Polarization Encoded Quantum Key Distribution Research Articles

Research on polarization compensation for practical satellite-based quantum key distribution

Satellite-based quantum key distribution (QKD) is one of the effective solutions for building a global-scale quantum network. For practical satellite-based polarization-encoding QKD systems, the polarization degradation introduced by the single-mode fiber and the relative rotation between satellites and ground stations will lead to a decrease in the polarization performance and need to be compensated. In this work, we propose two novel on-orbit satellite-to-ground joint polarization measurement and compensation methods, which can ensure high polarization fidelity while effectively saving hardware resources and reducing costs. By establishing a polarization compensation model and performing theoretical analysis, combined with desktop experimental verification, we demonstrated the effectiveness of the satellite-to-ground joint polarization compensation and achieved a low quantum bit error rate below 0.3% after background removal. The presented polarization compensation laid the foundation for the subsequent applications in satellite-based QKD and satellite-based quantum networks.

Numerical study of single-layer and interlayer grating polarizers based on metasurface structures for quantum key distribution systems

Polarization is an important property of light, which refers to the direction of electric field oscillations. Polarization modulation plays an essential role for polarization encoding quantum key distribution (QKD). Polarization is used to encode photons in the QKD systems. In this work, visible-range polarizers with optimal dimensions based on resonance grating waveguides have been numerically designed and investigated using the COMSOL Multiphysics Software. Two structures have been designed, namely a single-layer metasurface grating (SLMG) polarizer and an interlayer metasurface grating (ILMG) polarizer. Both structures have demonstrated high extinction ratios, ~1.8·103 and 8.68·104, and the bandwidths equal to 45 and 55 nm for the SLMG and ILMG, respectively.

Integrated silicon photonics chip fabricated by CMOS compatible technology for low-bit-error-rate quantum key distribution

Quantum communication has extremely high security and is a development direction of future communication networks. In this paper, an on-chip path, off-chip polarization encoded quantum key distribution (QKD) chip is designed and fabricated using advanced silicon photonics fabrication technology. Using a special two-dimensional grating, we can convert light of different path states into corresponding polarization states and propagate in fiber. The performance of the basic optics, waveguides, gratings, and Mach–Zehnder interferometers on the chip is characterized. In the demonstrated QKD experiment, the quantum bit error rate is as low as 1.08%. It provides an important device for the future development of advanced integrated quantum information.

Silicon-based decoder for polarization-encoding quantum key distribution

Silicon-based polarization-encoding quantum key distribution (QKD) has been widely studied, owing to its low cost and robustness. However, prior studies have utilized off-chip devices to demodulate the quantum states or perform polarization compensation, given the difficulty of fabricating polarized independent components on the chip. In this paper we propose a fully chip-based decoder for polarization-encoding QKD. The chip realizes a polarization state analyzer and compensates for the BB84 protocol without requiring additional hardware. It is based on a polarization-to-path conversion method that uses a polarization splitter-rotator. The chip was fabricated using a standard silicon photonics foundry; it has a compact design and is suitable for mass production. In the experimental stability test, an average quantum bit error rate of 0.59% was achieved through continuous operation for 10 h without any polarization feedback. Furthermore, using the developed feedback algorithm, the chip enabled the automatic compensation of the fiber polarization drift, which was emulated by a random fiber polarization scrambler. In the case of the QKD demonstration, we obtained a finite-key secret rate of 240 bps over a fiber spool of 100 km. This study represents an important step toward the integrated, practical, and large-scale deployment of QKD systems.

Cost-Effective ML-Powered Polarization-Encoded Quantum Key Distribution

Secure communications have become a requirement for virtually all kind of applications. Currently, two distant parties can generate shared random secret keys by using public key cryptography. However, quantum computing represents one of the greatest threats for the finite complexity of the mathematics behind public key cryptography. In contrast, Quantum Key Distribution (QKD) relies on properties of quantum mechanics, which enables eavesdropping detection and guarantees the security of the key. Among QKD systems, polarization encoded QKD has been successfully tested in laboratory experiments and recently demonstrated in closed environments. The main drawback of QKD is its high cost, which comes, among others, from: <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i> ) the requirements for the quantum transmitters and receivers; and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i> ) the need of carefully selecting the fibers supporting the quantum channel to minimize the environmental effects that could dramatically change the polarization state of photons. In this paper, we propose a Machine Learning (ML) -based polarization tracking and compensation that is able to keep shared secret key exchange to high rates even under large fiber stressing events. Exhaustive results using both synthetic and experimental data show remarkable performance, which can simplify the design of both quantum transmitter and receiver, as well as enable the use of aerial optical cables, thus reducing total QKD system cost.

