Switching speed is a critical factor in quantum computing because qubits are fragile and retain their quantum states for only a limited time. The faster individual operations can be completed, the more of them fit into that window, which matters for complex algorithms and for resource-intensive quantum error correction. Yet speed comes with a physical price. The shorter the microwave pulses used to switch a qubit, the stronger those pulses must be, and stronger pulses raise the risk of unwanted excitation. Superconducting qubits possess more than the two energy states used for computation, so if they are driven too hard they can unintentionally transition to higher energy levels.

The team found that strong, rapid microwave pulses can push information not only one level beyond its intended destination but also two or three levels higher through so-called multiphoton transitions, an effect the researchers report had previously been overlooked. Such multiphoton transitions are particularly treacherous in qubits: they can persist across multiple computational steps and impair quantum error correction. To suppress them, the experimental group of Yuan Gao and Prof. Rami Barends at PGI-13 worked closely with the theoretical group of José da Costa Jesus, Dr. Francisco Cárdenas-López and Prof. Felix Motzoi at PGI-8 to develop a control method that shapes the pulses with exceptional precision. Similar to a frequency filter, certain frequency components are suppressed, so the pulses continue to trigger the desired computational operation while barely exciting transitions to higher energy states.

The new method, called R2D, builds on the established DRAG correction technique, which has been standard practice for controlling qubits for more than a decade. Compared with the previous standard, the new approach reduces the probability of unwanted multiphoton transitions by a factor of more than 20. The method was tested on a single superconducting transmon qubit fabricated at the Helmholtz Nano Facility at Forschungszentrum Jülich. Two fundamental quantum operations were performed using microwave pulses, each lasting only 6.8 nanoseconds, with measured accuracies of 99.981 percent and 99.989 percent. The probability of the qubit unintentionally transitioning to a higher energy state remained below 0.002 percent per operation.

The researchers note that so far the method has only been tested on a single, isolated qubit, and further experiments are needed to determine whether it works just as well in larger quantum processors and during operations involving two qubits. Even so, the results set a new benchmark for quantum gates and deepen understanding of the underlying physics, providing insights for further improvements to the quantum system. The work demonstrates that extremely short switching times and high precision can be achieved simultaneously, a combination that has long been difficult to reconcile in superconducting qubit control.

Subject of Research: Ultrafast high-accuracy single-qubit gates in superconducting qubits using multiphoton transition suppression

Article Title: Fast but accurate – a new speed record for quantum gates

Article References: Fast but accurate – a new speed record for quantum gates. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum computing, qubit, quantum gates, superconducting qubits, multiphoton transitions, pulse shaping, DRAG, R2D, Forschungszentrum Jülich, Nature Communications, quantum error correction, transmon qubit