論文 / 著書情報
Article / Book Information
題目(和文)
Title(English)

A study of charge sensing techniques in physically defined siliconquantum dots

著者(和文)

溝口来成

Author(English)

Raisei Mizokuchi

出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第12144号,
授与年月日:2021年12月31日,
学位の種別:課程博士,
審査員:小寺 哲夫,波多野 睦子,宮本 恭幸,若林 整,鈴木 左文,河野 行雄,小田 俊理,小林 正治

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12144号,
Conferred date:2021/12/31,
Degree Type:Course doctor,Examiner:,,,,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

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（論文博士）(Dissertation Doctorate)

論文

要旨 （ 英 文 ）（800語程度）

Dissertation Summary (approx. 800 words in English)

報告番号乙 第号

氏名NameFor

溝口来成administrativeuse only（ 要旨 ）(Summary)

Qubits based on various quantum systems have been studied to build large-scale qubit systems. One
promising qubit for large-scale integration is a spin qubit in a silicon quantum dot, which has a small
structure, a long coherence time, and a compatibility with existing CMOS technology. For the readoutof a spin qubit, one measures spin-dependent charge distribution from the impedance change of thecharge sensor via spin-to-charge conversion techniques. For quantum error correction, a fast (wide bandwidth) measurement within the coherence time (about 10^-5 second) is of particular importance.
On the other hand, a wide bandwidth will inevitably increase the noise; hence, it is not easy to achieve
high-speed spin detection.
To build the large-scale qubit system, it is considered to be important to study a qubit system having
a balance among the following three ingredients: suitability to two-dimensional integration, goodreadability, and good controllability. Recently, integration of silicon quantum dot systems having goodreadability and controllability has been studied, however, where there is a lack of studies on twodimensionally scalable systems. In this thesis, we first focus on development of a minimum unit of twodimensional quantum dot array and then apply charge sensing techniques to the system as a first step
to demonstrate its readability. Here, we separately utilize digital and analog techniques to the readout
and demonstrate that both of them are useful to detect signal from noisy environment.
At first, we explore the low-temperature transport properties of two types of physically defined triple
quantum dot devices: linear and triangular ones. The linear device has three tunnel -coupled quantum
dots in line while the triangular device at each vertex of a triangle. Thi s quantum dot system has thefollowing advantages for large scale integration: 1) it is fabricated from an industry -standard silicon-oninsulator wafer; and 2) it is physically defined by the shape of the silicon channel layer itself, so that
there is no need to use gate electrodes to define the quantum dot structure. In experiment, we studied
charge transport through the systems by directly measuring the current and by charge sensing techniqueusing additional quantum dot structures. The results reveal the formation of each triple quantum dot
system without unintentional quantum dots as expected.
Next, we detect single tunneling events in a physically defined silicon triple quantum dot system. Inmeasurements, we observe single-shot tunneling events by using a charge sensor; however, it turned

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out that the signals are too weak to acquire tunneling statistics by binary thresholding. To overcome thisproblem, we apply numerical treatments, one of which utilizes the maximum log -likelihood ratio test,
and thereby, the step position is precisely indicated. Its superiority is confirmed by comparing a digital
filtering in the simulation. The results demonstrate its potential for spin readouts from noisy
environments.
Finally, we apply radio-frequency reflectometry technique to a physically defined silicon quantum dot
system. To readout spins in this system, as in many other quantum dot systems, the radio -frequency
reflectometry will be of a significant importance; however, measuring clear reflectometry signals
depending on quantum states has so far been elusive. This is likely attributed to the top gate structure
overlapping reservoirs, which causes leakage of radio-frequency signals. To avoid this common problem
of insensitivity, here we employ physically defined quantum dots without the top gate structure, leading
to successful reflectometry as expected. Moreover, we observe large phase shifts corresponding toconductance peak of quantum dot (about 45 degree). To reveal the mechanism, we analyze the data
based on an equivalent circuit consisting of an inductor, a capacitor, a quantum dot impedance, andtheir parasitic components which are typically neglected in previous reflectometry studies. By asimulation with the equivalent circuit, the measurement data is su ccessfully reproduced, which allows
us to conclude that reflectometry phase signals can be greatly enhanced by a good impedance matching
and an appropriate frequency detuning.
In conclusion, we have developed elemental technologies for fast charge detection using physicallydefined silicon quantum dots: detection of charge states in a triangular triple quantum dot structure,
which is the smallest unit of integration; single-shot measurement of single tunneling events of electrons
supported by log-likelihood ratio test; realization of radio-frequency reflectometry in physically definedquantum dot for wideband sensing and the analysis using equivalent circuit. We believe that this workpaves a way for fast spin readout in large-scale quantum dot system.

備考：論文要旨は、和文2000字と英文300語を1部ずつ提出するか、もしくは英文800語を1部提出してください。
Note: Dissertation summaries must be written in either of the following formats: (A) both in Japanese (approx. 2000 characters) and in
English (approx. 300 words), or (B) in English (approx. 800 words).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Important: Dissertation summaries will be published online on the Tokyo Tech Research Repository (T2R2). Do not include
information treated as confidential under certain circumstances.