Hello all,
In CV QKD, in general, there is two noise sources in the system. the shot-noise, which is the fundamental noise of the signal and arises from quantization of the electromagnetic field, and the excess noise, that includes all other noises present in the system and also the noise introduced by the eavesdropper. In CV QKD, in order to determine whether the eavesdropper detected the signal or not, it is important that the detector able to distinguishes the shot noise contribution to the total noise from the excess noise. To do so, it is proposed to utilized shot noise limited homodyne detection. Why?
-What is the different between the shot noise limited homodyne detector and usual homodyne detectors?
-Is it possible to consider a usual homodyne detector as a shot-noise limited one in special conditions? If yes, what is that conditions?
Bests
If N is the number of homodyne reference photons absrobed and detected in the photodetector in some given time interval , they typically give sigma noise of sqrt(N). That is called shot noise and comes from approx Poissonian statistics of such detections. The reference is usually much stronger than signal, therefore reference shot noise dominates over it. However, often if using detectors with no internal amplifications (e.g. PIN photodiodes) the electric noise of the preamplifier may prevail over shot noise. On another hand if using Photomultiplier tubes , SiPm detectors the shot noise likely to previal and to define noise floor of the system.
Thank you Nikolay Pavlov for your comprehensive answer.
Is it correct to say that: the electronic noise of a balance homodyne detector (BHD) relates to its NEP parameter (noise equivalent power) and its detection bandwidth B. So, it does not varies with the LO power. On the other hand, as you mentioned above, the shot noise scales with the LO power. Therefore, for small LO powers, the total BHD noise dominates with the electronic noise (which is independent of the LO power), and for large LO powers, the shot noise dominates the total noise of the BHD.
I saw in some references that a BHD is called shot-noise limited in LO power ranges in which the shot noise dominates the total noise of the detector, and therefore, the variance of the output voltage of BHD scales linearly with the LO power. For this reason, the definition of shot noise limited detection is related to the level of the electronic noise. as the electronic noise of the BHD is smaller, the the range over which the detector is shot noise limited becomes larger.
Yes. I would just add that electronic preamplifier needs be a bit of careful design to make sure its noise does not increase when taking significant photodetectors currents caused by LO reference. Also, if LO happened to be noisy = modulated by some non-fundamental noise that may contribute to the 'homodyne noise'. So it should be desined well and charcterized, in addition such non-fundamental noise is suppresed by balanced detection.
https://123.physics.ucdavis.edu/shot_files/ShotNoise.pdf
https://en.wikipedia.org/wiki/Shot_noise
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