02 Research
Cavity-QED Microlaser:
Generation of nonclassical radiation based on the enhanced vacuum field
P. A. M. Dirac proposed a vacuum electric field due to the vacuum energy in order to explain atomic spontaneous emission. According to Dirac, the spontaneous emission of an excited atom is just a photon emission phenomenon stimulated by the vacuum electric field (or the vacuum fluctuations). The vacuum energy and consequent vacuum fluctuations are considered as the physical origin of the cosmological constant, the Casimir force and the Lamb shift. The structure of enhanced vacuum fluctuations inside a cavity is expected to be determined by the geometry of the cavity. However, only indirect measurement of its partial structure has been demonstrated. Here we have directly imaged the three dimensional structure of enhanced vacuum fluctuations inside the cavity through atomic position localization done by a nanohole array. We utilize the fact that the atomic emission rate is proportional to the strength of the vacuum fluctuations. The capability of controlling atomic position/phase with a nanohole array allows us to realize the phase-correlated interaction bewteen a collection of atoms and the cavity field. Utilizing such capability, we are currently working towards realizing novel quantum radiation sources with much more nonclassical features than the existing microlasers.
(a) By utilizing single 138Ba atoms controlled by a nanohole aperture, the distribution of vacuum energy density is measured. (b) A nanohole aperture made of a silicon nitride membrane with the focused-ion-beam technique. A diameter of each hole is 170nm. (c) The iso-surface of the vacuum energy density, corresponding to 20% of its maximum. From M. Lee et al., Nat. Commun. 5, 3441 (2014).
[Newsletter articles on OPLI and CERN COURIER (Online, pdf) on this work]
By utilizing the coherent interaction between atoms and the cavity due to the enhanced vacuum field, it is possible to realize an intensity-squeezed light source. Particularly, if the damping of atoms and the cavity during their interaction is negligible, a coherent Rabi oscillation is maintained even when hundreds of atoms interact with the cavity field simultaneously. Consequently, generation of strong continuous-wave intensity-squeezed light is possible by means of enhanced photon number stabilization due to the coherent Rabi oscillation. The extent of the intensity squeezing is quantified by the Mandel Q parameter, which lies between -1(Fock state) and 0(coherent state) when the source is intensity-squeezed. In the recent research, we have succeeded in observing the generation of the intensity-squeezed light with its Mandel Q close to -0.45. This is the lowest Mandel Q value ever observed in continuous-wave intensity-squeezed sources so far. Because the intensity-squeezed light has less noise than coherent light sources like the conventional lasers, it has great applicability in precision measurements and high-efficiency low-noise optical communications.
A second-order correlation function g(2)(τ) is measured to investigate the photon statistics of the intensity-squeezed light. The amount of decrease below unity at zero time delay is equal to (Mandel Q)/(Photon number). In the recent result, 45% reduction of the photon number variance with respect to that of the Poisson distribution is observed with the average photon number of 600. The left plot shows the previous result [W. Choi et al., Phys. Rev. Lett. 96, 093603 (2006)] and the right plot shows the latest result (2014).
The strong interaction between atoms and the cavity is predicted to enhance the frequency pulling of a laser. We have performed the first observation of the quantum frequency pulling, which shows enhancement proportional to the number of atoms and square of the coupling constant [H.-G. Hong et al., Phys. Rev. Lett. 109, 243601 (2012)]. The quantum frequency pulling phenomenon can be utilized to make a more stable optical clock because it enables compensation of the detuning between a gain material and a resonator.
"A-Team" Members: From the front, Daeho Yang, Younghoon Song, Kyungwon An and Junki Kim. Collaborators: Dr. Hyun-Gue Hong (KRISS), Dr. Wontaek Seo (SAIT). Dr. Moonjoo Lee (U. Innsbruck), Prof. Wonshik Choi (Korea U.), Prof. Youngtak Chough (Kwangju U.).
Last updated: February 25, 2014