Analysis of phase noise effects in a coupled Mach–Zehnder interferometer for a much stabilized free-space optical link

Abstract

Recently, new physics for unconditional security in a classical key distribution (USCKD) has been proposed and demonstrated in a frame of a double Mach–Zehnder interferometer (MZI) as a proof of principle, where the unconditional security is rooted in MZI channel superposition. Due to environmental phase noise caused by temperature variations, atmospheric turbulences, and mechanical vibrations, free-space optical links have been severely challenged for both classical and quantum communications. Here, the double MZI scheme of USCKD is analyzed for greatly subdued environment-caused phase noise via double unitary transformation, resulting in potential applications of free-space optical links, where the free-space optical link has been a major research area from fundamental physics of atomic clock and quantum key distribution to potential applications of geodesy, navigation, and MIMO technologies in mobile communications systems.

Network-Compatible Unconditionally Secured Classical Key Distribution via Quantum Superposition-Induced Deterministic Randomness

“…The proposed NC-USCKD can be applied to conventional DWDM-based fiber-optic communications networks by allocating each address to each wavelength [34]. Because of the MZI robustness in phase fluctuations demonstrated in both optical fibers [24] and free space [28] for a few km ranges, the network extension to tens of km with large N is a simple technical issue with current locking technologies [27,28,33,34]. In a multi-core fiber, the MZI path length is potentially error-free due to the core-to-core proximity in a few microns [1].…”

“…To overcome the limitations of classical and quantum cryptographies, an entirely different method of unconditionally secured classical key distribution (USCKD) has been proposed for both wired [23] and wireless [24] transmissions using a pair of transmission channels forming a Mach-Zehnder interferometer (MZI) via quantum superposition between the MZI channels and its unitary transformation, resulting in deterministic randomness.This deterministic randomness represents no eavesdropping due to measurement indistinguishability caused by quantum superposition in the MZI channels, as well as the deterministic key distribution between two remote parties via unitary transformation. As demonstrated, the key generation determinacy in USCKD [23] is well understood in the coherence optics of MZI in terms of the directional determinacy [25].…”