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S. Oda

Publications and source records attributed to S. Oda.

18 recordsLinked to original sources
arXiv2018

Optimized electrical control of a Si/SiGe spin qubit in the presence of an induced frequency shift

Electron spins confined in quantum dots are an attractive system to realize high-fidelity qubits owing to their long coherence time. With the prolonged spin coherence time, however, the control fidelity can be limited by systematic errors rather than decoherence, making characterization and suppression of their influence crucial for further improvement. Here we report that the control fidelity of Si/SiGe spin qubits can be limited by the microwave-induced frequency shift of electric dipole spin resonance and it can be improved by optimization of control pulses. As we increase the control microwave amplitude, we observe a shift of the qubit resonance frequency, in addition to the increasing Rabi frequency. We reveal that this limits control fidelity with a conventional amplitude-modulated microwave pulse below 99.8%. In order to achieve a gate fidelity > 99.9%, we introduce a quadrature control method, and validate this approach experimentally by randomized benchmarking. Our finding facilitates realization of an ultra-high fidelity qubit with electron spins in quantum dots.

cond-mat.mes-hall↗
arXiv2017

A >99.9%-fidelity quantum-dot spin qubit with coherence limited by charge noise

Recent advances towards spin-based quantum computation have been primarily fuelled by elaborate isolation from noise sources, such as surrounding nuclear spins and spin-electric susceptibility, to extend spin coherence. In the meanwhile, addressable single-spin and spin-spin manipulations in multiple-qubit systems will necessitate sizable spin-electric coupling. Given background charge fluctuation in nanostructures, however, its compatibility with enhanced coherence should be crucially questioned. Here we realise a single-electron spin qubit with isotopically-enriched phase coherence time (20 microseconds) 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 (instead of conventional magnetic) noise featured by a 1/f spectrum over seven decades of frequency. The qubit nevertheless exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average. Our work strongly suggests that designing artificial spin-electric coupling with account taken of charge noise is a promising route to large-scale spin-qubit systems having fault-tolerant controllability.

cond-mat.mes-hall↗
arXiv2016

A fault-tolerant addressable spin qubit in a natural silicon quantum dot

Fault-tolerant quantum operation is a key requirement for the development of quantum computing. This has been realized in various solid-state systems including isotopically purified silicon which provides a nuclear spin free environment for the qubits, but not in industry standard natural (unpurified) silicon. Here we demonstrate an addressable fault-tolerant qubit using a natural silicon double quantum dot with a micromagnet optimally designed for fast spin control. This optimized design allows us to achieve the optimum Rabi oscillation quality factor Q = 140 at a Rabi frequency of 10 MHz in the frequency range two orders of magnitude higher than that achieved in previous studies. This leads to a qubit fidelity of 99.6 %, 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 from the industrial and quantum computing communities.

cond-mat.mes-hall↗
arXiv2013

GHz photon-activated hopping between localized states in a silicon quantum dot

We discuss the effects of gigahertz photon irradiation on a degenerately phosphorous-doped silicon quantum dot, in particular, the creation of voltage offsets on gate leads and the tunneling of one or two electrons via Coulomb blockade lifting at 4.2K. A semi-analytical model is derived that explains the main features observed experimentally. Ultimately both effects may provide an efficient way to optically control and operate electrically isolated structures by microwave pulses. In quantum computing architectures, these results may lead to the use of microwave multiplexing to manipulate quantum states in a multi-qubit configuration.

cond-mat.mes-hall↗
arXiv2013

Charge-noise-free Lateral Quantum Dot Devices with Undoped Si/SiGe Wafer

We develop quantum dots in a single layered MOS structure using an undoped Si/SiGe wafer. By applying a positive bias on the surface gates, electrons are accumulated in the Si channel. Clear Coulomb diamond and double dot charge stability diagrams are measured. The temporal fluctuation of the current is traced, to which we apply the Fourier transform analysis. The power spectrum of the noise signal is inversely proportional to the frequency, and is different from the inversely quadratic behavior known for quantum dots made in doped wafers. Our results indicate that the source of charge noise for the doped wafers is related to the 2DEG dopant.

