ABSTRACT Quantum key distribution (QKD) allows two users to exchange a provably secure key for cryptographic applications. In prepare‐and‐measure QKD protocols, the states must be indistinguishable to prevent information leakage to an eavesdropper performing a side‐channel attack. Here, we measure the indistinguishability of quantum states in a prepare‐and‐measure three‐state BB84 polarisation‐based decoy state protocol using resonant‐cavity light‐emitting diodes (RC‐LEDs) as the source in the transmitter. We make the spatial, spectral and temporal DOF of the generated quantum states nearly indistinguishable using a spatial filter single‐mode fibre, a narrow‐band spectral filter and adjustable timing of the electrical pulses driving the RC‐LEDs, respectively. The sources have fully indistinguishable transverse spatial modes. The measured fractional mutual information between an assumed eavesdropper and the legitimate receiver is due to the spectral distinguishability and for the temporal distinguishability. The source is integrated into a full QKD system operating in a laboratory environment, where we achieve a secure key rate of 41.5 kbits/s with an average quantum bit error rate of 2.9%. The low system size, weight and power make it suitable for mobile platforms such as uncrewed aerial vehicles (drones) or automobiles.
ABSTRACT Quantum key distribution (QKD) allows two users to exchange a provably secure key for cryptographic applications. In prepare‐and‐measure QKD protocols, the states must be indistinguishable to prevent information leakage to an eavesdropper performing a side‐channel attack. Here, we measure the indistinguishability of quantum states in a prepare‐and‐measure three‐state BB84 polarisation‐based decoy state protocol using resonant‐cavity light‐emitting diodes (RC‐LEDs) as the source in the transmitter. We make the spatial, spectral and temporal DOF of the generated quantum states nearly indistinguishable using a spatial filter single‐mode fibre, a narrow‐band spectral filter and adjustable timing of the electrical pulses driving the RC‐LEDs, respectively. The sources have fully indistinguishable transverse spatial modes. The measured fractional mutual information between an assumed eavesdropper and the legitimate receiver is due to the spectral distinguishability and for the temporal distinguishability. The source is integrated into a full QKD system operating in a laboratory environment, where we achieve a secure key rate of 41.5 kbits/s with an average quantum bit error rate of 2.9%. The low system size, weight and power make it suitable for mobile platforms such as uncrewed aerial vehicles (drones) or automobiles.
We present progress on demonstrating a side-channel-free, decoy-state quantum key distribution (QKD) source based on a polarization-modulator and a wavelength-stable attenuated pulsed laser. By modulating the polarization of the quantum state, a three-state QKD protocol is achieved. The polarization-modulator-based QKD source improves security by removing several sources of side-channel attacks that exist when multiple sources are used to generate different QKD states. Here we present our single-source design, as well as an evaluation of critical subsystems characterized by the quantum bit error rate (QBER), quantum state tomography, and achievable key rates. The QKD source is intended to operate using minimal size, weight, and power (SWaP). In our system, we use a custom KTP waveguide modulator designed and fabricated by AdvR. Our QKD source can be operated using both narrow-band and broadband lasers, and we present characterization data using a He-Ne laser, and a diode laser, respectively. The polarization-modulator-based QKD source has applications in future mobile quantum networks incorporating unmanned aerial vehicles (UAV) and autonomous vehicles, as well as fiber-based quantum networks.
Mastering the generation, propagation and detection of electro-magnetic waves has enabled a technological breakthrough that has changed our entire society. World-wide communication through the telephone and the internet has become an integral part of our daily-life, which is expected to grow even further with the emergence of the internet of things. While secure communication was of concern mostly for governmental and financial institutions, digital security has now caught the attention of the general public. The weaknesses of cur- rent encryption protocols, such as the existence of back-doors or the predicted breakdown of popular algorithms such as RSA, reveal the need for alternative encryption schemes ensuring unconditional security on all types of devices. Quantum Key Distribution (QKD) has emerged as a powerful option to ensure a private communication between two users. Based on the laws of quantum mechanics, this class of protocols offers the possibility to detect the presence of a third party trying to intercept the key during its distribution, and even to quantify the amount of leaked information. While most research projects focus on long distance applications, little attention has been devoted to short distance schemes such as wireless payment, network access and authentication, which could highly benefit from QKD-enhanced security. This thesis focuses on the development of a miniature QKD sender add-on that could be embedded either in mobile devices or in existing optical communication platforms, thus allowing for a secure key exchange with a shared dedicated receiver over a free- space link. The proposed optics architecture (35 × 20 × 8 mm 3 ) is optimised for BB84-like protocols and uses an array of four Vertical-Cavity Surface-Emitting Lasers with highly similar properties to generate 40 ps long near-infrared faint coherent pulses at 100 MHz repetition rate. Under strong modulation, the polarisation of the pulses is not well defined and enables an external control of each diode’s emission by a wire-grid polariser. The four beams are spatially overlapped in a polarisation-insensitive femtosecond laser written waveguide array, and combined with a red beacon laser using an external beamsplitter to ensure a stable, synchronised optical link with the receiver. The complete module is compatible with current smartphone technology, allowing to run the classical post-processing over WLAN in the future. First tests with a free-space receiver indicate an average error ratio of 3.3 % and an asymptotic secure key rate of 54 kHz under static alignment. For the first time, a secure key exchange between a mobile platform held by a user and a receiver equipped with a dynamic alignment system could be demonstrated with an error ratio of 4.1 % and a secure key rate of 31 Hz. The further optimisation of the experimental parameters and the implementation of a decoy protocol will enhance the key generation rate as well as the general security of the system. The results of this thesis pave the way towards unprecedented security in wireless optical networks, as examplified for the communication between a mobile device and a dedicated receiver.
