{"id":2220,"date":"2026-04-07T10:13:43","date_gmt":"2026-04-07T08:13:43","guid":{"rendered":"https:\/\/www.milq.info\/?page_id=2220"},"modified":"2026-04-07T12:31:57","modified_gmt":"2026-04-07T10:31:57","slug":"chapter3","status":"publish","type":"page","link":"https:\/\/www.milq.info\/en\/qti\/qcomm\/module2\/chapter3\/","title":{"rendered":"Quantum Communication &#8211; Module 2 Chapter 3"},"content":{"rendered":"<div id=\"bsf_rt_marker\"><\/div><p><\/p>\n<h1>Decoy-state Protocol<\/h1>\n<p>In the preceding chapters, two different protocols for quantum key distribution (QKD) were introduced. In the BB84 protocol, the sender, Alice, prepares a quantum object\u2014a photon\u2014and transmits it to the receiver, Bob. In the E91 protocol as well as in the BBM92 protocol, a source is located in the middle of the communication channel that sends an entangled photon pair to each of the communication partners, Alice and Bob. Both methods were initially considered secure\u2014but is this really the case in practice?<\/p>\n<p>&nbsp;<\/p>\n<p>Let us examine vulnerabilities in the BB84 protocol. Here, Eve must actively tap into the communication channel by intercepting, measuring, and forwarding the quantum objects, because the no-cloning theorem forbids the creation of an exact copy of a quantum state. In reality, however, the controlled generation and transmission of single photons is an enormous challenge. It often happens that not single but multiple photons are sent simultaneously. In such a case an eavesdropper, Eve, can obtain the encryption key:<\/p>\n<p><video src=\"\/QTIndu\/QComm\/en-2-3-video-1.mp4\" controls=\"controls\" width=\"1920\" height=\"150\"> Dein Browser unterst\u00fctzt dieses Videoformat nicht. Your browser does not support the video tag. <\/video><\/p>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-88\" class=\"h5p-iframe\" data-content-id=\"88\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-1\"><\/iframe><\/div>\n<hr \/>\n<p>This procedure is called a Photon-Number-Splitting attack (PNS attack). The photon pulse is split into individual photons\u2014some are intercepted and stored by Eve, while others are transmitted to Bob without disturbance. In this scenario Eve has far-reaching capabilities by selectively extracting individual photons from a multi-photon pulse, storing them, and later measuring them on the basis of the disclosed polarization bases.<br \/>\nIn the theoretical ideal case the BB84 protocol is secure as long as only single photons are used. However, the targeted generation of single photons remains a technical challenge.<\/p>\n<p>To counter a PNS attack, the so-called decoy-state protocol was developed. In addition to the actual information-carrying signal, auxiliary decoy states are sent in order to identify a possible eavesdropper:<\/p>\n<p><video src=\"\/QTIndu\/QComm\/en-2-3-video-2.mp4\" controls=\"controls\" width=\"1920\" height=\"150\"> Dein Browser unterst\u00fctzt dieses Videoformat nicht. Your browser does not support the video tag. <\/video><\/p>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-90\" class=\"h5p-iframe\" data-content-id=\"90\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-2\"><\/iframe><\/div>\n<hr \/>\n<p>Decoy-state protocols constitute a significant extension of QKD, particularly in the context of the BB84 protocol. This approach greatly increases robustness against sophisticated attacks, so that even elaborate attempts can be identified and neutralized.<\/p>\n<hr \/>\n<p>With the decoy-state protocol we conclude the module section dealing with discrete-variable QKD (DV-QKD). The protocols BB84, E91, BBM92, and finally the decoy-state protocol are based on the discrete nature of quantum objects and use their specific properties for key distribution. Measurements on individual quantum objects are performed to generate a secure key.<br \/>\nIn the next chapter, Chapter 4, we will turn to continuous-variable QKD (CV-QKD).<br \/>\nTo conclude the DV-QKD part, we will review your understanding of these topics, repeat the contents, and deepen them.<\/p>\n<hr \/>\n<h2>Exercises:<\/h2>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-92\" class=\"h5p-iframe\" data-content-id=\"92\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-3\"><\/iframe><\/div>\n<h3>Exercise:<\/h3>\n<p>The figure shows an excerpt of the communication between Alice and Bob. Analyze whether an eavesdropping attack has taken place. If not, give the key that can be extracted from the figure.<\/p>\n<p><img decoding=\"async\" src=\"\/QTIndu\/QComm\/en-2-3-image-1-puzzle1.png\" alt=\"Dein Browser unterst\u00fctzt dieses Bildformat nicht. Your browser does not support the image format.\" width=\"1280\" \/><\/p>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-94\" class=\"h5p-iframe\" data-content-id=\"94\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-4\"><\/iframe><\/div>\n<h3>Exercise:<\/h3>\n<p>The figure shows an excerpt of the communication between Alice and Bob. Analyze whether an eavesdropping attack has taken place. If not, give the key that can be extracted from the figure.<\/p>\n<p><img decoding=\"async\" src=\"\/QTIndu\/QComm\/en-2-3-image-1-puzzle2.png\" alt=\"Dein Browser unterst\u00fctzt dieses Bildformat nicht. Your browser does not support the image format.\" width=\"1280\" \/><\/p>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-96\" class=\"h5p-iframe\" data-content-id=\"96\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-5\"><\/iframe><\/div>\n<h3>Exercise:<\/h3>\n<p>The figure shows an excerpt of the communication between Alice and Bob. Analyze whether an eavesdropping attack has taken place. If not, give the key that can be extracted from the figure.<\/p>\n<p><img decoding=\"async\" src=\"\/QTIndu\/QComm\/en-2-3-image-1-puzzle3.png\" alt=\"Dein Browser unterst\u00fctzt dieses Bildformat nicht. Your browser does not support the image format.\" width=\"1280\" \/><\/p>\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-98\" class=\"h5p-iframe\" data-content-id=\"98\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"QTI-EN-QComm-2-3-6\"><\/iframe><\/div>\n<p style=\"text-align: right;\"><a class=\"button\" href=\"https:\/\/www.milq.info\/en\/qti\/qcomm\/module2\/chapter4\/\">Continue to Chapter 4<\/a><\/p>\n<p><\/p>","protected":false},"excerpt":{"rendered":"<p>Decoy-state Protocol In the preceding chapters, two different protocols for quantum key distribution (QKD) were introduced. In the BB84 protocol, the sender, Alice, prepares a quantum object\u2014a photon\u2014and transmits it to the receiver, Bob. In the E91 protocol as well as in the BBM92 protocol, a source is located in the middle of the communication&hellip; <a href=\"https:\/\/www.milq.info\/en\/qti\/qcomm\/module2\/chapter3\/\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">Quantum Communication &#8211; Module 2 Chapter 3<\/span><\/a><\/p>\n","protected":false},"author":8,"featured_media":0,"parent":2207,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-2220","page","type-page","status-publish","hentry","without-featured-image"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Quantum Communication - Module 2 Chapter 3 - milq<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.milq.info\/qti\/qcomm\/module2\/chapter3\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Quantum Communication - Module 2 Chapter 3 - milq\" \/>\n<meta property=\"og:description\" content=\"Decoy-state Protocol In the preceding chapters, two different protocols for quantum key distribution (QKD) were introduced. 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