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.
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.
Abstract Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
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) 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.
The impending realization of fault-tolerant quantum computers threatens the foundational public-key cryptography underpinning global telecommunications and financial infrastructure. While algorithmic Post-Quantum Cryptography (PQC) provides mathematical computational hardness against quantum cryptanalysis, Quantum Key Distribution (QKD) establishes an alternative information-theoretically secure physical paradigm based on the immutable laws of quantum mechanics: the No-Cloning theorem and Heisenberg's uncertainty principle. However, deploying scalable, wide-area quantum networks introduces formidable physical and systems engineering challenges: exponential optical fiber attenuation that restricts direct point-to-point QKD to beneath 150 km, high dark-count rates in superconducting nanowire single-photon detectors (SNSPDs), non-deterministic Bell-state entanglement swapping across quantum repeaters, and intense Raman scattering cross-talk when multiplexing quantum channels alongside multi-terabit classical DWDM optical networks. This paper delivers an exhaustive, mathematically formal technical investigation and empirical architectural evaluation of quantum networking and QKD systems. We formalize the asymptotic secret key rate bounds under the Decoy-State BB84 and Measurement-Device-Independent (MDI-QKD) protocols, model continuous entanglement routing over quantum repeaters utilizing bipartite Bell-pair purification, and establish optical co-existence bounds under Dense Wavelength Division Multiplexing (DWDM). Furthermore, we construct an empirical optical testbed evaluating an 8-node metropolitan quantum network co-propagating QKD over 100 Gbps classical optical traffic, paired with hybrid PQC-QKD key derivation functions (ML-KEM + QKD). Our empirical findings demonstrate that decoy-state BB84 achieves a sustainable secret key rate of 1.42 kbps across 100 km of standard single-mode fiber (SMF-28), while hybrid PQC-QKD integration eliminates single-point cryptographic failures with less than 2.4 ms handshake latency overhead. Finally, we formulate open challenges in memory-assisted atomic quantum repeaters, satellite-to-ground freespace entanglement links, and quantum internet transport protocols.
High-dimensional quantum key distribution (QKD) allows achievement of information-theoretic secure communications, providing high key-generation rates, which cannot, in principle, be obtained by QKD protocols with binary encoding. Nonetheless, the amount of experimental resources needed increases as the quantum states to be detected belong to a larger Hilbert space, thus raising the costs of practical high-dimensional systems. Here, we present an alternative scheme for fiber-based four-dimensional QKD, with time and phase encoding and one-decoy state technique. Quantum state transmission is tested over different channel lengths up to 145 km of standard single-mode fiber, evaluating the enhancement of the secret key rate in comparison to the three-state two-dimensional BB84 protocol, which is tested with the same experimental setup. Our scheme allows measurement of the four-dimensional states with a simplified and compact receiver, where only two single-photon detectors are necessary, thus making it a cost-effective solution for practical and fiber-based QKD.