Superradiance originally refers to the collective emission of densely packed quantum emitters. It is fundamentally different from ordinary spontaneous emission in that its emission power scales as the square of the number of emitters whereas it does linearly in the ordinary spontaneous emission. Such enhanced matter-light interaction has drawn much interest since it would enable efficient photonics devices and quantum interfaces. Recently several research groups have demonstrated new approaches to superradiance. A laser pulse is introduced to an ensemble of millions of atoms to imprint phase correlation and thus to induce an immediate superradiant output. However, the output occurred in the same direction as the input, making it difficult to separate them. In another approach, a few emitters are prepared in a cavity and their quantum states are individually manipulated to exhibit controlled collective emission. However, due to technical difficulties, the number of emitters participating in superradiance has been limited to two so far. Recently, we have observed coherent superradiance made by single atoms in a cavity in such a way that the output is completely separated from the input and tens of atoms participate in superradiance as they control atomic states individually. In our experiment, single two-level atoms are prepared in the same quantum superposition state and then made to traverse a cavity one by one. A single atom in the cavity then emits a photon collectively with the past atoms that have already gone through the cavity. Such collective interaction among time-separated atoms has never been observed before. The interaction is one-sided or chiral in that the preceding atoms can affect the following atoms, not vice versa. We observe the emission power increases as the square of the number N of the atoms traversing the cavity during the cavity-field decay time. The N-squared dependence occurs even when the number of photons in the cavity exceeds unity without exhibiting any lasing threshold. So, our results can also be viewed as thresholdless lasing. Interestingly, the so-called beta factor, the fraction of spontaneous emission directed to the cavity mode from one excited atom in the cavity, is only 0.03, much smaller than unity, and thus cannot explain our thresholdless lasing, which is made possible by the atomic collective emission. In fact, the smaller beta factor is the better for our thresholdless lasing: currently, the largest N is about 30, but it can be easily scaled up further by reducing the beta factor more. We prepared the single atoms in the same quantum superposition state by controlling their position in a nanometer resolution with a nanohole-array atom aperture, first developed in our previous work reported in Nature Communications in 2014. This novel form of superradiance is fundamentally interesting and may lead to important practical applications. The chiral interaction feature can be utilized to build a chiral atom-atom interaction system to study quantum many-body chirality physics. The phase-controlled many-atom superradiant interaction based on the nanohole-array technique can be used to generate non-classical field state such as Schrödinger cat states. Greatly enhanced emission of single atoms can be used to build efficient atom-photon quantum interfaces. Moreover, the thresholdless lasing property can be utilized in making more efficient lasers. For more details, see Junki Kim et al., "Coherent single-atom superradiance", Science 359 (6376), 66 (2018).

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.

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.

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.