wowt explains: what is a quantum computer?

Quantum computers are coming. But what exactly are they and what can they do for us?
neon circuit diagram

Quantum is a term we use frequently in pop culture, so one would think we would all know exactly what it means by now. But thanks to decades of marketing hype, televised woo and pseudoscience, and frequent use and abuse in sci-fi, the word has almost lost its meaning. In this light, Google’s announcement of achieving “quantum supremacy” seems murky at best and confusing at worst. What exactly did they achieve? Why does it matter? Does this mean we’re about to have access to quantum computers? And why do we need those in the first place? Let’s tackle these questions and more in this installment of WoWT Explains.

so, what is a quantum computer anyway?

Quantum computers are devices that tackle challenging problems with complex arrangements of tiny particles instead of fluctuations in electrical currents. Try and solve these problems on modern devices and it would take decades, if not millennia, to evaluate every possible answer. But quantum computers can use the weird behavior of atoms and subatomic particles to try out countless potential solutions at once. Instead of using ones and zeros created by the absence and presence of an electrical signal respectively as a basic computing unit called a bit, quantum devices use qubits, which are arrangements of tiny particles that can represent a whole range of bit values at the same time.

A qubit could be a one, a zero, both, neither, or a certain percent one vs. a certain percent zero and everything in between. This indeterminacy is known as superposition, and by collecting the final configurations of these qubits’ states, a quantum computer could quickly find answers to problems that would otherwise take quadrillions of iterations of trial and error if a bit could be only a one or a zero for each attempt. This is especially useful for tackling problems involving an answer for how probable something is given a vast array of potentially interconnected inputs, something digital computers just aren’t very efficient at doing.

how do quantum computers work?

To create the qubits we just discussed, quantum computers entangle particles, or create groups of interacting particles able to influence each other. The phenomenon of entanglement itself is very complex and still being studied, but the fact that it works and we can do it suffices for our purposes here. As some of those particles are altered during calculations, they create a cascade of changes which, when measured as a complete system, give us answers to problems they’re tasked with solving. However, qubits are very fragile and any unplanned wayward interactions, or noise, between them causes another cascade to ripple through the system that we need to measure, making the answer unreliable. In the quantum world, this is called “decoherence.”

Think of qubits like snowflakes on a steep mountain peak. By using very small, controlled blasts, we can move the mass of snow where we need it. But should one of those explosions trigger an echo that’s a little too loud, or the amount of explosive be just a little off, or the underlying rock is unstable, we’ll see an avalanche sending all that snow in the wrong direction, blocking paths we want to keep clear. And this is the balancing act quantum computers try to achieve. Using very precise tools to manipulate the fundamental building blocks of matter into the right set of shapes, they can find which answer is the best fit for a fiendishly complex problem and allow us to create extremely useful statistical and mathematical models of the world.

can you reduce decoherence in quantum computers?

We think so. There are three approaches showing promise. The first is to cool them to within a fraction of a degree above absolute zero, reducing entropy and making it a lot harder for noise to enter the quantum system. The second is to use larger particles, making the whole system more resilient to quantum noise. The third is to design quantum circuits to take advantage of something called “quantum scarring” and tricking the system inside into arranging itself into an energetic but very stable configuration that could be read to produce an accurate solution to a problem. All of these approaches have their limitations and still require a lot of research before they end up in anything even resembling consumer electronics.

will quantum computers replace those we use today?

No. It’s best to think of quantum computing as an addon rather than a replacement to digital devices. Because they use statistical probabilities and are prone to decoherence, they’re not a great fit for many typical computing tasks. In fact, they’d be much slower at opening your email, browsing the web, or downloading files. At the same time, they’d be much faster at analyzing the contents of those files, doing complex web searches, and keeping you safe from hackers snooping on your electronic activity. In a future where we can harness their power, our devices would have both regular processors and quantum ones switching between tasks depending on which ones are best suited for their capabilities.

for what would we use quantum computers and how?

Quantum computers would be extremely good at both breaking encryption and securing data, analyzing and processing images and video, tackling complex simulations for everything from astrophysics to economics, and training artificial intelligence models. We’d specify the problem and the inputs using classical computing, dump the data and definitions into a quantum circuit, and get a result we’d translate using classical computing methods, meaning that the only real difference we’d notice as users would be our devices doing more complicated things a lot faster than we’d expect. This is not to undersell what quantum computers could do. They could easily enable revolutionary new approaches to our software and how we use it. But those approaches would be very much behind the scenes.

how many quantum computers are out there?

There are prototypes of such devices in academia and a few large corporations. IBM, Google, NASA, defense contractor Lockheed Martin, and a number of universities all say they have one, but a device that’s unanimously and without question accepted to be a quantum computer in the sense we think of one doesn’t yet exist. The closest thing currently available for purchase is manufactured by Canada’s D-Wave Systems, meant to understand the applications of quantum computing in the real world and test ideas for improvements of existing prototypes. As you can imagine, these devices have price tags in the ten-figure realm and require a great deal of know-how from their future users.

so, what exactly did google accomplish with its quantum computer?

Google claims that it was able to solve a problem that should take a classical computer 10,000 years to solve in just 200 seconds. The problem itself has few practical applications but was a perfect way to jam up a digital system’s processing power. Of course there is a possibility that computer scientists will find classical algorithms to tackle the same problem more efficiently at some point in the future, but for now, Google can claim that it has the largest and fastest chip for quantum computations, and built the infrastructure to build even larger, more accurate ones in the near future. And the more powerful and faster quantum chips become, the faster they will pull away from classical computers in tackling problems, even with new algorithms to circumvent the limits of digital processing.

will we get access to quantum computers as they’re being built?

Very likely. Both Google and Microsoft are working on enabling software engineers to access their quantum computing efforts using cloud services. The end result would be a web API, or a set of commands that can be triggered remotely by code meant to be executed on a quantum system. In effect, it would be a black box access to which would be billed by sent command. It may seem rather early for those services to be really useful, but they may be a perfect vehicle to build an infrastructure for future quantum devices and get programmers thinking of how to incorporate quantum components into their software.

# tech // computer science / quantum computing / quantum mechanics


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