Aug 24 2021

Quantum Computers and Nanodots

Quantum computers are at the cusp of becoming an amazing technological breakthrough with many applications. Some heavy-hitters are working on the technology, including IBM and Google, and progress has been steady. IBM predicts it will have a 1000 qubit quantum computer by 2023. Quantum computers are essentially a direct application of quantum weirdness, and so now every time there is an advance in basic quantum mechanics the reporting is likely to mention how it might benefit quantum computing technology. In some cases, this is actually reasonable.

A quantum computer can be ridiculously more powerful than a traditional computer. Calculations that would take trillions of supercomputers billions of years to complete could be performed by a quantum computer in minutes. We don’t have such quantum computers yet, but when we do they will be able to perform specific computing tasks better than any supercomputer. They will essentially break any current encryption, can model the weather, and be used for things like drug development and many kinds of research. You won’t have one on your desktop, however. They do not function like ordinary computers and will not replace them.

The primary difference between a standard computer and a quantum computer is this – computers store data in bits, which is a binary unit of information, usually represented by a 0 or 1. Eight bits are chunked into bytes which have 256 states, representing every letter, number, and punctuation. Quantum computers, however, have qubits, which is a quantum bit. Qubits exploit the quantum phenomenon of superposition, the ability for a particle to be in more than one state at once. A qubit can therefore be in any state between 0 and 1 inclusive, and in fact can be in every state from 0 to 1 at the same time. Qubits are then entangled with each other and their interaction (specifically constructive and destructive interference) is used to produce the probable answer to a calculation. Yes – results are probabilistic, and they are typically checked against a regular computer to make sure the quantum computer is working.

This is a grossly oversimplified view. Here is a good discussion by an expert if you want to go into more detail. In terms of understanding the power of quantum computers the key concept is that when you increase the number of qubits in the system you increase the power exponentially. This is why adding more qubits makes quantum computers so fantastically powerful for the kinds of calculations they can do. A 1000 qubit computer, such as the one IBM is planning, is likely the threshold where many real-world applications start to be possible. But there are still technical hurdles to quantum computing besides just putting lots of qubits on a chip.

One major hurdle is that the superposition and entangled states are very fragile. They require supercooling to near absolute zero. We also need to connect every qubit to every other qubit, a problem which also increases geometrically, not linearly. And we need to interact with the whole system without breaking its fragile state. All of these problems have potential solutions that researchers are working on, and the pace of advancements seems to be accelerating. But until we cross over the finish line, it’s difficult to predict how long it will take. Also, it’s not so much a matter of crossing one finish line, but how many qubits can researchers get into one quantum computer. One thousand is powerful, but when we get up near a million qubits is where things start to get really interesting.

One recent experiment, for example, may be a path to more efficiently preserving the fragile quantum state of qubits. The researchers made what they call a semiconductor “lasagna” – 600 layers of alternating semiconductor materials. In between the layers they have nanodots (nanoscale bits of matter). The goal is to achieve a theoretical state of matter called many-body localization (MBL). In this state matter does not reach thermal equilibrium with its surroundings, because the waves of heat are blocked from progressing through the material. In short, this means that the material could theoretically stay ultracold for quantum computing for a much longer period of time. This material could also have other applications, such as trapping heat in electronics, or harvesting heat to convert to electricity.

When will a quantum computer perform a useful task? According to Google, it already has, just recently. They have used their quantum computer to replicate a quantum state known as a time crystal. Very briefly, this is a phase of matter in which the quantum material switches between two high energy states, while never reaching equilibrium at a low energy state. The switch between states does not expend energy, which is why it can be maintained indefinitely. This was a long theorized state of matter, first demonstrated by Google’s quantum computer.

What is amazing is that quantum computers are functioning based on the theoretical physics of quantum mechanics – they are a real-world demonstration that the ideas of quantum mechanics actually describe the way the universe works. I know that tons of research has already validated many aspects of quantum mechanics, but building a working machine based on those principles is still impressive and makes it all more real. This is often my challenge to cranks who thing they have discovered new physics – build something out of it and then I’ll be impressed. You have cold fusion? Run your house off of it. Generate some useful electricity. Real science delivers the goods – and now quantum computers are starting to do just that.

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