Polarization squeezing of light by single passage through an atomic vapor

Abstract

We have studied relative-intensity fluctuations for a variable set of orthogonal elliptic polarization components of a linearly polarized laser beam traversing a resonant 87 Rb vapor cell. Significant polarization squeezing at the threshold level (-3dB) required for the implementation of several continuous variables quantum protocols was observed. The extreme simplicity of the setup, based on standard polarization components, makes it particularly convenient for quantum information applications.PACS numbers: 42.50. Lc,42.50.Ct,32.80.Qk In recent years, large attention has been given to the use of continuous variables for quantum information processing. A foreseen goal is the distribution of entanglement between distant nodes. For this, quantum correlated light beams are to interact with separate atomic systems in order to build quantum mechanical correlations between them [1,2].A particular kind of quantum correlation between two light beams occurs when the intensity difference between them has fluctuations smaller than the standard quantum limit (SQL), that is smaller than the fluctuations of the intensity difference of two coherent states of the same intensity. The two beams are said to present relative-intensity squeezing (RIS). RIS has been generated through different nonlinear optics techniques. One of the most successful is parametric down conversion in a nonlinear χ (2) crystal. Up to 9.7 dB RIS has been obtained with this method [3]. Such experiments require a relatively elaborate and expensive setup. The resulting light beams are spectrally broad and usually far detuned from convenient alkali atoms D transitions. An alternative approach has considered the use of four-wave mixing in atomic samples [4][5][6][7].Reduced relative-intensity fluctuations have been observed between light beams of different frequency. However, RIS can also occur between orthogonal polarization components of a single light beam. In such case, the field is said to be polarization squeezed [8,9] and the noise reduction is described in terms of squeezing of the fluctuations of one of the Stokes operators:Here a x , a y are the field destruction operators for the orthogonal linear polarizations x and y. Polarization squeezing has been produced via propagation in optical fibers [10,11], through the combination on a polarizing beam splitter of two quadrature squeezed light beams [12] and through the interaction of linearly polarized light with cold atoms inside an optical cavity [9,13].It has been recently demonstrated that the single passage of a linearly polarized pump beam through a few-cmlong atomic vapor cell results in squeezing of the polarization orthogonal to that of the pump (vacuum squeezing) [14][15][16][17] as a consequence of the nonlinear optics mechanism known as polarization self-rotation (PSR) [18][19][20]. Vacuum squeezing via PSR has been observed for the D1 [15][16][17] and D2 [14] transitions using 87 Rb vapor. As noted in [9], the existence of polarization squeezing can be inferred from these results.In this ...

Quantum-enhanced magnetometer with low-frequency squeezing

“…We also demonstrate the transition from a shot-noise-limited magnetometer at lower atomic densities, to a region where the magnetometer is affected by the interaction of the light noise with the atoms at higher atomic densities. In contrast to a previously reported magnetometer, with squeezing generated via parametric down conversion in a nonlinear crystal [5], our setup uses an atomic squeezer based on the polarization self-rotation (PSR) effect [15][16][17][18][19][20][21]. Unlike its crystal counterpart, the PSR squeezer does not require a powerful pump laser, but uses a pump laser with only several milliwatts of power in a single-path configuration.…”

Propagation of quantum optical fields under the conditions of multi-photon resonances in a coherent atomic vapor

“…28,29 If the incoming field is linearly polarized, the same nonlinear interaction modifies the quantum fluctuations of a vacuum field with orthogonal polarization, which becomes squeezed. [30][31][32][33] Our group has recently demonstrated up to 2.5 dB of low-frequency (< 10 kHz) broadband vacuum squeezing at optical frequencies of the 87 Rb D1 line. 8,30 Here we experimentally investigate propagation of a squeezed vacuum or coherent vacuum optical probe through a hyperfine EIT Rb vapor.…”

Atomic resonant single-mode squeezed light from four-wave mixing through feedforward

“…Nevertheless, resonant vacuum single-mode squeezing has been reported using polarization self-rotation (PSR) in a Rb vapor cell. Although theoretical predictions suggest PSR can generate −6 dB of squeezing [18], only −2.9 dB of squeezing on resonance with the D1 lines of Rb has been generated [19]. A recent theoretical analysis shows that noise from the thermal vapor can limit the levels of squeezing that can be achieved with PSR to levels significantly below original predictions [20].…”