Secure optical communications using antisqueezing are a realistic application of quantum information technologies. Antisqueezed light with enhanced fluctuations was transmitted over 100 km. Greater difficulty of eavesdropping than in a coherent-state case was shown.
Antisqueezed light is a possible resource to apply quantum information technologies to the real world. When antisqueezed light is used in secure optical communications, an LD is a preferable light source from an engineering point of view. Although LD output power is low, LD light can be antisqueezed with the help of an EDFA in a reflection-type interferometer consisting of a standard single-mode fiber of typically 5 km. The ellipticity of the obtained antisqueezed light was 9 at maximum in a balanced interferometer case, and the angle that was subtended by antisqueezed fluctuations at the origin of phase space was 23 degrees at maximum. The feasibility of secure optical communications using antisqueezed light is demonstrated.
When considering an application of quantum-information technology, we should consider realistic conditions first. From this point of view, we propose using antisqueezed light as a resource for optical communication because antisqueezing maintains its nature even after loss and amplification. Applying antisqueezed light to cipher communication using randomized sender bases, we can increase, by high order of magnitude, the bit-error rate of eavesdroppers who do not have the basis information. Even at macroscopic optical intensity, an ordinary level of optical communication, we can cause the bit-error rate of eavesdroppers to be as high as it would be at low intensity, through increased antisqueezing depending on optical intensity, leading to more secure optical communication.
Abstract Quantum secure direct communication is a quantum communication technology that uses quantum states as carriers to transmit information securely and directly, first proposed by a team at Tsinghua University in 2000. Quantum communication broadly refers to information transfer processing tasks accomplished through the transmission of quantum states, which can be categorized into three types based on the content transmitted: quantum key distribution for transmitting random numbers, quantum secure direct communication for transmitting classical information, and quantum teleportation for transmitting quantum information. After more than two decades of development, quantum secure direct communication has matured and entered practical application: a theoretical framework tailored for scenarios with high loss, high noise, and stringent security requirements has been established; experimental demonstrations of quantum direct communication in optical fibers and free space have been completed; three milestone systems for quantum secure direct communication at the metropolitan, intercity, and network levels have been built; and successful trials have been conducted in various scenarios such as 5G private networks and banking applications. Quantum secure direct communication offers multiple applications: it prevents information leakage by detecting eavesdropping, ensuring information security; identifies exposed locations by monitoring surveillance activities, providing technical support for preventing targeted eliminations and safeguarding commanders' personal safety; and detects intrusion and tampering behaviors, offering novel protection for cybersecurity. Currently, quantum direct communication has been incorporated into relevant planning documents in China, the European Union, and the United States, and is undergoing rapid development. This article briefly outlines the historical development, fundamental principles, research advancements, and future prospects of quantum direct communication.
The secure distribution of the secret random bit sequences known as `key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions: Heisenburg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single- photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over `open' multi-km node-to-node optical fiber communications links is possible.
The secure distribution of the secret random bit sequences known as {open_quotes}key{close_quotes} material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over {open_quotes}open{close_quotes} multi-km node-to-node optical fiber communications links is possible. We have also constructed a quantum key distribution system for free-space, line-of-sight transmission using single-photon polarization states, which is currently undergoing laboratory testing. 7 figs.
As a direct result of the quantum nature of electromagnetic radiation, all forms of light exhibit inherent and unavoidable random fluctuations. Because these fluctuations reveal information about the nature of light and the underlying processes that generate it, they have received considerable attention since the time of Einstein. But they are also a source of noise that limits the accuracy with which information can be transmitted by a beam of light. Although experiments often still struggle with instrumental noise, some are quickly approaching the measurement limits set by the quantum-statistical nature of light. With the help of new techniques to “squeeze” the uncertainty of light, researchers can now conduct experiments with greater precision than possible with laser light. The generation of various forms of squeezed light in recent years has raised hopes for its application to such diverse areas as gravity-wave detection based on optical interferometry and reduced-error lightwave communications.