Optical front-end for a quantum key distribution cubesat

The progress of quantum computing and quantum information science (QIS) will be largely reliant on an emerging workforce prepared to enter the QIS industry. As a relatively new field, opportunities for teachers and youth to engage with QIS have historically been limited. Quantum Computing, Mathematics, and Physics (QCaMP) is a summer camp designed to address this gap. QCaMP serves both teachers and high school students, providing an entry point into the growing field and introducing participants to core quantum concepts. During the teacher camp, we offer a framework for educators to integrate QIS into their own classrooms. The student camp focuses on building a foundational understanding of QIS while highlighting its practical applications and relevance to everyday life. In this talk, I will focus on how we introduce application-based topics using a hands-on, experiential approach that enhances student engagement and comprehension. I will highlight our approach to teaching Quantum Key Distribution (QKD). We begin by introducing classical cryptography methods, such as the Caesar cipher, through interactive activities in which students write and exchange encrypted messages. We then use “coin boxes” to guide students through the BB84 protocol, offering a tactile and intuitive way to learn a quantum cryptographic method. The camp culminates in student projects, where participants demonstrate their understanding of chosen QIS topics. In the QKD project, for example, students successfully designed and conducted a BB84-based experiment and presented their findings through posters, showcasing their conceptual growth and understanding.

Remarkable progress has been made in satellite-based quantum key distribution (QKD), which can effectively provide QKD service even at the intercontinental scale and construct an ultralong-distance global quantum network. But there are still some places where terrestrial fiber and ground stations cannot be constructed, like harsh mountainous areas and air space above the sea. So the airborne platform is expected to replace the ground station and provide flexible and relay links for the large-scale integrated communication network. However, the photon transmission rate would be randomly reduced, owing to the randomly distributed boundary layer that surrounds the surface of the aircraft when the flight speed is larger than 0.3 Ma. Previous research of airborne QKD with boundary layer effects is mainly under the air-to-ground scenario in which the aircraft is a transmitter, while the satellite-to-aircraft scenario is rarely reported. In this article, we propose a performance evaluation scheme of satellite-to-aircraft QKD with boundary layer effects in which the aircraft is the receiver. With common experimental settings, the boundary layer would introduce a ∼31 dB loss to the transmitted photons, decrease ∼47% of the quantum communication time, and decrease ∼51% of the secure key rate, which shows that the aero-optical effects caused by the boundary layer cannot be ignored. Our study can be performed in future airborne quantum communication designs.

This survey article discusses the revolutions of wireless communication technologies right from the prehistoric man's fire signals to today's space optical communications. A brief description of evaluations of communication technologies from the ancient Greece to today's mature telecommunication fields is discussed in this review paper. Principle, significance, demonstration and development of free space optical (FSO) communication technology over different decades are discussed. Major FSO channel limitations, temporal and spatial challenges of FSO communication system and their state‐of‐art mitigation techniques are presented. Classical quantitative analysis of reliability of FSO communication, developments on radio over FSO (RoFSO) and hybrid FSO/RF systems are also explained. Advanced developments of FSO communication techniques such as optical free space wavelength division multiplexing (WDM), sub‐carrier multiplexing (SCM), worldwide interoperability for microwave access (WiMAX), visible light communications (VLC) and vehicular visible light communications (VVLC) are reported. Deep space optical communication systems and next generation FSO wireless terrestrial/global network architecture are summarised. Research challenges of FSO systems for internet of things/everything (IoT/IoE), 5G communication, mobile‐network, terahertz spectrum, quantum communication and underwater optical applications are presented. Based on the review, we outlined the challenges that need to be addressed in near‐future researches to realize full potential FSO communication systems.

This chapter presents state-of-the-art and future perspectives of quantum computing and communication. Timeline of relevant findings in quantum informatics, such as quantum algorithms, quantum cryptography protocols, and quantum computing models, is summarized. Mathematics of information representation with quantum states is presented. The quantum circuit and adiabatic models of quantum computation are outlined. The functionality, limitations, and security of the quantum key distribution (QKD) protocol is presented. Current implementations of quantum computers and principles of quantum programming are shortly described.

This chapter discusses quantum information science (QIS), quantum key distribution (QKD), and quantum teleportation. The new field of QIS encompasses several overlapping subdisciplines: quantum computing, quantum key distribution (also known as quantum cryptography), and quantum metrology (the measurement of small parameters and weak forces with ultra-high precision). In QIS, bits of information are encoded in quantum states, which themselves can be in superposition states and/or entanglement states. Generically, the 0 and 1 quantum bits are represented as qubits (quantum bits). QKD, or quantum cryptography, is a completely secure method of secret communication, and will remain so even if a quantum computer becomes available. Quantum teleportation is one of the more exotic features to have emerged from the field of QIS. It refers to the ability to transfer an unknown quantum state from one location to another, possibly very distant, location by using quantum entanglement.

