Satellite-ground quantum key distribution has embarked on the stage of engineering implementation, and a global quantum-secured network is imminent in the foreseeable future. As one payload of the quantum-science satellite which will be ready before the end of 2015, we report our recent work of the space-bound decoy-state optical source. Specialized 850 nm laser diodes have been manufactured and the integrated optical source has gotten accomplished based on these LDs. The weak coherent pulses produced by our optical source feature a high clock rate of 100 MHz, intensity stability of 99.5%, high polarization fidelity of 99.7% and phase randomization. A series of space environment tests have been conducted to verify the optical source’s performance and the results are satisfactory. The emulated final secure keys are about 120 kbits during one usable pass of the low Earth orbit satellite. This work takes a significant step forward towards satellite-ground QKD and the global quantum-secured network.
© 2014 Optical Society of America
In 1984, C. H. Bennett and G. Brassard proposed the first quantum key distribution (QKD) protocol, known as BB84 protocol . Different from classical communication, the fundamental laws of quantum mechanics guarantee the unconditional security of quantum communication. Nowadays QKD has reached the level of maturity sufficient for commercial implementation . However, current transmission distances are limited to the order of 200 km in fiber [3, 4] and 100 km in free space . On the other hand, the use of the low-loss free-space link between satellite and ground provides the most promising solution to overcome this limitation and realize the global quantum-secured network. Recently some experimental verifications towards satellite-ground QKD [6, 7] have been carried out. What’s even more exciting is that China is preparing to launch a satellite dedicated to quantum-science experiments . All this puts forward an urgent demand to design the desired optical source that works well in the harsh space environment.
The weak coherent optical source is the most common optical source for QKD experiments, which can be implemented for both phase encoding and polarization encoding. On the category of satellite-ground QKD, polarization encoding has a significant advantage as Earth’s atmosphere preserves polarization with high fidelity . Previous implementations of polarization-encoded optical source [10–13] are all based on ideal desktop working environment and do not meet the needs of space application. For a satellite mission, payloads have to withstand the harsh environment in outer space, such as severe vibration (typically 10 g root-mean-square shocks), extreme thermal vacuum environment (typically storage temperature between −30 °C and 70 °C, working temperature between 5 °C and 35 °C in vacuum environment) and long working life (typically 2 years), etc .
In this paper, we report our recent work of the space-bound optical source for satellite-ground QKD. Aimed at space application, specialized 850 nm laser diodes (LD) have been manufactured and the integrated optical source has gotten accomplished based on these LDs. Eight LDs are used to realize the optical source, as each LD prepares one of the four polarization states (|H〉, |V〉, |+〉, |−〉) with the two intensity states (signal state, decoy state). The 100 MHz pulse train based on random numbers is used to modulate these LDs through the specially designed driving circuits. The intensity states of the optical source are of high stability with the help of the closed loop power control. The output optical signals from these LDs feature the same central wavelength through precise temperature control of each LD. A series of space environment tests including random vibration, thermal cycling and life test have been carried out to verify the optical source’s performance in outer space. Based on the satisfactory results of above tests, we can draw the conclusion that the optical source is well prepared for space application. Successful in-door QKD experiments have been conducted to demonstrate the optical source’s performance. For the downlink 40 dB lossy channel , the emulated quantum bit error ratio (QBER) is 0.31% and the lower bound of secure key rate (LBSKR) is 574 bps. There are over 200 usable passes, of which the effective communication times are more than 150 s, over 1 year for the low Earth orbit satellite. The average communication time is about 210 s and the emulated final secure keys are 120 kbits during one usable pass of the satellite.
This paper is organized as follows: in section 2, we present a brief description of the LD manufacture, introduce the optical source design and give some performance measurements of the optical source; in section 3, we introduce the space experiment tests and compare the performance variations before and after these tests; in section 4, we give the experimental demonstration and theoretical analysis towards satellite-ground QKD; finally we conclude in section 5 and give the acknowledgements.
