A01 Proposed Research Projects (2016-2017)

Paper | Original Paper


*Jun Yoneda, Kenta Takeda, Tomohiro Otsuka, Takashi Nakajima, Matthieu R. Delbecq, Giles Allison, Takumu Honda, Tetsuo Kodera, Shunri Oda, Yusuke Hoshi, Noritaka Usami, Kohei M. Itoh, and Seigo Tarucha,
A quantum-dot spin qubit with coherence limited by charge noise and fidelity higher than 99.9%,
Nature Nanotechnology 13, 102-106 (2017).

[Summary] The isolation of qubits from noise sources, such as surrounding nuclear spins and spin–electric susceptibility, has enabled extensions of quantum coherence times in recent pivotal advances towards the concrete implementation of spin-based quantum computation. In fact, the possibility of achieving enhanced quantum coherence has been substantially doubted for nanostructures due to the characteristic high degree of background charge fluctuations. Still, a sizeable spin–electric coupling will be needed in realistic multiple-qubit systems to address single-spin and spin–spin manipulations. Here, we realize a single-electron spin qubit with an isotopically enriched phase coherence time (20 μs) and fast electrical control speed (up to 30 MHz) mediated by extrinsic spin–electric coupling. Using rapid spin rotations, we reveal that the free-evolution dephasing is caused by charge noise—rather than conventional magnetic noise—as highlighted by a 1/f spectrum extended over seven decades of frequency. The qubit exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average, offering a promising route to large-scale spin-qubit systems with fault-tolerant controllability.

*Akito Noiri, Tatsuki Takakura, Toshiaki Obata, Tomohiro Otsuka, Takashi Nakajima, Jun Yoneda, and Seigo Tarucha,
Cotunneling spin blockade observed in a three-terminal triple quantum dot,
Physical Review B 96, 155414/1-7 (2017).

[Summary] We prepare a triple quantum dot with a separate contact lead to each dot to study Pauli spin blockade in the tunnel-coupled three dots in a row. We measure the tunneling current flowing between the center dot and either the left or right dot with the left and right leads as a common source and the center lead as a drain. In the biased stability diagram, we establish Pauli spin blockade in the respective neighboring dots, with features similarly obtained in double-quantum-dot systems. We further realize Pauli spin blockade with two different conditions by tuning the interdot coupling gates: strong and weak interdot tunnel coupling regimes. In the strong-coupling regime we observe significant suppression of cotunneling through the respective double dots due to Pauli spin blockade. We reveal the influence from the third dot in the triple-dot device on this cotunneling Pauli spin blockade and clarify that the cotunneling Pauli spin blockade is lifted by the resonant coupling of excited states to the third dot level as well as spin exchange of the left and right dots with the adjacent reservoir.

*Tomohiro Otsuka, Takashi Nakajima, Matthieu R. Delbecq, Shinichi Amaha, Jun Yoneda, Kenta Takeda, Giles Allison, Peter Stano, Akito Noiri, Takumi Ito, Daniel Loss, Arne Ludwig, Andreas D. Wieck, and *Seigo Tarucha,
Higher-order spin and charge dynamics in a quantum dot-lead hybrid system,
Scientific Reports 7, 12201-1-7 (2017).

[Summary] Understanding the dynamics of open quantum systems is important and challenging in basic physics and applications for quantum devices and quantum computing. Semiconductor quantum dots offer a good platform to explore the physics of open quantum systems because we can tune parameters including the coupling to the environment or leads. Here, we apply the fast single-shot measurement techniques from spin qubit experiments to explore the spin and charge dynamics due to tunnel coupling to a lead in a quantum dot-lead hybrid system. We experimentally observe both spin and charge time evolution via first- and second-order tunneling processes, and reveal the dynamics of the spin-flip through the intermediate state. These results enable and stimulate the exploration of spin dynamics in dot-lead hybrid systems, and may offer useful resources for spin manipulation and simulation of open quantum systems.

*Hironori Hoshino, Shin Nakamura,
Phenomenological construction of new dictionaries for holographic conductors,
Physical Review D 96, 066006/1-10 (2017).

[Summary] We propose new dictionaries for holographic conductors that enable us to compute carrier densities andmean velocities of charge carriers in the system. The carrier density, which differs from the charge density,is the total number density of both the positive and the negative charge carriers. The mean velocity is themean value of the velocities of all charge carriers. These quantities are not conjugate to the sources that aregiven by boundary values of bulk fields, and we cannot compute them by using the conventional method inholography. In the present work, we introduce a phenomenological model of charge transport, and weestablish the dictionary by comparing the results of the phenomenological model and those from theholography. We show that the mean velocity agrees with the velocity of an analog black hole on theworldvolume of the probe D-brane, and it can be read from the spectrum of the fluctuations.

*Akito Noiri, Kento Kawasaki, Tomohiro Otsuka, Takashi Nakajima, Jun Yoneda, Shinichi Amaha, Matthieu Delbecq, Kenta Takeda, Giles Allison, Arne Ludwig, Andreas D. Wieck, and *Seigo Tarucha,
A triangular triple quantum dot with tunable tunnel couplings,
Semiconductor Science and Technology 32, 084004-1-5 (2017).

