Single-photon-level atomic frequency comb storage in room temperature alkali vapor

We have demonstrated the coherent storage and retrieval of single-photon-level light using the atomic frequency comb protocol in a room temperature rubidium vapor. Velocity-selective optical pumping is used to prepare the comb within the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>F</a:mi> <a:mo>=</a:mo> <a:mn>2</a:mn> </a:mrow> </a:math> hyperfine ground state of rubidium, with the spacing between peaks coinciding with half the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>F</b:mi> <b:mo>=</b:mo> <b:mn>2</b:mn> <b:mo>−</b:mo> <b:mi>F</b:mi> <b:mo>=</b:mo> <b:mn>3</b:mn> </b:mrow> </b:math> hyperfine splitting of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>5</c:mn> <c:mmultiscripts> <c:mi mathvariant="normal">P</c:mi> <c:mrow> <c:mn>3</c:mn> <c:mo>/</c:mo> <c:mn>2</c:mn> </c:mrow> <c:none/> <c:mprescripts/> <c:none/> <c:mn>2</c:mn> </c:mmultiscripts> </c:mrow> </c:math> excited state. Weak coherent states of average photon number <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi>μ</e:mi> <e:mi>in</e:mi> </e:msub> <e:mo>=</e:mo> <e:mn>0.083</e:mn> <e:mrow> <e:mo>(</e:mo> <e:mn>5</e:mn> <e:mo>)</e:mo> </e:mrow> </e:mrow> </e:math> are stored with preprogrammed recall time of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mn>7.5</f:mn> <f:mspace width="0.16em"/> <f:mi>ns</f:mi> </f:mrow> </f:math> with an efficiency of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:msub> <h:mi>η</h:mi> <h:mtext>AFC</h:mtext> </h:msub> <h:mo>=</h:mo> <h:mn>6.59</h:mn> <h:mrow> <h:mo>(</h:mo> <h:mn>5</h:mn> <h:mo>)</h:mo> </h:mrow> <h:mo>%</h:mo> </h:mrow> </h:math> , while two temporally distinct modes have been stored and recalled with <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:msub> <i:mi>η</i:mi> <i:mtext>AFC</i:mtext> </i:msub> <i:mo>=</i:mo> <i:mn>2.6</i:mn> <i:mrow> <i:mo>(</i:mo> <i:mn>1</i:mn> <i:mo>)</i:mo> </i:mrow> <i:mo>%</i:mo> </i:mrow> </i:math> , allowing for time-bin qubit storage. Finally, the storage efficiency is observed to be independent of the input pulse polarization, satisfying a necessary condition for polarization qubit storage.