Free-space quantum key exchanges between ground stations and low earth orbiting satellites will be characterized by high link losses, typically of the order of 30 dB or higher. These losses, together with the need to transmit weak Poissonian laser pulses containing on average substantially less than 0.1 photons per transmitted bit to preserve security, will result in exceedingly low channel efficiencies, typically of the order of 10-5. In order to achieve even a relatively modest secure key bit rate of 100 kbps, it will therefore often be necessary to key the transmitter at rates in excess of 10 Gbps. In this paper we outline several different methods of achieving such fast polarization keying including the use of dual drive Mach-Zehnder intensity modulators on lithium niobate in a hybrid fibre-guided modulator structures. We then propose a total integrated polarization keying structure in LiNbO3. We describe the fabrication and testing of such ultra-broadband polarization keyers suitable for use in high loss, short-wave free-space quantum key distribution systems employing silicon photon counters. We also indicate how these devices may be incorporated into quantum key satellite courier payloads and ground station terminals.
Free-space quantum key exchanges between ground stations and low earth orbiting satellites will be characterized by high link losses, typically of the order of 30 dB or higher. These losses, together with the need to transmit weak Poissonian laser pulses containing on average substantially less than 0.1 photons per transmitted bit to preserve security, will result in exceedingly low channel efficiencies, typically of the order of 10-5. In order to achieve even a relatively modest secure key bit rate of 100 kbps, it will therefore often be necessary to key the transmitter at rates in excess of 10 Gbps. In this paper we outline several different methods of achieving such fast polarization keying including the use of dual drive Mach-Zehnder intensity modulators on lithium niobate in a hybrid fibre-guided modulator structures. We then propose a total integrated polarization keying structure in LiNbO3. We describe the fabrication and testing of such ultra-broadband polarization keyers suitable for use in high loss, short-wave free-space quantum key distribution systems employing silicon photon counters. We also indicate how these devices may be incorporated into quantum key satellite courier payloads and ground station terminals.
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.
In practical satellite-based quantum key distribution (QKD) systems, the preparation and transmission of polarization-encoding photons suffer from complex environmental effects and high channel loss. Consequently, the hinge to enhancing the secure key rate (SKR) lies in achieving robust, low-error, and high-speed polarization modulation. Although the schemes that enable self-compensation demonstrate remarkable robustness, their modulation speed is limited to around 2 GHz to prevent the interaction between the electrical signal and the reverse optical pulses. Here, we utilize the non-reciprocity of the lithium niobate modulators and eliminate the modulation on the reverse optical pulses. This characteristic is widely available in the radio-frequency band, allowing the modulation speed to no longer be limited by the self-compensating optics and enabling further increases. The measured average intrinsic quantum bit error rate of the four polarization states at 10 GHz system repetition frequency is as low as 0.53% over 10 min without any compensation. The simulation results show that our scheme can maintain a SKR of about 5 kbps even at the extreme communication distances between the satellite and the earth. Our work can be efficiently applied in high-speed, high-loss satellite-based quantum communication scenarios.
To implement the BB84 decoy-state quantum key distribution (QKD) protocol over a lossy ground-satellite quantum uplink requires a source that has high repetition rate of short laser pulses, long term stability, and no phase correlations between pulses. We present a new type of telecom optical polarization and amplitude modulator, based on a balanced Mach-Zehnder interferometer configuration, coupled to a polarization-preserving sum-frequency generation (SFG) optical setup, generating 532 nm photons with modulated polarization and amplitude states. The weak coherent pulses produced by SFG meet the challenging requirements for long range QKD, featuring a high clock rate of 76 MHz, pico-second pulse width, phase randomization, and 98% polarization visibility for all states. Successful QKD has been demonstrated using this apparatus with full system stability up to 160 minutes and channel losses as high 57 dB . We present the design and simulation of the hardware through the Mueller matrix and Stokes vector relations, together with an experimental implementation working in the telecom wavelength band. We show the utility of the complete system by performing high loss QKD simulations, and confirm that our modulator fulfills the expected performance.
Photonic integration in quantum communication holds significant potential for miniaturization and enabling commercial applications. Among various platforms, thin-film lithium niobate (TFLN) stands out due to its exceptional combination of high electro-optical efficiency, low propagation loss, and compact footprint. Here, we demonstrate a 2.5 GHz chip-to-chip fully integrated quantum key distribution (QKD) system based on a TFLN platform, which incorporates high-speed dual-polarization time-bin phase encoding and decoding functionalities. We achieve an extremely low quantum bit error rate of 0.53% and a secret key rate exceeding 10 Mbps over 25 km fiber spools. The design of cascaded Mach–Zehnder modulators effectively suppresses the patterning effect in high speed QKD. Notably, the TFLN chips used in both the transmitter and receiver share a similar architecture, highlighting the potential for creating a homogeneous transceiver. This work paves the way for high-speed, miniaturized QKD systems based on the lithium niobate integrated platform.
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.
We have designed and built a free space quantum cryptography system. It operates using weak laser pulses with polarization modulation by acousto-optic switching. The system also incorporates full key sifting and error correction between computers connected by a serial port. We have used this system to exchange keys over ranges up to 1.9km with absolute security. The system is robust against transmission (and diffraction) losses up to 20 dB. Building from this initial result we analyse the feasibility of exchanging keys to a low earth orbit satellite.