[Summary] A two-dimensional arrangement of quantum dots (QDs) with finite inter-dot tunnel coupling provides a promising platform for studying complicated spin correlations as well as for constructing large-scale quantum computers. Here, we fabricate a tunnel-coupled triangular triple QD with a novel gate geometry in which three dots are defined by positively biasing the surface gates. At the same time, the small area in the center of the triangle is depleted by negatively biasing the top gate placed above the surface gates. The size of the small center depleted area is estimated from the Aharonov–Bohm oscillation measured for the triangular channel but incorporating no gate-defined dots, with a value consistent with the design. With this approach, we can bring the neighboring gate-defined dots close enough to one another to maintain a finite inter-dot tunnel coupling. We finally confirm the presence of the inter-dot tunnel couplings in the triple QD from the measurement of tunneling current through the dots in the stability diagram. We also show that the charge occupancy of each dot and that the inter-dot tunnel couplings are tunable with gate voltages.

Takahiro Ishida, Yohann Duguet, *Takahiro Tsukahara,
Turbulent bifurcations in intermittent shear flows: from puffs to oblique stripes,
Physical Review Fluids 2, 073902/1-18 (2017).

[Summary] Localized turbulent structures such as puffs or oblique stripes are building blocks of the intermittency regimes in subcritical wall-bounded shear flows. These turbulent structures are investigated in incompressible pressure-driven annular pipe flow using direct numerical simulations in long domains. For low enough radius ratio η, these coherent structures have a dynamics comparable to that of puffs in cylindrical pipe flow. For η larger than 0.5, they take the shape of helical stripes inclined with respect to the axial direction. The transition from puffs to stripes is analyzed statistically by focusing on the axisymmetry properties of the associated large-scale flows. It is shown that the transition is gradual: as the azimuthal confinement relaxes, allowing for an azimuthal large-scale component, oblique stripes emerge as predicted in the planar limit. The generality of this transition mechanism is discussed in the context of subcritical shear flows.

*Takashi Nakajima, Matthieu R. Delbecq, Tomohiro Otsuka, Peter Stano, Shinichi Amaha, Jun Yoneda, Akito Noiri, Kento Kawasaki, Kenta Takeda, Giles Allison, Arne Ludwig, Andreas D. Wieck, Daniel Loss, and *Seigo Tarucha,
Robust single-shot spin measurement with 99.5% fidelity in a quantum dot array,
Physical Review Letters 119, 017701-1-6 (2017).

[Summary] We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon.


*Takumi Ito, Tomohiro Otsuka, Shinichi Amaha, Matthieu R. Delbecq, Takashi Nakajima, Jun Yoneda, Kenta Takeda, Giles Allison, Akito Noiri, Kento Kawasaki, and Seigo Tarucha,
Detection and control of charge states in a quintuple quantum dot,
Scientific Reports 6, 39113 (2016).

[Summary] A semiconductor quintuple quantum dot with two charge sensors and an additional contact to the center dot from an electron reservoir is fabricated to demonstrate the concept of scalable architecture. This design enables formation of the five dots as confirmed by measurements of the charge states of the three nearest dots to the respective charge sensor. The gate performance of the measured stability diagram is well reproduced by a capacitance model. These results provide an important step towards realizing controllable large scale multiple quantum dot systems.

*Tomohiro Otsuka, Takashi Nakajima, Matthieu R. Delbecq, Shinichi Amaha, Jun Yoneda, Kenta Takeda, Giles Allison, Takumi Ito, Retsu Sugawara, Akito Noiri, Arne Ludwig, Andreas D. Wieck, and Seigo Tarucha,
Single-electron Spin Resonance in a Quadruple Quantum Dot,
Scientific Reports 6, 31820 (2016).

[Summary] Electron spins in semiconductor quantum dots are good candidates of quantum bits for quantum information processing. Basic operations of the qubit have been realized in recent years: initialization, manipulation of single spins, two qubit entanglement operations, and readout. Now it becomes crucial to demonstrate scalability of this architecture by conducting spin operations on a scaled up system. Here, we demonstrate single-electron spin resonance in a quadruple quantum dot. A few-electron quadruple quantum dot is formed within a magnetic field gradient created by a micro-magnet. We oscillate the wave functions of the electrons in the quantum dots by applying microwave voltages and this induces electron spin resonance. The resonance energies of the four quantum dots are slightly different because of the stray field created by the micro-magnet and therefore frequency-resolved addressable control of each electron spin resonance is possible.

*Kenta Takeda, Jun Kamioka, Tomohiro Otsuka, Jun Yoneda, Takashi Nakajima, Matthieu R. Delbecq, Shinichi Amaha, Giles Allison, Tetsuo Kodera, Shunri Oda, and Seigo Tarucha,
A fault-tolerant addressable spin qubit in a natural silicon quantum dot,
Science Advances 2, e1600694 (2016).

[Summary] Fault-tolerant quantum computing requires high-fidelity qubits. This has been achieved in various solid-state systems, including isotopically purified silicon, but is yet to be accomplished in industry-standard natural silicon, mainly as a result of the dephasing caused by residual nuclear spins. This high fidelity can be achieved by speeding up the qubit operation and/or prolonging the dephasing time, that is, increasing the Rabi oscillation quality factor Q. In isotopically purified silicon quantum dots, only the second approach has been used, leaving the qubit operation slow. We apply the first approach to demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet that is designed for fast spin control. This optimized design allows access to Rabi frequencies up to 35 MHz, which is two orders of magnitude greater than that achieved in previous studies. We find the optimum Q = 140 in such high-frequency range at a Rabi frequency of 10 MHz. This leads to a qubit fidelity of 99.6% measured via randomized benchmarking, which is the highest reported for natural silicon qubits and comparable to that obtained in isotopically purified silicon quantum dot–based qubits. This result can inspire contributions to quantum computing from industrial communities.