Long-Lived Coherence on a μHz Scale Optical Magnetic Quadrupole Transition

We report on the coherent excitation of the ultranarrow <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mrow><a:mmultiscripts><a:mrow><a:msub><a:mrow><a:mi>S</a:mi></a:mrow><a:mrow><a:mn>0</a:mn></a:mrow></a:msub></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>1</a:mn></a:mrow></a:mmultiscripts></a:mrow><a:mi>–</a:mi><a:mrow><a:mmultiscripts><a:mrow><a:msub><a:mrow><a:mi>P</a:mi></a:mrow><a:mrow><a:mn>2</a:mn></a:mrow></a:msub></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>3</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:mrow></a:math> magnetic quadrupole transition in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mmultiscripts><c:mrow><c:mi>Sr</c:mi></c:mrow><c:mprescripts/><c:none/><c:mrow><c:mn>88</c:mn></c:mrow></c:mmultiscripts></c:mrow></c:math>. By confining atoms in a state insensitive optical lattice, we achieve excitation fractions of 97(1)% and observe linewidths as narrow as 58(1) Hz. With Ramsey spectroscopy, we find coherence times of 14(1) ms, which can be extended to 266(36) ms using a spin-echo sequence. We determine the lifetime of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mmultiscripts><e:mrow><e:msub><e:mrow><e:mi>P</e:mi></e:mrow><e:mrow><e:mn>2</e:mn></e:mrow></e:msub></e:mrow><e:mprescripts/><e:none/><e:mrow><e:mn>3</e:mn></e:mrow></e:mmultiscripts></e:mrow></e:math> level for spontaneous emission of magnetic quadrupole radiation to be 110(31) min, confirming long-standing theoretical predictions. These results establish an additional clock transition in strontium and pave the way for applications of the metastable <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mmultiscripts><g:mrow><g:msub><g:mrow><g:mi>P</g:mi></g:mrow><g:mrow><g:mn>2</g:mn></g:mrow></g:msub></g:mrow><g:mprescripts/><g:none/><g:mrow><g:mn>3</g:mn></g:mrow></g:mmultiscripts></g:mrow></g:math> state in quantum computing and quantum simulations. Published by the American Physical Society 2024