As quantum communications (QC), in particular quantum key distribution (QKD) transitions from lab curiosity to commercial implementation, cost reduction will become essential to increase the user number and thus make commercialisation viable. QC is hardware intensive, meaning new and novel techniques and technologies are essential to that cost reduction strategy. We present an experimental demonstration of a polarisation-based decoy-state BB84 QKD protocol utilising a single laser source (enabled by application of cascaded electro-optic modulator to perform the pulse-carving and encoding) and single single-photon avalanche diode (enabled by the application of space-to-time multiplexing), which reduced the hardware requirements for the QKD transmitter and receiver, is presented.
The security provided by Quantum Key Distribution (QKD) can be strongly compromised by interception of the raw and sifted key through side channels, such as the practical characteristics of electronic components used in the transmitter and receiver modules. Some out-of-band electromagnetic attacks have already been identified and tested in components used in QKD, such as quantum random number generators. In this presentation, we explore out-of-band electromagnetic attacks of other components used in a quantum receiver, such as Single Photon Avalanche Diodes (SPADs), and the time-correlated single-photon counting module. We measured the electromagnetic (EM) radiated emissions of the components to quantify the emanation levels and evaluate the vulnerability that this QKD side channel may present. The test was conducted in an anechoic chamber up to 1 GHz, at 3 m distance, and rotating the SPAD to provide radiation from four azimuth angles. Results show that measurable radiated pulses are generated by the SPAD in this frequency range due to dark count pulses and due to incoming Single-Photon level pulses. Dark counts of few kHz and Single-Photon level counts of hundreds of kHz were considered in the tests. EM radiation frequency bands with main emissions and electric field strengths are identified for both operation conditions.
The timing jitter of a single-photon avalanche diode (SPAD) can be a limiting factor for the performance of quantum key distribution (QKD). Within QKD receiver modules, single mode fiber coupling has been extensively studied due to optical fiber-based demonstrations. However, multimode fiber, which is utilized for free-space demonstrations because it reduces the coupling losses, has been less extensively studied, but it is understood that multimode fibers will increase the timing jitter and decrease the performance of the detector as the fiber core diameter increases. Here, we present a collection of experimental analysis over the relationships between characteristics of multimode fiber such as length, core diameter and incident spot size and how this impacts the timing jitter of the detector. Additionally, we present analysis on the use of graded-index multimode fibers, mitigating for a portion of the impact on quantum bit error rate (QBER) due to multimode characteristics which can improve implementation of these receiver systems.
Quantum Key Distribution (QKD), a technology for growing mathematically secure key encryption keys, is now on the verge of becoming widely commercially available. Due to lack of standardization, multiprotocol QKD receivers are particularly beneficial for satellite QKD, so that an optical ground station is not limited to a sub-set of satellites. Moreover, if both transmitter and receiver can operate with different protocols, they will be able to actively adapt to specific conditions by choosing the most suitable protocol. In this work, we present the design and performance of a multiprotocol reconfigurable free-space QKD receiver. The reconfigurability relies on polarization-based optical routing, which can also be used to optimize the performance of time-bin QKD protocols.
Quantum key distribution (QKD) is a quantum communications protocol which provides the growth of encryption keys under guaranteed security. Due to the single-photon nature of many QKD protocols, QKD systems can be optically jammed by overwhelming a receiver with many photons at wavelengths at which the single photon detectors are responsive, causing a prohibitively high quantum bit error rate (QBER). In satellite QKD (SatQKD), which relies on communication during brief satellite visual contact, short jamming periods could prevent access to secure communications for much longer periods of time. Optical jamming (OJ) can be achieved both from within line-of-sight by targeting the receiver with a light source, or, in the case of downlink SatQKD, from without line-of-sight by reflecting OJ light off the transmitting satellite.1 The latter attack can be effective 1000km from the ground station, which presents challenges to the deployment of SatQKD in mission-critical applications. In this work, we present two investigations for OJ attacks on SatQKD. Firstly, we present an experimental demonstration utilizing SPAD array technology to locate and mitigate within line-of-sight OJ at long range. Secondly, we present simulations quantifying the effectiveness of without line-of-sight OJ against SatQKD systems and outline mitigation techniques inspired by RF communications. Implementation of the mitigation techniques will be essential for defence applications.