Practical underwater quantum key distribution based on decoy-state BB84 protocol.

Polarization encoding quantum key distribution has been proven to be a reliable method to build a secure communication system. It has already been used in an inter-city fiber channel and near-Earth atmosphere channel, leaving an underwater channel the last barrier to conquer. Here we demonstrate a decoy-state BB84 quantum key distribution system over a water channel with a compact system design for future experiments in the ocean. In the system, a multiple-intensity modulated laser module is designed to produce the light pulses of quantum states, including signal state, decoy state, and vacuum state. Classical communication and synchronization are realized by wireless optical transmission. Multiple filtering techniques and wavelength division multiplexing are further used to avoid cross talk of different lights. We test the performance of the system and obtain a final key rate of 245.6 bps with an average quantum bit error rate of 1.91% over a 2.4 m water channel, in which the channel attenuation is 16.35 dB. Numerical simulation shows that the system can tolerate up to 21.7 dB total channel loss and can still generate secure keys in 277.9 m Jerlov type I ocean channel.

Real-time polarization compensation system for wavelength division multiplexing in low noise fiber channel based on single photon counting feedback

The physical effects such as random birefringence of fiber optic channels due to environmental influences make the optical signals transmitted in them have sensitive polarization variations, which seriously affects the performance of polarization biased code quantum key distribution systems. In this paper, a low-noise fiber channel wavelength division multiplexing real-time polarization compensation system is presented, where single photon counting is used as a feedback signal. The system can acquire the fiber channel polarization change information by detecting the photon counting of the conjugate reference light. In the system, the compensation algorithm is designed to control the electric polarization controller to calibrate the polarization state of the quantum signal light under the corresponding polarization base in real time, and the stable fiber channel polarization compensation is successfully achieved. In order to verify the effectiveness of the compensation system, a quantum key distribution test based on BB84 protocol with a transmission distance of 25.2 km is conducted, and stable test results of up to 8 hours are obtained in the laboratory environment and the simulated metropolitan area network buried fiber environment, with the average quantum bit error rate being 0.52% and 1.25%, respectively. The experimental results show that this system can guarantee the stable operation of polarization-encoded quantum key distribution in the buried fiber environment in urban areas.

High-speed robust polarization modulation for quantum key distribution.

Polarization modulation plays a key role in polarization-encoding quantum key distribution (QKD). Here, we report a new, to the best of our knowledge, polarization modulation scheme based on an inherently stable Sagnac interferometer. The presented scheme is free of polarization mode dispersion and calibration as well as insensitive to environmental influences. Successful experiments at a repetition frequency of 1.25 GHz have been conducted to demonstrate the feasibility and stability of the scheme. The measured average quantum bit-error rate of the four polarization states is as low as 0.27% for 80 consecutive minutes without any adjustment. This high-speed intrinsically stable polarization modulation can be widely applied to many polarization-encoding QKD systems, such as BB84, MDI, etc.

Eavesdropper's ability to attack a free-space quantum-key-distribution receiver in atmospheric turbulence

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum key distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver, or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack.

Analysis of polarization fluctuation in long-distance aerial fiber for QKD system design

Abstract The polarization basis misalignment caused by polarization fluctuation is the main limiting factor in the deployment of polarization-encoding quantum key distribution (QKD) in aerial fibers. In this study, we conduct further research on polarization fluctuations in aerial fibers from the perspective of QKD. We propose a method to evaluate polarization fluctuation in aerial fibers. The relation connecting the quantum bit error rate (QBER) to the components of the Stokes vector is derived. Through a field experiment, long-time data on the polarization fluctuations in a 68-km-long aerial fiber are obtained. A statistical analysis of the time-cost corresponding to a typical QBER caused by polarization misalignment is conducted. The results can be used to evaluate the required speed of polarization compensation, and they indicate that the time-cost of the polarization compensation module should ideally be lower than 1 ms. This can provide guidance for the design of polarization compensation modules and algorithms for polarization-encoding QKD systems based on long-distance aerial fibers, aiding in its deployment.

Energy Conservation in Distributed Interference as a Guarantee for Detecting a Detector Blinding Attack in Quantum Cryptography

An avalanche single-photon detector blinding attack is one of the methods for quantum hacking of quantum key distribution (QKD) systems. The attack was experimentally demonstrated for both phase- and polarization-encoded QKD systems. During this attack, an eavesdropper knows the entire key, does not produce errors and саnnot be detected. However, the phase encoding has neglected some significant features of the photocount statistics in the receiving party. It is shown in the paper at the level of fundamental principles that this attack changes the photocount statistics and leads to the detection of an eavesdropper. Expressions for the secret key length are obtained for this attack. This does not require any changes in the design and control electronics of the phase-encoded QKD system, and only changes in processing the results of registration of quantum states are sufficient. At the same time, the secret key vulnerability and compromise in polarization-encoded QKD systems is an existing fact rather than a potential menace.

