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By employing a novel reduction to the \"Population Recovery\" problem, we give an extremely simple algorithm that learns the Pauli error rates of an <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>n<\/mml:mi><\/mml:math>-qubit channel to precision <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>&amp;#x03F5;<\/mml:mi><\/mml:math> in <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:msub><mml:mi>&amp;#x2113;<\/mml:mi><mml:mi mathvariant=\"normal\">&amp;#x221E;<\/mml:mi><\/mml:msub><\/mml:math> using just <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>O<\/mml:mi><mml:mo stretchy=\"false\">(<\/mml:mo><mml:mn>1<\/mml:mn><mml:mrow class=\"MJX-TeXAtom-ORD\"><mml:mo>\/<\/mml:mo><\/mml:mrow><mml:msup><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mn>2<\/mml:mn><\/mml:msup><mml:mo stretchy=\"false\">)<\/mml:mo><mml:mi>log<\/mml:mi><mml:mo>&amp;#x2061;<\/mml:mo><mml:mo stretchy=\"false\">(<\/mml:mo><mml:mi>n<\/mml:mi><mml:mrow class=\"MJX-TeXAtom-ORD\"><mml:mo>\/<\/mml:mo><\/mml:mrow><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mo stretchy=\"false\">)<\/mml:mo><\/mml:math> applications of the channel. This is optimal up to the logarithmic factors. Our algorithm uses only unentangled state preparation and measurements, and the post-measurement classical runtime is just an <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>O<\/mml:mi><mml:mo stretchy=\"false\">(<\/mml:mo><mml:mn>1<\/mml:mn><mml:mrow class=\"MJX-TeXAtom-ORD\"><mml:mo>\/<\/mml:mo><\/mml:mrow><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mo stretchy=\"false\">)<\/mml:mo><\/mml:math> factor larger than the measurement data size. It is also impervious to a limited model of measurement noise where heralded measurement failures occur independently with probability <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mo>&amp;#x2264;<\/mml:mo><mml:mn>1<\/mml:mn><mml:mrow class=\"MJX-TeXAtom-ORD\"><mml:mo>\/<\/mml:mo><\/mml:mrow><mml:mn>4<\/mml:mn><\/mml:math>.We then consider the case where the noise channel is close to the identity, meaning that the no-error outcome occurs with probability <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mn>1<\/mml:mn><mml:mo>&amp;#x2212;<\/mml:mo><mml:mi>&amp;#x03B7;<\/mml:mi><\/mml:math>. In the regime of small <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>&amp;#x03B7;<\/mml:mi><\/mml:math> we extend our algorithm to achieve multiplicative precision <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mn>1<\/mml:mn><mml:mo>&amp;#x00B1;<\/mml:mo><mml:mi>&amp;#x03F5;<\/mml:mi><\/mml:math> (i.e., additive precision <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mi>&amp;#x03B7;<\/mml:mi><\/mml:math>) using just <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:mi>O<\/mml:mi><mml:mrow class=\"MJX-TeXAtom-OPEN\"><mml:mo maxsize=\"1.2em\" minsize=\"1.2em\">(<\/mml:mo><\/mml:mrow><mml:mfrac><mml:mn>1<\/mml:mn><mml:mrow><mml:msup><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mn>2<\/mml:mn><\/mml:msup><mml:mi>&amp;#x03B7;<\/mml:mi><\/mml:mrow><\/mml:mfrac><mml:mrow class=\"MJX-TeXAtom-CLOSE\"><mml:mo maxsize=\"1.2em\" minsize=\"1.2em\">)<\/mml:mo><\/mml:mrow><mml:mi>log<\/mml:mi><mml:mo>&amp;#x2061;<\/mml:mo><mml:mo stretchy=\"false\">(<\/mml:mo><mml:mi>n<\/mml:mi><mml:mrow class=\"MJX-TeXAtom-ORD\"><mml:mo>\/<\/mml:mo><\/mml:mrow><mml:mi>&amp;#x03F5;<\/mml:mi><mml:mo stretchy=\"false\">)<\/mml:mo><\/mml:math> applications of the channel.<\/jats:p>","DOI":"10.22331\/q-2021-09-23-549","type":"journal-article","created":{"date-parts":[[2021,9,23]],"date-time":"2021-09-23T14:25:19Z","timestamp":1632407119000},"page":"549","update-policy":"https:\/\/doi.org\/10.22331\/q-crossmark-policy-page","source":"Crossref","is-referenced-by-count":27,"title":["Pauli error estimation via Population Recovery"],"prefix":"10.22331","volume":"5","author":[{"given":"Steven T.","family":"Flammia","sequence":"first","affiliation":[{"name":"AWS Center for Quantum Computing, USA"},{"name":"IQIM, California Institute of Technology, USA"}]},{"given":"Ryan","family":"O&apos;Donnell","sequence":"additional","affiliation":[{"name":"Computer Science Department, Carnegie Mellon University, USA"}]}],"member":"9598","published-online":{"date-parts":[[2021,9,23]]},"reference":[{"key":"0","doi-asserted-by":"publisher","unstructured":"F. 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