Recently quantum computation devices have been intensively studied in various systems. Spins in silicon quantum dots (Si QDs) are one of the promising candidates for implementing quantum bits (qubits). Since only 4.7% of Si atoms are isotopes with nuclear spins, long coherence times of electron spins reflecting small hyperfine interactions were demonstrated [1-3]. Moreover, using isotopically-engineered Si [4], qubits with extended coherence time and gate operation with high fidelity were demonstrated [5-7]. Si qubit research has accelerated following the demonstration of these high fidelity single qubit operations and two-qubit logic gates [8]. The major remaining issue is large scale integration of qubits.

The advantage of Si qubits is that matured CMOS technologies can be applied for integration of qubits. At Tokyo Tech, we have studied physically-defined coupled QDs in MOS structures prepared on Si-on-insulator (SOI) substrates [9-20], which are promising for high density integration. The physically-defined devices do not require gates to create confinement potentials for the QDs. These devices thus represent a technological simplification owing to the reduced number of gates. Common fabrication processes normally used to produce Si MOS was applied to make the physically-defined Si QDs.

We have developed various device structures of physically-defined Si QDs. By measuring double QD devices, we succeeded in charge sensing of few-electron regime [11, 15, 16] and observation of Pauli-spin blockade [12]. In triple QDs arranged in an equilateral triangle, we demonstrated for the first time in the world a silicon-based two-dimensional arrayed QD structure [20]. We also fabricated and characterized Si double QDs in p-channel MOS and few-hole regime and Pauli-spin blockade were observed [14, 18, 19]. Furthermore, we formed Si single QDs on ultrathin (~6 nm) SOI and obtained comparatively large charging energy (~ 20 meV). We performed three-dimensional calculations of capacitance matrix and transport properties through the QD and found a good quantitative agreement with experiment [17]. These achievements are the important steps for realizing quantum computation devices.

We have also studied gate-defined QDs in Si/SiGe heterostructures to demonstrate high-fidelity gate operation through joint research with the University of Tokyo [3,6,7]. The fusion of Si devices and established GaAs qubit technology was advanced. Manipulation of spin states in Si QDs and single-shot readout using a measurement system constructed through development of GaAs qubits, demonstrating that the coherence time is about 100 times longer than that of GaAs systems [3]. The fidelity of single-qubit gate operation has been demonstrated to be 99.6% for native Si QDs [3] and 99.93% for isotopically-engineered Si QDs [6]. Both fidelity values are above the quantum error correction threshold of 99%.

This work was financially supported by Q-LEAP of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and CREST (JPMJCR1675) of Japan Science and Technology Agency (JST) in Japan.

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