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Scientific Reports

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

Byoung Seung HAM

Published Jan 21, 2021• 4 citations
Publisher

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.

DOI
10.1038/s41598-021-81522-y

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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.
Recently, new physics for unconditional security in a classical key distribution (USCKD) in a frame of a double Mach-Zehnder interferometer has been proposed and demonstrated as a proof of principle, where the unconditional security is unaffected by the no-cloning theorem of quantum key distribution protocols. Due to environmental phase fluctuations caused by temperature variations, atmospheric turbulences, or mechanical vibrations, active phase locking seems to be necessary for the two-channel transmission layout of USCKD. Here, the two-channel layout of USCKD is demonstrated to be an environmental noise-immune protocol especially for free space optical links, where the transmission distance is potentially unlimited if random phase noises are considered.
The unconditionally secured classical key distribution (USCKD) has been presented for an air turbulent free-space optical link. Even in the noisy air channels, the USCKD has shown nearly unconditionally secure communications in data transmission owing to double unitary transformation in a round-trip scheme, where the absolute phase noises in both air channels are replaced by a relative phase noise between them. Unlike classical cryptography, USCKD is based on basis randomness and thus resistant to the attack of quantum computers. Unlike quantum key distributions, USCKD is free from nonclassical light requirement such as a single photon, which is sensitive to quantum loopholes of channel loss and imperfect detectors.
So far, unconditional security in key distribution processes has been confined to quantum key distribution (QKD) protocols based on the no-cloning theorem of nonorthogonal bases. Recently, a completely different approach, the unconditionally secured classical key distribution (USCKD), has been proposed for unconditional security in the purely classical regime. Unlike QKD, both classical channels and orthogonal bases are key ingredients in USCKD, where unconditional security is provided by deterministic randomness via path superposition-based reversible unitary transformations in a coupled Mach-Zehnder interferometer. Here, the first experimental demonstration of the USCKD protocol is presented.
Free-space quantum communications offers a promising route for securely transmitting information over long distances. However, a major challenge for these systems is atmospheric turbulence, which distorts the spatial structure of light and can severely limit the amount of information that can be reliably transmitted. In this work, we propose and demonstrate a turbulence-resilient scheme for free-space quantum communication. By leveraging the phase conjugation property of stimulated parametric down-conversion, our scheme enables all-optical dynamic correction of spatial-mode distortion induced by atmospheric turbulence, thereby enhancing the secure key rate in high-dimensional quantum key distribution. We develop a theoretical model that provides detailed guidelines for selecting the optimal basis and spatial properties needed to maximize the efficiency of the proposed scheme. Both numerical simulations and experimental results show that, even under strong turbulence, our scheme can reduce the quantum error rates well below the security threshold. These results highlight the potential of nonlinear optical approaches as powerful tools for robust quantum communication in realistic free-space environments.
We propose and experimentally demonstrate an approach to generate and distribute secret keys over optical fiber communication infrastructure. Mach-Zehnder interferometers (MZIs) are adopted for key generation by transferring the environmental noise to random optical signals. A novel combination of wideband optical noise and an asymmetric MZI structure enables the secret keys to be securely transmitted and exchanged over public fiber links without being detected. We experimentally demonstrate this system and show reliable performance: keys are generated at the rate of 502 bit/s, and are successfully exchanged between two parties over a 10 km optical fiber with a bit error of ∼ 0.3%. System security analysis is performed by corroborating our experimental findings with simulations. The results show that our system can protect the key distribution under different attacks, attributed to wideband optical noise and asymmetric MZI structures. Compared to the previous schemes based on distributed MZIs, our scheme exploits localized MZI which provides twofold advantages. Firstly, the key generation rate can be increased by a factor of 5.7 at a negligible additional cost. Secondly, the system becomes robust to, in particular, active intrusion attack. The proposed system is a reliable and cost-effective solution for key establishment, and is compatible with the existing optical fiber communication infrastructure.
This article offers a solution to the fragmentation problem through a unified model and simulation showing how Adaptive Optics (AO) improves both classical and quantum communications. Free-space optical links can carry Quantum Key Distribution (QKD) over distances that cannot be reached with fiber; however, atmospheric turbulence corrupts the wavefront. For a single-mode quantum receiver, this drives the quantum bit error rate above the security threshold before detector noise can even be considered. AO addresses this by restoring the wavefront and recovering the coupling loss. The theory, mitigation techniques, and quantum security analysis needed to understand this are spread across three research communities that rarely cite one another: atmospheric turbulence and AO theory, classical modulation over fading channels, and BB84 quantum key distribution. We consolidate and develops all three topics in order to trace the chain from wavefront error to bit error rate and secret key rate. We survey six families of turbulence mitigation and provide an assessment of their suitability for single-mode quantum links. In addition, we survey the free-space QKD experimental record, from terrestrial hops to satellite downlinks. A wave optics case study of a representative 500-km Low Earth Orbit (LEO) downlink, corrected by a closed-loop Shack–Hartmann and deformable-mirror receiver at 1.55 μm ties the review together. Across a turbulence sweep, the correction stabilizes single-mode coupling near its physical ceiling, while the uncorrected channel varies by more than an order of magnitude. When carried into the downlink budget, this lifts the quantum link across the 11% security bound at 12 of 15 turbulence conditions where the uncorrected link yields no key, thereby restoring a secure quantum bit error rate in a narrow 4.2–4.9% band throughout and a secret key rate of 7.6–11.2 kbit/s while simultaneously widening the margin of the co-propagating classical channel above its forward error correction threshold.