System and method for intensity monitoring
A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.
1. A detection system for monitoring the intensity of a stream of modulated pulses, the detection system comprising:
a pulse modulator configured to output the modulated pulses, comprising an input for receiving a control signal and an output for outputting the modulated pulses;
a controller configured to provide the control signal to the pulse modulator, the control signal controlling a level of modulation for each pulse exiting the pulse modulator; and
a detector configured to measure the intensity of the pulses in the stream of modulated pulses exiting the pulse modulator;
wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector;
wherein said controller is configured to output the gating signal to vary a gain of the detector with the control signal,
wherein the detector detects pulses with a selected modulation level.
2. A detection system according to claim 1 , wherein the controller is configured to group the pulses into frames with each frame comprising a plurality of pulses, the controller being configured to control the gain of the detector to detect pulses modulated by a selected modulation level in each frame.
3. A detection system according to claim 2 , wherein the controller is configured to control the pulse modulator to vary the modulation between J different levels where J is an integer of at least 2, the controller being configured to control the detector to measure over at least J frames, such that the gain of the detector is set to detect pulses of intensity level J for each of the J frames.
4. A detection system according to claim 2 , wherein an output of the detector is integrated over each frame.
5. A detection system according to claim 1 , wherein the detector is an avalanche photodiode.
6. A detection system according to claim 2 , wherein each of the frames comprises at least 10 2 pulses.
7. A detection system according to claim 1 , wherein the controller is configured to vary the control signal sent to the pulse modulator dependent on the measured intensities.
8. A sending unit for a quantum communication system, comprising a source of pulses and the detection system of claim 1 .
9. A sending unit according to claim 8 , wherein the pulse modulator is selected from a component configured to set a basis or bit value of the pulses, an intensity modulator and a variable intensity source of pulses.
10. A method of monitoring the intensity of a stream of modulated pulses, the method comprising:
modulating a stream of pulses at a pulse modulator;
outputting the stream of the modulated pulses from the pulse modulator;
generating a control signal at a controller;
providing the control signal to said pulse modulator to modulate pulses, the control signal controlling a level of modulation applied to each pulse by the pulse modulator;
generating a gating signal at the controller which varies a gain of a gated detector with the control signal, wherein pulses with a selected modulation level are detected at the gated detector; and
measuring an intensity of the modulated pulses exiting the pulse modulator using the gated detector.