Continuous variable quantum key distribution with locally local oscillator

Abstract

Quantum cryptography key distribution (QKD) is a technique for distributing a completely secure key using quantum mechanics such as Heisenberg's uncertainty principle and the impossibility of duplicating quantum states. Among them, the continuous variable quantum cryptographic key distribution (CV QKD) protocol encodes the key information into the amplitude and phase of the quantum state. CV QKD utilizes homodyne detection which requires phase-synchronized local oscillator. In general, local oscillator pulses are transmitted along with a quantum state having information. Here, the role of the local oscillator pulses is to provide the phase reference of a transmitter (Alice) to a receiver (Bob). During transmission, phase between the quantum state and the local oscillator can be varied by a channel or device imperfections, which provides corrupted information to Bob. This can severely degrade the performance of a CVQKD system. This paper presents two methods for synchronizing the phase between the local oscillator and the data signal, which affect the performance of CV QKD. Both methods generate a local oscillator pulse at the receiving end and synchronize the phase difference with the data signal through post-processing. As a first method, we propose a method to synchronize the phase by homodyne detection using pilot pulse of a specific pattern. It eliminates the requirement for phase locking for an interferometer, while the previous works required critically precise phase control between I and Q detection paths. For validation of the proposed scheme, we conduct experiments using a high-performance laser and a low-performance telecom-grade laser with the proposed scheme. As a second method, we proposed a phase synchronization method using multidimensional reconciliation (MDR). The above method also has an advantage that stabilization of the interferometer is unnecessary, and there is no security threat that arises because the pilot pulse is not used. The second method also shows that the key can be generated through experiments and simulations.