Introduction to Quantum Technologies

Introduction to Quantum

Quantum physics relates to laws governing matter at the fundamental level, the atomic world.
Classical physics fails to explain realities as we go deeper and deeper at the atomic level.
Quantum mechanics evolved to understand the world of atoms, electrons, quanta of light etc.

Failure of classical physics to explain the blackbody radiation.
According to classical physics the amount of energy in a radiation increases if you increase its intensity. However, the observations found that it is not the intensity that determines energy level but frequency of light/radiation. This is called Plank's law.
In addition, the amount of energy occurred in discreet lumps or packets or bags or quanta (origin of the word quantum). Accordingly, light behaves like particles.
The quantum nature of light was for the 1st time demonstrated by the famous photo-electric effect of Einstein.
When you shine a beam of light on metal surface you could eject/excite an electron from its place.
The amount of energy in light packets determines its ability to eject electrons. This is what differentiates conductors, semi-conductors, insulators. The amount of energy required to excite an electron is very high in case of insulator and relatively lower in semi-conductors.
With Plank's law and Einstein's photo-electric effect quantum physics took birth.

The wave nature of light results in interference pattern as shown in the figure when passed through two small openings separated by a distance(slits). Alternate light bands and can be seen indicating constructive and destructive interference.
If light behaves like particles expected pattern on the screen is as shown in figure 2(a). However, pattern in the Fig 2(b) is what we observed. Why is it so? To understand this let us look at some quantum mechanical principles.

Interference pattern is made by a flash of light which is made of trillions and trillions of photons.
What if you emit a single photon? Where would find the photon on the screen?
Turns out you cannot precisely predict the position of this photon until you measure it.
The photon could be anywhere on the screen. You can only find out the probability of the photon's position. This ability of the photon to be anywhere at the same time is called superposition.
In other words, the ability of the photon to be I composite states (many states at the same time with varying probability) is called superposition. Superposition is the principle behind quantum computers.

Further this applies to all properties of photons not just the path it takes. Eg: polarization states, energy, momentum (its properties are not important to future civil servants)
Not only that this applies to all fundamental particles be it photon, electron, quark, neutrino etc.

Schrodinger's wave equation is akin to newton's laws but that in the quantum world.
It explains behaviour of objects (their properties) in the quantum world.

In the double-slit experiment consider trillions of photons are working in tandem to make the interference pattern as shown in the figure.
In other words ,they are correlated to each other.
The ability of photons to be correlated to each other to act in tandem is called entanglement.
Illustration: Say out of the 1000 photons I had some way to hold 1 of the 1000 photons and let go of other 999. If I want to predict the position of the last photon, I have to know where on the screen 999 photons fell. (because 1000 photons put together have to make the same interference pattern) So it means that where the last photon would hit depends on where the 1st photon or 5th photon or 999th photon fell. This property of the photons is called entanglement.
Like superposition, entanglement also applies to all properties of photons.
It also applies to all fundamental particles like electrons, neutrinos, muons etc.
Einstein called this weird property 'spooky action at a distance'.
Entanglement is very important in quantum cryptography and quantum communication.

According to uncertainty principle you cannot measure two related characteristics like velocity and position of a quantum particle at the same time with precision.
Further the error in measurement is distributed between the two characteristics that is being measured. (it’s a zero-sum game)
If you want to measure one with accuracy, the precision with which you measure the other takes a hit. This is called squeezed state.
In other words, you squeeze the uncertainty of measurement of one characteristic at the cost of the other.
Squeezed states have very important application in next-gen sensing and measurement.