Demystifying Opto-Quantum Communication — PART I

Hi everyone. After a long time I am going to start a series on Opto-Quantum Communication where I will start from the basics and then go to deeper concepts with also state of the art research.

This blog series is part of my self-study which I am doing to understand photonics and it’s integration with opto-quantum communication. So after reading many things I realized if I need to actually get complete grasp of photonic quantum computing then I need to understand optics first at a macroscopic level, then at a microscopic level.

I am planning to complete this series within 8 to 10 post but it may go longer. Today I will start with the fundamentals of optical communication which is at a macroscopic level of photons.

In any communication system let it be standard radio communication or optical communication there is always a transmitter which will encode our data in the form of 0’s and 1’s with either of the following ways:

1. Amplitude difference
2. Phase difference
3. Combination of both phase and amplitude difference.

Now let’s look into a source aka transmitter in optical communication. Some of the critical parameter for choosing an ideal optical source are

1. Emission Wavelength (because different have different level of attenuation)
2. Spectral Width (whether it is a broadband emitter or a tunable emitter),
3. Modulation Capabilities
4. Ability to interface with optical fiber (the channel by which our information will be sent)
5. Finally the compactness, reliability and the price.

So the possible sources are LED and Laser. Let’s deep dive into few concepts before we go further:

Absorption: An external photon is absorbed to generate electron-hole pair. Ex: photodetectors

Spontaneous Emission: A current is injected and photon is generated. Eg: LED

Stimulated Emission: A current is injected to generate charge carriers and then external photon is used to generate more photons. Ex: Laser Diode

Simple Diagram to show the above process (taken from web)

Note: To create any type of emission we need to have excess electrons in the conduction band or excess holes in the valence band popularly known as population inversion and to have population inversion we need to have a forward bias current. The fundamental difference between spontaneous and stimulated emission is in spontaneous emission photon are generated at a random phase but in stimulated emission photons can be in phase. For example, in stimulated emission a photon is used to generate multiple photons simultaneously which can be assumed that these photons are in phase.

Now you may be thinking these are basic physics, how they are related to Optical Communication.

So please think a simple red led, you connect a switch to it and do on or off. Now suppose you have a black-box which can see observe the timing of your on/off. And boom you have a transmitter because this on/off resembles 1 and 0. Now the question arise how do we encode and decode our data and we will come to it in the future blogs.

Few important points and scenarios:

Does all electron-hole recombination generate photons?

No, all of them does not generate photons. The ones which generate photons are known as radioactive recombination. So what happens to the E-H recombination which does not generate photon, so basically it generates phonons and it is most due to defect states and auger recombination. And this gives us an important factor i.e. internal quantum efficiency which is the ratio of radioactive recombination to total recombination.

Now a question for the reader why it is not possible to generate photons in indirect band gap materials? (Please put your answer in the comment box)

I think that’s it for today’s, the next blog will be on complete analysis of laser and led.

I am a software consultant who believes in interdisciplinary research and loves to integrate various domain to others. Feel free to connect me on Linkedin and all type of reviews are more than welcome.