Single negatively charged nitrogen-vacancy (NV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> ) defect centres in diamond are attractive systems for the implementation of solid state quantum information processing (QIP) and quantum key distribution (QKD). In both approaches, the coupling of an individual NV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> centre to a resonant microcavity is of interest: it would improve the optical readout of the NV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> centre electron spin state in QIP and enhance the collection efficiency of the single photon fluorescence in QKD while increasing the coherence length of the emission. We theoretically demonstrate that, at room temperature, the coupling to a single-mode Fabry-Perot microcavity can be used to obtain a fluorescence spectrum narrowing without loss of total intensity. We also experimentally demonstrate the control of the fluorescence spectrum of a single NV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> centre by adjusting the thickness of the microcavity. Finally, we report a doubling of the emission spectral density with respect to the single mirror configuration.

Quantum Key Distribution (QKD) is increasingly pivotal in securing communication channels against the looming threats posed by quantum computing. However, existing QKD protocols encounter challenges related to efficiency and transmission capabilities. In response, this research investigates the integration of metamaterials into QKD systems, aiming to fortify security and enhance practicality. In the current landscape of quantum communication, where the vulnerability of classical encryption methods is magnified by rapid advancements in quantum computing, finding innovative solutions is imperative. This study is motivated by the need to strengthen the security and viability of QKD protocols to meet the demands of evolving cryptographic threats. By integrating metamaterials, the authors optimise quantum state control, improve signal‐to‐noise ratio (SNR), and enable longer transmission distances. Through mathematical modelling and simulations, the authors demonstrate how metamaterials reduce errors and enhance the robustness of QKD systems. Our findings show significant improvements in transmission efficiency and security, making Metamaterial‐Based Quantum Key Distribution (MQKD) a promising approach for future quantum communication networks. The study not only advances the understanding of the theoretical foundations, but also presents simulated results illustrating the practical effectiveness of MQKD. The exploration of these innovative techniques contributes to the ongoing efforts to secure quantum communication channels.

Quantum communication is brand new way of communication in which quantum entanglement is used to transmit information. It is an interdisciplinary subject combining quantum informatics with modern communication theory. Motivated by the communication requirements for underwater sensor networks, submarines, etc., underwater optical communication has been developing rapidly in recent years due to the ideal information security of quantum communication. However, the research on the performance of underwater quantum communication in sea has not yet been fully developed because of a series of factors such as surge, salinity and seaweed and so on. In this paper, the influence of surge in non-uniform water flow on the underwater quantum communication is studied theoretically and experimentally. Firstly, a new Boussinesq equation with a given flow function is derived based on the horizontal and vertical wave velocity of the free surface to represent the free surface boundary conditions. On the other hand, In view of the nonlinear motion of movement, the complexity of change and the randomness of the distribution, the spectrum is used for numerically calculating the surge. The characteristics of wave motion are described by wave height, period and wavelength. Secondly, the influence of surge on the entanglement of underwater quantum channel is analyzed. It is proved that the wave height of surge and the change of the cycle affect quantum communication due to the destruction of the quantum coherence and the reduction in quantum entanglement degree. Thirdly, the influence of surge motion on the quantum channel capacity is studied. The influence of the relation between the wavelength and the transmission cycle on the quantum channel capacity is simulated. The relationship between the physical characteristics of surge wave and the capacity of depolarized channel is established. Fourthly, the influence of surge motion on error rate in quantum key distribution is studied. The simulation results show that when the sea surface wind speed changes in a range of 0-20.5 m/s, the propagation cycle is increased gradually. The channel entanglement is increased from 0.0012 to 0.8426, and the channel capacity is reduced from 0.8736 to 0.1024. In the key distribution process, the quantum bit error rate increases from 0.1651 to 0.4812. Therefore, in underwater quantum communication, the parameters of the system should be adjusted adaptively according to the varying degree of the surge movement.

Quantum communication, based on the quantum physics, can provide information theoretical security. Building a global quantum network is one ultimate goal for the research of quantum information. Here, this talk will review the progress for quantum communication in China, including quantum key distribution over metropolitan area with untrustful relay, field test of quantum entanglement swapping over metropolitan network, the 2000 km quantum key distribution main trunk line, and satellite based quantum communication.

