Observation of broadband entanglement in microwave radiation from the dynamical Casimir effect.

A fundamental property of the quantum vacuum is that a time-varying boundary condition for the electro-magnetic field can generate photon pairs from the vacuum; this is called the Dynamical Casimir Effect (DCE)¹. Quantum theory predicts that these photon pairs are entangled and that the resulting electromagnetic field is in a nonclassical state. We use a SQUID at the end of an open transmission line to realize a magnetic-flux-tunable boundary condition, and generate wideband DCE radiation² by modulating the magnetic flux. The circuit noise and gain is calibrated by using the same SQUID as a shot noise tunnel junction. After calibration, we measure and analyze the covariance matrix for the DCE radiation at two different frequencies which add up to the pump frequency. We calculate the logarithmic negativity, and show two-mode squeezing below the vacuum, indicating entanglement. We furthermore estimate a potentially usable generation rate of 90 mega-entangled bits per second (Mebit s^-1) at the sample, applicable for detection schemes³ or to entangle quibits⁴.

[1] G. T. Moore, J. Math. Phys. 11, 2679 (1970).
[2] C. M. Wilson et al.: Nature 479, nature10561 (2011)
[3] P. B. Dixon et al.: Phys. Rev. A 83, 051803 (2011)
[4] S. Felicetti et al.: Phys. Rev. Lett. 113, 093602 (2014)