What Is It and Why Do We Want It?

With every passing day, quantum computers are getting closer to being practical computers that can be put to use in various industries and walks of life, but what are scientists and engineers actually aiming for? When will these computers be ready?

One measure is that of “quantum supremacy”. Once quantum supremacy is demonstrated, it will usher in the age of quantum computers for real, but what does it mean for quantum computers to have supremacy?

Understanding Quantum Supremacy

In principle, quantum supremacy is something that has to be demonstrated over “classical” computers. That is, the computer you’re using right now to read this. It uses binary logic to perform computations. At its core, it’s all ones and zeros.

A quantum computer has “quantum supremacy” when it can do a calculation that’s impractical for a classical computer to do because it would take too long to be useful. We know from Alan Turing’s Universal Turing Machine that you can compute the answer to anything you can express mathematically with a classical computer. It’s just that the answer might take several thousand times as long to compute as the age of the universe!

This is actually a good time to stop and watch this brief explanation of Universal Turing Machines from the Computerphile channel.

It seems counterintuitive that such a simple machine could effectively compute any algorithm, but the math has been checked out by people who are orders of magnitude smarter than I am, so I guess I’ll have to take their word for it.

So the core of supremacy is demonstrating that quantum computers can solve certain problems that classical computers can also solve, but can’t solve for practical purposes.

How Classical and Quantum Computers Are Different

How can quantum computers (in theory) seemingly skip all that work and get to the answer so much faster than a classical computer? It all comes down to how these different computer systems work.

A classical computer processes information in the form of binary code. Everything in a computer boils down to data represented in some way as a one or a zero. From these ones and zeros, we can build symbol manipulation codes. For example, if you have eight bits (binary digits) you have 256 possible permutations of ones and zeros. Which means you can represent 256 numbers, which can each stand for something. There are 128 standard ASCII characters represented by a seven-bit binary number. The extended eight-bit ASCII set has 256 characters.

As you increase the bit-length, you exponentially increase the number of values you can represent, which is why a 32-bit computer can only address 4GiB worth of RAM, but a 64-bit computer can address (in theory) 16 Exabytes of the stuff.

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This is really useful since you can represent these two states in so many ways, such as magnetic field strengths on tape, pits and lands on a CD, or the levels of electrical charge inside a CPU or the memory cells of an SSD.

Qauntum computers fundamentally differ from classical computers. Instead of binary digits called bits, they use quantum digits called “qubits”. A qubit makes use of quantum phenomena and can be a one or a zero at the same time. Because every bit is in “superposition” the quantum computer can compute all possible answers at the same time. Then, when you measure the value of the qubits, they collapse into the correct answer.

Yes, I don’t really know how it works either.

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Nonetheless, based on how they work, and that they’ve been shown to work, these quantum computers have the potential to computer in seconds what would take the fastest classical computers in the world millions of years and of the energy in our solar system. So why aren’t they supreme already?

Well, for one thing, qubits are incredibly fragile, and the number of them has to be scaled up dramatically for a quantum computer to actually be useful. Existing quantum computers only have a handful of qubits and are good for little more than proving the concept works.

The main thing scientists and engineers are working on is creating robust qubits, figuring out how to cram as many of them into a quantum computer as possible, and perhaps making it all work at room temperature.

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Claims of Quantum Supremacy Are Controversial

Google claimed in 2019 that its 53-qubit Sycamore processor had achieved quantum supremacy by solving a complex problem in 200 seconds—a task they estimated would take the world’s most powerful classical supercomputer thousands of years. This was for one specific problem known as “random circuit sampling” and not general supremacy.

IBM, a leader in both classical and quantum computing, argued that their classical supercomputers could, with clever optimizations, solve the same problem much faster than Google’s initial estimates suggested—casting doubt on whether true supremacy had been reached.

It’s also interesting that there’s been pushback against the term “supremacy”, which some academics feel has problematic overtones that needlessly evoke unrelated social issues based on race or ethnicity. Which is why some people prefer to use “quantum advantage” to mean the same thing, but you’ll find uses of both terms all over the place.

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What Happens When We Achieve Supremacy for Real?

So what happens when quantum computers finally get their act together and can solve problems that take too long and are too expensive for current computers? The truth is that the world will probably change in some profound ways, and that’s without taking into account how the recent revolution in AI has already blown the doors off some problems, such as predicting practically every protein structure we can think of.

The big one everyone’s likely heard of is that all our current strong encryption will be useless overnight. These encryption algorithms work specifically on the principle that they are fast and cheap to apply using a classical computer, but they’d take thousands or millions of years to crack.

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Well, a quantum computer with sufficient qubits could open these virtual locks as if they weren’t there. Of course, we’ve already designed post-quantum encryption methods that are hard even for quantum computers to break, in theory. Still, a lot of data is going to be retroactively cracked open and that could be bad for obvious reasons.

While AI has done a tremendous job accelerating materials science and medical research, quantum computers could amp this up even more, helping us find new medicines and other chemicals quickly.

Speaking of AI, adding some qubits into the mix could truly supercharge the mental states of our best AI today, allowing them to consider a huge number of possibilities almost instantly.

If nothing else, quantum computers will likely be excellent at logistics, making our use of energy and resources much better, and maybe getting rid of traffic jams forever. Now that’s a cause I think we can all get behind.