Ghost Polarization Communication

Abstract

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincar\'e sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient ${g}^{(2)}(\ensuremath{\tau})$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for ${g}^{(2)}(\ensuremath{\tau})$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.