We consider main methods for detecting single photons used in quantum communications, including the quantum key distribution (QKD) technology. Two most promising single photon detectors (SPDs) based on superconducting nanowires and on a single-photon avalanche diode are described. The most effective SPD designs are presented and their advantages and disadvantages are analysed from the point of view of the possibility of their use in QKD devices. The results of the work of various scientific groups conducting research on QKD are discussed, which makes it possible to trace the trends in the global technological development of this industry over the past five years.

Security of modern classical data encryption often relies on computationally hard problems, which can be trivialized with the advent of quantum computers. A potential remedy for this is quantum communication which takes advantage of the laws of quantum physics to provide secure exchange of information. Here, quantum key distribution (QKD) represents a powerful tool, allowing for unconditionally secure quantum communication between remote parties. At the same time, microwave quantum communication is set to play an important role in future quantum networks because of its natural frequency compatibility with superconducting quantum processors and modern near-distance communication standards. To this end, we present an experimental realization of a continuous-variable QKD protocol based on propagating displaced squeezed microwave states. We use superconducting parametric devices for generation and single-shot quadrature detection of these states. We demonstrate unconditional security in our experimental microwave QKD setting. The security performance is shown to be improved by adding finite trusted noise on the preparation side. Our results indicate feasibility of secure microwave quantum communication with the currently available technology in both open-air (up to ~ 80 m) and cryogenic (over 1000 m) conditions.

In recent years, quantum-based techniques have attracted significant research attention because of its unique advantages on satellite communications, especially for security problem. Security guarantee is one of the most important requirements in QoS-driven 3D satellite networks. Quantum key distribution (QKD) is a methodology for generating and distributing random encryption keys using the principles of quantum physics, which enables two distant communications parties to securely communicate in a way that cannot be eavesdropped on without being detected. Although the QKD method can ensure the absolute security transmission over 3D satellite networks, it imposes many new implementation challenges due to the various limitations on quantum communication over long distances via 3D free space, including quantum channel attenuation, photon-state disruption and vulnerability to noise/interference, laser-beam widening, and constrained security-key generation rate. These problems get even more challenging when QoS provisioning is required for the applications over the 3D satellite networks. To overcome the aforementioned difficulties, we propose the framework to efficiently implement the QKD for security guarantees over quantum-repeater-based QoS-driven 3D satellite networks. First, we develop the quantum-repeater-based QKD satellite network architecture. Then, we design the quantum repeater including the purification scheduling algorithm and the optimal QoS-based repeating-router selection scheme in quantum-repeater-based QKD satellite networks. Finally, the obtained simulations evaluation validate and evaluate our proposed algorithms and schemes.

Quantum cryptography can provide almost complete security of the data transmitted in optical telecommunication systems by single photons based on the laws of quantum mechanics. The paper presents a brief overview of the element base and experimental investigations in the field of quantum cryptography and data transmission by single photons in atmospheric and optical fiber quantum communication lines. Two experimental setups for the single-photon quantum key distribution in the atmospheric and optical fiber quantum channels are described. The results of the quantum key distribution experiments performed on them are given.

Preserving entanglement is a crucial dynamical process for entanglement-based quantum computation and quantum-information processes, such as one-way quantum computing and quantum key distribution. However, the problem of quantifying the ability of an experimental process to preserve two-qubit entanglement in experimentally feasible ways is not well understood. Accordingly, herein, we consider the use of two measures, namely composition and robustness, for quantitatively characterizing the ability of a process to preserve entanglement, referred to henceforth as entanglement preservability. A fidelity benchmark is additionally derived to identify the ability of a process to preserve entanglement. We show that the measures and introduced benchmark are experimentally feasible and require only local measurements on single qubits and preparations of separable states. Moreover, they are applicable to all physical processes that can be described using the general theory of quantum operations, e.g., qubit dynamics in photonic and superconducting systems. Our method extends the existing tools for analyzing channels, e.g., channel resource theory, to quantify entanglement preservability for non-trace-preserving quantum processes. The results are of significant interest for applications in quantum-information processing in which entanglement preservation is required.

We propose a terahertz continuous-variable quantum key distribution protocol using pulse position modulation, which enables simultaneous quantum and classical communication for inter-satellite links. In this scheme, classical information is conveyed through the relative positioning of encoded pulses while quantum information is encoded using Gaussian modulation to generate thermal terahertz (THz) signals. Coherent detection is employed to measure all the relevant parameters. Extensive multi-parameter simulations were conducted to validate the protocol, examining the bit error rate, secret key rate, and transmission distance under varying pulse position modulation (PPM) settings and other parameters. The results demonstrate that the protocol is feasible and represents an effective method for quantum key distribution. The proposed approach offers a promising pathway for the deployment of inter-satellite quantum communication networks.