Experimental Generation of Multiple Quantum Correlated Beams from Hot Rubidium Vapor

Quantum correlations and entanglement shared among multiple quantum modes are important for both fundamental science and the future development of quantum technologies. This development will also require an efficient quantum interface between multimode quantum light sources and atomic ensembles, which makes it necessary to implement multimode quantum light sources that match the atomic transitions. Here, we report on such a source that provides a method for generating quantum correlated beams that can be extended to a large number of modes by using multiple four-wave mixing (FWM) processes in hot rubidium vapor. Experimentally, we show that two cascaded FWM processes produce strong quantum correlations between three bright beams but not between any two of them. In addition, the intensity-difference squeezing is enhanced with the cascaded system to $-7.0\pm 0.1\mathrm{dB}$ from the $-5.5\pm 0.1/-4.5\pm 0.1\mathrm{dB}$ squeezing obtained with only one FWM process. One of the main advantages of our system is that as the number of quantum modes increases, so does the total degree of quantum correlations. The proposed method is also immune to phase instabilities due to its phase insensitive nature, can easily be extended to multiple modes, and has potential applications in the production of multiple quantum correlated images.

Figure 1

Proposed system for the generation of multiple quantum correlated beams. (a) Single PA configuration for generating quantum correlated twin beams; (b) Cascaded two-PA configuration for generating quantum correlated triple beams; (c) Cascaded $n$-PA configuration for generating $n+1$ quantum correlated beams. ${\mathrm{PA}}_i$, the $i$th parametric amplifier; $P_i$, the $i$th pump beam; ${\mathrm{Pr}}_i$, the $i$th probe beam, with ${\mathrm{Pr}}_0$ as the initial probe beam; $C_i$, the $i$th conjugate beam; SA, spectrum analyzer.

Figure 2

Experimental layout for generating and detecting quantum correlated triple beams. (a) Double-$\mathrm{\Lambda }$ scheme in the $D1$ line of ${}^{85}\mathrm{Rb}$. $P$: pump; Pr: probe, $C$: conjugate. (b) Experimental setup. ECDL, external cavity diode laser; HW, half wave plate; PBS, polarization beam splitter; ${\mathrm{TA}}_1$, ${\mathrm{TA}}_2$, tapered amplifiers; AOM, acousto-optic modulator; GL, Glan-Laser polarizer; GT, Glan-Thompson polarizer; ${\text{cell}}_1$, ${\text{cell}}_2$, first and second rubidium vapor cell; $D_1$, $D_2$, $D_3$, photodetectors; $S_1$, $S_2$, subtractors; SA, spectrum analyzer; ${\mathrm{Pr}}_0$, initial probe beam; ${\mathrm{Pr}}_1$, ${\mathrm{Pr}}_2$, first and second probe beam; $C_1$ and $C_2$, first and second conjugate beam.

Figure 3

Quantum correlations of the triple beams. (a) Normalized noise power of ($A$) $i_1$; ($B$) $i_2$; ($C$) $i_3$; ($D$) $i_3-i_2$; ($E$) $i_3-i_1$; ($F$) $i_2-i_1$; ($G$) $i_3-i_2-i_1$; ($H$) the corresponding SNLs of trace $A\sim G$; (b) Normalized intensity-difference noise power of the twin beams generated from ${\text{cell}}_1$ ($A$) and the corresponding SNL ($B$); (c) Normalized intensity-difference noise power of the twin beams generated from ${\text{cell}}_2$ ($A$) and the corresponding SNL ($B$). The electronic noise floor and background noise are all about 6 dB below the corresponding SNLs at 1 MHz and have been subtracted from all of the traces.

Figure 4

Enhancement of quantum correlation. Relative intensity noise power at different total optical power for ($A$) triple beams from the cascaded configuration (green diamonds); ($B$) twin beams from ${\text{cell}}_1$ (red circles); ($C$) twin beams from ${\text{cell}}_2$ (blue triangles), and ($D$) SNL (black squares). All these four noise power curves fit to straight lines. The electronic noise floor and background noise are subtracted from all of the data points.