1 Introduction

Quantum communication [1] has been attracting more and more attention in the field of secure communication, due to its unconditionally secure property. The pace of practical application is also accelerating [2, 3]. The BB84 protocol [4] based key distillation, which is extensively used in quantum key distribution (QKD) systems, requires synchronization between the transmitter and the receiver to complete the key generation. The synchronization system is critical, for it can affect the running of the basis sifting directly and influence the efficiency of key distillation.

Generally, we use optical synchronization in practical implementation. There are two types of signals in the QKD system, quantum signal and synchronization signal. Quantum signal adopts single photons as carriers of quantum information to generate a secure key [5], while the synchronization signal is some kind of classical periodic signal, which is mainly used for synchronization of the two sides [6].

Common system synchronization uses one quantum signal following one synchronization signal. This scheme is simple and reliable in low-speed QKD systems, but it has an obvious disadvantage in supporting high-speed QKD systems. On the one hand, during the transmission, since the synchronization signal has a strong intensity light, while the quantum signal has a very weak intensity light, the strong light will have influence over the weak light in simultaneous transmission. This may result in failure of the high-speed QKD system. On the other hand, a more complex synchronization scheme is required, along with the increase in system frequency [7-9].

To solve the problems mentioned above, we put forward a novel synchronization scheme based on TDC [10-13], which allows multiple quantum signals following one synchronization signal, and verify the feasibility of this scheme in an actual QKD system. The schemenot only reduces the influence of the synchronization signal on the quantum signal during transmission, but also simplifies the synchronization system.

2 Principle of Scheme

According to BB84 protocol, the two parties (the transmitter is called "Alice" and the receiver is called "Bob") of QKD need to have the same temporal order of quantum signal before executing basis sifting. The synchronization is to map the temporal order information of the quantum signal detected by the receiver with that sent by the transmitter.

The temporal order of the quantum signal in the transmitter can be predetermined, therefore, we only need to obtain the temporal order of the quantum signal in the receiver. To achieve this goal, we use high precision time measurement technology, which can convert the time information of the quantum signal into the corresponding digital temporal information, and match it with the transmitter.

The synchronization scheme focuses on two aspects: the frequency of the synchronization signal of the transmitter, and the time interval between the two adjacent quantum signals.

The module of the receiver used to detect the time information [14, 15] is called the "high precision time measurement module". As the core unit of the whole synchronization system, its structure and performance directly determines the process of synchronization and the precision of system. The module mentioned above adopts a "start-and-stop" measuring method, that is, a single "start" signal and multiple "stop" signals. Each "stop" signal refers to its previous "start" signal, and the module obtains the time interval information between the "start" and "stop" signals.

The illustrative diagram of the time measurement based synchronization scheme is shown in Fig. 1.

Fig. 1.Full-screen View

(Color online) Schematic diagram of the time measurement based synchronization scheme.

Firstly, Alice sends the sequence of synchronization signal pulses S0, S1,, Sn (time interval for adjacent synchronization signal is t1), and the sequence of quantum signal pulses AP0, AP1,, APn (time interval for adjacent quantum signal is t2).

Secondly, Bob detects the synchronization signals and quantum signals. The sequence of synchronization signals can all be detected, while only a few of the quantum signals can be detected, for most are lost due to path attenuation. The supposed quantum signals, BPi and BPk, are detected by Bob, and the two detected quantum signals correspond to APi and APk from Alice, respectively.

Then, Bob calculates the time information of each quantum signal. The calculation process is as follows (take Bpi as an example):

(1) Record the number of synchronization signals ahead of the BPi quantum signal as m, then multiply by the cycle time t1. The product is regarded as coarse timing. The coarse timing is m× t1.

(2) Record the time interval between the BPi quantum signal and its previous synchronization signal as T1. This serves as fine timing.

(3) Obtain the sum of the coarse timing and fine timing as total time T, T=m× t1+T1.

Finally, Bob converts the total time to the corresponding temporal order for each quantum signal, temporal order is T/t2. The temporal order of each quantum signal that is figured out by Bob, corresponds to one of the values in the quantum signal sequence, AP0, AP1,, APn, of Alice.

3 Experimental

In order to verify the feasibility of this synchronization scheme in QKD system, we designed a time measurement module and complete the relevant experimental work on an actual platform of the QKD system.

3.1 Time measurement module test

The main chip of the time measurement module is TDC-GPX, which is produced by the ACAM Company of Germany. It has eight independent channels and its maximum sampling rate is 40 MHz [16]. Figure 2 is a test block diagram of the time measurement module. We used a signal generator as a source which can output two LVTTL signals with the same frequency. One serves as the "start" signal and the other serves as the "stop" signal. Moreover, the "start" signal is ahead of the "stop" signal and the delay time between the "stop" and the "start" is adjustable. The TDC-GPX chip is in I mode and the time BIN is set as 100 ps. FPGA is used to read the data from TDC-GPX and upload it to a PC.

