Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence. For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death (ESD). Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing, quantum cryptography, and quantum metrology. In this talk, I will describe a new approach for tackling decoherence by making use of weak quantum measurement and quantum measurement reversal. Unlike projection measurement in which the quantum state is irreversibly collapsed, weak quantum measurement only slightly perturbs the quantum state and it is possible to reverse the measurement so that the pre-measurement state is recovered probabilistically. I will first discuss an entanglement protection scheme in which weak and reversing measurements are used to effectively circumvent even ESD-causing decoherence. I will then discuss how this effect can be combined with the delayed-choice concept, leading to delayed-choice decoherence suppression in which the decision to suppress decoherence on an entangled two-qubit state is delayed until after the decoherence and even after the detection of a qubit.
 “Protecting Entanglement from Decoherence using Weak Measurement and Quantum Measurement Reversal” Y.-S. Kim, J.-C. Lee, O. Kwon, and Y.-H. Kim, Nature Physics 8, 117 (2012).
 “Experimental demonstration of delayed-choice decoherence suppression” J.-C. Lee, H.-T. Lim, K.-H. Hong, Y.-C. Cheong, M.S. Kim, and Y.-H. Kim, Nature Communications 5:4522 (2014).
 “Experimental realization of a delayed-choice quantum walk” Y.-C. Jeong, C. Di Franco, H.-T. Lim, M.S. Kim, and Y.-H. Kim, Nature Communications 4:2471 (2013).
Prof. Yoon-Ho Kim received Ph.D. in quantum optics from the University of Maryland, Baltimore County (USA) in 2001. He then held the prestigious Eugene P. Wigner Fellowship at the Oak Ridge National Laboratory (USA). Since 2004, he has been on the faculty of the Pohang University of Science and Technology (POSTECH) in Korea. His recent interests include weak quantum measurement in quantum information, multi-photon quantum systems, quantum/ghost imaging, and atom-photon interactions. He is currently serving as an Associate Editor for Optics Express and as an editor for the Journal of the Korean Physical Society.