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Mara had not come downstairs for a milestone . A molecule in her notebook would not settle on the cluster her group already trusted , and Ade had offered the quantum processor for an evening . She wanted one sketch of how that molecule held its energy , honest enough that she could choose which variant to make on the bench . If the evening produced only a performance , she would rather find out before midnight . Ade did not open her file . He opened the result the field still uses as its reference point . In 2012 John Preskill named a threshold with unusual clarity : quantum supremacy , a quantum machine doing work that sits out of practical reach for classical computers . The demonstration people still cite came in 2019 , when Google's Sycamore processor sampled the output of a random circuit . It was not a request from Mara's kind of work . It was offered as evidence that classical simulation would struggle , and the struggle was the point . Later classical methods narrowed the gap those first announcements described . Ade refused to treat the year as a finish line . Mara set her pen down . She did not need a harder puzzle . She needed an answer she could check against a vial . That pressure made the distinction plain . Supremacy , in the sense Preskill named , can be a real crossing and still say nothing about her molecule . Practical quantum advantage would be the day a machine of this kind wins on a problem she already had , and wins in a form the bench can contradict . He turned the monitor toward her so she could see how thin the trustworthy part was . The qubits on their processor do not keep a state for long . Each added layer of gates gives noise another entrance , and a circuit deep enough to follow the electrons she cares about starts to lose the question . Error correction is the proposed remedy : protect one logical qubit by spreading it across many physical ones that fail more easily . Ade could describe the remedy . He could not switch it on before she had to leave . A sampling run can be compared with a theoretical pattern . Her molecule cannot . It either matches the flask or it does not . Mara put the notebook away and left the Sycamore result where it belonged , in the record , not in her vial . The calibration kept running after she climbed the stairs . The famous task was already finished . Hers was not .
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