Quantum Light from a Whispering-Gallery-Mode Disk Resonator

Optical parametric down-conversion has proven to be a valuable source of nonclassical light. The process is inherently able to produce twin-beam correlations along with individual intensity squeezing of either parametric beam, when pumped far above threshold. Here, we present for the first time the direct observation of intensity squeezing of $-1.2\mathrm{dB}$ of each of the individual parametric beams in parametric down-conversion by use of a high quality whispering-gallery-mode disk resonator. In addition, we observed twin-beam quantum correlations of $-2.7\mathrm{dB}$ with this cavity. Such resonators feature strong optical confinement and offer tunable coupling to an external optical field. This work exemplifies the potential of crystalline whispering-gallery-mode resonators for the generation of quantum light. The simplicity of this device makes the application of quantum light in various fields highly feasible.

Figure 1

PDC setup with WGM resonator (disk radius 1.9 mm, rim radius 0.25 mm, height 0.5 mm).

Figure 2

Twin-beam measurement. (a) Variances of the signal and idler photocurrent sum and difference compared to the SNL vs laser frequency detuning from the pump WGM center. (b) Twin-beam squeezing vs pump power for stronger coupling (circle) and weaker coupling (square). Error bars are determined from the raw data, as seen in (a). Horizontal lines indicate the SNL and the mean squeezing values, for weak coupling $-1.3\mathrm{dB}$ and for strong coupling $-2.7\mathrm{dB}$.

Figure 3

Single-beam intensity noise measurement. (a) Intensity noise compared to SNL vs laser frequency detuning from the pump WGM center. (b) Intensity noise in SNU vs pump power (measurement and theoretical fit). Error bars are estimated as the mean variance of two measurements at every point. The horizontal lines indicate the SNL (black) and the asymptotic limit of 0.9 for the normalized intensity variance (orange). The vertical line visualizes the OPO threshold of $12.3\mu \mathrm{W}$. The gray area marks the range of the pump power, where relaxation oscillations may be excited.