cond-mat.mes-hall↗
arXiv2013

Characterization and Suppression of Low-frequency Noise in Si/SiGe Quantum Point Contacts and Quantum Dots

We report on the effects of a global top gate on low-frequency noise in Schottky gate-defined quantum point contacts (QPCs) and quantum dots (QDs) in a modulation-doped Si/SiGe heterostructure. For a relatively large top gate voltage, the QPC current shows frequent switching with 1/f2 Lorentzian type charge noise. As the top gate voltage is decreased, the QPC pinch-off voltage becomes less negative, and the 1/f2 noise becomes rapidly suppressed in a homogeneous background 1/f noise. We apply this top-gating technique to double QDs to stabilize the charge state for the electron number down to zero.

cond-mat.mes-hall↗
arXiv2012

Localization effects in the tunnel barriers of phosphorus-doped silicon quantum dots

We have observed a negative differential conductance with singular gate and source-drain bias dependences in a phosphorus-doped silicon quantum dot. Its origin is discussed within the framework of weak localization. By measuring the current-voltage characteristics at different temperatures as well as simulating the tunneling rates dependences on energy, we demonstrate that the presence of shallow energy defects together with an enhancement of localization satisfactory explain our observations. Effects observed in magnetic fields are also discussed.

cond-mat.mes-hall↗
arXiv2011

Magnetic field dependence of Pauli spin blockade: a window into the sources of spin relaxation in silicon quantum dots

We investigate spin relaxation in a silicon double quantum dot via leakage current through Pauli blockade as a function of interdot detuning and magnetic field. A dip in leakage current as a function of magnetic field on a \sim 40 mT field scale is attributed to spin-orbit mediated spin relaxation. On a larger (\sim 400 mT) field scale, a peak in leakage current is seen in some, but not all, Pauli-blocked transitions, and is attributed to spin-flip cotunneling. Both dip and peak structure show good agreement between theory and experiment.

cond-mat.mes-hall↗
arXiv2011

Detection of variable tunneling rates in silicon quantum dots

Reliable detection of single electron tunneling in quantum dots (QD) is paramount to use this category of device for quantum information processing. Here, we report charge sensing in a degenerately phosphorus-doped silicon QD by means of a capacitively coupled single-electron tunneling device made of the same material. Besides accurate counting of tunneling events in the QD, we demonstrate that this architecture can be operated to reveal asymmetries in the transport characteristic of the QD. Indeed, the observation of gate voltage shifts in the detector's response as the QD bias is changed is an indication of variable tunneling rates.

cond-mat.mes-hall↗
arXiv2010

Early B-physics at ATLAS

The B-physics program at the ATLAS experiment, which covers the mid-rapidity region, complements that at the dedicated LHCb experiment, which covers the forward rapidity region. At the early stage of the LHC operation, the program concentrated on understanding of detector performance and measurements of quarkonia and D mesons. This article presents recent results of the B-physics program at ATLAS.

hep-ex↗
arXiv2007

Bottom-Up Approach to Silicon Nanoelectronics

This paper presents a brief review of our recent work investigating a novel bottom-up approach to realize silicon based nanoelectronics. We discuss fabrication technique, electronic properties and device applications of silicon nanodots as a building block for nanoscale silicon devices.

cond-mat.mtrl-sci↗
arXiv1999

Introducing Dynamical Triangulations to the Type IIB Superstrings

In order to consider non-perturbative effects of superstrings, we try to apply dynamical triangulations to the type IIB superstrings. The discretized action is constructed from the type IIB matrix model proposed as a constructive definition of superstring theory. The action has the local N=2 supersymmetry explicitly, and has no extra fermionic degrees of freedom. We evaluate the partition function for some simple configurations and discuss constraints required from the finiteness of partition functions.

hep-lat↗
arXiv1999

Type IIB Random Superstrings

We consider random superstrings of type IIB in $d$-dimensional space. The discretized action is constructed from the supersymetric matrix model, which has been proposed as a constructive definition of superstring theory. Our action is invariant under the local N=2 super transformations, and doesn't have any redundant fermionic degrees of freedom.

