The quantum-key-distribution (QKD) system by ThinkQuantum QuKy was functionally tested at the JRC. It is based on a polarisation-encoded three-state BB84 protocol with decoy states, a protocol proven to be secure but for distances shorter than for the standard four-state BB84. The tested QKD system (QuKy) proved robust, reliable and easy to install and operate. Measurements performed over a dark-fibre link with incremental attenuation up to 28 dB demonstrated the secret-key rate up to 12 kbit/s and quantum-bit-error-rate (QBER) below 2 %. A proof-of-concept video transmission encrypted with the generated keys ran continuously for several hours without interruption. These results confirm that the QuKy platform can provide secure key material under realistic loss and polarisation-drift conditions.
The quantum-key-distribution (QKD) system by ThinkQuantum QuKy was functionally tested at the JRC. It is based on a polarisation-encoded three-state BB84 protocol with decoy states, a protocol proven to be secure but for distances shorter than for the standard four-state BB84. The tested QKD system (QuKy) proved robust, reliable and easy to install and operate. Measurements performed over a dark-fibre link with incremental attenuation up to 28 dB demonstrated the secret-key rate up to 12 kbit/s and quantum-bit-error-rate (QBER) below 2 %. A proof-of-concept video transmission encrypted with the generated keys ran continuously for several hours without interruption. These results confirm that the QuKy platform can provide secure key material under realistic loss and polarisation-drift conditions.
Quantum key distribution (QKD) is a well-established quantum technology with the potential to address growing security concerns associated with advancements in quantum computation. While the theoretical foundation of QKD is sufficiently understood, challenges persist in establishing long-distance connections without the necessity of costly satellite missions or the complexities of deploying quantum repeaters.We present a demonstration of QKD over a fibre with 21 dB signal attenuation, utilising a link between St. Pölten and Vienna. Overall, we achieved a secret key rate of 4 kbps, matching our prior test measurements under laboratory conditions.The implemented protocol was a four-state, one-decoy BB84 at 1550 nm, using a desktop transmitter-receiver pair configured as a flexible setup. The quantum signal was prepared via time-bin encoding, a technique that has been demonstrated to be reliable and robust against depolarisation and polarisation mode dispersion. This work demonstrates the capability of quantum communication in conventional fibre networks by showing the functionality of performing QKD over an existing single-mode fibre (SMF) link.
In recent years, the feasibility of quantum key distribution (QKD) in a water channel has been verified by theory and experiment. Here, we present an experimental investigation of QKD and decoy-state QKD based on the BB84 protocol. The experiment was carried out in a 10 m water tank. The attenuation coefficient of tap water is 0.08/m, which is close to Jerlov Type II seawater. We measured the probability-of-detection matrix of polarization states, and the average fidelity of the four polarization states is up to 98.39%. For the 10 m underwater QKD experiment, 20 MHz optical pulses are generated by modulating the laser diode (LD) and attenuated to an average of 0.1 photons per pulse. The security key rate can reach 563.41 kbits/s and the quantum bit error rate (QBER) is 0.36%. Two decoy states (one of which is the vacuum state) was used in the 10 m underwater decoy-state QKD experiment, and the average QBER of signal state is 0.95%, the security key rate reaches 711.29 kbits/s. According to the parameters of the decoy-state experiment, the maximum secure transmission distance of the underwater decoy-state QKD is predicted to be 19.2 m, while it can be increased to 237.1 m in Jerlov Type I seawater with a lower dark count single photon detector (SPD).
Quantum Key Distribution (QKD) offers a revolutionary method for secure key exchange by exploiting the principles of quantum mechanics. This paper comprehensively reviews the pioneering BB84 protocol, its experimental realization using IBM's Qiskit framework, and the underlying quantum computing concepts. We detail the methodology including qubit encoding, measurement, basis reconciliation, and key sifting, and discuss performance metrics such as the Quantum Bit Error Rate (QBER) and key generation rate. Experimental results obtained on IBM Quantum hardware are presented and analyzed, highlighting the robustness of the protocol and its security implications. The review underscores the importance of integrating QKD with future-proof encryption schemes in a quantum-enabled era.
