Quantum Computing Series -1
Quantum Computing- The Physics behind it
Basics of the Physics required in QC!
Quantum Computing
Sounds fancy doesn’t it? This writer (being a physics major) felt crazy excited to dive into this field. I really hope I can convey my same, excitement to y’all too🙃
Let’s begin with what is quantum computing?
So clearly, to understand what quantum computing is, it is important to know quantum theory.
Hmm.
Well, this writers logic is simple. Dive head first, so you can’t take any backsies. Let’s do exactly that.
Polarization of light
Wait, You just said you will be talking about Quantum, why are you bringing highschool trauma back?
You know what they say, trauma builds character 😂 (jkjk)
We will need few basics of highschool physics to truly understand the concept of Quantum computing. Don’t worry if you don’t remember them, this blog will jog your memory.
Polarization of light is studied because it’s a quantum system with everyday experiences. Egs — Polarised sunglasses reduce glare, Polarised lensfilters reduce reflections in pictures.
Polarisation
A light wave that is vibrating in more than one plane is known as unpolarized light. All normal sources of light , tubelights to torches emit unpolarised light. Polarized waves are light waves in which the vibrations occur in a single plane. Plane polarized light consists of waves in which the direction of vibration is the same for all waves. A polarizing filter converts unpolarised light into polarised light.
The below images will help in visualisation —
A polarising filter only allows the light in the plane parallel to its own. For example, if a polarising filters angle of polarisation is 45 degrees, It only allows those waves which are in 45 degree to the filter. Imagine a polarising filter to be like a sieve.
Quantum Behaviour of Polarizers.
- Deterministic
Deterministic behavior is the phenomenon which will definitely happen or definitely not happen. There is no in between.
If a photon ( here we will consider photon instead of wave as it makes relating quantum behavior easier — imagine photons to be small packets of energy ) is aligned to the polarising angle of the polarising filter it will definitely pass through.
If a photon and filter are anti aligned ( 90 degrees to eachother), it will definitely not pass through.
2. Probabilitistic
Photon and filter at an angle between 0 and 90. Then the photon may or may not pass through.
Probability of pass through is between 0 and 1.
Polarising filters are not the only one used for the following setup.
We can use calcite crystals too
In a calcite crystal, all photons pass through but light beam is split into two different light — one stream polarised to be aligned. Another stream polarised to be anti-aligned.
So when I direct light aligned with the calcite through the crystal, all the photons appear in the aligned stream and none in the anti-aligned stream. This is deterministic behaviour of photons
Now if light is neither aligned or anti-aligned, some photons appear in the aligned beam and rest in anti-aligned beam. This is probabilistic behaviour.
When a single photon of unknown polarisation is sent through, it will exit either aligned or anti-aligned , all information about its polarisation before entering the calcite is LOST.
Now,
Information (the angle of polarisation) of a single photon is a real number, after passing through a calcite crystal, it converts into single bit of information ( either aligned or anti-aligned) or binary form. Here if this information are to be stored, space is reduced. Drastically.
But one major negative is that information is lost, the info about its state before entering the calcite is lost.
This is one very important thing to be noted in quantum science.
Every measurement of a quantum system can change the state of the system, hence the previous information seems to be lost.
In a nutshell, measurement changes the state of system being measured.