differentiallydiscriminateminutesignals.Intheirtechnique,ashortinfraredpulseisusedtocoherentlyexcitevibrationsinadissolvedchi-ralcompound,therebycreatingamacroscopicassemblyofoscillatingdipoles.Thisinduced‘polarization’radiatesthevibrationalpropertiesofthetargetmoleculesintheformofasignalknownasafree-inductiondecay.Thissignalhaschirospecificinformationencodedintheamplitudeandphaseofitselectricfield—inthesamewaythatthecharacteristicsoundofaringingbellreflectsitssizeandshape.Sohowistheweakchiropticalsignaldiscrim-inatedfromthestrongachiralbackground?Theauthorsrelyonthefactthatlinearlypolar-izedlightalwaysdevelopsanotherpolarizedcomponent(perpendiculartotheoriginaldirectionofpolarization)asitcrossesachiralmedium(Fig.1b,c).Usingpolarizationanalys-ersofexceptionalqualityalignedtodetectonlysignalsorthogonaltotheimpinging(linearlypolarized)infraredbeam,theauthorswereabletoisolatethedesiredoptical-activityresponseintheirexperiments.Toamplifyandinterprettheresponses,Rheeetal.exploitedthephenomenonofinterfer-ence,inwhichthesuperpositionofoscillatingwavesgivesrisetodistinctivepatternsofnewwaves.Theythuscombinedthefree-inductiondecayfromthechiralsamplewithaninfraredreferencepulseofmuchgreaterintensity.Thecoherentlybeatingportionoftheresultantpattern—theproductoftheweaksignalfieldandthestrongreferencefield—canbeiso-latedandanalysedusingFourier-transformmanipulations6,yieldinginformationaboutthesample.Ultimately,theauthorswereabletoascertainapropertyofchiralmoleculesknownasthecomplexsusceptibility.ThisquantityconsistsofapartthatisrelatedtoΔnandanotherpartthatisrelatedtoΔκ.Inthisway,VORDandVCDspectracanbeacquiredforeachincidentpulseofinfraredradiation,withthetimescaleofeachmeasurementessentiallybeinggivenbythepulseduration(lessthan100femtoseconds).Rheeetal.validatedtheirapproachbyexam-iningtheenantiomersofasimplechiralmol-ecule(limonene)dissolvedinsolution.Theyshowedthattheresulti