In practicable CV-QKD systems one major challenge is to synchronise the time scales of the free running electronics at Alice (A) and Bob's (B) site and the free running true local oscillator (TLO) in the coherent receiver. For the correct phase estimation of the discrete symbols and the mitigation of the phase noise of the lasers some phase reference is required. Reference signals can be multiplexed with the quantum signal in time, frequency or polarisation domain [1]. In some CV-QKD experiments this is achieved by using an extra wavelength which requires additional hardware [2]. In [3] we proposed the use of auxiliary signals in an 8-PSK 40 MBd system. The advantage of this pilot-assisted clock recovery is that it works independently of the quantum signal and without training symbols. On the other hand, the necessary second pilot occupies DAC and ADC bandwidth. In the receiver DSP constructive interference of both pilots must be realized. This requires knowledge of the clock ratio before phase noise compensation. To simplify the signal processing, we propose to use two very stable Rubidium clocks at A and B which can be disciplined with the time reference broadcast by GPS [4].
In practicable CV-QKD systems one major challenge is to synchronise the time scales of the free running electronics at Alice (A) and Bob's (B) site and the free running true local oscillator (TLO) in the coherent receiver. For the correct phase estimation of the discrete symbols and the mitigation of the phase noise of the lasers some phase reference is required. Reference signals can be multiplexed with the quantum signal in time, frequency or polarisation domain [1]. In some CV-QKD experiments this is achieved by using an extra wavelength which requires additional hardware [2]. In [3] we proposed the use of auxiliary signals in an 8-PSK 40 MBd system. The advantage of this pilot-assisted clock recovery is that it works independently of the quantum signal and without training symbols. On the other hand, the necessary second pilot occupies DAC and ADC bandwidth. In the receiver DSP constructive interference of both pilots must be realized. This requires knowledge of the clock ratio before phase noise compensation. To simplify the signal processing, we propose to use two very stable Rubidium clocks at A and B which can be disciplined with the time reference broadcast by GPS [4].
We present a pilot-assisted coherent intradyne reception methodology for CV-QKD with true local oscillator. An optically phase-locked reference tone, prepared using carrier-suppressed optical single-sideband modulation, is multiplexed in polarisation and frequency to the 250 Mbaud quantum signal in order to provide optical frequency- and phase matching between quantum signal and local oscillator. Our concept allows for high symbol rates and can be operated at an extremely low excess-noise level, as validated by experimental measurements.
In the domain of continuous variable quantum key distribution (CV-QKD), a significant challenge arises in achieving precise frequency synchronization, an issue commonly termed as frequency locking. This involves matching the optical frequencies of both the quantum signal laser and the local oscillator laser for accurate symbol demodulation during the exchange of quantum keys. As such, implementations today still grapple with maintaining precise synchronization between sender and receiver frequencies, occasionally hindering the efficiency and reliability of the information exchange. Addressing this challenge, we present and empirically validate a novel approach to CV-QKD by incorporating a pilot tone-assisted frequency locking algorithm to enhance stability when using a locally generated local oscillator (LLO) at the receiver. The proposed design leverages software-based optimization techniques, thereby eliminating the need for high-speed electronic stabilization devices and achieving efficient performance at typical repetition rates. Specifically, the introduction of the pilot tone algorithm allows us to effectively mitigate phase fluctuations and preserve the integrity of the quantum signals during transmission without resorting to time-multiplexed reference pulses or fast-locking electronics in the lasers. Our results suggest the potential for achieving secure key rates of up to 1 Mb/s over a 50 km single-mode fiber when using these techniques, offering promising insights into the feasibility of high-rate, low-complexity CV-QKD implementations under realistic conditions.
Continuous-variable quantum key distribution (CV-QKD) is realized with coherent detection and is therefore very suitable for a cost-efficient implementation. The major challenge in CV-QKD is mitigation of laser phase noise at a signal to noise ratio of much less than 0 dB. So far, this has been achieved with a remote local oscillator or with auxiliary signals. For the first time, we experimentally demonstrate that CV-QKD can be performed with a real local oscillator and without auxiliary signals which is achieved by applying Machine Learning methods. It is shown that, with the most established discrete modulation protocol, the experimental system works down to a quantum channel signal to noise ratio of -19.1 dB. The performance of the experimental system allows CV-QKD at a key rate of 9.2 Mbit/s over a fiber distance of 26 km. After remote local oscillator and auxiliary signal aided CV-QKD, this could mark a starting point for a third generation of CV-QKD systems that are even more attractive for a wide implementation because they are almost identical to standard coherent systems.
Phase recovery is a key enabling process in continuous-variable quantum key distribution (CV-QKD) systems operating at ultra-low optical power, where phase noise from the finite linewidths of the transmitter and local oscillator lasers critically impacts system performance. Here, we investigate the performance of a phase recovery scheme for a prepare-and-measure CV-QKD setup using the Viterbi–Viterbi carrier phase estimation algorithm. We experimentally evaluate the scheme by varying the power of a binary phase shift keying (BPSK)-modulated pilot tone, using laser sources with nominal linewidths of 1 kHz and 25 kHz. Our results reveal a well-defined operational regime in which stable phase recovery is achieved, as well as a critical threshold below which pilot tone power becomes insufficient, leading to signal distortion and phase recovery failure. As expected, narrower-linewidth lasers exhibit improved phase stability and reduced variance, confirming the combined impact of pilot tone power and laser linewidth on phase recovery performance. These findings provide practical guidelines for CV-QKD receiver synchronization.
Quantum key distribution (QKD) enables information-theoretic-secure secret key exchange thanks to the laws of quantum mechanics. Since it can easily be incorporated into existing telecom networks, continuous-variable QKD (CV QKD) using coherent states and coherent detection has recently emerged as a promising candidate. However, the secret key rate and link distance in CV QKD systems are still limited by the imperfections of used components. In this work, through simulations, we study the use of different digital filtering techniques for optical phase recovery, aided by pilot symbols, to mitigate the phase noise induced by the coherent mixing of two free-running lasers in a CV QKD system. Moving average, Butterworth and unscented Kalman filters are evaluated for different phase drift conditions. Results show that phase recovery from small amplitude pilot symbols, along with further digital filtering, can considerably reduce phase noise and measurement error, while increasing the secret key rate and link distance of the CV QKD system.
Continuous-variable quantum key distribution (CV-QKD) has been increasingly studied, which offers the advantage of compatibility with modern coherent optical communication systems. In contrast to CV-QKD with a transmitting local oscillator, the local local oscillator CV-QKD avoids the security vulnerabilities of a local oscillator by generating a local oscillator at the receiver. In practice, the frequency offset of the two lasers introduces extra phase noise, which is generally suppressed by various carrier phase recovery algorithms. However, the accuracy of carrier phase recovery can be influenced by the power of the pilot tone, particularly as the transmission distance increases. To further improve accuracy, we propose a method based on the unscented particle filter algorithm, to increase the accuracy of phase estimation, effectively restore the quantum signal and reduce excess noise. In our work, we demonstrated a local local oscillator CV-QKD experiment with a finite-size block of 1×108 under a transmission distance of 50 km. Through our method, we achieved a secret key rate of 525 kbps, which represents a 28% improvement. These results confirm that our proposed method not only improves the accuracy of carrier phase recovery, but also provides a new approach for future research on algorithms for long-distance CV-QKD. Furthermore, our study improves the phase compensation performance, enabling the orthogonal components of the quantum signal to exhibit enhanced symmetry in phase space.