Review of representative free-space quantum communications experiments

Physical implementations of quantum key distribution (QKD) protocols, like the Bennett-Brassard (BB84), are forced to use attenuated coherent quantum states, because the sources of single photon states are not functional yet for QKD applications. However, when using attenuated coherent states, the relatively high rate of multi-photonic pulses introduces vulnerabilities that can be exploited by the photon number splitting (PNS) attack to brake the quantum key. Some QKD protocols have been developed to be resistant to the PNS attack, like the decoy method, but those define a single photonic gain in the quantum channel. To overcome this limitation, we have developed a new QKD protocol, called ack-QKD, which is resistant to the PNS attack. Even more, it uses attenuated quantum states, but defines two interleaved photonic quantum flows to detect the eavesdropper activity by means of the quantum photonic error gain (QPEG) or the quantum bit error rate (QBER). The physical implementation of the ack-QKD is similar to the well-known BB84 protocol.

This paper presents the development, comparison, and analysis of several implementations of the B92 Quantum Key Distribution (QKD) protocol. To achieve this goal three prototypes which consists of traditional simulators (non-quantum) were created, one for the B92 protocol, one for the B92 protocol with an eavesdropper, and one for the B92 protocol with the Quantum Bit Travel Time (QBTT) eavesdropper detection method. The principles of quantum mechanics were studied, as a foundation of quantum cryptography, for the implementation of the simulation programs that were written in C++, focusing mainly on the B92 protocol and QBTT eavesdropper detection method. We compared the Quantum Bit Error Rate (QBER) for the implementation of the B92 protocol without an eavesdropper, the B92 protocol with an eavesdropper and the B92 protocol with the QBTT eavesdropper detection method and found that the latest one significantly reduces the QBER from the final key.

In the BB84 quantum key distribution (QKD) protocol, the communicating parties do a quantum bit error rate (QBER) test to determine whether there is an eavesdropper trying to gain information about the secret key. However, the QBER is not only influenced by the eavesdropper’s strategies, but also by the imperfections of the physical devices and the channel through which the quantum states propagate. We developed a simple channel model with error parameters describing the channel and the potential polarization switching in the transmitter, to see how those effects influence the QBER in a polarization-qubit BB84 implementation. Certain well-defined probabilistic channel models are compared to see which is responsible for the highest error probability.

Quantum key distribution (QKD) is a cornerstone of secure communication in the quantum era, yet most existing protocols are designed for point-to-point transmission, limiting their scalability in networked environments. In this work, we introduce Loop-Back QKD, a novel QKD protocol that supports both two-party linear configurations and scalable multiuser ring topologies. By leveraging a structured turn-based mechanism and bidirectional pulse propagation, the protocol enables efficient key distribution while reducing the quantum bit error rate (QBER) through a multi-pulse approach. Unlike trusted-node QKD networks, Loop-Back QKD eliminates intermediate-node vulnerabilities, as secret keys are never processed by intermediate nodes. Furthermore, unlike Measurement-Device-Independent (MDI-QKD) and Twin-Field QKD (TF-QKD), which require complex entanglement-based setups, Loop-Back QKD relies solely on direct polarization transformations, reducing vulnerability to side-channel attacks and practical implementation challenges. Additionally, our analysis indicates that multi-pulse Loop-Back QKD can tolerate higher QBER thresholds. However, this increased robustness comes at the cost of a lower key rate efficiency compared to standard QKD schemes. This design choice enhances its robustness against real-world adversarial threats, making it a strong candidate for secure multiuser communication in local and metropolitan-scale quantum networks.

Quantum secure direct communication (QSDC) enables the direct transmission of secret messages over a quantum channel without prior key sharing. QSDC implemented in the continuous-variable (CV) regime allows high-capacity information transmission using simple optical communication technologies. In this work, we propose a novel CV-QSDC protocol with a quantum one-time pad, which achieves secure message transmission based on coherent states and unitary operations, and the receiver can directly obtain the secret messages after measuring the coherent states. The security of this protocol is jointly guaranteed by one-time pad encryption and a CV quantum key distribution (QKD) protocol. Performance analysis shows that the proposed scheme can achieve a quantum bit error rate (QBER) lower than 1% by adjusting the modulation variance and unitary operation parameters. With the optimal modulation variance, the secure transmission distance of the proposed protocol can exceed 150 km.