2. Integrated optical source
2.1. Laser diode manufacture
Because the commercial products can hardly survive the harsh space environment, we must manufacture the tailored LDs suitable for space application. Previous research shows that the optimal wavelength of a low Earth orbit downlink QKD is closer to 700 nm, mainly due to lower diffraction losses . However, current technology is more mature in the 850 nm band, which providing higher reliability and more flexibility for space application. Meanwhile, 850 nm is also within the atmospheric window for the downlink QKD. Although the final secure keys at the 850 nm band are a little less than 700 nm, it is also sufficient for everyday application. Based on the above considerations, we choose the LDs of 850 nm.
All the components of the LD are integrated in a standard butterfly box (Fig. 1), including a vertical-cavity surface-emitting laser (VCSEL) chip, a thermoelectric cooler (TEC), a thermistor (TR) and a photodiode (PD). The temperature control system composed of a thermistor and a TEC is used to manage the environment temperature of the entire cavity. Thus, the internal laser chip and PD can work in a stable environment, which helps guarantee the output power, polarization state and other performance parameters of the LD. The specially designed gas cell greatly reduces the damage suffered from various external factors such as water vapour and extreme temperature. Optical signals emitted from the laser chip are divided into two parts with a 50 : 50 beam splitter (BS). One half irradiates on the PD and helps monitor the LD’s output power, and the other half is coupled into the polarization maintaining fiber (PMF) using a double-lens optical system. We use the UV-curable glue and hot glue to solidify these lens, which survives the LD up to 30 groot-mean-square (grms) shocks. As for the external coupling, we use several stainless steel components and the laser welding technology to ensure the stability of the optical path.
2.2. Optical source design
The integrated optical source consists of five main parts (Fig. 2): a physical random number generator (RNG), a field programmable gate array (FPGA), peripheral circuits, eight LDs and a polarization-encoding module. The FPGA outputs trigger signals based on random numbers generated from the RNG. The peripheral circuits use fast differential clock D Flip-Flops (DFF) to produce the 1 ns electrical pulses and drive the LDs through several high-speed bipolar junction transistors (BJT) switches. Using the gain-switching method, optical pulses with width of around one hundred picoseconds can be obtained . The specially designed polarization-encoding module is of small size and easy to use, with eight PMF inputs and one free-space output. Optical signals from the eight LDs respectively represent one of the four polarization states (|H〉, |V〉, |+〉, |−〉) with the two intensity states (signal state, decoy state) of decoy state BB84 protocol. Vacuum state is prepared when there comes no trigger signal.
The bias current of the LD is kept far bellow threshold, thus the generated optical pulses do not have any phase coherence from one pulse to the subsequent one . This idea helps produce a random phase for each pulse, and get a high extinction ratio at the same time. The output optical power of the LD has a linear correlation with the photocurrent of the internal PD. We use an operational amplifier (OA) to transform the weak photocurrent into a voltage signal and implement data acquisition by an analog-to-digital-converter (ADC). The temperature-stable power-monitoring PD provides a reference for the optical intensities in the harsh space environment. Through the closed loop power control, the intensity states of the optical source can get accurate monitoring and stable control. It is worth mentioning that although we can use the scheme of four LDs with different driving voltages to modulate signal and decoy pulses, for more precise close-loop power control we use the scheme of eight LDs.
The TR’s resistance is correlated to the surrounding environment temperature and get transformed into a voltage signal using a constant current source. A dual power metal oxide semiconductor field effect transistor (MOSFET) driver is used to drive a typical H-Bridge circuit consist of four MOSFET switches. The direction of the TEC’s current determines it to be heating or cooling. Based on the proportion-integral (PI) algorithm, the LD’s temperature can be controlled precisely and steadily. Since the optical spectrum of one LD is closely correlated to its temperature, we are able to realize spectral consistency of the entire optical source by controlling the temperatures of the eight LDs separately.
