IEEE Photonics Journal (Jan 2024)
Entanglement Based Detection, Networking, Sensing, and Radars
Abstract
Entanglement based (EB) communication, networking, and sensing represent excellent alternatives to corresponding classical counterparts in particular in either low-brightness or high-attenuation regime and when the signal photons are buried in noise. In EB applications idler photons are stored in memory while the signal photons, carrying information, are transmitted over lossy, noisy, and for communication/networking over free-space optical (FSO) channels signal photons are also affected by the atmospheric turbulence and scattering effects. In optical phase conjugation (OPC)-based EB receiver, the OPC operation is performed on signal photons, which are severely affected by FSO channel impairments, with many signal photons being either absorbed or scattered by the FSO channel. Here we propose to perform the OPC operation on idler photons instead. The OPC operation is performed using low-cost, C-band components including tunable laser, periodically polled lithium niobate (PPLN) waveguides, tunable filter, and WDM demultiplexer; and this implementation is suitable for the photonic integrated circuit (PIC) realization. A similar approach is applied to implement the C-band devices-based entanglement source. Given that the idler photons are brighter compared to the signal photons transmitted over either FSO channel or fiber-optics channel, the OPC operation is much more efficient. In the OPC-based receiver for FSO communications, we need to couple the signal photons to the single-mode fiber (SMF) before the OPC takes place, making the OPC process sensitive to the FSO-to-SMF coupling inefficiency. By performing the OPC on the idler photons instead, the OPC process is insensitive to poor coupling efficiency. In addition to being superior in terms of spectral efficiency compared to classical counterparts, in particular in highly lossy and noisy environment, the proposed EB scheme with the phase-conjugation on idler photons is superior to the corresponding scheme performing the phase-conjugation on signal photons in terms of receiver sensitivity making it an attractive option for radar applications. To validate the proposed concept at the University of Arizona campus we have developed a terrestrial FSO testbed. For detection probability of 0.99 the proposed EB radar outperforms the classical counterpart by 6.7 dB. We experimentally demonstrate that the target detection probability of the proposed entanglement based radar over turbulent FSO channel is significantly better than that of corresponding classical detection scheme.
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