matsko et al, Phys. Rev. A 63, 043814 (2001).PACS numbers: 42.50. Gy, 42.65.Ky The sensitivity of many experiments is ultimately limited by quantum fluctuations of electromagnetic field. This stimulates the development of various methods for generation of light with non-classical statistics ("squeezed light"). Usage of such light field in an experiment often allows measurement noise reduction below shot noise limit [1,2]. Photon statistics modification often requires nonlinear optical medium; the degree of change depends on the strength of the nonlinear effects and linear losses in such medium [2,3].Recent proposals of photon statistics control based on coherent atomic vapor look quite promising from that point of view. Atomic coherence created between two hyperfine ground states of an alkali metal by a strong classical control field eliminates linear absorption and simultaneously produces strong nonlinear dispersion for a weak probe field (classical or quantized). This effect is known as electromagnetically induced transparency (EIT) [1]. Third-order nonlinear susceptibility in an EIT medium is substantial and comparable with linear susceptibility, which enhances coupling between electromagnetic fields participating in such nonlinear processes [4]. Ref. [5] demonstrated effective projection of photon quantum state on collective spin excitation of an atomic ensemble under EIT condition, long storage time, and the ondemand retrieval of quantum information.Matsko et al.[6] predicted squeezing of vacuum quantum fluctuations for laser light propagating through an EIT medium with coherence between Zeeman states of the same hyperfine atomic sublevel. Theoretical calculations estimated squeezing up to 8dB under realistic experimental conditions.The main purpose of this work was to study the polarization quantum noise of laser radiation for the D 1 line of Rb and to explore the possibility for squeezed vacuum observation on that transition. In particular, we studied the relation between laser intensity and the bandwidth of laser-induced quantum noise.The mechanism of polarization quantum noise squeezing proposed in [6] is the following: linearly polarized laser radiation is decomposed into two circularly polarized components σ + and σ − , as shown in Fig. 1(a), which form a Λ system and create coherence between |m F = −1 and |m F = +1 Zeeman sublevels, producing EIT. In an ideal three-level Λ system phase difference for two circularly polarized components does not change after interaction with atoms. However, under realistic conditions interaction of light with additional atomic levels (in particular with the other far-detuned hyperfine sublevel(s) of the excited state), Doppler effect, optical losses, etc., causes...

Vacuum squeezing via polarization self-rotation and excess noise in hot Rb vapors

“…3(a) shows that for our experimental parameters VS occurs in a very small "island" of the parameter space, and only excess noise is measured elsewhere. This may explain the inconsistency between earlier experimental works [7,8,9,10], since it is not hard to miss the optimal conditions for squeezing detection. We also noticed that the observation is quite sensitive to various experimental settings: for example, small longitudinal displacement of the laser focus inside the cell has changed the position of the "squeezing island" in the laser power/atomic density parameter plane.…”

“…A few recent experiments reported observation of VS via PSR squeezing on different transitions in Rb [7,8]. However, the amount of observed squeezing was significantly smaller than predicted, and some experimental efforts failed to observe any squeezing at all [9,10]. All these measurements were degraded by excess noise caused by interaction of light and atoms, which was not accounted for in the original theory.…”

“…Recently, several research groups [7,8,9,10,11] explored another method for generation of squeezed vacuum that is based on the rotation of elliptically polarized light as a result of propagation through an isotropic optical material (when no external electric or magnetic field present). Such polarization self-rotation (PSR) is a nonlinear optical effect that occurs in many optical media, but it is particularly strong near resonant optical transitions in alkali metal atoms, where even a small difference in resonant frequency due to different AC-Stark shifts of two circular components produces detectable variation in their relative refractive index [12,13,14].…”

Observation of optical Rabi oscillations in transmission signal of atomic vapor under continuous-wave laser excitation

“…Noise spectroscopy was applied in [22,23] to study nonlinear magneto-optical resonances in Rb vapor. Experimental observation of quantum noise in the polarization of laser light passing through a dense Rb vapor using a homodyne detection technique was re-ported in [24].…”

Intensity correlations in resonance nonlinear magneto-optical rotation

“…The amplitude noise of optical fields increases after the fields interact with an atomic media (atomic excess noise) [27,28,29,30,31]. Several processes, such as the conversion of phase noise to amplitude noise and the four-wave mixing process, can be associated with the generation of atomic noise [27,28,29,30,31,32]. The strong coupling of optical fields in Λ-type atoms provides a way of controlling amplitude fluctuations.…”