Figure 1

(Color online) Experimental details. (a) Double-$\Lambda$ scheme in ${}^{85}\mathrm{Rb}$, $\mathrm{P}=\text{pump}$, $\mathrm{C}=\text{conjugate}$, $\mathrm{Pr}=\text{probe}$. The width of the excited state represents the Doppler-broadened profile. The probe and the conjugate are cross-linearly polarized with the pump. (b) Experimental setup, $\mathrm{PBS}=\text{polarizing}$ beam splitter, $\mathrm{SA}=\text{spectrum}$ analyzer.Reuse & Permissions

Figure 2

(Color online) Intensity-difference noise versus total optical power at $1\mathrm{MHz}$. Blue circles, SQL; red squares, 4WM. The ratio of the two slopes is $-8.8\mathrm{dB}$.Reuse & Permissions

Figure 3

(Color online) Distributed gain and loss model. The medium is modeled with a large number of interleaved gain and loss stages (see text). The loss stages affect only the probe beam, since only this beam is tuned close enough to an atomic resonance to experience absorption. The detection efficiency is modeled by a beam splitter (BS) on both beams. $\mathrm{SA}=\text{spectrum}$ analyzer.Reuse & Permissions

Figure 4

(Color online) One-photon 4WM resonance. Intensity-difference noise measured at $1\mathrm{MHz}$ (red squares) and gain (green circles) versus one-photon detuning, for a two-photon detuning of $4\mathrm{MHz}$ and a cell temperature of $112°\mathrm{C}$. The blue curve is the theoretical squeezing computed from the output power levels of the probe and the conjugate. The top and bottom of the shaded area indicate the variation of the theoretical curve for the uncertainty range of our detection efficiency.Reuse & Permissions

Figure 5

(Color online) Simulated and measured intensity-difference squeezing as a function of the probe transmission $T$ and gain of the medium $G$. The theory takes into account the detection efficiency $(\eta =0.9)$. The squeezing (corrected for the system noise) measured at $1\mathrm{MHz}$ is shown for different cell temperatures, $109°\mathrm{C}$ (diamonds), $112°\mathrm{C}$ (squares), and $114°\mathrm{C}$ (circles), as the one-photon detuning of the pump laser is scanned. The crosses indicate the projection of the measured squeezing onto the theoretical surface while the lines connecting the points and crosses give an indication of the vertical distance between them. Most of the points are very near the surface. The projection onto the $x\text{−}y$ plane shows contour lines of the theoretical squeezing at $2\mathrm{dB}$ intervals from $+4\text{to}-8\mathrm{dB}$, and the projections of the data points. The arrow indicates the direction of increasing one-photon detuning.Reuse & Permissions

Figure 6

(Color online) Low-frequency squeezing. Noise spectra (RBW $0.3\mathrm{kHz}$, VBW $3\mathrm{Hz}$) for the electronic noise, the pump scattering, the intensity difference, and the standard quantum limit. The inset shows a larger frequency span (RBW $1\mathrm{kHz}$, VBW $10\mathrm{Hz}$).Reuse & Permissions