| Hardly a week goes by without breathless claims about the problems quantum computers will one day solve: cancer, climate change, misinformation and — my personal favorite — the inconvenience of automobile crash testing.
Don't believe the hype. It's not just that these predictions are premature or wildly optimistic. They all stem from the false premise that quantum computers are general-purpose computing machines, like laptops but far more powerful. In fact, researchers expect that quantum computers will only outperform ordinary "classical" ones for a few specific tasks.
This widespread misconception originates from a simplified account of the inner workings of quantum computers — one with a grain of truth in it. Ordinary computers process information in discrete chunks called bits, each of which has a value of either 0 or 1. Their quantum counterparts instead work with quantum bits, or "qubits," which can take on special combinations of their 0 and 1 states, called superpositions. In superpositions involving many qubits, all possible combinations of their individual 0 and 1 states can coexist.
So far, so good. But some people go on to claim that quantum computers can use superposition to rapidly solve any problem, because they can simply try all possible solutions at once. To be clear: They can't. To get a dramatic speedup, a quantum computer must weave together many strands of a superposition in just the right way, and this delicate choreography isn't always possible.
There are a few known problems that sufficiently powerful quantum computers will be able to solve far faster than classical ones. Quantum computers should excel at breaking down large numbers into their prime factors, a task whose difficulty is often used as the basis for encryption (as I explained in the June 17 Fundamentals newsletter). And they can also speed up the simulation of quantum systems like molecules, which could lead to many applications in chemistry, materials science and drug discovery.
Decades after these applications were first discovered, they remain the most compelling motivations for quantum computing. But researchers have continued to explore new areas where quantum computers might shine, and in the past few years they've made some exciting progress.
What's New and Noteworthy Much of the recent theoretical work on quantum computing has focused on its application to the study of quantum systems. Researchers have known how to speed up simulations of these systems since the 1990s. But for many other important tasks, like calculating the energy associated with a system's specific configurations, quantum speedups are harder to come by. Last year, a team of computer scientists proved that quantum computers can excel at this energy estimation task for certain configurations that often occur in nature. Lakshmi Chandrasekaran wrote about the breakthrough for Quanta.
The procedure that future quantum computers will use to factor large numbers has also guided researchers seeking other applications of quantum computing. That factoring algorithm works by exploiting special mathematical symmetries of the problem it's designed to solve, and many other quantum algorithms do something similar. In 2022, Mordechai Rorvig reported on the discovery of a dramatic quantum speedup for a qualitatively different problem that lacks this kind of mathematical structure. That problem was an artificial one without practical applications, but it's a sign that the landscape of quantum computation may be richer than researchers have imagined.
Not all researchers studying the theory of quantum computing focus on specific problems. Some instead search for general principles that might explain why some problems admit quantum speedups and others don't. In the 1990s, they thought this question would have a single answer, but reality quickly proved more complicated. Researchers have now identified three necessary conditions for quantum computers to outperform classical ones, and there may be many others. Charlie Wood covered the ongoing quest to quantify quantumness last year.
Beyond these theoretical developments, physicists are hard at work improving quantum computing technology, and they've made lots of progress in recent years. But no matter how fast technology advances, you shouldn't expect to upgrade to a quantum laptop. Ordinary classical computers aren't going anywhere. | 
