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International Conference on Mobile, Military, Maritime IT Convergence (ICMIC)

An Experimental Analysis of Several Variants of CASCADE Protocol for QKD*

Heejeung Won, Jieun Ryu, Ju-Sung Kang, Yongjin Yeom

Published Aug 27, 2025• 1 citation
Publisher

Abstract

In a quantum key distribution (QKD) system, the shared secret, called sifted key between the participants may not match due to the inherent properties of the noisy quantum channel. Thus, error correction protocols are indispensable. CASCADE is one of the most popular (currently the standard) error correction protocol in QKD based on parity exchange followed by binary search. In this study, we compare four Trace-Back algorithms in CASCADE and experimentally analyze their efficiency. Up to QBER 5%, we recommend the variant, TraceBack2First, as the most optimized approach, and comparatively analyze its efficiency in terms of communication with CASCADE. Through experiments, we examine that TraceBack2First is effective in reducing communication passes and memory usage, and expect it to be applicable to commercial QKD systems.

DOI
10.1109/icmic66299.2025.11257827

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In a quantum key distribution (QKD) system, the shared secret, called sifted key between the participants may not match due to the inherent properties of the noisy quantum channel. Thus, error correction protocols are indispensable. CASCADE is one of the most popular (currently the standard) error correction protocol in QKD based on parity exchange followed by binary search. In this study, we compare four Trace-Back algorithms in CASCADE and experimentally analyze their efficiency. Up to QBER 5%, we recommend the variant, TraceBack2First, as the most optimized approach, and comparatively analyze its efficiency in terms of communication with CASCADE. Through experiments, we examine that TraceBack2First is effective in reducing communication passes and memory usage, and expect it to be applicable to commercial QKD systems.
CASCADE protocol is an error detection and correction (EDC) method proposed firstly for use in quantum key distribution (QKD) systems. It is used to detect and correct all the errors in keys transmitted over a noisy quantum channel. This is done by sending some redundant information about the key to receiver as usual. However, just as differently, this extra information is sent over another noiseless classical channel after the quantum transmission is completely finished. Briefly, all the errors in noisy quantum communication are detected and corrected by a later noiseless classical communication using CASCADE protocol. In QKD literature, this EDC process is also called as information reconciliation (IR) or secret key reconciliation (SKR). For an IR protocol in QKD, one of the main performance measures is efficiency which depends on the amount of redundant information sent to make EDC possible. Since this extra information is transmitted over public channels, everyone can get it easily. Because this can damage the secrecy of keys that must be kept secret from third parties, more efficient, that is revealing less information about keys, IR methods are needed. In this paper, we present more efficient implementations of CASCADE protocol, using some inherent information already available in the protocol, exactly known bits and already known parities. This information is used in error detection and correction steps of the protocol to decrease the redundancy in redundant information. Our experiments have shown that the resulting protocols have higher efficiency than both all the previous CASCADE versions and several other more recently proposed IR methods.
CASCADE protocol is an error detection and correction (EDC) method proposed firstly for use in quantum key distribution (QKD) systems. It is used to detect and correct all the errors in keys transmitted over a noisy quantum channel. This is done by sending some redundant information about the key to receiver as usual. However, just as differently, this extra information is sent over another noiseless classical channel after the quantum transmission is completely finished. Briefly, all the errors in noisy quantum communication are detected and corrected by a later noiseless classical communication using CASCADE protocol. In QKD literature, this EDC process is also called as information reconciliation (IR) or secret key reconciliation (SKR). For an IR protocol in QKD, one of the main performance measures is efficiency which depends on the amount of redundant information sent to make EDC possible. Since this extra information is transmitted over public channels, everyone can get it easily. Because this can damage the secrecy of keys that must be kept secret from third parties, more efficient, that is revealing less information about keys, IR methods are needed. In this paper, we present more efficient implementations of CASCADE protocol, using some inherent information already available in the protocol, exactly known bits and already known parities. This information is used in error detection and correction steps of the protocol to decrease the redundancy in redundant information. Our experiments have shown that the resulting protocols have higher efficiency than both all the previous CASCADE versions and several other more recently proposed IR methods.
Quantum key distribution (QKD) enables secure communication by using the principles of quantum mechanics. A critical stage in QKD protocols such as BB84 is information reconciliation, where the communicating parties correct errors in their sifted sequences without revealing extra information to a potential eavesdropper. The CASCADE protocol is a widely used interactive error-correction algorithm for this purpose. The classical CASCADE and its known modifications perform well under the assumption of uniformly distributed errors in the quantum channel. However, this assumption can break down in real conditions, where short-term noise bursts create clusters of errors. This paper proposes a modification of the CASCADE designed to improve its efficiency in channels with time-varying, non-uniform error distributions. The main idea is to identify and discard blocks with a relatively high concentration of errors during reconciliation, preventing inefficient correction attempts and reducing the number of communication rounds. Two new modifications based on existing CASCADE variants are presented. The proposed approach reduces the number of interactions, which is a critical performance metric for reconciliation protocols.
This article addresses the development of quantum communication methods in the context of emerging quantum computing threats and emphasizes the importance of key reconciliation in quantum communication systems. The study focuses on the CASCADE protocol and the design of a software prototype intended for research and educational purposes. A parallel error-correction algorithm based on the actor model was implemented, improving the efficiency of key reconciliation and reducing the amount of exchanged data. Evaluation of the prototype revealed limitations, including the computational cost of message passing, complexity of error handling, and code redundancy due to iterative development. Experimental results confirmed the correct implementation of the core CASCADE algorithms and informed the design of future improvements. Proposed enhancements include redesigning the system architecture, developing interfaces for exporting intermediate data, defining the communication channel as a separate component, and expanding tools for systematic verification and comparative analysis of blind key-reconciliation methods.
Quantum Key Distribution (QKD) provides unconditional security, but requires a robust information reconciliation step to eliminate errors introduced by the quantum channel. The efficiency of this step directly determines the net secret key generation rate. The paper presents a detailed comparative analysis of three major reconciliation techniques: CASCADE, LDPC (Low-Density Parity-Check), and Polar Codes. The analysis focuses on seven essential metrics, including reconciliation efficiency (f), maximum error tolerance (QBERmax,), final secure key rate (R), frame error rate (FER), and computational complexity. Simulations are performed on a realistic QKD channel model (BB84 with depolarization), with QBER varying between 0.1% and 16%, covering experimental conditions. Keywords: QKD, Information Reconciliation, LDPC, CASCADE, Polar Codes, Secret Key Rate