Polarization-encoded photonic qubits are a key resource for quantum communication, particularly for space-based quantum key distribution (QKD). Practical polarization-based QKD transmitters require the combination of multiple polarization states into a single spatial mode, a task that is typically implemented using bulk or fiber-based optics. Both demand precise alignment and are highly sensitive to motion and temperature. Here we demonstrate an ultra-low-birefringence, silica waveguide, which generates different polarization states at the output depending on the input channel. This is achieved by aligning an array of polarization-maintaining (PM) fibers at the interface with the waveguide, with the slow axis of the fiber angled to produce H, V, D, and A polarization states. The eight input channels are combined into one single-mode output via a cascade of adiabatic Y-junction couplers. We demonstrate real-time free-space decoy-state QKD across a 3 m link, at a clock rate of 1 GHz. We also analyze the performance of the waveguide under various temperature conditions (15°C–60°C). The photonic integrated circuit (PIC)-based design is readily suitable for satellite implementation, due to low-SWaP, ease of alignment, and minimal effects from temperature changes.
Polarization-encoded photonic qubits are a key resource for quantum communication, particularly for space-based quantum key distribution (QKD). Practical polarization-based QKD transmitters require the combination of multiple polarization states into a single spatial mode, a task that is typically implemented using bulk or fiber-based optics. Both demand precise alignment and are highly sensitive to motion and temperature. Here we demonstrate an ultra-low-birefringence, silica waveguide, which generates different polarization states at the output depending on the input channel. This is achieved by aligning an array of polarization-maintaining (PM) fibers at the interface with the waveguide, with the slow axis of the fiber angled to produce H, V, D, and A polarization states. The eight input channels are combined into one single-mode output via a cascade of adiabatic Y-junction couplers. We demonstrate real-time free-space decoy-state QKD across a 3 m link, at a clock rate of 1 GHz. We also analyze the performance of the waveguide under various temperature conditions (15°C–60°C). The photonic integrated circuit (PIC)-based design is readily suitable for satellite implementation, due to low-SWaP, ease of alignment, and minimal effects from temperature changes.
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.
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.
Deployable quantum key distribution (QKD) for free-space link must be both reference frame tolerant and hardware compact, yet existing free-space implementations still rely on bulk optics and active alignment. To address this gap, we develop the first reference frame independent (RFI) QKD transmitter fully integrated on a photonic-integrated circuit (PIC) and packaged in a CFP2 optical-transceiver form factor. The PIC incorporates a six-channel laser diode array, a six-channel variable optical attenuator array, and a polarization beam combiner, providing wavelength indistinguishability and high polarization fidelity. Combined with a micro-optics based polarization decoder and a bidirectional two-stage pointing, acquisition, and tracking (PAT) system, the PIC transmitter forms a complete free-space RFI QKD system. Based on the system, RFI QKD is performed with 10 m free-space channel, providing that QBERs measured in the same basis are below 3.81 % and the security parameter is1.77±0.02. This leads a stable secret key rate of77.33±1.65kbpsfor 100 s. These results verify the feasibility of PIC-based RFI QKD transmitter and establish a clear pathway toward fully deployable PIC-based QKD systems.
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.
We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.
In this work we introduce a complete Quantum Key Distribution device that implements a decoy-state protocol [1].Qubits are coded in the polarization state of weak optical pulses, produced by infrared LEDs [2].The link between Transmission (Alice) and Reception (Bob) stages is open air, which is the preferred channel to eventually through low-orbit satellites establish quantum communications.Both terminals are placed on different rooms, separated by approximately 8 meters.In order to compensate misalignments produced by small mechanic displacements or atmospheric thermal gradients, the system implements a closed-loop for automatic alignment based on a counter-propagating laser, a servo-actuated mirror and a CMOS image sensor.The apparatus runs in an autonomous way, and in the above described conditions it is able to generate raw cryptographic key at a rate of 185 bits/s, with a QBER of 6.15%.