Quantum key distribution relies on quantum mechanics to securely distribute cryptographic keys, offering security but necessitating complex infrastructure and significant resources for practical implementation. Quantum keyless private communication ensures information-theoretic security in free-space communication, with simpler setups, and without the need for secret keys by leveraging the wiretap channel model. Here we propose a variant of quantum keyless private communication using polarization encoding and experimentally validate both the original on-off keying method and the polarization-multiplexed approach using time-multiplexed threshold single-photon detectors as photon counting detectors. Our analysis highlights the advantages of polarization-multiplexed schemes for daylight operation. This work paves the way towards practical and scalable quantum communication systems, with potential applications extending to space-based communication.
Quantum key distribution relies on quantum mechanics to securely distribute cryptographic keys, offering security but necessitating complex infrastructure and significant resources for practical implementation. Quantum keyless private communication ensures information-theoretic security in free-space communication, with simpler setups, and without the need for secret keys by leveraging the wiretap channel model. Here we propose a variant of quantum keyless private communication using polarization encoding and experimentally validate both the original on-off keying method and the polarization-multiplexed approach using time-multiplexed threshold single-photon detectors as photon counting detectors. Our analysis highlights the advantages of polarization-multiplexed schemes for daylight operation. This work paves the way towards practical and scalable quantum communication systems, with potential applications extending to space-based communication.
Quantum key distribution (QKD) is the most established approach in quantum communication. However, long-distance free-space implementations, particularly satellite links, remain challenging, especially during the day due to daylight background noise. Quantum keyless private communication (QKPC) is a quantum communication protocol that enables information-theoretic security with simpler system requirements, improved robustness against noise, and without the need for secret key distribution. QKPC and QKD are complementary, with QKPC enabling free-space links where QKD is impractical, while QKD provides channel monitoring for applications that require eavesdropping detection. Here, we report a complete implementation of QKPC in a daylight free-space experiment over a 90 m rooftop link, using an experimentally simple setup. Our demonstration includes all stages of the protocol, from encoding and synchronization to message decoding, operates entirely without auxiliary classical synchronization channels, and is implemented offline through post-processing. This work demonstrates the feasibility of practical and scalable quantum communication over high-noise daylight links and highlights the potential of QKPC as a complementary solution to QKD for future ground-based and space-based communication systems.
Quantum technologies have been proved useful for space applications, such as for the well-known application of quantum key distribution (QKD). However, QKD shows a number of serious practical limitations and therefore we are interested in different ways of using quantum resources and readily available quantum technology for secure communication in space. Towards this aim, we first of all provide an overview of quantum resources and state-of-the art quantum technologies, specifically sources and detectors. Then, we identify two realistic and related applications for the short-term. The first one is quantum-enhanced communication as quantified by Shannon rates using single-photon detection technology and lasers. In this case, we show illustrative potential gains for Optical On/Off Keying (O3K) communications. The second one is quantum physical layer cryptography, which builds upon quantumenhanced communications. In this case, we show how to realize secure communication in space with quantum resources, cryptographic primitives and without the use of any key, which we call Quantum Keyless Privacy (QKP). It is a variant of the quantum wiretap channel principle hybridized with primitives and technology of QKD. In this case, we discuss the security of QKD and of our method and show that both methods provide information theoretical security.
We study information-theoretical security for space links between a satellite and a ground station. Quantum key distribution (QKD) is a well-established method for information-theoretical secure communication, giving the eavesdropper unlimited access to the channel and technological resources limited by only the laws of quantum physics. But QKD for space links is extremely challenging, the achieved key rates are extremely low, and daytime operating impossible. However, eavesdropping on a channel in free space without being noticed seems complicated, given the constraints imposed by orbital mechanics. If we also exclude the eavesdropper's presence in a given area around the emitter and receiver, we can guarantee that he has access to only a fraction of the optical signal. In this setting, quantum keyless private (direct) communication based on the wiretap channel model is a valid alternative to provide information-theoretical security. Like for QKD, we assume that the legitimate users are limited by state-of-the-art technology, while the potential eavesdropper is only limited by physical laws: either by specifying her detection strategy (Helstrom detector) or by bounding her knowledge, assuming the most powerful strategy through the Holevo information. Nevertheless, we demonstrate information-theoretical secure communication rates (positive keyless private capacity) over a classical-quantum wiretap channel using on-off keying of coherent states. We present numerical results for a setting equivalent to the recent experiments with the Micius satellite and compare them to the fundamental limit for the secret key rate of QKD. We obtain much higher rates compared with QKD with an exclusion area of less than 13 m for low earth orbit satellites. Moreover, we show that the wiretap channel quantum keyless privacy is much less sensitive to noise and signal dynamics and daytime operation is possible.
The goal of cryptography is to encode data in such a way that no one can decode it unless they are the intended recipient. With the emergence of quantum computing, classical cryptography is not reliable enough to protect sensitive information. Nevertheless, there are alternatives. Quantum Key Distribution (QKD) is a method to secure communication based on the laws of quantum physics. Due to being impossible to measure the quantum state of a system without disturbing it, QKD provides a way to communicate between two parties while being, in theory, immune to eavesdropping. In this work, a free-space high-speed QKD setup is proposed, with the objective of implementing the polarization three-state one-decoy variant of the BB84 protocol. An assessment of the best type of laser diode to use for free-space QKD is made between several options. The system’s performance was assessed, incorporating a quantitative analysis of system losses and an estimation of the number of photons traversing the quantum channel. The system achieved a Quantum Bit Error Rate (QBER) of 1.58% for the Z basis and 1.46% for the X basis, for an average reception of two photons per pulse. Preliminary QBER measurements were also undertaken in the context of Quantum Keyless Private Communication protocols, focusing on the average number of photons per pulse.
The rapid increase on the information sharing around the world, leads to an utmost requirement for capacity and bandwidth. However, the need for security in the transmission and storage of information is also of major importance. The use of quantum technologies provides a practical solution for secure communications systems. Quantum key distribution (QKD) was the first practical application of quantum mechanics, and nowadays it is the most developed one. In order to share secret keys between two parties can be used several methods of encoding. Due to its simplicity, the encoding into polarization is one of the most used. However, when we use optical fibers as transmission channels, the polarization suffers random rotations that may change the state of polarization (SOP) of the light initially sent to the fiber to a new one at the output. Thus, in order to enable real-time communication using this encoding method it is required the use of a dynamic control system. We describe a scheme of transmission of quantum information, which is based in the polarization encoding, and that allows to share secret keys through optical fibers without interruption. The dynamic polarization control system used in such scheme is described, both theoretically and experimentally. Their advantages and limitations for the use in quantum communications are presented and discussed.
High-dimensional entanglement has been shown to provide significant advantages in quantum communication. One of its most promising implementations is available in the time domain routinely produced in spontaneous parametric down-conversion . While advantageous in the sense that only a single detector channel is needed locally, it is notoriously hard to analyze, especially in an assumption-free manner as required for quantum key distribution (QKD) applications. We develop a complete analysis of high-dimensional entanglement in the polarization-time domain and show how to efficiently certify relevant density matrix elements and security parameters for QKD. In addition to putting past experiments on rigorous footing, we also develop a physical noise model and propose a setup that can further enhance the noise resistance of free-space quantum communication.