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Frontiers in Optics

Squeezed vacuum states from a whispering gallery mode resonator

Alexander Otterpohl, Florian Sedlmeir, Golnoush Shafiee, Thomas Dirmeier, Ulrich Vogl, Gerhard Schunk, Dmitry Strekalov, Harald G. L. Schwefel, Tobias Gehring, Ulrik L. Andersen, Gerd Leuchs, Christoph Marquardt

Published Jan 1, 2018• 9 citations
Publisher

Abstract

We demonstrate the generation of squeezed vacuum states of light in a crystalline whispering gallery mode resonator. We utilize a millimeter-sized lithium niobate resonator with strong optical second order nonlinearity. This system allows us to reach 1.2 dB of squeezing at a pump power of only 300 dW and offers hereby a compact and power efficient source of squeezed vacuum states of light.

DOI
10.1364/fio.2018.fw7b.3

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We demonstrate the generation of squeezed vacuum states of light in a crystalline whispering gallery mode resonator. We utilize a millimeter-sized lithium niobate resonator with strong optical second order nonlinearity. This system allows us to reach 1.2 dB of squeezing at a pump power of only 300 dW and offers hereby a compact and power efficient source of squeezed vacuum states of light.
Squeezed vacuum states enable optical measurements below the quantum limit and hence are a valuable resource for applications in quantum metrology and also quantum communication. However, most available sources require high pump powers in the milliwatt range and large setups, which hinders real world applications. Furthermore, degenerate operation of such systems presents a challenge. Here, we use a compact crystalline whispering gallery mode resonator made of lithium niobate as a degenerate parametric oscillator. We demonstrate about 1.4 dB noise reduction below the shot noise level for only 300 μWμ\text{W}μW of pump power in degenerate single mode operation. Furthermore, we report a record pump threshold as low as 1.35 μWμ\text{W}μW. Our results show that the whispering gallery based approach presents a promising platform for a compact and efficient source for nonclassical light.
We demonstrate for the first time natural phase matching for optical frequency doubling in a high-Q whispering-gallery-mode resonator made of lithium niobate. A conversion efficiency of 9% is achieved at 30 microW in-coupled continuous wave pump power. The observed saturation pump power of 3.2 mW is almost 2 orders of magnitude lower than the state-of-the-art value. This suggests an application of our frequency doubler as a source of nonclassical light requiring only a low-power pump, which easily can be quantum noise limited. Our theoretical analysis of the three-wave mixing in a whispering-gallery-mode resonator provides the relative conversion efficiencies for frequency doubling in various modes.
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 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 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.