超導磁通量元之去相干時間量測

Decoherence time measurements in a superconducting flux qubit

吳承恩(Cheng-En Wu)

指導教授 ： 齊正中

國立清華大學/理學院/物理學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00149

摘要

在本實驗中，我們製作了數個超導磁通量元，並選擇其中一個好的樣品來量測去相干時間。我們的製程和其他實驗室的不同點是，我們的超導磁通量元的環路大小是50微米*70微米，而其他實驗室則是10微米*10微米。使用大環路的超導磁通量元的好處是，我們可以利用晶片上的微米線作精確且獨立的操控。[1]

本論中文，我們報告了單個磁通量元的一些量測結果。這些量測都在0.05度K的溫度下進行的。我們透過能譜的量測，成功的觀測到巨觀量子相干現象。這正是巨觀態的量子線性疊加的一個證據。我們同時也量測了拉比震盪（Rabi oscillation）作為實現操控單一磁通量元的証明。關於去相干時間的量測，鬆弛時間（relaxation time）和去相位時間（dephasing time）分別是150奈秒和7奈秒。這些量測結果均小於spin-boson模型的估計。我們在論中文將有詳細討論。

關鍵字

量子電腦 ； 超導量子干涉儀 ； 約瑟芬結 ； 磁通量元

並列摘要

In this experiment, we fabricated the three-Josephson-junction(3JJ) superconducting °ux
qubits with a large loop size(50¹m £ 70¹m) that the self-inductance(150pH) is much larger
than previous studies from other groups(10pH). The reason for choosing a large inductance
is to facilitate the on-chip bias scheme through the on-chip superconducting lines to control
the qubit's energy levels and to increase the SQUID's readout sensitivity. The on-chip bias
scheme we proposed[1] here is a ‾rst step of realizing scalability by utilizing the quantum
engineering of Josephson devices.
In this dissertation, we present several single qubit measurements at 50mK bath temper-
ature. First, the quantum superposition of macroscopically distinct °ux states is observed
by measuring the level anti-crossing energy spectrum. The measured tunneling splitting is
4GHz. This is an indirect prove of macroscopic quantum coherence(MQC).
The single qubit manipulation is also demonstrated by measuring the Rabi oscillation
which is driven by microwave photons. Through this measurements, we can de‾ne the pulse
con‾gurations of ¼-pulse and ¼=2-pulse, or more speci‾cally, any single qubit unitary oper-
ation. The measured Rabi decay time can vary from a few nanosecond to several tens of
nanosecond depending on the qubit energy level spacing. A clear Rabi oscillation with 40ns
decay time was observed around 10GHz energy level spacing.
The life time of the excited state, or the relaxation time Tr, is about 150ns at 10GHz
level spacing. The phase coherence time, or the dephasing time TÁ, is measured in several
di®erent ways. One method is to measure the Ramsey fringe which is similar to free-induction
decay(FID) in NMR system. The other one is to apply the spin echo technique to see the
decay of echo signal. The former one gives the dephasing time 7ns and the latter one is 30ns.
Both Tr and TÁ are much shorter than the estimation of the formula derived from spin-boson
model.

並列關鍵字

quantum computer ； SQUID ； Josephson junction ； flux qubit

參考文獻

J. Clarke. Flux qubits and readout device with two independent °ux lines. Phys. Rev.

[2] P. W. Shor. Algorithms for quantum compuation: Discrete log and factoring. In Proc.

35 Annual Symp. On Foundations of Computer Science, page 124, 1994.

Chuang. Experimental realization of shor's quantum factoring algorithm using nuclear

magnetic resonance. Nature, 414:883, 2001.