# Quantum Computing: The Qubits

Post By Saksham Garg on 21-July-2015

In today’s world, the computers keep getting smaller as well as faster. From the first computers which fitted in whole rooms to computers today which fit in our hands, technology has progressed immensely. What remains same though, is the basic working principle – computers are electronic devices which work using digital information. That is they work with 1’s and 0’s. Currently China's supercomputer is the world's fastest: it can run at a sustained 2.5 petaflops (a petaflop is a thousand trillion floating point operations per second) thanks to its 186,368 cores and 229,376 GB of RAM. And you thought your computer was fast with a 4 GB RAM! But then again, supercomputers have different objectives than your home computer. Still, they play with binary numbers to do all their processing.

So why did we need a different kind of computer to being with? Is there a limit to the work the modern day computer can perform? What alternative does Quantum Computers provide? How do they work? Are they a replacement for modern day computers? Is there life outside Earth? Let’s answer all these questions, except the last one (for now), definitely not in the above sequence.

**What is Quantum Computing?**

Quantum computing is essentially harnessing and exploiting the amazing laws of quantum mechanics to process information. A traditional computer uses long strings of “bits,” which encode either a zero or a one. A quantum computer, on the other hand, uses quantum bits, or ** qubits**. A qubit is a quantum system that encodes the zero and the one into two distinguishable quantum states. Thus instead of being either a 0 or 1, a qubit is a 0 AND 1 at the same time. Confusing? Thank the quantum part in it for that.

So what are qubits? We know that electrons have spin and thus act as small bar magnets. When a magnetic field is applied, they align themselves parallel to it, and this is the lowest energy state or the 0 state.

If they align themselves anti parallel to the magnetic field, they are in the highest energy state or the 1 state.

Before we measure a qubit, it’ll be present in spin up (1) or spin down (0). But when we measure it, the qubit exists in what is known as a quantum superposition. Superposition is essentially the ability of a quantum system to be in multiple states at the same time — that is, something can be “here” and “there,” or “up” and “down” at the same time.

Entanglement is an extremely strong correlation that exists between quantum particles — so strong, in fact, that two or more quantum particles can be inextricably linked in perfect unison, even if separated by great distances, say they opposite ends of the universe. Superposition and Entanglement are two parameters which help us explain qubits.

Let us take two classical bits (0 and 1), which gives us four numbers-

00

01

10

11

To determine any number, I just need two ‘information’, the first bit and the second. But with qubits, I need four of them because of superposition. It means one will have to give the value of the four coefficients to give full information about the number. As a result, for the same number I can store more information using qubits.

To be more precise, two qubits actually contain 4 bits of information. If we make 3 different spins, there will be 8 different states and thus 8 bits of information, whereas there would be only 3 corresponding classical bits. On generalising, we find out that the equivalent amount of **information contained by ‘N’ qubits is 2 ^{N }classical bits. **Doesn’t sound that impressive yet? Here’s some interesting food for thought – suppose we have 300 of these qubits. This is equivalent to 2

^{300 }classical bits – as many as the number of particles in the observable universe!