Mobile quantum cryptography enhances secure communications
Recently, studies conducted on potential applications of quantum technology utilizing the optical and electrical domains have attracted a considerable amount of interest. Quantum technology possesses the characteristics necessary to become the fundamental functional architecture for future computer processing and communication technologies. In particular, researchers have studied quantum cryptography, quantum teleportation, and quantum dense coding technology.1–3 Quantum cryptography is a recently developed technique that uses single photons to transmit quantum information securely. Only certain quantum cryptographic protocols, such as BB84, which Bennett and Brassard demonstrated in 1984, guarantee unconditional security.4
At present, numerous fiber-based quantum cryptography systems are commercially available.5–7 However, optical fiber quantum key distribution (QKD) is limited to a distance of approximately 100km due to transmission losses and nonlinear polarization in the optical fibers.1 Free-space QKD enables long-distance transmission of photons; therefore, ground-to-satellite quantum cryptography is an ideal application for global QKD.8 QKD systems use photon polarization states to transmit information. One of the drawbacks of polarization-based ground-to-satellite quantum cryptography is that the attitudes of the transmitter and the receiver rotate relative to each other. Therefore, it is necessary to continuously compensate for the resulting change in polarization direction.
The National Institute of Information and Communications Technology (NICT, formerly Communications Research Laboratory, or CRL) has developed a prototype free-space quantum cryptography system. This system uses weak coherent light5 with polarization modulation. NICT has also developed a polarization tracking method. Figures 1 and 2 show the configurations of the transmitter (Alice) and the receiver (Bob), respectively. The wavelength of the lasers for the single photon quantum channel is 0.86μm. A 1.5μm wavelength laser is used for the synchronization of the 0.8μm quantum photons and also for data transmission at 20MHz. The transmitter synchronizes the global positioning system (GPS) clock, generates weak coherent pulses, and produces the random signal for polarization modulation. A field programmable gate array (FPGA) performs signal processing. The receiver synchronizes data clock recovery, generates gate pulses for the single photon counting modules (SPCMs), and generates random signals for polarization modulation. After processing by the FPGA, the data is recorded on storage devices.
For polarization tracking, a waveform at the transmitter side modulates the polarization of the 1.5μm laser beam.9 The polarization axis of this beam is adjusted so that it is oriented along the same direction as the axes of the polarization bases of 0.8μm photons. A 1.5μm photodiode (PD) placed after the polarizer measures the received signal, as shown in Figure 2(a). The amplitude of the optical output signal increases to its maximum value at the minimum angular difference. The received signal has the same phase as that of the modulated waveform for the minimum angular difference. The waveforms of the output optical signal and the modulated waveform are inverted with the maximum angular difference. Thus, the polarization axis of the receiver can be adjusted to maximize or minimize the 1.5μm output signal.
We evaluated two devices for the polarization rotator. One, a half-wave plate (HWP) rotator, uses a mechanical rotation motor. The other device uses a polarization rotator with an electro-optic (EO) modulator. The polarizations for the 0.8 and 1.5μm signals can be rotated by the same HWP placed in front of the receiver. However, when using the EO modulator, the polarizations of both the 0.8 and 1.5μm signals are difficult to control.
In conclusion, we proposed a polarization tracking scheme that uses a polarization rotator. The polarization direction could be determined when two mobile terminals were moved relative to each other by using a 1.5μm reference polarization modulated signal. The application of this method did not require information on the attitude of the transmitter. Therefore, polarization tracking was necessary only at the receiver side. The use of quantum cryptography in mobile networks will increase the security of communications. In particular, future applications of quantum cryptography in long-distance transmission systems will be significant in space communications.
Morio Toyoshima joined the National Institute of Information and Communications Technology (NICT) in 1994. He was involved in the development of the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) at the Japan Aerospace Exploration Agency (JAXA) from 1999 to 2003. He spent one year as a guest scientist at Vienna University of Technology, Austria, in 2004. In 2006 he conducted ground-to-OICETS laser communication experiments at NICT. His research interests include laser beam propagation through atmospheric turbulence, optical space communications, and quantum cryptography.