Time-bin QKD

Team Members: Wang Ziyan, Wu Jinbin

Summary

The project is to implement time-bin QKD on the encoding and decoding ends.

List of Equipment

Equipment	Manufacturers	Quantities
Function Generator AFG1022	Tektronix, Inc	1
Oscilloscope HDO6104	Teledyne LeCroy	1
NIM FAN-OUT Module	S-Fifteen Instruments	1
NIM Anti-coincidence Module	S-Fifteen Instruments	2
NIM cables		multiples
BNC cables		multiples

Experiment Setup

1. Producing a short pulse

The shortest pulse that can be generated by the function generator is about 40 nanoseconds, which is too long for our purpose. To create a shorter pulse, we performed pulse shortening using delayed Anti-Coincidence Logic.

A short electrical pulse was first generated using function generator with high level as 0V and low level as -1.0V, following NIM (Nuclear Instrumentation Module) Standard.

The input pulse was first passed through a fanout module to produce two identical copies, B and B'. A relative time delay, $\Delta t$, was introduced between the two signals before they were sent to the signal and veto inputs of the anti-coincidence module. The anti-coincidence module produces an output only when B is present while the delayed signal B' is absent. This operation can be represented as

$V_{out}=B\wedge \overline{B^{\prime }}$.

When the leading edge of B arrives, the output becomes high. After a delay $\Delta t$, the leading edge of B' reaches the veto input and suppresses the output. Consequently, the original pulse is converted into a much shorter pulse whose width is approximately determined by the relative delay:

$T_{\mathrm{p}\mathrm{u}\mathrm{l}\mathrm{s}\mathrm{e}}\approx \Delta t$.

Thus, by controlling the delay between the two copies of the input signal, the output pulse width can be adjusted independently of the width of the original pulse. The short pulse is generated at the rising edge of the input signal.