Fig. 2.Full-screen View

Test block diagram of the time measurement module.

We set the "start-stop" signal time interval value from 1 μs to 9 μs on the signal generator. The module obtains the measured data of each time event and uploads it to a PC for analysis. The linearity between the time interval value and the measured data is shown in Fig. 3. The horizontal axis is the set time interval value between the "start" signal and "stop" signal on the signal generator. The axis unit is μs. The vertical axis is the measured data from the time measurement module. The axis unit is time BIN. We can see that the measured data increases with the length of the time interval value. The data show good linearity.

Fig. 3.Full-screen View

(Color online) The linearity between time interval value and measured data.

Then, we set a typical fixed time interval 5 μs between the "start" and "stop" on the signal generator. The distribution of output data is shown in Fig. 4. The unit of the horizontal axis is 100 ps, and that of vertical axis is the number of counts. Meanwhile, the total sample number is 50000. From Fig. 4, we can see that the output data of TDC-GPX basically meets the law of normal distribution: the central value is 5.0940 μs; the bottom width (the total data of 99%) is five times the time unit, that is, 5×100 ps= 500 ps; the Full Width Half Maximum (FWHM) is 1.35×100 ps= 135 ps; and the sigma is 0.57100 ps=57 ps.

Fig. 4.Full-screen View

(Color online) Distribution of the test data.

According to the test results, if the frequency of the quantum signal is 200 MHz, the minimum time interval between the two adjacent signals that arrived at Bob is 5 ns. The bottom width of the TDC-GPX’s output is far less than the time interval between two adjacent quantum signals, so TDC-GPX can distinguish the arrival time of the two quantum signals accurately.

3.2 Validation of the QKD system

On an actual QKD hardware platform, the frequency of the quantum signal is 200 MHz and the frequency of the synchronization signal is 200 kHz. The hardware block diagram of the platform of the QKD system is shown in Fig. 5.

Fig. 5.Full-screen View

Hardware block diagram of the platform of QKD system.

This is a typical QKD system [17, 18]. The FPGA of Alice receives random data from the PC and generates driving signals for the "synchronization laser" and the "quantum laser". The "synchronization laser" generates synchronization optical signals and the "quantum laser" generates quantum optical signals. Both of the two types of optical signal should get through an attenuator (ATT) respectively before going to the optical fiber for transmission. The optical fiber length is 50 km and the path attenuation between the transmitter and the receiver is adjustable. When the synchronization signal and quantum signal arrived at the receiver, they respectively go to the detector for the synchronization signal and the detector for the quantum signal. The synchronization detector converts the optical signal into an electrical signal which goes into a fan-out chip to become double signals. One goes to the time measurement module and serves as the "start" signal, while the other enters into FPGA for calculating coarse timing. The detector for the quantum signal is a kind of single photon detector (SPD), which can convert the quantum signal into an electrical signal. The converted signal goes into the time measurement module and serves as the "stop" signal. After that, the FPGA of Bob reads the data from the module and uploads it to the PC. Finally, the key distillation process is done on the PC from both sides.

In the actual system, since the synchronization signal and the quantum signal have different transmission paths, the receiver needs to do a time-base offset compensation between the two signals. Figure 6 shows the principle of the time-base offset compensation.

Fig. 6.Full-screen View

Principle of the time-base offset compensation.

(1) Alice sends a low frequency pulse sequence of the synchronization signal S0, S1, …, Sn, and the same frequency pulse sequence of the quantum signals AP0, AP1, …, APn. Each synchronization signal follows a quantum signal, and the time interval between the two signals is fixed, which is recorded as Ta;

(2) Bob records the time intervals between each arrived quantum signal and its previous synchronization signal, and then figures out the average value as Tb.

(3) Bob calculates the D-value between Tb and Ta as Δt, so, Ta=Tb+Δt. Δt is the value of the time-base offset compensation.

The data from the time measurement module should add a Δt bias as the "correct time". Key distillation will begin after the FPGA of Bob converts the "correct time" to a corresponding temporal order. The QKD system can output sifted keys continuously without increasing the quantum bit error rate. The test result shows that this synchronization scheme can accurately map the temporal order information of the receiver with that of the transmitter.

4 Conclusion

The time measurement based synchronization scheme proposed in this paper adopts multiple quantum signals following one synchronization signal. From the above experimental data, the following conclusions can be obtained: the measured data of the time measurement module shows good linearity. Moreover, the test data shows that the module has a good time resolution. These good performance allow the module to be applied in an actual QKD system. In the actual QKD platform, the frequency of the quantum signal is set to 200 MHz, with the frequency of the synchronization signal set to 200 kHz, the time information can be accurately converted to the corresponding temporal order.

The synchronization scheme is novel and practical, it can support a wide frequency range of the QKD system. It is not only applicable in low-speed QKD systems, but also proved to be feasible in high-speed QKD systems. Furthermore, this scheme is not sensitive to the distance of the two sides and as result is a good choice in a long-distance QKD system.