NASA's X-59 research jet is designed to travel faster than sound while muffling the resulting sonic boom. The space agency is exploring whether the plane's tech could also apply to future commercial airliners - allowing travellers to cross the Atlantic at speeds greater than 1,500mph.
There exists a research paper claiming that a much more fuel efficient plane is possible: https://arc.aiaa.org/doi/10.2514/6.2022-3314
Problem is, it is locked behind a paywall that I cannot circumvent. But even if the paper is only half-true, it still implies a large efficiency gain is possible.