In　this　paper,　we　investigate　the　optimal　decay-in-time　rate　of　solutions　to　the　Cauchy　problem　of　the　compressible　quantum　Navier-Stokes-Poisson　(abbr.　QNSP)　equations,　which　had　been　used　to　model　the　motion　of　electrons　in　the　semiconductor　devices　on　the　nanometer　scale.　Since　the　quantum　effect　in　the　QNSP　equations　is　described　by　the　third-order　spatial　derivatives　of　density,　this　fact　results　in　some　essential　difficulties　in　the　investigation　of　the　optimal　decay-in-time　rate　of　solutions　by　the　spectral　analysis　method.　To　avoid　the　essential　difficulties,　we　use　the　method　of　pure　energy　estimates　to　substitute　the　spectral　analysis　method　as　in　[1],　and　thus　establish　the　decay-in-time　rates　of　solutions.　We　believe　that　the　decay-in-time　rates　obtained　in　this　paper　are　optimal,　since　they　are　consistent　with　the　ones　of　the　linear　equations.　©　Published　under　licence　by　IOP　Publishing　Ltd.