What happens when materials are cooled down close to absolute zero temperature? It sounds a boring question because everything freezes at T = 0. It is NOT true, however, because quantum fluctuations persist even at absolute zero temperature. The richness of low-temperature physics was first demonstrated by Heike Kamerlingh Onnes at 1911, who was the first to liquify Helium and reach ~ 1 K. He discovered that the resistance of mercury suddenly vanished at low temperature. Followed by this discovery of the superconducting transition, many amazing quantum phenomena – superfluid transition of Helium, Bose-Einstein condensations of Alkali Bose gases – were found at low temperatures. We are interested in these quantum condensed states at low temperatures where the thermal fluctuation is negligible. Especially, we are now challenging measurements of correlated electron systems at ultra-low temperature (below 20 mK) where many interesting phenomena have remained unexplored due to technical difficulties. Further, we are studying thermal-transport properties of quantum spin liquids emerged in frustrated magnetic materials.