A Beginner’s Guide to Quantum Computing

Quantum computing is in the news more and more as the technology evolves and scales. The day is fast approaching when their impact will be widespread and major new discoveries will be made. Quantum computers are an entirely new technology and instead of transistors and bits, quantum computers manipulate quantum particles. Things like captured spinning electrons and photons of light are used to form quantum bits or ‘qubits’ for short. It sounds pretty amazing and it is. But why bother going to all these lengths when we already have pretty good computers?

Classical Computers

Computers commonplace now are referred to as classical computers. They typically have billions of transistors sitting on a chip called a Central Processing Unit or CPU. Transistors are used to return a binary outcome such as a 0 or 1 or true/false and this value is called a binary digit or ‘bit’ for short. With billions of bits all working together, computers are able to achieve what we see today. You’re almost certainly reading this on a computer. Be it a desktop, laptop, phone or TV, they’re all computers in one form or another. All built with silicon processing chips etched with transistors returning bits.

Unfortunately, classical computers are limited on at least two fronts. The first issue is that we’re starting to hit a physical boundary in terms of transistor size. Classical processors are built by etching transistors into wafers of silicon with current transistors now as small as 7 nanometres (nm). With an atom of silicon being only 0.2 nm, transistors won’t be able to be shrunk down much further. In fact, transistors are getting so small that they’re not able to securely hold the electron in the transistor. This is something known as quantum tunnelling. Instead of staying in the intended logic gate, the electron continues through making it impossible for the transistors to have an off state. Just to put these numbers into perspective, a sheet of paper is about 100,000 nm thick so would hold over 14,000 transistors across its edge!

The second problem with classical computers is that there are some problems they’re just not good at solving. Difficult problems include optimisation problems, factorisation and simulating molecule interactions to name a few.

Silicon CPU
Close up of a silicon CPU

Quantum Computers

If humanity wishes to progress further along the road of technological advancement the need is clearly there for a new approach to computing. Cue quantum computers. They will be able to solve a lot of problems that classical computers simply cannot. They’re excellent simulators and will no doubt usher in many new advances. Quantum computers gain their enormous power from two quantum state properties
called superposition and entanglement which are not possible in classical computers.

A Quantum Computer - Source: IBM Research
A Quantum Computer – Source: IBM Research

Superposition

Just like the bits in classical computers, qubits can only return a 0 or 1 answer. But crucially they can also be in a third unmeasured state called superposition which is a combination of the 0 and 1 states. This is one of the main advantages a quantum computer has.

Superposition sounds pretty complicated but an easy way to start to understand it is by considering a coin toss. When the coin lands its either heads or tails but when its spinning in the air what is it? When it’s spinning in the air it’s kind of in a combination (superposition) of heads and tails but we won’t know the outcome until the coin lands and the outcome is observed. Quantum superposition is much more complicated. For now, the coin toss example nicely illustrates how a system can temporarily be in a third hybrid state while still only having a binary outcome of either heads or tails.

Quantum Computers typically measure the spin direction of an electron to determine whether the qubit is a 0 or 1, upward spin indicates a 0 and downward spin indicates a 1. For the coin, before it lands all we can do is assign probabilities as to the outcome e.g. a 50% chance of heads and a 50% chance of tails. An electron in superposition is similar but way more versatile as we can manipulate the spin angle and phase to change the likelihood of getting a 0 or a 1 when measured.  

Now, as a qubit can be in a superposition of both states, we can design software to test out both the true and false cases at the same time. Crucially, this changes everything because when you have many true or false decisions to make you will no longer have to cycle through them one by one. Now you can assign probabilities to each state and then simulate all the combinations in one go!

Quantum Entanglement

Entanglement is a second property of qubits which give quantum computers their power. When two qubits are entangled and the state of the first qubit is measured, we can then infer the state of the second qubit. For example, if one entangled qubit is measured as 0 we may then automatically know that the other qubit will show a particular value if measured in the same way. Astoundingly, this even holds true regardless of distance, so the second qubit could be on the other side of the galaxy and the outcomes would still be correlated. Einstein famously referred to this as ‘spooky action at a distance’.

Not only is the entanglement effect independent of distance but it is also instantaneous. You immediately know the state of the second qubit after having measured the first. You may think that this could lead to new instantaneous communication capabilities and it may. However, researchers must first find a way to work around the fact that the measurement of the first qubit always results in a random answer.

The above is a very simplified explanation and barely scratches the surface of Quantum Computing but it should give you a rough idea why Quantum Computing is different and why it will be a game changer when the technology matures.

See IBM’s publicly available Quantum Computer here if you want to try out a quantum computer for yourself!

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