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We theoretically study a parallel configuration of two interacting whispering-gallery-mode optical resonators and show a narrowband modal structure as a basis for a widely tunable delay line. For the optimum coupling configuration the system can possess an unusually narrow spectral feature with a much narrower bandwidth than the loaded bandwidth of each individual resonator. The effect has a direct analogy with the phenomenon of electromagnetically induced transparency in quantum systems for which the interference of spontaneous emission results in ultranarrow resonances.

We investigate the quantum properties of fields generated by resonantly enhanced wave mixing based on atomic coherence in Raman systems. We show that such a process can be used for generation of pairs of Stokes and anti-Stokes fields with nearly perfect quantum correlations, yielding almost complete (i.e. 100%) squeezing without the use of a cavity. We discuss the extension of the wave mixing interactions into the domain of a few interacting light quanta.One of the intriguing and potentially useful aspects of nonlinear optical phenomena is their ability to suppress intrinsic quantum fluctuations [1]. However, the efforts to exploit these properties were hindered, either by the small values of nonlinearities in available optical crystals, or by absorption losses and the associated noise in resonant atomic systems with large nonlinearities. For example, four-wave mixing is known to result, in principle, in squeezed-state generation or non-classical correlations [2], but all experimental realizations reported to date showed rather limited noise reduction and required the use of cavities [3].The work of the past few years has shown that substantial improvements in resonant nonlinear optics can be achieved by utilizing the concepts of quantum coherence and interference [4,5]. The aim of the present contribution is to demonstrate the usefulness of this regime of nonlinear optical enhancement for applications involving quantum correlations and reduction of quantum noise. As an example, we consider here four-wave mixing in resonant Raman systems [5], where atomic phase coherence can be used to generate a large nonlinearity and at the same time suppress resonant absorption. Recent theoretical [6] and experimental work [7] demonstrated that the efficient nonlinear interactions in this system can lead to mirrorless parametric oscillation, where pairs of counter-propagating Stokes and anti-Stokes photons are generated spontaneously from noise. We here show that under certain, very realistic conditions this process can be considered as ideal from the viewpoint of quenching of quantum noise. As a result, the generated Stokes and anti-Stokes field components can possess practically perfect quantum correlations, leading e.g. to an almost complete suppression of the quantum fluctuations in one quadrature of a combined mode (i.e. 100% squeezing). We point out that this can be achieved even in the case when the intensity of the driving fields approaches, under realistic experimental conditions, the few-photon level. These results, together with recent studies on strongly interacting photons [9], single-photon switching [10] and few photon quantum control [11], show that a truly new regime of nonlinear optics involving just a few interacting light quanta is feasible.Physically, such a performance of the nonlinear media is due to the possibility of eliminating the resonant absorption and associated noise processes via atomic coherence. Furthermore, the associated large linear dispersion is very important for achieving phase matching...

Ultrastable high-spectral-purity lasers have served as the cornerstone behind optical atomic clocks, quantum measurements, precision optical microwave generation, high-resolution optical spectroscopy, and sensing. Hertz-level lasers stabilized to high-finesse Fabry-Pérot cavities are typically used for these studies, which are large and fragile and remain laboratory instruments. There is a clear demand for rugged miniaturized lasers with stabilities comparable to those of bulk lasers. Over the past decade, ultrahigh-Q optical whispering-gallery-mode resonators have served as a platform for low-noise microlasers but have not yet reached the stabilities defined by their fundamental noise. Here, we show the noise characteristics of whispering-gallery-mode resonators and demonstrate a resonator-stabilized laser at this limit by compensating the intrinsic thermal expansion, allowing a sub-25 Hz linewidth and a 32 Hz Allan deviation. We also reveal the environmental sensitivities of the resonator at the thermodynamical noise limit and long-term frequency drifts governed by random-walk-noise statistics.