Despite its widespread use in fiber optics, encoding quantum information in photonic time-bin states is usually considered impractical for free-space quantum communication as turbulence-induced spatial distortion impedes the analysis of time-bin states at the receiver. Here, we demonstrate quantum key distribution using time-bin photonic states distorted by turbulence and depolarization during free-space transmission. Utilizing a novel analyzer apparatus, we observe stable quantum bit error ratios of 5.32 %, suitable for generating secure keys, despite significant wavefront distortions and polarization fluctuations across a 1.2 km channel. This shows the viability of time-bin quantum communication over long-distance free-space channels, which will simplify direct fiber/free-space interfaces and enable new approaches for practical free-space quantum communication over multi-mode, turbulent, or depolarizing channels.

Covert communication methods are used in the communication with high security level. When it turns to quantum communication, covertness is also an important concern which is firstly discussed by Arrazola and Scarani (Phys Rev Lett, 117:250503, 2016). To make quantum key distribution (QKD) protocol more suitable in the scenarios need high security, we propose a covert QKD protocol with decoy-state method in this paper. The secure key rate and covertness of the covert decoy-state QKD are proved. We compare the performance of the covert decoy-state QKD with those of the original decoy-state QKD and covert QKD without decoy states in numerical simulations. It shows that (1) the covert decoy-state QKD can have a performance comparable to the original decoy-state QKD protocol besides its covertness; (2) the covert decoy-state QKD can have a considerable improvement of transmission distance over covert QKD without decoy states at the cost of a small change of covertness parameter. Furthermore, the statistical fluctuation due to the finite length of data is also taken into account based on the Gaussian analysis method.

The full time-jitter response of a single-photon detector can make a significant contribution to the quantum bit error rate (QBER) of high repetition rate quantum key distribution (QKD) implementations. Although there have been studies into understanding the contribution for single-mode optical fiber coupled single-photon detectors, the contribution of larger, multimode core diameters to the QBER have not been explored in detail. With the growing importance of free-space QKD, which typically use multimode fibers to reduce coupling loss, it is vitally important to understand how the multimode fiber coupling will impact the total QBER. This work studies the impact of the time-jitter contribution to QBER when coupling a commercial off-the-shelf silicon single-photon avalanche diode with various multimode fibers while simulating operating at 1 GHz with empirical measurements taken at 1 MHz repetition rate. It was found that step-index multimode fibers can significantly increase the QBER, while graded-index fibers can provide an QBER contribution similar to a single-mode fiber. The results highlight that there is a significant benefit in using graded index multimode fibers for a free-space QKD receiver, particularly for high repetition rate applications.

The security of quantum communication using a weak coherent source requires\nan accurate knowledge of the source's mean photon number. Finite calibration\nprecision or an active manipulation by an attacker may cause the actual emitted\nphoton number to deviate from the known value. We model effects of this\ndeviation on the security of three quantum communication protocols: the\nBennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol without\ndecoy states, Scarani-Acin-Ribordy-Gisin 2004 (SARG04) QKD protocol, and a\ncoin-tossing protocol. For QKD, we model both a strong attack using technology\npossible in principle, and a realistic attack bounded by today's technology. To\nmaintain the mean photon number in two-way systems, such as plug-and-play and\nrelativistic quantum cryptography schemes, bright pulse energy incoming from\nthe communication channel must be monitored. Implementation of a monitoring\ndetector has largely been ignored so far, except for ID Quantique's commercial\nQKD system Clavis2. We scrutinize this implementation for security problems,\nand show that designing a hack-proof pulse-energy-measuring detector is far\nfrom trivial. Indeed the first implementation has three serious flaws confirmed\nexperimentally, each of which may be exploited in a cleverly constructed\nTrojan-horse attack. We discuss requirements for a loophole-free implementation\nof the monitoring detector.\n