The security of sensitive information exchange has become a major topic in recent years. Quantum Key Distribution (QKD) provides a highly secure approach to share random encryption keys between two communication terminals. In contrast with traditional public cryptography methods, QKD security relies on the foundations of quantum mechanics and not on computational capabilities. This makes QKD unconditionally secure (if properly implemented) and it is envisaged as a main component in the next–generation cryptographic technology. QKD has already been successfully demonstrated in different contexts such as fibre-to- fibre, and free-space ground-toground as well as ground-to-air communications. However, Size, Weight and Power (SWaP) constraints have prevented previous implementations to be demonstrated on small form airborne platforms such as Unmanned Aircraft Systems (UAS) and High Altitude Pseudo-Satellites (HAPS). Project Q-DOS aims to deliver a QKD module using compact, cutting-edge photonic waveguide technology, which will allow low-SWaP aerospace requirements to be met. This module uses 1550 nm single photons to implement a BB84 protocol, and will enable the demonstration of a secure, high-speed optical communication data link (~0.5 Gbps) between a drone and a ground station. The targeted link range is 1 km. The airborne communications module, including the QKD terminal, tracking modules, traditional communications systems, optics and control electronics, must not exceed a mass of 5 kg and a power consumption of 20 W.
Abstract Quantum key distribution (QKD) offers information‐theoretic security by leveraging the principles of quantum mechanics. This means the security is independent of all future advances in algorithm or computational power. However, due to the non‐availability of single‐photon sources, most traditional QKD protocols are vulnerable to various attacks, such as photon number‐splitting (PNS) attacks. Also, the imperfections in the measuring devices open a loophole for side channels that an eavesdropper may exploit to launch attacks such as large‐pulse attacks. As a result, this compromises the security of transmitted information. To address these challenges, the authors present a QKD protocol that is secure against both large‐pulse attacks and PNS attacks at zero‐error, in which the eavesdropper does not introduce any error, but still, the legitimate users of the system cannot distil a secure key. A notable feature of the proposed protocol is that it promotes greater robustness against both attacks than the Bennett‐Brassard 1984 (BB84) protocol or the Scarani‐Acin‐Ribordy‐Gisin 2004 (SARG04) protocol.
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.
To enable practical real-time quantum key distribution (QKD) over free-space links, such as for satellite-to-ground secure communications, the encoding rate of the system should be sufficiently high to distribute long keys (>106bits) in a short period of time (< 300 seconds) [1]. Here, we report an optical assembly and driving electronics designed to form a low-size}, weight and power (SWaP) QKD transmitter, which can operate at gigahertz clock rates. The QKD transmitter employs multiple vertical cavity surface emission lasers (VCSELs), which provide a low power and high polarization extinction ratio (PER) source of weak coherent pulses of light. These light pulses are combined using an intrinsically stable optical combining module utilising polarization-maintaining fiber. In the experiments presented we employ discrete variable QKD via the decoy-state BB84 protocol to achieve high secure key rates over large channel losses [2]. We characterise the source using a free-space polarization decoder over short free-space links within the laboratory and explore the performance of the system with commercially available single photon detectors. Fig. 1 Plot showing the effect of varying the channel loss over the free-space link. We show the measured quantum bit error rate (blue circles), sifting rate (green triangles) and secure key rate (red squares) alongside the rates simulated using the measured system parameters (solid lines). We characterise the system up to ~38 db of channel loss, revealing positive key rates at relatively high channel loss.