KYANWAR KWANTAM DA KE ZAMA A WURI ƊAYA TSAWON MINTUNA

Kwamfutar kwantam za ta yi amfani ne kawai idan za ta iya gyara kurakuranta da kanta. Hakan yana buƙatar kayan aiki (hardware) masu yawa sosai — sai dai idan kurakuran ba su da daidaito (lopsided). Idan wani nau’in kuskure kusan ba ya taɓa faruwa, lambar gyara (correcting code) tana yaƙi da ɗayan nau’in kawai, kuma za ta iya zama mafi sauƙi sosai.

Abin da ake kira qubits na kyanwa (cat qubits) an gina su don su zama marasa daidaito. Suna adana bayanai a cikin da’irar superconducting da ke riƙe filin microwave, a matsayin kai-da-komo biyu masu saɓani na wannan filin. Mayar da 0 zuwa 1 — juyewar bit (bit flip) — yana nufin tsalle daga wani kai-da-komo zuwa ɗayan. Yayin da photon ke ƙaruwa a cikin filin, nisan da ke tsakanin yanayin biyu yana ƙaruwa: juyewar bit tana ƙara ƙaranci a hanyar exponential tare da yawan photon, yayin da ɗayan kuskuren, juyewar phase (phase flip), ke girma kawai daidai gwargwado.

Kwaruruka biyu, rauni ɗaya

Akwai hanyoyi biyu na riƙe kyanwa a wurinta. Kyanwoyi na “dissipative” (masu zubar da makamashi) ana ci gaba da tura su zuwa yanayinsu biyu ta hanyar wani abokin tarayya mai asara (lossy); sun riga sun riƙe fiye da daƙiƙa 100. Kyanwoyin “Kerr” kuma suna zaune a cikin yanayin makamashi (energy landscape) mai kwaruruka biyu da shinge ya raba su, wanda aka ƙirƙira ta hanyar tuƙa da’ira mara layi (nonlinear). Fa’idarsu ita ce sarrafawa mai sauri. Raunin su: juyewar bit bayan millisecond kawai — wani ci gaba na kwanan nan ya kai 3.6 ms, har yanzu da sauran darajoji biyar na girma (orders of magnitude).

Dalilin shi ne babu abin da ke cire makamashin da ya ɓace. Hayaniya tana harba kyanwar zuwa yanayi masu ɗaukaka (excited states) a cikin kwarinta, kuma daga can, zamewa zuwa ɗayan kwarin tana zama da sauƙi sosai. Mafi muni, a tuƙi mai ƙarfi, qubit ɗin yawanci yana shiga resonance (daidaituwar girgiza) da da’irar karatunsa, abin da ke lalata kariyar sosai.

Da’irar karatu a matsayin magudana

Sergey Hazanov, Mikhail Svintsov da abokan aiki a Weizmann Institute of Science da ke Isra’ila sun juya wannan matsala. Mu’amala mara layi ɗaya da ke haƙa kwaruruka biyu tana kuma haɗa ɗaukakar (excitations) da ke cikin kowane kwari da resonator na karatu (readout resonator), wata da’ira da ba ta daina zubar da photon zuwa layin aunawa (measurement line).

Lokacin da makamashin wani excitation ya yi daidai da tazara tsakanin tuƙin (drive) da resonator na karatu — resonance na “autoparametric” — ana miƙa excitation ɗin a matsayin photon na karatu, wanda ke tserewa. 0 ko 1 ɗin kansa ba a taɓa shi ba. Magudana ta biyu tana bayyana ta hanyar matatar (filter) da ke kan layin fitarwa.

Ƙungiyar ta auna shi kai tsaye: a kan resonance, yanayin farko mai ɗaukaka yana zubewa da sauri gwargwadon yadda da’irar karatu ta yarda, kuma yanayi na sama suna zubewa a jere-jere (cascade). Guntunsu (chip) — aluminium a kan silicon, an sanyaya shi zuwa millikelvin 10 — bai buƙaci ƙarin tuƙin sanyaya ko ƙarin sashen da’ira ba.

Darajoji bakwai na girma

Nesa da resonance, juyewar bit har yanzu tana zuwa cikin millisecond. Kusa da su, kariyar ta yi tashin gwauron zabi:

• millisecond 519 a kan resonance na karatu (photon 22.4);
• daƙiƙa 15.4 tsakanin resonance biyu (photon 27.3);
• babu raguwar da za a iya auna ta yanayin da aka adana a cikin tagar aunawa ta daƙiƙa 100 a kan resonance na matata (photon 33.8), ma’ana lokacin juyewar bit ya fi daƙiƙa 100.

Ta hanyar ƙirga juyewa da ba kasafai ba a cikin dogayen bayanan karatu da aka maimaita — juyewa 62 cikin awa 15 — ƙungiyar ta ƙiyasta lokacin juyewar bit a wannan wuri na ƙarshe: daƙiƙa 448 ± 57, fiye da darajoji biyar na girma sama da kyanwoyin Kerr na baya.

Juyewar bit da ke raguwa a hanyar exponential tare da yawan photon babban hasashe ne da ke bayyana kyanwoyin Kerr, amma ba a taɓa ganin sa ba. Ga shi nan: kowane ƙarin photon ya ninka lokacin juyewar bit da 1.82 ± 0.06, daga microsecond 52 a photon 7.4 zuwa daƙiƙa 448 a 33.8, kusan darajoji bakwai na girma, ba tare da alamar tsayawa ba.

Ƙofofi (gates) masu sauri, karatu mai taushi

Kariya ba ta da amfani idan ba za a iya amfani da qubit ɗin ba kuma. Wata zagayawa ta asali (rotation) ta qubit ɗin da aka sanyaya ta ɗauki nanosecond 8, kuma barin tuƙin zagayawar a kunne a duk lokacin bai rage lokacin juyewar bit ba (daƙiƙa 0.65 da shi, daƙiƙa 0.64 ba tare da shi ba). Karanta qubit ɗin cikin microsecond 4 ya haifar da juyewa kawai sau 5 cikin miliyan 10 na karatu.

Ɗayan kuskuren

Farashin shi ne juyewar phase. Tana bin iyakar da ake tsammani wadda asarar photon guda-guda ke sanyawa, kusan T₁/(2n̄), inda T₁ shi ne tsawon rayuwar photon na da’irar (kusan microsecond 8) kuma n̄ yawan photon. A photon 24.2, kyanwar ta riƙe daƙiƙa 3.55 a kan juyewar bit amma nanosecond 113 kawai a kan juyewar phase: rashin daidaito (bias) na kusan miliyan 30 zuwa ɗaya.

Marubutan sun nuna hanyoyi biyu na rufe wannan gibi. An riga an nuna tsawon rayuwar photon na microsecond 55.7 a wani wuri, kusan sau bakwai fiye da na nan. Kuma da’irori masu sanyi fiye da haka za su bar ƙarancin ɗaukaka da suka ɓace, mai yiwuwa su ba da kariya iri ɗaya da ƙarancin photon — saboda haka ƙarancin juyewar phase.