The emergence of quasiparticles in strongly interacting matter represents one of the cornerstones of modern physics. However, when different phases of matter compete near a quantum critical point, the very existence of quasiparticles comes under question. Here we create Bose polarons near quantum criticality by immersing atomic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. Using locally-resolved radiofrequency spectroscopy, we probe the energy, spectral width, and short-range correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities form well-defined quasiparticles. However, their inverse lifetime, given by their spectral width, is observed to increase linearly with temperature at the Planckian scale k B T , a hallmark of quantum critical behavior. Close to the BEC critical temperature, the spectral width exceeds the binding energy of the impurities, signaling a breakdown of the quasiparticle picture. arXiv:1904.02685v3 [cond-mat.quant-gas]

Coherence, the stability of the relative phase between quantum states, is central to quantum mechanics and its applications. For ultracold dipolar molecules at sub-microkelvin temperatures, internal states with robust coherence are predicted to offer rich prospects for quantum many-body physics and quantum information processing. We report the observation of stable coherence between nuclear spin states of ultracold fermionic sodium-potassium (NaK) molecules in the singlet rovibrational ground state. Ramsey spectroscopy reveals coherence times on the scale of 1 second; this enables high-resolution spectroscopy of the molecular gas. Collisional shifts are shown to be absent down to the 100-millihertz level. This work opens the door to the use of molecules as a versatile quantum memory and for precision measurements on dipolar quantum matter.

We demonstrate coherent microwave control of rotational and hyperfine states of trapped, ultracold, and chemically stable 23 Na 40 K molecules. Starting with all molecules in the absolute rovibrational and hyperfine ground state, we study rotational transitions in combined magnetic and electric fields and explain the rich hyperfine structure. Following the transfer of the entire molecular ensemble into a single hyperfine level of the first rotationally excited state, J ¼ 1, we observe lifetimes of more than 3 s, comparable to those in the rovibrational ground state, J ¼ 0. Long-lived ensembles and full quantum state control are prerequisites for the use of ultracold molecules in quantum simulation, precision measurements, and quantum information processing. DOI: 10.1103/PhysRevLett.116.225306 Ultracold molecules with large electric dipole moments hold great promise as a novel platform for quantum stateresolved chemistry [1], precision measurements of fundamental constants [2-4], quantum computation [5], quantum simulation [6,7], as well as for the realization of new states of dipolar quantum matter [6][7][8]. Essentially all anticipated applications depend on the ability to coherently control the quantum state of molecules, implying full control over electronic, vibrational, rotational, and nuclear spin degrees of freedom [1]. With the recent production of dipolar molecules at submicrokelvin temperatures [9-13], this full quantum control has come into experimental reach for an entire ensemble of trapped molecules [14].Controlling the rotational states of molecules is directly linked to the control over long-range dipolar interactions [15][16][17][18][19][20][21]. Indeed, no state of definite parity can possess a dipole moment, but creating a superposition of oppositeparity rotational states induces one. Such a superposition can be achieved either via applying electric fields or by coherently driving a microwave transition between rotational states. The potential applications for such coherent control range from quantum simulation of spin Hamiltonians [22][23][24][25][26] to the realization of topological superfluidity [27]. Also, interaction control is expected to facilitate direct evaporative cooling of ultracold molecules [17,19,28].In particular, for quantum information applications and many-body physics with dipolar molecules, a long lifetime of molecules in their individual quantum states is a key requirement. This is a prerequisite for having a large number of possible gate operations and for equilibration into novel phases, respectively. For ultracold chemically reactive molecules, losses can be prevented by isolating molecules in individual wells of an optical lattice [29].