Quantum key distribution (QKD) is a promising technology to enable secure cryptography after quantum computers have been developed. It allows for a key growing protocol that permits creating absolutely random keys to be used in the onetime pad codification scheme. Enabling a global QKD network is one of the final goals of the field. However, to do this with conventional optical fibres presents a fundamental limitation due to their intrinsic loss. Free-space, and specifically satellite links, have been proposed as an alternative and have gathered a lot of interest in recent years. They are considered one of the best candidates to enable a global network. Free-space QKD implementations are dominated by polarisation encoding protocols due to the relative transparency of the atmosphere to polarization. Nonetheless, time-bin and phase codifications offer some advantages and can be practical thanks to new passive interferometer designs. In this paper, the first free-space Coherent One-Way (COW) implementation is reported, some design considerations are commented, and the results of the experiment are shown. These show how time-bin/phase codifications are interesting candidates for free-space QKD.
Applications of quantum key distribution are becoming more diverse due to the increase in interest in the secure key
sharing protocol. Transmission through free-space channels has risen in popularity in recent years, primarily due to
global coverage using satellite platforms. However, free-space channels come with challenges that need to be addressed,
such as; diffraction loss, background noise, pointing-and-tracking, and atmospheric aberration. Novel design and use of
state-of-the-art detector technologies in quantum receivers can help alleviate these difficulties.
This paper presents and discusses the implementation of 2D single-photon sensor technology for free-space quantum key
distribution. We present an experimental method that utilizes independent single-photon avalanche diode pixel read-out
to reduce background noise contributions while simultaneously increasing optical field-of-view. Finally, we show and
discuss single-photon level beaconing capabilities for pointing and tracking.
The analysis of link loss is one of the first and most important steps for the design of an optical communication system. This is particularly vital in quantum communications systems where the information is encoded at the single photon level, and the quantum optical signal cannot be amplified deterministically. In most cases, the desired quantum bit error rate and secure key rate can only be achieved by minimizing the link attenuation and the background noise level in the quantum communication system.
In optical fiber implementations, the transmission distance is inherently limited by the loss per unit distance of the optical fiber, meaning fully global coverage is not readily achievable with the current optical fiber backbone networks. To overcome the terrestrial link limitations for quantum communications, long-distance free-space links, using low-Earth orbit satellites are being proposed and implemented. Due to the optical link length, the main contributors to link losses are geometric loss, atmospheric attenuation, and losses associated with pointing and tracking errors. The total link loss is dominated by the geometric loss, therefore, it is important to analyze its importance in relation to the quantum communications link.
In this paper, the loss of a low-Earth orbit satellite-to-ground (downlink) quantum communication link is analyzed. The analysis includes losses associate with the channel (geometric and atmospheric) and the receiver system. This paper also compares the data of a known satellite quantum communications mission, highlighting trade-offs in investment for satellite platform and optical ground station. Based on the link loss analysis, decoy state BB84 and E91 protocols were chosen to demonstrate the link performance under an example scenario. The work contributes to the design of the optical ground station for a CubeSat mission.
Quantum key distribution is a quantum communication protocol which seeks to address potential vulnerabilities in data transmission and storage. One of the main challenges in the field is achieving high rates of secret key in lossy and turbulent free-space channels. In this scenario, most experimental demonstrations have used the polarization of photons as their qubit carriers, due to the relative robustness of polarization in free-space propagation. Time-bin or phase-based protocols are considered less practical due to the wave-front distortion caused by atmospheric turbulence. However, demonstrations of novel free-space interferometer designs are enabling interferometers to measure multimodal signals with high visibility. That means it is now viable to consider the prospects of implementing time-bin or phase-based protocols, which have demonstrated high key rates and long transmission distances in optical fiber. In this work, we present the possibilities of implementing time-bin protocols in turbulent free-space channels, using the coherent one-way protocol as the example. We present an analysis of the secret key rate and quantum bit error rate of the system, providing the errors due to noise counts, and the extinction ratio of the pulses. Finally, we developed a model to quantify the expected losses for a turbulence free-space channel, specifically for a free-space satellite-to-ground station channel.
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