Generating and verifying entangled itinerant microwave fields with efficient and independent measurements
H. S. Ku, William Kindel, F. Mallet, Scott Glancy, K. D. Irwin, Gene C Hilton, Leila R. Vale, Konrad W. Lehnert · Physical Review A · 2015
By combining a squeezed propagating microwave field and an unsqueezed vacuum field on a hybrid (microwave beam splitter), we generate entanglement between the two output modes. We verify that we have generated entangled states by making independent and efficient single-quadrature measurements of the two output modes. We observe the entanglement witness ${E}_{\mathrm{W}}=\ensuremath{-}0.{263}_{\ensuremath{-}0.036}^{+0.001}$ and the negativity $N=0.{0824}_{\ensuremath{-}0.0004}^{+0.01}$ with measurement efficiencies at least $26\ifmmode\pm\else\textpm\fi{}0.1%$ and $41\ifmmode\pm\else\textpm\fi{}0.2%$ for channels 1 and 2, respectively. These measurements show that the output two-mode state violates the separability criterion and therefore demonstrates entanglement. This shared entanglement between propagating microwaves provides an important resource for building quantum networks with superconducting microwave systems.