One Way Weak Pulse System

Toshiba Research Europe Ltd.

The QKD-system by Toshiba Research Europe Ltd. is a one-way fiber-optic, decoy-state one with phase encoding [1]. It employs a decoy protocol which involves one weak decoy pulse and a vacuum pulse.

The system is schematically shown in Figure 1. It uses two asymmetric Mach-Zender interferometers for encoding and decoding. During development, the sending (Alice) and receiving (Bob) units are connected with a ~25.0km fibre spool. Both the signal and decoy pulses are generated by a 1.55 Ám pulsed laser diode operating at 8.333MHz. The pulses are modulated with an intensity modulator to produce the desired ratio of signal pulse strength to decoy-pulse strength, and are then strongly attenuated to the desired level by an attenuator before leaving Alice's apparatus. Synchronization is realized by multiplexing a 1.3 Ám clock signal with the quantum signals so that clock and quantum signals can be sent through the same fibre. An active stabilization technique (not shown in Figure 1) is used to ensure continuous operation. Both Alice's and Bob's electronics and optics were housed in 19 inch rack units.



Figure 1: Schematic showing the fibre optic decoy-state quantum key distribution system with active phase & polarisation compensation. LD1: signal laser diode at 1.55 Ám, LD2: clock laser diode at 1.3 Ám, PC: polarization controller, IM: intensity modulator, Attn: attenuator, F: phase modulator, ?: fibre stretcher, D0 & D1: Bob's single photon InGaAs avalanche photodiodes, WDM: wavelength division multiplexer;

Figure 2 shows photographs of the fully integrated devices.



Figure 2: The fully integrated devices.

1. J. F. Dynes, Z. L. Yuan, A. W. Sharpe, A. J. Shields, Optics Express, 15, 8465-8471 (2007