A Brief Introduction to Quantum Computing

by Shivani Srinivasan

First, a story about quantum computing, kind of…

A king had once asked the inventor of chess to choose a reward for his creativity. The inventor humbly requested that one grain of rice be placed on the first tile, two on the second, four on the third, eight on the fourth and so on. The king laughed at the easy task without knowing that by the 64th tile he would have promised more than any kingdom could produce. The one grain with which he started would turn into 9,223,372,036,854,775,808 grains!!

Wait, but what does this even have to do with quantum computing?

Quantum computers harness the power of exponentiality allowing them to be so much more powerful than quantum computers. They do this by embracing the weird but amazing laws of quantum physics discussed later in this article.

Less is Moore…(haha, get it?😂)

Moore’s Law (transistors vs. years)

From the development of the first computer (which was about 1800 square feet!) to our iPhones today, there has been an exponential pattern in the change in size and power.

Moore’s Law essentially states that the number of transistors that can be integrated into a silicon microchip doubles every two years. This means that the size of transistors must keep decreasing to be able to exponentially fit more in a chip.

So, as the transistors keep getting smaller and smaller, at one point they end up storing information in an atomic level using tiny particles like photons or atoms. Quantum means small and at the atomic level quantum mechanics literally overwrite everything we already know even Moore’s law too!

The Laws of Quantum Physics

1. Superposition:

A classical computer transports and supplies information in the form of bits which can either be in a form of a 0 or a 1. Different arrangements of 1’s and 0’s are used to represent data in a computer.

On the contrary, a quantum computer uses qubits (qubit = quantum + bit) to supply and transport information. Qubits can be an electron, photon or the nucleus of a phosphorus atom.

What sets qubits apart is their ability to be 0 and 1 at the same time because of a quantum mechanic: superposition.

Note that the superposition only exists until we measure the qubit. When we measure it, it is forced to collapse to one state. It has a 50% chance of being a 0 and a 50% chance of being a one. Measurement of qubits is done by observing their spin, rotation or direction of the magnetic field.

Superposition: A qubit exists as a 0 and 1

When there is one qubit, it exists is a possibility of 2 states. But a complex system of qubits with more than one qubit exists in an exponentially increases state. For example, 5 qubits can be in a superposition of 36 and 10 qubits can be in a superposition of 1024 states! This exponentiality ties back to the story of the chess inventor because with about 50 qubits quantum supremacy can be reached; when a quantum computer is more powerful than a classical computer.

2. Entanglement

When two qubits interact physically, they form a connection and can be entangled. So, they are correlated and any action performed on one instantaneously affects the other. If one qubit of the entangled pair is measured as a 0, we can say that the other qubit must be a 1. Entanglement works from any distance, even one end of space to another. Einstein had called it a “spooky action at a distance.”


A combination of superposition and entanglement allow quantum computers to powerfully process huge amounts of data in a really short time. Recently Google has achieved quantum supremacy; a quantum solved a problem faster than a classical computer. A Google-designed quantum processor called Sycamore completed a task in 200 seconds that, by Google’s estimate, would take 10,000 years on the world’s fastest supercomputer.

To explain why a quantum computer may be faster, imagine a maze. While a classical computer would have to go through one path at a time to find the way out, the quantum computer can go through all of the possibilities at the same time. This is because qubits can be in multiple states at the same time.

The red line is a classical computer trying out all the paths, and the green line is by qubits which instantaneously choose the optimal path.

The red line is a classical computer trying out all the paths, and the green line is by qubits which instantaneously choose the optimal path.

The limitations of quantum computing

Although quantum computers are really powerful, one shortcoming is decoherence. Decoherence is the inability of a qubit to stay in the quantum system. This can happen by even the smallest disturbance from outside the system including radiation, light, sound, vibrations, heat, magnetic fields or even the act of measuring a qubit.

We can think of this as balancing a spinning coin on a ledge. It keeps spinning in a superposition of values. If it is pushed by an external force, then the coin falls down with a definite state of heads or tails. In a quantum computer, the qubit collapses to either a 0 or 1. Eventually, this affects processing and ultimately, the output of the computer.

To prevent decoherence, qubits are stored in a really cold place of about -273 degrees Celcius.

What a quantum computer actually looks like. 😍

Applications of quantum computing
So why should quantum compting even matter?

It matters because it plays a huge role in optimizations problems. These are problems where there are many possibilities of solution but the most optimal solution must be found.

Application: Drug discovery- ProteinQure

A Toronto based start-up that uses quantum computing to speed up protein-modeling, accelerating drug design and new cures for diseases.

The process of protein modelling takes months. Starting from prototyping a protein structure to testing it to see if it works and restarting the whole design if the structure is faulty it consumes a lot of time and money.

But luckily, ProteinQure’s quantum-powered solutions has the potential to completely disrupt the way that protein modelling happens now. Quantum algorithms are being created so it will be possible to digitally model protein folding. A quantum computer rapidly executes the algorithms for protein prototyping.

Application: Quantum cryptography

Quantum computers could go through all possible combinations of digits rapidly until they find a password. This makes the current password system completely obsolete.

With all passwords breakable does that mean we have no more security? No, because while the old system might be brought down, quantum computing holds the promise of secure communications channels for key distribution. The quantum key distribution is safe because of the characteristic of qubits losing their properties when they are observed. This means if any hacker tries to read the key they would change its properties.

Quantum computers could disrupt many processes making them happen exponentially faster including traffic control, weather prediction and complementing AI. Many companies are already embracing the change this technology could bring like Google, Rigetti, Xanadu and IBM.

5 Key Takeaways about Quantum Computing

  1. Transistors keep getting smaller and smaller, at one point they end up storing information in an atomic level using tiny particles like photons or atoms. This is the basis of quantum computing.
  2. Qubits are used to store and transport information
  3. Qubits harness quantum mechanics including superposition and entanglement.
  4. Qubits can be in multiple states at the same time allowing them to be exponentially faster
  5. Decoherence is the inability of a qubit to stay in the quantum system. This is one of the biggest challenges in this technology.

Read More Articles by Shrivani on Medium


Author: TKS