Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Discrete-variable quantum key distribution (QKD) requires the use of highly sensitive single-photon detectors (SPD) to detect decoded quantum states. These can form a significant portion of the cost and the physical layer vulnerabilities in a QKD system. Previously, a single-detector architecture which reduced the cost of simple, polarization-encoded fiber QKD systems was proposed, which decoded polarization states into varying fiber delays before reaching a single SPD. We describe the parameter space available for a single detector receiver and, using simulation, optimise performance for a given example. It is demonstrated that the order of state delays can be changed to reduce quantum bit error rate (QBER), and that delay change between states need only exceed some small minimum value in the ns range, which saves on fiber length, weight and cost, as well as loss. We also explore time gating and efficient (asymmetric basis selection) BB84 to reduce QBER. This work improves the understanding and performance of single-SPD-based QKD receivers, allowing simpler and more robust receivers with fewer physical loopholes and a lower cost, reducing the barrier to entry of fiber QKD.
Integrated photonics has the advantages of miniaturization, low cost, and CMOS compatibility, and it provides a stable, highly integrated, and practical platform for quantum key distribution (QKD). While photonic integration of optical components has greatly reduced the overall cost of QKD systems, single-photon detectors (SPDs) have become the most expensive part of a practical QKD system. In order to circumvent this obstacle and make full use of SPDs, we have designed and fabricated a QKD receiver chip for multiple users. Our chip is based on a time-division multiplexing technique and makes use of a single set of SPDs to support up to four users' QKD. Our proof-of-principle chip-based QKD system is capable of producing an average secret key rate of 13.68 kbps for four users with a quantum bit error rate (QBER) as low as 0.51% over a simulated distance of 20 km in fiber. Our result clearly demonstrates the feasibility of multiplexing SPDs for setting QKD channels with different users using photonic integrated chip and may find applications in the commercialization of quantum communication technology.
Due to the fast-paced advancements of the quantum computing field, current public key cryptographic protocols need to be rethought. One promising solution is Quantum Key Distribution (QKD), whose security is robust to any computational advancement. The security of QKD relies on the laws of quantum mechanics, ensuring that any eavesdropper attempting to access the data will be detected. In particular, polarization-encoding QKD systems allow to generate secret keys in two remote locations to encode/decode information, using the State of Polarization (SOP) of single photons. Although its security has been theoretically proven, its practical implementation poses some challenges, part of which are the main focus of this PhD thesis. One relevant process in QKD systems is the synchronization between the transmitter and receiver, namely, the frame synchronization. This way, in this thesis, a frame synchronization method is developed and implemented, showing its potential for the use in prepare-and-measure QKD systems. The developed method has been shown to be effective, even with a low number of photons reaching the receiver. Furthermore, its architecture allows it to be adjusted for different attenuation regimes of the QKD system. The ability to encode data on the physical properties of single photons, in this case, polarization, highly depends on the capacity to accurately generate the required polarization states on demand. This way, an automatic SOP generation method, comprising an optical polarization monitoring system and a dedicated algorithm was developed and implemented, capable of generating four SOPs. This method reached an average Quantum Bit Error Rate (QBER) of 0.75%, which is well below the security threshold. After the SOP generation, and their transmission through the quantum channel, polarization alignment algorithms are required to compensate for the random rotations induced by birefringence in the optical fiber. In this thesis, a non-iterative polarization compensation method was implemented. This method requires only three QBER estimations to re-align the polarization, reaching average QBERs of 1.8%. Additionally, multiple iterative methods were implemented for comparison purposes, with the results clearly showing a better efficiency of the deterministic method. Beyond fiber-based transmissions, free-space transmissions were analysed due to their ability to reach sites without installed fiber and due to its potential for satellite-based quantum communications. This thesis demonstrates the feasibility of polarization-encoding QKD over free-space channels, by subjecting quantum signals to atmospheric turbulence to study its impact. In that sense, polarization variations over a 1.8 km outdoor free-space channel were monitored, to understand the effect of atmospheric turbulence on polarization. These results have shown that SOP variations in free-space links are slow, allowing their compensation. Furthermore, it was identified that, under conditions tested, the Secret Key Rate (SKR) could reach values up to 8.7×10ˉ⁴ bits per second, allowing QKD over free space for distances up to 2.31 km.
Single photon detector(SPD) problems arise in most quantum tasks, especially for measuring states going through high-lost channels. They are particularly prominent in quantum key distribution(QKD), which could be the most significant application in quantum information theory. In recent years, QKD distance has been improved dramatically but is still restricted because the bit error rate(QBER) caused by SPD dark counts will be out of control as the distance increases. If this problem can be solved, QKD can be implemented over arbitrarily long distances. However, previous solutions often result in impractical requirements such as superconductors while they can only reduce the dark count rate to finite low levels. In this paper, we solve SPD problems with today's technologies only. Although it is the no-cloning theorem that prevents a state from being measured multiple times to obtain a more reliable result, we propose a scheme circumventing the no-cloning theorem in certain tasks to allow a single state to be employed several times. The scheme demonstrates that imperfect detectors can provide nearly perfect results, namely, the QBER caused by dark counts can be reduced to arbitrarily low while in the meantime, detective efficiency can be improved to arbitrarily high. Consequently, QKD distance is not limited by the imperfect SPD anymore and can be improved from hundreds of kilometers to thousands without high-technology detectors. Furthermore, similar schemes can be applied for reducing measurement errors or improving the performance of sources. Finally, it is worth noting that although the paper is mainly discussed in the context of QKD, our scheme is an independent scheme that could be employed in other protocols wherever SPD are employed.
We report the development of an optical assembly and driving electronics for a low-SWaP polarization encoder design for use in satellite-to-ground quantum communication. The optical design multiplexes multiple lasers, which are selectively excited to produce a polarization encoded output. This implementation is intrinsically stable due to the use of only polarization maintaining fiber in the combining optics. The transmitter, provides a low-cost, low-power and high-speed platform to produce polarization encoded pulses. We use the transmitter to generate 4 polarization states with 2 intensity levels via multiplexing of 8 pulsed light sources. The module can generate the polarization states H, V, D and A, which correspond to polarization angles of 0, 90, 45 and -45 degrees respectively, forming two mutually unbiased bases. The transmitter is characterized via a polarization decoder over a free-space link within a laboratory setting. We characterize the source for varying optical channel loss which is introduced between the transmitter and receiver. The transmitter employs the T12 decoy-state BB84 protocol. We explore the performance of the system with commercially available single photon detectors for two clock rates of 500 MHz and 1 GHz. We find a similar secure key rate for both repetition rates, despite the expected 3 dB gain at 1 GHz. This is a result of detector jitter hindering the performance of the QKD system, resulting in a larger QBER when detection events leak into the adjacent time bins and ultimately reduces the secure key rate.