Understanding Quantum Computers comparing them to Classical Computers


Varun Rustomji

3 years ago | 5 min read


Computers use bits to process data which can either be 0 or 1. Bits are made from transistors that have electric current flowing through them. They work like a switch, they can either be On or Off.

Quantum computers are made up of quantum bits or qubits. A qubit can be made from anything that exhibits quantum behaviour like an electron, atom or molecule in a controlled environment. What makes a qubit unique is that it can be in a state of 0, 1 or a special in-between state where it can be 0 and 1 simultaneously.

Modified from IBM
Modified from IBM

What makes Qubits really unique is that they follow the laws that govern quantum particles like superposition and entanglement. We will see how these properties can help solve problems that classical computers can’t.


Superposition is essentially the ability of a quantum system, qubits in case of a quantum computer, to be in multiple states at the same time. This is counter intuitive because in our normal computers, a bit can be 0 or 1 however superposition allows the qubit to be both 0 and 1 simultaneously. An important attribute of superposition is that quantum states when measured, immediately collapses to 0 or 1.

We can think about it simply by considering a coin. When we flip a coin, the final result at the end of the flip is either heads or tails however when the coin is in the air, it has a chance of being heads or tails. Until you measure it by stopping the coin, it can be either. Superposition is like a flipping coin.

Superposition of quantum states (0 and 1 simultaneously) collapse into a classical state (either 0 or 1) upon measurement.

Source: IBM Research
Source: IBM Research


Entanglement is an extremely strong correlation that exists between quantum particles. In case of quantum computers, when two qubits interact with each other they get entangled.

This means that knowing the state of one qubit will give you precise informations about the state of the other, regardless of the distance between them. That basically means that two quantum particles will depend on each other even if there are placed on the extreme ends of the universe.

A simpler way to think about it would be to consider flipping two coins. Generally flipping one coin will have no effect on the result of the other, however in case of entanglement, flipping one coin and getting heads guarantees that flipping the other coin will also produce heads even if the coins are on two different continents.

If you are finding it difficult to wrap your head around this phenomenon, you are not alone. Einstein described this phenomenon as spooky action at a distance.

Source: IBM Research
Source: IBM Research

How do quantum computers harness the power of entanglement and superposition?

  1. One of the applications in quantum computing are related to modelling molecules. Complex molecules can not be modelled on a classical computer because electrons can exist in multiple states at once. We could use probabilities to model this but the numbers soon become impossible for classical computers. Auto manufacturers are using quantum computers to simulate the chemical composition of electrical batteries to improve their performance. Pharmaceutical companies are leveraging them to analyze and compare compounds that could lead to the creation of new drugs.
  2. Another application is to solve optimization problems that can not be solved by classical computers due to a large number of variables or large number of possible outcomes. Financial services companies are using them to optimize and manage risk in their portfolios as well as speeding up classical trading algorithms. Aviation and Transportation companies are using them for route optimization and limiting congestion.
Source: ColdFusion
Source: ColdFusion

Building a quantum computer

So now that we understand the fundamental properties that distinguish quantum computers from classical computers, let’s understand some of the obstacles that stand in the way of building quantum computers.

A quantum computer at its core can be thought of as a specialized co-processor attached to a classical computer for performing certain types of computations, similar to a GPU that is a co-processor for parallel calculations in computer graphics.

The hurdles in building a quantum computer

The basic building block for a quantum computer is a Qubit. These qubits could be made of photons, atoms, electrons, molecules or any other particle that obeys the laws of quantum physics.

Manipulating qubits is tricky because even the slightest disturbance can cause them to fall out of their quantum state. This phenomenon is known as decoherence. The field of quantum error correction is working on ways to combat decoherence and other errors that may occur.

For qubits to express their quantum states, they need to be free of all radiations and kept at a temperature of almost absolute zero.

The quantum computers that exist today

IBM is one of the prominent companies doing research in the field of quantum computing. Currently they provide quantum computing capabilities through the cloud-based IBM Q Experience, for researchers and industry professionals. More information can be found here.

D-wave is another company that is building quantum annealing architecture which is not as powerful as the universal quantum computers and can only be used for solving optimization problems. They have a platform called Leap that allows people to run quantum algorithms and learn about them. More information can be found here.

Rigetti is another company that provides access to quantum computing resources. They Forest SDK allows users to spin up a quantum virtual machine (QVM) on their local machines. It is the best way to get started with quantum programming. You can find the documentation here.

IBM Q’s System One | Credit : IBM
IBM Q’s System One | Credit : IBM

Google made an announcement in 2019 that it had achieved quantum supremacy. This basically meant that their 53-qubit Sycamore processor was able to perform a calculation in 200 seconds that would have taken the world’s most powerful supercomputer 10,000 years. This indicates the type of calculation that would be impossible for a classical computer to compute.

Source: Google


As we can see from the section above, even the most powerful supercomputer today comprises only of 53 qubits. It could take a few years for quantum computers to realize their full potential. If these technologies live up to their promise, they could potentially transform industries and enhance our understanding of the physical world as we know it today.


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Varun Rustomji







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