A beginner’s guide to quantum computing

A picture of one of IBM’s quantum computers

Optimization

The first major use of quantum computing is optimization, as quantum computers can optimize much more efficiently than classical computers. This can include the best route for delivery trucks, the best takeoff schedules for planes, all the way to maximizing profits in the stock market. Quantum computers are great at optimization because unlike normal computers who have to check every possible solution, quantum computers can instantaneously check every single solution! Imagine a book with many pages, and one page was chosen at random then marked. A classical computer would have to flip through every page and check if it is marked. While a quantum computer, on the other hand, would be able to check every page at once. This means that even though they perform each operation more slowly than classical computers, they can do exponentially more with each operation performed on specific tasks. This is due to superposition (further explained below) which allows a qubit to exist in multiple states. Optimization will be one of the most important and major uses of quantum computers.

Simulation

Quantum computers are also great at simulation compared to classical computers. They can simulate chemistry exponentially better than classical computers, which will significantly speed up the process of drug discovery, vaccine manufacturing, and generally helps anything that can make use of simulation. As Richard Feynman states,

Factorization

Peter Shor, the creator of Shor’s algorithm

Taking advantage of quantum weirdness

How do quantum computers work? And what quantum properties do they take advantage of?

Qubits

Representation of a qubit vs a classical bit

Superposition

The “closer” the probability of the qubit (the black arrow in the picture above) is to 0, the more likely it will collapse into the 0 state, and vice versa for the 1 state. You can imagine a qubit as the result of a coin flip; when it’s spinning it has a chance of landing on both heads and tails. The only way to measure this is to stop the coin, thereby “collapsing” it into a state of heads or tails. Imagine this coin as also possibly being weighted on one side, making it have a higher probability of landing on that side. There is also no way to determine the weighted side unless the coin is repeatedly flipped again and again. The same thing occurs to a qubit in superposition, meaning that there is no way to directly measure the probability of a qubit without collapsing it into a 1 or 0 state, much like the coin.

Entanglement

A representation of two entangled qubits in different states

Interference

Interference is a very important aspect of quantum computing. With clever use of superposition and entanglement, you can get your answer, but this begs the question of outputting your results. For a quantum computer, that’s not as simple as a classical computer. Going back to the book analogy, imagine your quantum computer has figured out what page your marking is on. The quantum computer is now in a superposition of every page and its result (page 1: no marking, page 2: no marking, page 3: marking, etc…). If you were to directly measure the state, then the state would collapse, giving you a random answer! (such as page 45: no marking) This is just as slow, if not worse, than a classical computer, and it solves none of our problems. That’s why we have to cleverly make use of interference to destructively interfere with all the wrong answers and constructively interfere with the right answers.

Wave interference in water
Constructive vs deconstructive interference visualized

Quantum computing today

D-wave

A D-wave 2000 qubit computer

IBM

IBM is also developing quantum computers, and they are one of the industry leaders. They also let average people test drive their quantum computers, so that’s a great place to get started in the field of quantum computing if you’re interested. They also have developed their own quantum language called Qiskit, which can be used to program their quantum computers. Qiskit is based on python, but it has a lot of custom commands and changes that make it suitable for quantum computing.

Google

Google is on this list because they were the first to achieve quantum supremacy. This means that it was demonstrated that a quantum computer could actually beat a classical computer at solving a problem by a lot in fact! Google claims that the same problem would have taken 10 000 years to solve on the most powerful supercomputer and was solved in 200 seconds, although that claim is disputed by IBM, who claims that the problem could have been solved in 2.5 days on their supercomputer. Even if IBM is right though, that’s still a noticeable improvement. The problem unfortunately wasn’t useful in any way, and it was heavily optimized for the quantum computer specifically for this purpose. But never the less, it is still a great achievement on Google’s part.

Xanadu

Xanadu is one of my personal favourite companies that is working on quantum computing, mostly because they’re the closest to breaking the barrier to room temperature quantum computing. They use a photonic quantum computer, not needing cryogenically cooled superconductors to function. Most of the energy draw of a quantum computer actually comes from the cooling (which needs to get colder than deep space, to a fraction of a degree kelvin), and most of the mass is actually the cryogenic freezer and radiation shielding! This means that Xanadu computers can be many times smaller — small enough to fit on a server rack in fact. The only catch is that the photon detectors, the parts which detect the result of the calculation, still need to be supercooled. But this means that the power draw is exponentially reduced, and the freezer doesn't have to be as cold, as well as making the entire computer much more resistant to outside factors and reducing the size dramatically.

A Xanadu tabletop cryogenic freezer, which fits snugly in a standard server rack

Quantum in the future

I hope that you already have a decently firm understanding of how the future of quantum will look like based on the information above, but I’ll also give my input on the future.

Security

I believe that in the future we will need to switch over to quantum encryption, or at least change our methods as it will be increasingly difficult to secure our data from attackers.

Personal quantum computers

We are currently working on making quantum computers smaller and smaller, as well as making them work without the need for cooling. In the future, you might buy a laptop with a quantum subprocessor to help you do specific problems, much like you could buy a floating-point subprocessor in the 90s to help you better calculate floating-point numbers.

Quantum in education

Much like schools are teaching kids the basics of coding now, in the future courses teaching quantum mechanics and quantum computing will be much more widespread and easily available, as well as giving kids a high-level understanding of how quantum computing works starting from a young age.

Quantum commonplace

In the future (albeit probably a few decades into the future) we will end up taking quantum computing for granted, much like we do the telephone or the electric lightbulb. It will be so ingrained in our lives that we wouldn’t be able to ever imagine a world without quantum computing.

TL;DR

  • Quantum computing is only good at specific tasks, notably optimization, simulation, and factorization
  • They use qubits (quantum bits) which have really weird properties instead of normal bits
  • They take advantage of superposition (can be in the state of 1, 0 or both), entanglement (two particles are inexplicably “linked” together, no matter how long the distance), and interference (to destructively interfere with the wrong answers and constructively interfere the right ones)
  • Quantum computers are admittedly not that good in the status quo, only barely being able to beat classical computers in specific tasks and requiring an immense amount of effort to cool and maintain
  • In the future, quantum computers will be extremely common and accessible for everyone

Extra resources:

How entanglement is actually created:
https://www.forbes.com/sites/chadorzel/2017/02/28/how-do-you-create-quantum-entanglement/?sh=25bab7551732

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Victor Feng

Victor Feng

I’m a 14-year-old innovator at TKS, and my interests are quantum computing and machine learning, but I’m constantly gaining new interests.