Improving quantum key distribution by adaptive optics

Abstract

Sending a quantum secret through a free-space channel is challenging. This is because quantum optics is affected by atmospheric turbulence, which induces an inhomogeneous media between the sending and receiving ends. The wavefront of the signal is distorted after passing through the free-space channel. As a result, light beam spreading and wandering occur, and thus the signal-to-noise ratio is reduced. Adaptive optics have been used to solve these kinds of problems in astrophotography and classical optical communication. It attempts to compensate for wavefront aberrations by using either a deformable mirror or a spatial light modulator. However, it is very challenging to correct signals sent from a fast-moving object such as a low Earth orbit satellite to a ground station. This is because the motion of the object causes temporal anisoplanatism, which increases the phase difference between the beacon beam and the signal beam. This introduces errors in the AO correction.

In this thesis, I discuss a new AO setting in which the beacon beam is installed in front of the signal beam along the moving direction. This can effectively reduce the temporal angle due to the movement. As the beams are spatially multiplexed, the wavelengths of the beams can be the same, thus further reducing the chromatic errors in the system. By using a semi-empirical analysis, I calculate the decoy-state BB84 key rate and compare the scheme with other AO setups. The results show that in the daytime, the key rate of the systems using a pioneer beam can reach up to 3.79 times and 25 times of the key rate of systems that use pure time-division multiplexing with 1×HV and 2×HV profiles, respectively. For the 3×HV profile, the pioneer beam method even enables decoy-state BB84 QKD. I also study an alternative system, which upgrades the existing wavelength-division multiplexing systems with a pioneer beam. When the zenith angle is less than about 30◦ , the key rate of the alternative is at least 93.9% and 50.1% of the original pioneer beam proposal with 1×HV and 2×HV profiles, respectively. For the 3×HV profiles, the WDM systems are not able to transmit any secret key while the key rate of the alternative design can be non-zero.