LASER MAFI TSAYAYYA DA AKA TAƁA GINAWA

Mafi kyawun agogunan zarra (atomic clocks) suna ƙirga girgizar hasken da zarra ke sha. Don yin haka, suna buƙatar laser wanda mitarsa da ƙyar take motsawa, don ya iya binciken sauye-sauyen zarra masu ƙunci matuƙa. Lasers mafiya tsayayya a yau an ɗaure su ne a kan rami na silicon mai tsananin sanyi (cryogenic silicon cavity): madubai biyu da sandar silicon mai crystal guda ke raba su, da aka sanyaya ƙasa da yanayin zafin ɗaki nesa ba kusa ba. Hasken da ke jujjuyawa tsakanin madubai yana ƙara ƙarfi ne kawai a mitoci daidai sosai, kuma ana tilasta laser ya bi su.

Irin waɗannan ramuka sun riga sun kai daidaito na kaso a matakin 10⁻¹⁷ — mitar tana yawo da kusan kashi ɗaya cikin miliyan ɗari na kashi ɗaya cikin biliyan na ƙimarta. A cikin gwaje-gwaje da yawa, laser har yanzu yana ɗaya daga cikin manyan tushen hayaniya — don haka mafi tsayayya yana inganta aunawa nan take.

Madubai masu girgiza

Abin da ke iyakance mafi kyawun ramukan silicon shi ne hayaniyar zafi ta Brown (Brownian thermal noise) a cikin shafen madubai: siraran yadudduka masu nuna haske ba sa taɓa tsayawa cik, saboda zafi yana sa zarrorinsu su girgiza. Wannan ƙaramin motsi yana sauya tsawon ramin, kuma tare da shi launin hasken.

Ben Lewis, Jun Ye da abokan aikinsu a JILA (NIST da Jami’ar Colorado, Boulder), tare da masana kimiyyar lissafi daga Physikalisch-Technische Bundesanstalt (PTB) a Braunschweig, Jamus, sun yi sauye-sauye biyu ga ramukan silicon na santimita 21 na baya:

• Ƙarin sanyi. Tsarukan baya suna aiki a K 124, ɗaya daga cikin yanayin zafin da silicon ke daina faɗaɗawa ko ƙanƙancewa da zafi. Sabon ramin, mai suna Si7, yana aiki ko dai a K 16, ɗayan irin wannan yanayin zafi, ko a K 6, mafi sanyin da na’urar sanyaya ta (cryostat) za ta iya kaiwa.
• Haske mai faɗi. Ta amfani da madubi mai ƙarancin lanƙwasa (radius na lanƙwasa mita 10.2 maimakon mita 1), tabon haske a kan madubai yana girma daga micrometer 480 zuwa 850 a radius — ƙara girman hasken yana ɗaya daga cikin hanyoyin da aka sani na rage hayaniyar zafi.

Tare, sun rage hayaniyar zafi sau uku zuwa biyar idan aka kwatanta da tsarukan baya.

Yaƙi da girgiza a cikin sanyi

Duk sauye-sauyen biyu yawanci suna sa rami ya fi saurin jin girgiza, kuma kaiwa K 6 ko 16 yana buƙatar na’urar sanyaya ta helium (cryocooler) — injin da ke girgiza. Ƙungiyar ta ajiye na’urar sanyayar a ƙasa kusa da teburin haske, tana tura helium mai sanyi ta bututu zuwa gwajin. Ramin, wanda aka riƙe a tsakiyarsa da ƙwallayen ruby uku masu faɗin milimita 6, yana kan teburin ware girgiza mai aiki (active vibration-isolation table).

Sanyi yana kawo wata matsala: abubuwa suna riƙe da zafi kaɗan sosai kusa da cikakken sifili, don haka sauye-sauyen yanayin zafi suna da wuyar kwantarwa. Mafita ita ce erbium, wanda a K 16 yake adana zafi ninki 6 a kowane girma fiye da bakin ƙarfe (stainless steel) da ninki 13 fiye da tagulla. Yadudduka na garkuwa da guntayen erbium suna rage gudun kowane canjin yanayin zafi zuwa ma’aunin lokaci na kusan kwanaki huɗu. Ana riƙe na’urar jin sarrafawa a K 16.183, kuma wata na’urar ji mai zaman kanta tana kasancewa cikin kashi 100 cikin miliyan na digiri daga abin da aka nufa.

Tarihin

Idan aka kwatanta shi da wasu ramukan silicon biyu da agogon zarra na strontium, laser ɗin yana kaiwa:

• a K 6, daidaito na kaso na 7 × 10⁻¹⁸ daga daƙiƙa 2 zuwa 50 na matsakaici — sabon tarihi: jijjiga ta kusan kashi 1.4 cikin dubu na hertz kawai — da faɗin layi (linewidth, wato yaɗuwar mitar laser ɗin kansa) na millihertz 2.5 a kan mitar haske ta terahertz 194;
• a K 16, 9 × 10⁻¹⁸, faɗin layi na mHz 2.9, amma hali mafi kyau a cikin kwanaki.

Hayaniyar tana raguwa idan ramin ya ƙara sanyi, wanda ke nuna cewa laser ɗin ya kai iyakar da aka nufa: hayaniyar zafi ta shafen, ba girgiza ko na’urorin lantarki ba.

A cikin gudu biyu na kwanaki 14 da 7, agogon strontium ya bi karkacewar ramin a hankali: ƙasa da microhertz 4 a kowane daƙiƙa, mai kama da hydrogen maser (na’urar mizanin girgiza ta ɗakunan gwajin lokaci) kuma ninki goma ƙasa da ramukan silicon da ke aiki a K 124. Ramin ya kai can watanni tara bayan sanyayawa; ramukan K 124 sun ɗauki shekaru goma.

Kallon zarra na daƙiƙa da yawa

Laser ɗin ya riga ya fara aiki. Yana tuƙa agogon lattice na strontium (zarrori da aka kama a cikin raga ta hasken laser), yana ba da damar binciken zarrorin na fiye da daƙiƙa 10 — kuma yayin da binciken ya daɗe, layin zarrorin yana ƙara siriri, don haka agogon yana iya gano tsakiyarsa daidai sosai. Bayan daƙiƙa 12, siginar tsangwama (Ramsey fringes) har yanzu tana riƙe da kashi 75% na bambancinta (contrast), kuma bugun guda ɗaya na daƙiƙa 12.4 yana ba da layi mai faɗin millihertz 64 kawai. Yanayin da aka tayar (excited state) na strontium-87 da agogon ke amfani da shi yana rayuwa kusan daƙiƙa 170 — iyaka ta gaba ga lokutan bincike.

Zuwa ga 10⁻¹⁸

Sabon laser ɗin zai maye gurbin tsohon mizanin JILA kuma, a cewar marubutan, zai ba dukan agogunan haske na hanyar sadarwar Boulder damar kai wani matakin daidaito da aka ƙayyade har sau goma da wuri. Shafen madubai na crystal na iya rage hayaniyar zafi zuwa kusan 1.5 × 10⁻¹⁸ a K 6; ramuka masu sanyi da tsawo fiye da haka ko haske mai faɗi na iya tura ta zuwa 1 × 10⁻¹⁸ ko ƙasa da haka. Wani asiri ya rage: ƙarin hayaniya bayan daƙiƙa 100, wadda babu wata na’urar ji a cikin cryostat da za ta iya gano tushenta tukuna.