This report reviews two quantum key distribution (QKD) protocols: the BB84 protocol and the measurement device independent (MDI) QKD protocol. The goal of this report is to recreate the security proof of the BB84 protocol, to generate the secret key rate for a practical application of the BB84 protocol, both with and without decoy states, and to review the MDI-QKD protocol and look at the advantages it has for practical QKD. The proof security of the BB84 protocol is done by designing an equivalent theoretical protocol and proving its security by bounding the information of the eavesdropper to an exponentially small number. The best distance over which secret key rate can be generated with the practical model of the BB84 protocol that is presented in this report, is 165 km. This is achieved by employing decoy states and is more than three times as far as the model can achieve without decoy states, which gave a distance of 52 km at best. The zero distance key rate of the model with decoy states is R = 1.21 · 10-2 bits per pulse (bpp), at best. This is an order of magnitude larger than R =1.02 · 10-3 bpp, which is what the model without decoy states could produce. The decoy state method is therefore an improvement for practical QKD. By reviewing the MDI-QKD protocol it can be concluded that because it eliminates the measurement device side channels, it is very useful for practical QKD, since it makes security analysis simpler and more precise. It also necessarily employs decoy states, which improves the secret key rate. MDI-QKD therefore is a promising protocol to use for future quantum communications.
Quantum Key Distribution (QKD) is increasingly recognized as a robust solution to the vulnerabilities posed by quantum computing to classical encryption systems. This paper presents a comprehensive investigation of the BB84 protocol, the foundational QKD scheme, through simulated quantum communication using IBM’s Qiskit framework. The study models quantum key exchange under both ideal and adversarial conditions, including channel noise and intercept-resend attacks. The Quantum Bit Error Rate (QBER) was measured as a function of noise intensity and eavesdropping, confirming that QBER increases predictably in compromised channels. A threshold of 11% QBER was used to determine the viability of secure key generation. Final key retention rates were analyzed across multiple channel scenarios, revealing BB84’s sensitivity to both photon loss and adversarial interference.\n\nSecurity vulnerabilities such as photon-number-splitting and detector blinding were assessed, and countermeasures including decoy-state protocols and authenticated classical channels were proposed. Four key performance graphs were generated to visualize system behavior under realistic operational conditions. The results support BB84’s theoretical advantage in ensuring information-theoretic security while acknowledging real-world limitations in transmission range and hardware reliability. The paper concludes by addressing future improvements in QKD systems, including satellite-based distribution, quantum repeaters, and device-independent implementations. Through simulation-driven analysis, this study validates BB84 as a practical defense against quantum-enabled attacks and underscores the need for continued development and standardization in post-quantum cryptographic infrastructure.
Quantum key distribution (QKD) enables remote users to establish secret keys secured by quantum mechanics. Polarisation encoding offers simple state preparation and measurement; however, fibre birefringence, component imperfections, and high-speed polarisation modulation make stable, low-error operation challenging. This thesis investigates the development of a high-speed polarisation-based QKD transmitter. The optical components, including intensity and phase modulators, interferometric systems, and electronic polarisation controllers (EPCs), are characterised experimentally. Gradient descent, coordinate gradient descent, and simulated annealing are compared for EPC-based state preparation. With a 50 mV step, coordinate gradient descent is approximately six times faster than conventional gradient descent, while simulated annealing achieves 100% success over the tested runs at the selected settings. The polarisation-control approach is evaluated in a proof-of-concept BB84 feasibility experiment over a deployed 224 km undersea fibre link between the United Kingdom and Ireland, with a measured attenuation of 38.6 dB. Channel loss and polarisation stability are characterised, and the QBER is measured using SPAD and SNSPD detector configurations. At a mean photon number of one, the SNSPD configuration produces average QBERs of 2.98% and 1.55% for the horizontal and vertical states, respectively, substantially below the corresponding SPAD results. Separately, a field-trial-informed asymptotic decoy-state model projects a maximum secret-key yield of 2.93*10^-6 bit/clock. Finally, several iterations of high-speed polarisation-encoders are developed and assessed, identifying extraction efficiency and fibre-axis alignment as key practical constraints. The resulting hybrid encoder prepares four polarisation states at 40 MHz, with measured overall system QBERs below 3%. Overall, the work establishes practical methods for polarisation control, long-distance feasibility assessment, and stable high-speed state preparation in polarisation-based QKD systems.