2.3. Performance measurements of the optical source
A high-resolution, low-jitter time-correlated single photon counting (TCSPC) module is used to measure the optical pulse width. The output optical pulses are detected by a low-jitter single photon avalanche diode (SPAD), and the generated electrical signals enter the stop input port of the TCSPC module (PicoHarp 400, PicoQuant GmbH). The 10 kHz synchronous electrical signals, which are used to generate the 10 kHz time-synchronization optical signals, enter the sync input port. The obtained coincidence counts show a full width at half maximum (FWHM) of 128 ps (Fig. 3(a)). Considering the jitter of the SPAD (PDM series, Micro Photon Devices) is about 30 ps and the jitter of the TCSPC module is neglectable, the FWHM of the optical pulses is about t = (1282 − 302)1/2 = 125 ps. The intensity states of the optical source are of high stability and accuracy with the help of the closed loop power control. The fluctuation of the optical power emitted by one LD is below 0.5% over consecutive 3600 seconds (Fig. 3(b)). Through independent power control of the eight LDs, we can achieve the optical source consistent in intensity states (signal state and decoy state have different intensities), which is also the biggest advantage of eight LDs over four LDs.
There is an approximate linear correlation between the optical spectrum and the temperature of a LD. As the temperature increases by one centigrade, the central wavelength of the optical signals shifts towards longer wavelength by 0.06 nm for the 850 nm LDs. The temperature control precision of the eight LDs is better than 0.05 °C and the wavelength deviation is less than 0.003 nm. The temporal overlap of the optical pulses is guaranteed better than 96% through carefully matching the length of each LD’s driving cable, after calibration of the signals’ temporal deviation. The spatial overlap of the eight LDs is guaranteed better than 95% through fine manufacturing and quantities filtrating process of the polarization-encoding module. In order to ensure communication security from side-channel information , the full set of eight LDs must perfectly overlap in time, spectra and spatial mode. We can further guarantee the temporal, spatial and spectral consistency of the optical source using proper filters in our future work.
To measure the polarization performance of the optical source, optical signals of one polarization state are sent out each time and the four polarization sensitive avalanche diodes (APD) give their counts. Test results show that polarization extinction ratios of the four polarization states are all better than 25 dB (Table 1) and the polarization fidelity is better than 99.7%. Considering the background counts of the APDs (SPCM-AQRH, Perkin Elmer) and the imperfection of the polarization-decoding module, the actual data would be even better. We have also measured the extinction ratios (ER) by calculating the signal counts and background counts, and the results are better than 25 dB.
3. Space environment tests
Optical source for space application has to withstand harsh environmental conditions such as severe mechanical vibration, extreme thermal vacuum environment and long working life during the launch and on-orbit working. The structure of our optical source is carefully designed for strengthening and heat dissipation (Fig. 4). To simulate the harsh space environment expected during a satellite mission, several terrestrial tests involving random vibration, thermal cycling and life test have been conducted to identify whether vibration and extreme temperature will impair the performance of the LDs as well the integrated optical source.
For the random vibration test, the LDs are fixed to a vibration table instrumented with accelerometers, and vibrated with the desired profile. The vibration test continues for two minutes as random vibrations applied along three perpendicular directions to 25 and 30 grms over a frequency range of 10–2000 Hz. The output optical power and the photocurrent of the LDs before and after the vibration are compared at the same working condition of 5 mA driving current and 20 °C working temperature in a room-temperature environment. The mean deviation of the optical power is 1.95% and the mean deviation of the photocurrent is 0.38%. For the thermal cycling test, the LDs are fixed into a temperature-controlled cabinet and cycle between −40 °C and 85 °C for 10 times. The speed of temperature change is about 10 °C per minutes, while the temperature is on hold for 30 minutes at −40 °C and 85 °C. Test results show that the mean deviation of the optical power is 1.48% and the mean deviation of the photocurrent is 2.17% at the same working condition as before. For the life test, the LDs are fixed into a temperature-controlled cabinet and keep working for continuous 240 hours in a 60 °C environment at one atmospheric pressure. The performances of the LDs before and after the life test are also compared at the same working condition. Test results show that the mean deviation of the optical power is 2.16% and the mean deviation of the photocurrent is 1.54% (Table 2). The performances of these LDs in the extreme 60 °C and −10 °C environment have also been measured, and the results show that deviations of the output optical power and the PD current are all less than 9%.