With the increasing information being shared online, the vast potential for cybercrime is a serious issue for individuals and businesses. Quantum key distribution (QKD) provides a way for distribution of secure key between two communicating parties. However, the current Quantum Key Distribution method, BB84 protocol, is prone to several weaknesses. These are Photon-Number-Splitting (PNS) attack, high Quantum Bit Error Rate (QBER), and low raw key efficiency. Thus, the objectives of this paper are to investigate the impacts of BB84 protocol towards QBER and raw key efficiencies in single quantum channel. Experiments were set up using a QKD simulator that was developed in Java NetBeans. The simulation study has reaffirmed the results of QBER and raw key efficiencies for the single quantum channel BB84 protocol.

This paper presents theoretical analysis of Quantum Bit Error Rate (QBER) for Quantum Key Distribution (QKD) in mobile networks (5G/6G). The considered configuration of the underlying optical fiber network that is available for the establishment of the QKD in multi-site 5G networks assumes that the 5G sites are connected by a single optical fiber. Thus, multiplexing of both quantum and classical channels in a single optical fiber is thus necessary. This, in turn, results in a cross talk between classical and quantum channels, which increases QBER and imposes limitations on the maximum distance that can be reached. The approach followed in this paper provides an estimate of the maximum quantum link budget for the targeted QBER. The results of the analysis show that the QKD solution considered here can be potentially deployed using current Operators’ 5G infrastructures to replace currently used asymmetric key exchange algorithms if needed.

Security of data transported in telecommunication networks is of primary importance for internet service providers (ISP) as data safety is at the basis of the mutual trust with customers. People were looking for theoretically secure transmission techniques since a long-time to ensure long-term confidentiality of data flows exchanged on these networks. Besides advanced encryption algorithms and the recently proposed post-quantum cryptography (PQC), a physics-based technology using quantum key distribution (QKD) was proposed a long time ago to approach this objective. QKD recently gained a renewed interest with many research programs worldwide supported by public authorities, and some pre-industrial QKD systems delivered to the market. However, some constraints of optical transport networks make sometimes complicated the deployment of such QKD systems in the field, such as the need of a dedicated fiber infrastructure for QKD, or the impossibility for a quantum key to pass through an optical amplifier. In this paper, we report the transmission of a coherent 400-Gbps dual-polarization 16QAM channel that transports QKD-secured 100-GbEthernet data stream with other fifty-four WDM channels at 100-Gbps over 184-km of standard single mode fiber (SSMF) through three QKD links (of 66-km, 50-km and 66-km, respectively) and two trusted nodes. On the two longest sections of the link, the quantum key and WDM signals are propagated on two different fibers, while on the shortest section of 50-km the QKD signal and WDM comb co-propagate on a same fiber. Secret key rates (SKR) and quantum bit error rate (QBER) are measured and reported on each of the three fiber sections. Various analyses are carried out on the co-propagation section of 50-km: the SKR / QBER sensitivity to the optical power of the WDM comb, or to the optical signal-to-noise ratio (OSNR) of the WDM channels co-propagating with the quantum signal is studied. The unique arrangement of the most advanced QKD and WDM technologies reported here constitutes, in our opinion, a world first, and demonstrates that QKD and WDM solutions can coexist in already deployed optical transport networks.

