![]() Faced with the huge gap between theory and practice, physicists in the early days worried that quantum computing would remain a scientific curiosity. Yet today’s best machines make an error every 1,000 gates. That feat demands they make at most a single error every billion gates. The trouble is that many proposals to solve useful problems require quantum computers to perform billions of logical operations, or “gates,” on hundreds to thousands of qubits. These capabilities give quantum computers their power to perform certain functions extremely efficiently and potentially speed up a wide range of applications: simulating nature, investigating and engineering new materials, uncovering hidden features in data to improve machine learning, or finding more energy-efficient catalysts for industrial chemical processes. Because qubit states are in the form of waves, they can interfere, just as light waves do, leading to a much richer landscape for computation than just flipping bits. By combining qubits through a quantum phenomenon called entanglement, we can store vast amounts of information collectively, much more than the same number of ordinary computer bits can. There I devoted myself to improving operations among multiple linked qubits and exploring how to correct errors. I helped run several quantum-computing research programs at IARPA and later joined IBM. My experience with superconductivity was suddenly in demand. Another decade later, in 2007, they invented the basic data unit that underlies the quantum computers of IBM, Google and others, known as the superconducting transmon qubit. Although Paul Benioff of Argonne National Laboratories had proposed them in 1980, it took physicists nearly two decades to build the first one. Quantum computers were in their earliest stages then. There I sought to employ the fundamentals of nature to develop new technology. Department of State, which next led me to the Defense Advanced Research Projects Agency (DARPA) and the Intelligence Advanced Research Projects Activity (IARPA). That came later when I took a hiatus to work on science policy at the U.S. I began as a condensed-matter theorist investigating materials’ quantum-mechanical behavior, such as superconductivity at the time I was oblivious to how that would eventually lead me to quantum computation. I am a physicist working in quantum computing at IBM, but my career didn’t start there. Quantum computers suffer types of errors that are unknown to classical computers and that our standard correction techniques cannot fix. ![]() But with great power comes great vulnerability. The implications for science and business could be profound. These emerging machines exploit the fundamental rules of physics to solve problems that classical computers find intractable. This law of inevitability applies equally to quantum computers. ![]() Errors may be inevitable, but they are also fixable. Error correction is one of the most fundamental concepts in information technology. Images from space probes can travel hundreds of millions of miles yet still look crisp. QR codes can be blurred or torn yet are still readable. Mitigating and correcting them keeps society running. They are everywhere: in language, cooking, communication, image processing and, of course, computation. The larger logical qubit had a lower error rate, about 2.9 percent per round of error correction, compared to the smaller logical qubit’s rate of about 3.0 percent, the researchers found.It is a law of physics that everything that is not prohibited is mandatory. After making steady improvements to the performance of the original physical qubits that make up the device, the researchers tallied up the errors that still slipped through. Using Google’s Sycamore quantum chip, the researchers studied two different sizes of logical qubits, one consisting of 17 qubits and the other of 49 qubits. But if the original qubits are too faulty, adding in more of them will cause more problems than it solves. Ideally, the larger the logical qubit, the smaller the error rate should be. That redundancy allows a quantum computer to check if any mistakes have cropped up and fix them on the fly. Logical qubits store information redundantly in multiple physical qubits. ![]() Quantum computers like Google’s require a dilution refrigerator (pictured) that can cool the quantum processor (which is installed at the bottom of the refrigerator) to frigid temperatures.
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