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Journal of Physics Photonics

Error-free secure key distribution with decoy mechanism via spectral encoding in Raman fiber laser

Yang Zhang, Jun Ye, Tianfu Yao, Jiangming Xu, Pu Zhou

Published Jul 30, 2025• 0 citations
Publisher

Abstract

Abstract Secure key distribution is a vital yet challenging task for most secure communication systems. Here, we demonstrate an error-free secure key distribution scheme with a decoy mechanism by establishing Raman lasing between the parties. At the transmitter port, true keys mixed with fakes keys are spectrally encoded into Raman signals at different wavelengths. A spectral manipulable feedback ring is adopted to provide weak feedback for the true signal and strong feedback for the fake one. Over the transmission link, true Raman signals are suppressed and the eavesdropper can only detect the fake ones. At the receiver port, the user can reveal the hidden true signals and identify the fake signals by turning on a magic switch. Moreover, the distributed Raman gain along the fiber can compensate for the fiber loss, thereby enabling error-free secure key distribution over 50 km. The structural simplicity, error-free property, and novel decoy mechanism render this scheme a promising alternative for long distance secure key distribution.

DOI
10.1088/2515-7647/adf617

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Abstract Secure key distribution is a vital yet challenging task for most secure communication systems. Here, we demonstrate an error-free secure key distribution scheme with a decoy mechanism by establishing Raman lasing between the parties. At the transmitter port, true keys mixed with fakes keys are spectrally encoded into Raman signals at different wavelengths. A spectral manipulable feedback ring is adopted to provide weak feedback for the true signal and strong feedback for the fake one. Over the transmission link, true Raman signals are suppressed and the eavesdropper can only detect the fake ones. At the receiver port, the user can reveal the hidden true signals and identify the fake signals by turning on a magic switch. Moreover, the distributed Raman gain along the fiber can compensate for the fiber loss, thereby enabling error-free secure key distribution over 50 km. The structural simplicity, error-free property, and novel decoy mechanism render this scheme a promising alternative for long distance secure key distribution.
Summary form only given. The distribution of the secret key is the weakest link of many data encryption systems. Quantum key distribution (QKD) schemes provide attractive solutions [1], however their implementation remains challenging and their range and bit-rate are limited. Moreover, practical QKD systems, employ real-life components and are, therefore, vulnerable to diverse attack schemes [2]. Ultra-Long fiber lasers (UFLs) have been drawing much attention recently because of their fundamentally different properties compared to conventional lasers as well as their unique applications [3]. Here, we demonstrate a 100Bps, practically secure key distribution, over a 500km link, employing Raman gain UFL. Fig. 1(a) depicts a schematic of the UFL system. Each user has an identical set of two wavelength selective mirrors centered at l0and l1. In order to exchange a key-bit, each user independently choose one of these mirrors and introduces it as a laser reflector at their end. If both users choose identical mirrors, a clear signal develops and the bits in these cases are discarded. However if they choose complementary mirrors, (1, 0 or 0, 1 states), the UFL remains below lasing threshold and no signal evolves. In these cases, an eavesdropper can only detect noise and is unable to determine the mirror choice of the users, where the choice of mirrors represent a single key bit (e.g. Alice's choice of mirror is the key-bit). These bits are kept and added to the key. The absence of signal in the secure states faxilitates fast measurements to distinguish between the non-secure and the secure states and to determine the key-bit in the later case, Sequentially reapeating the single bit exchange protocol generate the entire keys of any desirable length.
Abstract In traditional communication systems the transmission medium is considered as a given characteristic of the channel, which does not depend on the properties of the transmitter and the receiver. Recent experimental demonstrations of the feasibility of extending the laser cavity over the whole communication link connecting the two parties, forming an ultra‐long fiber laser (UFL), have raised groundbreaking possibilities in communication and particularly in secure communications. Here, a 500 km long secure key distribution link based on Raman gain UFL is demonstrated. An error‐free distribution of a random key with an average rate of 100 bps between the users is demonstrated and the key is shown to be unrecoverable to an eavesdropper employing either time or frequency domain passive attacks.
The secure distribution of a secret key is the weakest point of shared-key encryption protocols. While quantum key distribution schemes could theoretically provide unconditional security, their practical implementation remains technologically challenging. Here we provide an extended analysis and present an experimental support of a concept for a classical key generation system, based on establishing laser oscillation between two parties, which is realized using standard fiber-optic components. In our Ultra-long Fiber Laser (UFL) system, each user places a randomly chosen, spectrally selective mirror at his/her end of a fiber laser, with the two-mirror choice representing a key bit. We demonstrate the ability of each user to extract the mirror choice of the other using a simple analysis of the UFL signal, while an adversary can only reconstruct a small fraction of the key. The simplicity of this system renders it a promising alternative for practical key distribution in the optical domain.
In ultralong fiber laser key distribution, two sides use standard optical equipment to create kilometer long fiber lasers in a communication link to establish a secret key. Its security rests on the assumption that any attack to learn the generated key would be unpractical while the legitimate users only need to use standard equipment. We present a challenge to that assumption with a hidden probe attack in which the eavesdropper hides a weak signal in the unavoidable noise floor that appears in the laser during amplification and probes with it the configuration of one or both communication parties. We comment how this attack can compromise different proposals for ultralong laser key distribution and propose possible countermeasures.
Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demands are rather tough because they require single photon level interference of two remote independent lasers. Here, we adopt the technology developed in the frequency and time transfer to lock two independent laser wavelengths and utilize additional phase reference light to estimate and compensate the fiber fluctuation. Further, with a single photon detector with a high detection rate, we demonstrate twin field quantum key distribution through the sending-or-not-sending protocol with a realistic phase drift over 300 km optical fiber spools. We calculate the secure key rates with the finite size effect. The secure key rate at 300 km (1.96×10^{-6}) is higher than that of the repeaterless secret key capacity (8.64×10^{-7}).