Experiment Research On High-Speed Serdes Based Dual Phase Modulating Quantum Key Distribution

Cryptography is an ancient and emerging topic.It has been a long time since people learned to use a variety of encryption to encrypt the plaintext for information hiding. It was not until modern times cryptography rose to the height of the theory as a strictly scientific system. Nowadays, traditional cryptography is widely used, whose security is based on the complexity of mathematical algorithms. However, with the development of distributed computing and quantum computers, the traditional encryption system is facing serious challenges. People began to explore new means of encryption."One-time Pad" is a theoretically unbreakable arithmetic. Although it is very simple, it has little use in practice for its difficulty of distributing large number of random key securely in real-time.1984, Bennett, who put forward the basic principles of quantum key distribution (QKD), pointed out that the absolute security of key distribution could be achieved through single-photon quantum properties. Since then, based on the combination of quantum key distribution tehnolgoy and the principle of "One-time Pad", truly safe quantum cryptography came into being.Within the multiple protocols of quantum key distribution, BB84 is one of the widely used and deeply reasearched protocols, whose security has been fully proved. According to its encoding method, BB84 protocol can be divided into polarization encoding and phase encoding, of which, phase encoding is used more often in optical fiber communication for much more stable transmission than polarization.Our research target is BB84 phase encoding high-speed QKD system based on Faraday-Michelson Interferometer, which can automatically compensate for polarization changes caused by the fiber birefringence. The system is relatively stable due to polarization-independent phase modulation. In this system, both the transmitter and receiver need to randomly generate four different phase throught the phase modulator (PM). Using the Digital-to-Analog Converter (DAC) to produce four different voltages is the traditional approach of phase modulation. But for high-speed QKD system, the establish time of stable voltage levels (settling time) of current DAC is a big problem. In Faraday-Michelson scheme, four voltage levels with random selection are required as well as the unchanged time between photon running in to and out of the PM, which demands that the analog voltage transition(ie, settling time) must be very short, however, even ultra-high-speed DACs expect this time of nanosecond order of magnitude.We put forward the SERDES-based dual phase modulating quantum key distribution scheme. SERDES is serial transceivers, which is made of two words:Serializer and Deserializer. SERDES can generate and receive high-speed digital signals, which features up to Giga-Hertz data rates, sub-nanosecond transition edges, small overshoots, stable levels and perfect signal integrity.The high-speed transceiver in the FPGA is a kind of high-speed SERDES, which can be easily and flexibly used in QKD system to produce high-speed random phase modulating signals.Due to the requirements of the BB84 protocol, both sides must produce four different phases, while one channel of SERDES can only generate two levels to modulating two phases. To this end, the paper presents the structure of a dual phase modulator, in which two electrodes are drived by two channels of SERDES to generate phase modulation respectively. One is producing 0 and π phases, while the other is producing 0 and π/2 phases, therefore two two together, four phases can be obtained. This is the basic idea of our high-speed program.The specific technique route of this method is to have FPGA’s built-in high-speed SERDES signals amplified by variable gain RF amplifer to generate the phase modulator drive signals. We design the FPGA for the ability of 200Mbps-2Gbps phase modulating data rate, and have the 6GHz wide band RF amplifier to produce as large amplitude as 4V signals from low-voltage swing digital signals. The measured results of circuits show that the 800Mbps phase modulating signal has clear eye diagram with good eye height and width, which meet the phase modulation requirments in current high-speed quantum key distribution system.Based on SERDES technology, we also develop a high-precision time-screening method for the single-photon detector (SPD). It can tell the specific location information of the arrival photon, which is used in the basis reconciliation. We design mulit-level processing buffer, fan-out and delay circuits to have SPD signals received by SERDES. SPD signal is converted to low speed parallel data stream which is processed through the screening techonology embeded in the PFGA to capture the hopping edge, and then changed into location information.At present, limited by the phase modulator and detector, the total high-speed electronics system reduce its frequency to 200MHz, which means the repetition frequency of photon pulse is 200MHz. We put forward new parameters determination methods for AC-coupled scheme, as well as the BiasT hardware scan techonology for real-time phase compensation. Through the test of critical signals, we find the bandwidth issue of high-speed random phase modulation, then point out that the stability of phase modulating signals is ensured by improving the low-frequency response of amplifier ciruits. Through the analysis of prototype test results, we present the main factors affecting the quantum bit error rate (QBER), and monitor the long term QBER and sifted key rate. Affected by the extinction ratio of light source, the QBER is about 4.3%, and the sifted key generation rate is about 100Kbps at 50km distance based on estimation that the mean signal photon number is 0.5 using the decoy state protocol.The system software is designed based on the multi-thread technology, therefore, basis reconcillation, error correction and privacy amplify do not affect the efficiency of the system. The average efficiency of the system is about 90%. Finally, the main factors limiting the next step of high-speed QKD are stated briefly, together with some possible solutions.High-speed quantum key distribution is a complex system which eject much stress on the optical system, electronics hardware and computer software. We have gived some ideas and practices on high-speed phase modulation scheme, but there are more to do in the future. We should be active to take part in the research and development of high-speed quantum cryptography, to contribute the security of national and public information.