Coherent Light Sources

Coherent Light Sources

Coherent light sources are lasers that emit light with a high degree of coherence, meaning the light waves are in phase both spatially and temporally. In the context of CV-QKD and quantum optics, coherent light sources have the following characteristics:

1. Coherence: The emitted light waves maintain a constant phase relationship over time and space. This coherence is essential for precise measurements and interference effects in quantum optics experiments. Beam Quality can be measured with it’s M^2 Factor.
2. Monochromatic: Coherent light sources typically emit light at a single wavelength or a very narrow range of wavelengths, providing a well-defined frequency.
3. Stable Phase and Amplitude: The stability of the phase and amplitude of the coherent light is crucial for accurate homodyne detection and other quantum measurements.
4. Noise characteristics: Phase noise, or random fluctuations in the phase of the light wave over time can have an impact in the coherence of a light wave, creating deviations in the expected phase relationship between successive wavefronts. This appears as a broadening or spreading of the spectral line around a carrier frequency, and is often caused by mechanical vibrations, temperature fluctuations and electrical noise. Amplitude Noise is random fluctuations in intensity or power of a light wave over time, which appears as side bands or fluctuations in the power spectrum. This is commonly caused by power supply variations, spontaneous emission noise, and instabilities in the gain medium of the laser. This generally reduces the SNR of a transmission. In terms of CV-QKD exchange Phase noise can lead to inaccuracies in the estimation of quantum states, and Amplitude noise affect the reliability of key exchange.
5. Laser Sources: Common types of coherent light sources used in CV-QKD include:
   • Diode Lasers: Compact and efficient, often used in telecommunications.
   • HeNe Lasers: Known for their high coherence and stability.
   • Fiber Lasers: Highly stable and compatible with fiber optic systems, making them suitable for CV-QKD over fiber networks.

References

M^2 Factor – M squared, laser beam, quality factor, beam divergence, caustic, ISO Standard 11146 (rp-photonics.com)

https://www.rp-photonics.com/coherence_length.html

https://www.rp-photonics.com/coherence.html