# Quantum Computing Principles in a Nutshell

Khyatee Atolia

|

30 June 2022

|

## What are Classical Computers?

Computing itself is the process of using computers, traditional classical computers to complete a particular task, usually involving mathematical calculations and logic. Classical computers here refer to the computers that we use on a daily basis, all the way up to the mainframe and supercomputers. Classical computers work, simply put, on the Von Neumann architecture, and are built using special and general-purpose resistors and binary logic gates, like NAND or NOR gates. These binary logic gates use 0s or 1s (a bit) to crunch numbers and make “decisions”. A quantum computer, however, has a third state- it can be a 0 AND 1 at the same time.

## What are Quantum Computers?

Quantum computers use the principles of quantum mechanics as the basis of their calculations. In classical computing, a bit is a term to represent information by computers. Quantum computing uses quantum bits or qubits for a unit of memory. Qubits are comprised of a two-state quantum-mechanical system. In the real world where bits are perceived as "0" o "1," for a system of two bits, only one of the four possible states can exist at any time in space. However, in a quantum superposition state, all four of the possible states can co-exist in time and space simultaneously.

## How do They do This?

The 3 major principles used by quantum computers are:

### 1. Superposition:

This is the ability of a quantum system to be in multiple states simultaneously as explained above. A famous example of superposition or uncertainty is Schrodinger's cat thought experiment- there is a cat in a box. In that box, there is also a vial of a radioactive substance that has an exactly 50% chance of decaying and killing the cat. The cat in the box, hence, is thought to be dead AND alive unless the box opens. Another go-to example of superposition is the flip of a coin, which consistently lands as heads or tails, one in two. However, when that coin is spinning in the air, it is both heads and tails and until it lands, heads and tails simultaneously. Before measurement, the electron exists in quantum superposition. Upon measurement, however, the wavefunction of the electron collapses, and the electron assumes a definite state.

### 2. Entanglement:

This quantum property is taking objects and connecting them by permanently entangling them together. This has been explained as being able to pass information faster than the speed of light. Think of it like going to a restaurant and ordering two pizzas that come in identical boxes: if one of them is pepperoni, the other box must contain your second order, mushrooms. Now, take one of the boxes to the other end of the galaxy and open it. If the box you opened was a mushroom, then you know, immediately, that the pizza on the other end of the galaxy is pepperoni. This is entanglement. Electrons, too, have a property called spin, which can be either up or down, so interference can take place. When adding an additional qubit to a quantum computer, a 50-qubit quantum machine can examine two to the power of 50 states simultaneously. The increase in power plus the entanglement of qubits allows quantum computers to solve problems efficiently, finding a solution faster, with many fewer calculations.

### 3. Interference:

Interference is a phenomenon observed in all waves and particles with a wavelength. In this, two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. This only happens when waves are coherent or have the same or very similar frequencies. When an electron is passed through a double slit, it behaves as a wave and passes through both the slits at the same time. The two waves hence emerging from the different slits have a path difference, which leads to either constructive or destructive interference. Where the “phase” is the same, i.e. the troughs align with the troughs and the crests align with the crests, the resultant amplitude is greater.

Interference can be used to control quantum states and amplify the signals leading toward the right answer while canceling signals leading to the wrong answer.

These are the basic principles used in quantum computing which significantly increase the amount of data that can be simultaneously analyzed, overtaking classical computers by a large margin. While classical computers crunch data sequentially, a quantum computer does not have to wait for one process to end before it can begin another, it can do them at the same time.

Imagine you had lots of doors that were all locked except for one, and you needed to find out which one was open. A traditional computer would keep trying each door, one after the other, until it found the one unlocked. It might take five minutes to a million years, depending on how many doors there were, and the possible combinations. But a quantum computer could try all the doors at once, reducing the processing time by 158 million times. Quantum computers open one more door- into the future of computing and technology.