hep-th↗
arXiv1998

TeV gamma-ray observations of southern BL Lacs with the CANGAROO 3.8m Imaging Telescope

Observational and theoretical results indicate that low-redshift BL Lacertae objects are the most likely extragalactic sources to be detectable at TeV energies. In this paper we present the results of observations of 4 BL Lacertae objects (PKS0521-365, EXO0423.4-0840, PKS2005-489 and PKS2316-423) made between 1993 and 1996 with the CANGAROO 3.8m imaging Cherenkov telescope. During the period of these observations the gamma-ray energy threshold of the 3.8m telescope was ~2TeV. Searches for steady long-term emission have been made, and, inspired by the TeV flares detected from Mkn421 and Mkn501, a search on a night-by-night timescale has also been performed for each source. Comprehensive Monte Carlo simulations are used to estimate upper limits for both steady and short timescale emission.

astro-ph↗
arXiv1998

Discovery of TeV Gamma Rays from SN1006: Further Evidence for the SNR Origin of Cosmic Rays

This paper reports the first discovery of TeV gamma-ray emission from a supernova remnant made with the CANGAROO 3.8 m Telescope. TeV gamma rays were detected at the sky position and extension coincident with the north-east (NE) rim of shell-type Supernova remnant (SNR) SN1006 (Type Ia). SN1006 has been a most likely candidate for an extended TeV Gamma-ray source, since the clear synchrotron X-ray emission from the rims was recently observed by ASCA (Koyama et al. 1995), which is a strong evidence of the existence of very high energy electrons up to hundreds of TeV in the SNR. The observed TeV gamma-ray flux was $(2.4\pm 0.5(statistical) \pm 0.7(systematic)) \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ ($\ge 3.0\pm 0.9$ TeV) and $ (4.6\pm 0.6 \pm 1.4) \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ ($\ge 1.7\pm 0.5$ TeV) from the 1996 and 1997 observations, respectively. Also we set an upper limit on the TeV gamma-ray emission from the SW rim, estimated to be $ 1.1 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ ($\ge 1.7\pm 0.5$ TeV, 95% CL) in the 1997 data. The TeV gamma rays can be attributed to the 2.7 K cosmic background photons up-scattered by electrons of energies up to about 10$^{14}$ eV by the inverse Compton (IC) process. The observed flux of the TeV gamma rays, together with that of the non-thermal X-rays, gives firm constraints on the acceleration process in the SNR shell; a magnetic field of $6.5\pm2$ $μ$G is inferred from both the synchrotron X-rays and inverse Compton TeV gamma-rays, which gives entirely consistent mechanisms that electrons of energies up to 10$^{14}$ eV are produced via the shock acceleration in SN1006.

astro-ph↗
arXiv1997

Grand-canonical simulation of two-dimensional simplicial gravity

The string susceptibility exponents of dynamically triangulated 2-dimensional surfaces with various topologies, such as a sphere, torus and double-torus, were calculated by the grand-canonical Monte Carlo method. These simulations were made for surfaces coupled to $d$-Ising spins ($d$=0,1,2,3,5). In each simulation the area of surface was constrained to within 1000 to 3000 of triangles, while maintaining the detailed-balance condition. The numerical results show excellent agreement with theoretical predictions as long as $d \leq 2$.

hep-lat↗
arXiv1997

Very High Energy Gamma Rays from the Vela Pulsar Direction

We have observed the Vela pulsar region at TeV energies using the 3.8 m imaging Cherenkov telescope near Woomera, South Australia between January 1993 and March 1995. Evidence of an unpulsed gamma-ray signal has been detected at the 5.8 sigma level. The detected gamma-ray flux is (2.9 +/- 0.5 +/- 0.4) x 10^-12 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV and the signal is consistent with steady emission over the two years. The gamma-ray emission region is offset from the Vela pulsar position to the southeast by about 0.13 deg. No pulsed emission modulated with the pulsar period has been detected and the 95 % confidence flux upper limit to the pulsed emission from the pulsar is (3.7 +/- 0.7) x 10^-13 photons cm^-2 sec^-1 above 2.5 +/- 1.0 TeV.

astro-ph↗