Specific tests for the entire integrated optical source include 10.4 grms random vibration in three perpendicular directions, 13 times of thermal cycling between −25 °C and 60 °C as well as 7 times of thermal vacuum cycling between 0 °C and 40 °C with an average barometric pressure below 10−4 Pa. After the vibration test, the optical source appears completely unaffected. The integrated optical source, primarily the internal electronics, functions well during the thermal cycling and the thermal vacuum test. The rest of the other space environment tests such as electro magnetic compatibility (EMC) test have also been carried out according to plan.
The integrated optical source is designed to be in a 15 °C to 25 °C vacuum environment when the quantum-science satellite is on-orbit working. All these space environment tests have demonstrated that the harsh space environment makes very little influence on the LDs’ performance, and the integrated optical source is robust enough and well prepared for space application.
4. Experimental demonstration and discussion
We implement a polarization-encoded protocol with vacuum + weak decoy states . The lower bound of the secure key rate per pulse is given by
Alice and Bob share the 10 kHz time-synchronization optical signals. The probability of signal state, decoy state and vacuum state are 50%, 25% and 25% on the average. Towards the satellite-ground free-space downlink channel, we adopt a background count rate of 200 counts / s mainly due to detectors’ dark count and stray light’s contribution. Accounting for 620 ps jitter of the synchronous optical signals, 350 ps jitter of APDs and 125 ps of the width of the generated optical pulses, We adopt a 2 ns time window during data processing to reduce background counts. The emulated results of LBSKR and QBER are shown in Fig. 5. As expected, the LBSKR decreases and the QBER increases while the attenuation increases. Simulation results give a QBER of 0.20% and a LBSKR of 26.1 kbps for a 23.5 dB lossy channel. Successful in-door QKD experiments have been carried out to demonstrate the optical source’s performance, and we have achieved a QBER of 0.31% and a LBSKR of 25.9 kbps for an equivalent attenuation of 23.5 dB. The disparity between the experiment results and simulation results is due to the optical misalignments, measurement deviation and imperfection of the polarization-decoding module.
The first quantum-science satellite will be launched into a low Earth orbit with an altitude around 600 km, and the equivalent downlink channel loss is about 40 dB. The emulated QBER is 0.31% and the LBSKR is 574 bps based on our optical source. As the low Earth orbit satellite makes one usable pass, the effective communication time is about 210 seconds and the emulated final secure keys are 120 kbits. In the case of such a big final key size, the finite-key-size effect can be neglected . In the simulation and the experiment, we set the average photon numbers of the signal state and decoy state to be 0.6 and 0.2 per pulse respectively for simplification. More final secure keys will be achieved if the decoy-state protocol parameters are optimized.
We have presented the design and implementation of the integrated optical source for satellite-ground QKD built on specially designed LDs. The optical source is capable of generating pulses of four polarization states with three intensity states for decoy state BB84 protocol and obtaining high intensity stability and polarization fidelity at the same time. Several space environment tests involving random vibration, thermal cycling and life test have been conducted to verify the optical source’s performance for a satellite mission and the results are satisfactory. Towards the satellite-ground 40 dB downlink channel, the emulated LBSKR is 574 bps and the final secure keys are 120 kbits during one usable pass of the low Earth orbit satellite. The performance, integration and space qualification, make it particularly suitable for space application.
We acknowledge Prof. Q. Zhang and Prof. Y.-A. Chen for their helpful comments and suggestions. We also acknowledge D.-D. Li, B. Li, F.-Z. Li and Y. Xu for their useful discussions during the course of this article. This work has been supported by CAS Center for Excellence and Synergetic Innovation Center in Quantum Information and Quantum Physics, Shanghai Branch, University of Science and Technology of China, by the National Fundamental Research Program (under grant no. 2011CB921300 and 2013CB336800), by the National Natural Science Foundation of China, and by the Strategic Priority Research Program on Space Science, the Chinese Academy of Sciences.
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