<sec>Measurement-device-independent quantum key distribution (MDI-QKD) protocol can effectively resist all possible attacks targeting the measurement devices in a quantum key distribution (QKD) system, thus exhibiting high security. However, due to the protocol’s high sensitivity to channel attenuation, its key generation rate and transmission distance are significantly limited in practical applications.</sec><sec>To improve the performance of MDI-QKD, researchers have proposed quantum-memory (QM)-assisted MDI-QKD protocol, which has enhanced the protocol's performance to a certain extent. Nevertheless, under finite-size conditions where the total number of transmitted pulses is limited, accurately estimating the relevant statistical parameters is still a challenge. As a result, existing QM-assisted MDI-QKD schemes still encounters issues such as low key rates and limited secure transmission distances.</sec><sec>To solve these problems, this work proposes a novel improved finite-size QM-assisted MDI-QKD protocol. By utilizing quantum memories to temporarily store early-arriving pulses and release them synchronously, the protocol effectively reduces the influence caused by channel asymmetry. Additionally, the protocol introduces a four-intensity decoy-state method to improve the estimation accuracy of single-photon components. Meanwhile, to mitigate the influence of finite-length effects on QM schemes, the proposed protocol combines a collective constraint model and a double-scanning algorithm to jointly estimate scanning error counts and vacuum-related counts. This approach enhances the estimation accuracy of the single-photon detection rate and phase error rate under finite-size conditions, thereby significantly improving the secure key rate of the MDI-QKD system.</sec><sec>Simulation results show that under the same experimental conditions, compared with the existing QM-assisted three-intensity decoy-state MDI-QKD protocol and the four-intensity decoy-state MDI-QKD protocol based on Heralded Single-photon Source (HSPS), the proposed protocol extends the secure transmission distance by more than 30 km and 100 km, respectively. This proves that under the same parameter settings, the proposed scheme exhibits significant advantages in both key rate and secure transmission distance. Therefore, this research provides important theoretical references and valuable benchmarks for developing long-distance, high-security quantum communication networks.</sec>

Most quantum communications are resilient against eavesdropping attacks. But most protocols fail to address the problems of the poor rates of data transmission and key generation with the constraints of transmission distance and misattribution errors. To address these issues, we propose a new round-robin-differential-phase-shift (RRDPS) quantum key distribution (QKD) protocol to improve the transmission and key generation rates, secure transmission distance and misattribution errors. The new protocol named WDM-OAM-MDI-RRDPS QKD protocol is based on a combination of the wavelength division multiplexing (WDM) technique, Fibonacci-valued orbital angular momentum (OAM) theory, decoy state and measurement-device independent (MDI) method. It utilizes a series of coherent weak sources simultaneously and high-capacity Fibonacci-valued encoding technology. In our protocol, WDM technology can greatly increase the amount of information transmitted in optical fiber communication, and improve the transmission efficiency, the total secure bit rate and the key generation rate of QKD. The OAM theory can achieve basis independence and increase the information capacity per photon from one to five. Fibonacci-valued OAM states can be used to reduce misattribution errors. The decoy state and MDI method can be used to increase the transmission distance resilient to the Photon-number-splitting and detector blinding attacks.

Subject of study. Technology for quantum key distribution in fiber-optic communication lines under the influence of electromagnetic fields generated by atmospheric discharges is studied. Aim of study. A test bench for testing experimental techniques to be used in studying the effect of atmospheric discharges on transmission of a quantum key via fiber-optic communication lines is developed. Method. The characteristics of a quantum communication channel upon exposure of a coil containing an optical fiber to a magnetic field from a pulsed current are experimentally studied. Main results. An experimental test bench has been created and methods have been developed to evaluate the influence of magnetic fields arising from atmospheric discharges (lightning) on the transmission speed of a quantum key in fiber-optic communication lines. It was shown that quantum key distribution blocks using the BB84 phase-encoding protocol continue to show operational stability when the optical cable is exposed to alternating magnetic fields of up to 1.2 mT. However, the very fact of exposure to magnetic fields above 0.8 mT can be detected by a systematic decrease in the quantum key transmission rate and an increase in the quantum bit error rate (QBER) by 0.5%–1%. Practical significance. The laboratory test bench and methods developed herein support magnetic-field testing of various optical cables used for transmission of qubits. Fiber-optic networks through which qubits are transmitted can serve as a distributed lightning sensor; an increase in the level of quantum errors and a decrease in quantum key generation rate will be observed in any segment of an optical network where a lightning discharge generates strong magnetic fields.