Yosep Kim

Direct quantum process tomography via measuring sequential weak values of incompatible observables.

The weak value concept has enabled fundamental studies of quantum measurement and, recently, found potential applications in quantum and classical metrology. However, most weak value experiments reported to date do not require quantum mechanical descriptions, as they only exploit the classical wave nature of the physical systems. In this work, we demonstrate measurement of the sequential weak value of two incompatible observables by making use of two-photon quantum interference so that the results can only be explained quantum physically. We then demonstrate that the sequential weak value measurement can be used to perform direct quantum process tomography of a qubit channel. Our work not only demonstrates the quantum nature of weak values but also presents potential new applications of weak values in analyzing quantum channels and operations.Weak measurements constitute a powerful tool for metrology, tomography and even foundations, but in most cases could be explained classically. Here, the authors characterise a quantum channel using sequential weak value measurements of two incompatible observables utilising two-photon quantum interference.

Second-Order Temporal Interference with Thermal Light: Interference beyond the Coherence Time

We report an experimental demonstration for a counter-intuitive phenomenon in multi-path interferometer with thermal light. The intensity correlation between the outputs of two unbalanced Mach-Zehnder interferometers (UMZIs) with two correlated beams of thermal light at the input exhibits genuine second-order interference with the visibility of 1/3. Here, the second-order interference does not degrade at all no matter how much the path length difference in each UMZI is increased beyond the coherence length of the thermal light.

Experimental characterization of quantum polarization of three-photon states

We experimentally investigate various quantum polarization features of three-photon quantum states, including product and entangled states with varying purity. The three-photon quantum states are c ...

Protecting entanglement from decoherence via weak quantum measurement

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. 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, quantum teleportation, and quantum metrology. Here, we propose and demonstrate a novel scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death (ESD).

Publisher Correction: Direct quantum process tomography via measuring sequential weak values of incompatible observables

In the original version of this Article, the penultimate sentence of the second paragraph of the “Schematic and theory” section of the Results originally incorrectly read as “For instance, choosing θA = π/4, 0, −π/8 and π/8 rotates the bases |0>, |1>, |+>, and |−> into |1>, thereby implementing the observableÂ=|0><0|, |1><1|, |+><+| and, respectively.” In the corrected version, “|−><−|” has been added before “, respectively”. This error has been corrected in both the PDF and HTML versions of the Article.