Silicon photonic transmitter for polarization-encoded quantum key distribution

Silicon (Si) photonics is forming a fabless ecosystem, which is enabling low-cost and densely integrated components for optical communications and quantum information. We present a Si optical transmitter for polarization-encoded quantum key distribution (QKD). The chip was fabricated in a standard Si photonic foundry process and integrated together a pulse generator, intensity modulator, variable optical attenuator, and polarization modulator in a 1.3 mm×3 mm1.3 mm×3 mm die area. The devices in the photonic circuit meet the requirements for QKD. The transmitter was used in a proof-of-concept demonstration of the BB84 QKD protocol over a 5 km long fiber link. This work shows the potential of using foundry Si photonics for low-cost, wafer-scale fabricated components for quantum information.

Experimental demonstration of polarization encoding quantum key distribution system based on intrinsically stable polarization-modulated units.

A proof-of-principle demonstration of a one-way polarization encoding quantum key distribution (QKD) system is demonstrated. This approach can automatically compensate for birefringence and phase drift. This is achieved by constructing intrinsically stable polarization-modulated units (PMUs) to perform the encoding and decoding, which can be used with four-state protocol, six-state protocol, and the measurement-device-independent (MDI) scheme. A polarization extinction ratio of about 30 dB was maintained for several hours over a 50 km optical fiber without any adjustments to our setup, which evidences its potential for use in practical applications.

Attacking a practical quantum-key-distribution system with wavelength-dependent beam-splitter and multiwavelength sources

Unconditional security of quantum key distribution protocol can be guaranteed by the basic property of quantum mechanics. Unfortunately, the practical quantum key distribution system always have some imperfections, and the practical system may be attacked if the imperfection can be controlled by the eavesdropper Eve. Applying the fatal security loophole introduced by the imperfect beam splitter's wavelength dependent optical property, we propose wavelength-dependent attacking model, which can be applied to almost all practical quantum key distribution systems with the passive state modulation and photon state detection after the practical beam splitter. Utilizing our attacking model, we experimentally demonstrate the attacking system based on practical polarization encoding quantum key distribution system with almost 100% success probability. Our result demonstrate that all practical devices require tightened security inspection for avoiding side channel attacks in practical quantum key distribution experimental realizations.

QBER Estimation in QKD Systems With Polarization Encoding

A model for quantum BER estimation in polarization encoded quantum key distribution systems is presented. Both TDM and WDM based polarization control schemes are analyzed. It is shown that TDM presents some important advantages when compared with the WDM control scheme. In WDM, the polarization decorrelation between the reference and data signals is an intrinsic and very limitative impairment. This effect has a contribution to quantum BER that increases with the propagation distance, and is highly dependent on the fiber polarization mode dispersion. In the TDM control scheme, the polarization decorrelation is less critical and other issues, like the single photon detector and feedback polarization control system performance tend to dominate. We show that for long distances the fiber losses represent the main contribution to the total quantum BER. Nevertheless, for distances shorter than 70 km and frequencies higher than 5 MHz the after pulse detections provide an important contribution to the total quantum BER.

Stable quantum key distribution with active polarization control based on time-division multiplexing

Polarization-encoding provides a promising approach for the practical quantum key distribution (QKD) system due to the simple encoding and decoding method. Here we present a long-term stable polarization-encoding QKD with a real-time polarization control. The polarization of the signal photons in the optical fiber was maintained stable by using time-division-multiplexed reference pulses for feedback control. We implemented this technique in the QKD experiment in 50 km fiber to show that it could facilitate polarization-encoded quantum communication. The system operated stably for ∼460 min, and the quantum bit error rate was ∼5.27%. The raw key generating rate in this system was kept stable at ∼500 bits s-1. The advantages and disadvantages of the previous polarization-control schemes were also rigorously analyzed.

Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40π rad s−1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

Active polarization stabilization in optical fibers suitable for quantum key distribution

Polarization feedback control of single-photon pulses has been achieved in long-distance fibers for more than 10 hours, which facilitated "one-way" polarization-encoded quantum key distribution with long-term stabilities. Experimental test of polarization encoding in 75 km fibers demonstrated that the single-photon polarization transformation in long-distance fibers could be controlled to provide a typical QBER of (3.9+/-1.5)% within a long-term operation of 620 minutes.