Secure communication through free-space channel using quantum illumination

Abstract

Secure communication based on quantum illumination (QI) provides high-speed direct communication in the presence of loss and noise and is secure against passive eavesdropping. Recently, the QI based communication protocol has been demonstrated in a fiber channel. In this thesis, we extend the QI secure communication protocol to a free-space propagation channel. Unlike a fiber channel, a free-space channel is susceptible to air turbulence. Because a single spatial mode of light is essential in the QI protocol, if the beam path is affected by air turbulence, the power and phase may fluctuate, which can affect the interferometric measurement performance of QI. In order to fix this issue, we have designed and implemented a servo system to stabilize the coupling of the free-space propagating beam into a single-mode fiber. The servo system utilizes the X and Y tilts of a single piezoelectrically driven mirror mount, together with a quadrant detector, to stabilize the beam location at the collimation optics of the free-space path. To demonstrate the QI-based secure communication with a free-space path, we use a heat source to simulate air turbulence. We have demonstrated that the free-space secure communication using quantum illumination is still possible in an environment with air fluctuation, by using a servo system to counteract the deleterious effect the effect from air turbulence. Without air tubulence, we have demonstrated that BERA ~~ 5.8 x 10-⁴ for a free-space channel implemented in the Bob-to-Alice path or Alice-to-Bob path with Ns ~~ 5.6 x 10--⁴. When we introduce a heat source at a known setting, the effective attenuation h for the Bob-to-Alice or Alice-to-Bob channel transmissivity is found to be 0.63. Without the servo system, BERA drops to 2.40 x 10-⁴ and 9.8 x 10-³ for the free-space Bob-to-Alice and Alice-to-Bob channels, respectively, for the same amount of Ns. With both heat source and the servo system on, we have successfully operated the QI-based secure communication protocol and obtained the same level of Alice's BER as that without the heat source.