In their next set of experiments, that effect is exactly what they saw. "As the energy spreads, the disturbance would lead to qubit flips that are correlated across the entire chip." "If our model about particle impacts is correct, then we would expect that most of the energy is converted into vibrations in the chip that propagate over long distances," says Chris Wilen, a graduate student and lead author of the study. The bigger concern is what could happen next. This local effect can be easily mitigated with simple design changes. When one of these particles hits the chip, it frees up charges that affect nearby qubits. These sudden changes were most likely caused by cosmic rays or background radiation in the lab, which both release charged particles. The closer two qubits were together, the more likely they were to jump at the same time. The team observed long periods of relative stability followed by sudden jumps in offset charge. The fluctuating offset charge is effectively a change in electric field at the qubit. To assess whether qubit flips were correlated, the researchers measured fluctuations in offset charge for all four qubits. The scientists cool the chip to nearly absolute zero, which makes it superconduct and protects it from error-causing interference from the outside environment. The research team designed a chip with four qubits made of the superconducting elements niobium and aluminum. In this new study, they directly asked: are these flips independent, or are they correlated? In earlier experiments, McDermott's group had seen hints that something was causing multiple qubits to flip at the same time. However, the surface code protocol works reliably only if events that cause errors are isolated, affecting at most a few qubits. The surface code involves a large array of connected qubits - some do the computational work, while others are monitored to infer errors in the computational qubits. But the laws of quantum physics say that only one error type can be monitored at a time in a single qubit, so a clever error correction protocol called the surface code has been proposed. To fix errors, computers must monitor them as they happen. Qubits, however, can make two types of error: bit flips or phase flips, where a quantum superposition state changes. Classical bits, then, can only make bit flip errors, such as when a 1 flips to 0. The bits in a classical computer can either be a 1 or a 0, but the qubits in a quantum computer can be 1, 0, or an arbitrary mixture - a superposition - of 1 and 0. "Our experiments show absolutely that errors are correlated, but as we identify problems and develop a deep physical understanding, we're going to find ways to work around them." "I think people have been approaching the problem of error correction in an overly optimistic way, blindly making the assumption that errors are not correlated," says UW-Madison physics Professor Robert McDermott, senior author of the study. The researchers report their findings in a study published June 16 in the journal Nature, Importantly, their work also points to mitigation strategies. Now, researchers at the University of Wisconsin-Madison have found evidence that errors are correlated across an entire superconducting quantum computing chip - highlighting a problem that must be acknowledged and addressed in the quest for fault-tolerant quantum computers. Quantum computers could outperform classical computers at many tasks, but only if the errors that are an inevitable part of computational tasks are isolated rather than widespread events. Image: In this artistic rendering, a high-energy cosmic ray hits the qubit chip, freeing up charge in the chip substrate that disrupts the state of neighboring qubits.ĮMBARGOED UNTIL 11 A.M.
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