Publications (16)112.76 Total impact
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ABSTRACT: Devices to generate ondemand nonlocal spin entangled electron pairs have potential application as solidstate analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasiparticles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this nonlocal transport is confirmed through study of the nondissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair.Nature Communications 07/2015; 6:7446. DOI:10.1038/ncomms8446 · 11.47 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Threedimensional anisotropy of the Lande gfactor and its electrical modulation are studied for single uncapped InAs selfassembled quantum dots (QDs). The gfactor is evaluated from measurement of inelastic cotunneling via Zeeman substates in the QD for various magnetic field directions. We find that the value and anisotropy of the gfactor depends on the type of orbital state which arises from the threedimensional confinement anisotropy of the QD potential. Furthermore, the gfactor and its anisotropy are electrically tuned by a sidegate which modulates the confining potential.Physical review. B, Condensed matter 03/2013; 87(16). DOI:10.1103/PhysRevB.87.161302 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We use a superconducting quantum interference device (SQUID) to detect the phase of an InAs quantum dot Josephson junction (QDJJ) in the Kondo regime. The QDJJ phase is derived from the measurement of superconducting interference. The πjunction behavior is observed for a QD with an odd electron number and the phase transition from π to 0 is observed by changing the parity of the electron number. In the Kondo regimes, we find that the QDJJ becomes a 0junction even though the electron number is odd.Journal of Physics Conference Series 12/2012; 400(4):2027. DOI:10.1088/17426596/400/4/042027  [Show abstract] [Hide abstract]
ABSTRACT: We study supercurrent in a single InAs selfassembled quantum dot contacted with superconducting leads and demonstrate that for regions where energy level spacing and charging energy are smaller than tunnel coupling, the supercurrent may be controlled by the degree of overlaps between energy levels, which is tunable using a sidegate electrode. In such regions, we find strong correlation between the supercurrent and the normal state conductance when the device parameters are tuned. In a Kondo regime with low Kondo temperature, we find that the scaling of the supercurrent and normal state conductance varies when the sidegate voltage is changed.Applied Physics Letters 05/2012; 100(20):202109. DOI:10.1063/1.4719072 · 3.30 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Electrical control over electron spin is a prerequisite for spintronics spinbased quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spinorbit interaction in twodimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spinorbit interaction in the range 50150 µeV while maintaining the electron occupation of a single selfassembled InAs quantum dot. We determine the spinorbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.Nature Nanotechnology 12/2011; 7(1):75. DOI:10.1038/nnano.2011.228 · 34.05 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Electrical control over electron spin is a prerequisite for spintronics spinbased quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spinorbit interaction in twodimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spinorbit interaction in the range 50150 µeV while maintaining the electron occupation of a single selfassembled InAs quantum dot. We determine the spinorbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.Nature Nanotechnology 07/2011; 6(8):5116. DOI:10.1038/nnano.2011.103 · 34.05 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the allelectrical tuning of the Landé g tensor in a single uncapped InAs quantum dot contacted with a nanogap electrode technique and electrically gated with both back and sidegate electrodes. Magnetotransport measurements allow extraction of the g tensor components from measurements of the Zeeman energy for magnetic fields applied in the plane of the sample. The sidegate electrode allows tuning of the anisotropy of the inplane g tensor components and is suitable for the manipulation of the quantumdot spin states using g factor modulation resonance schemes.Physical Review B 07/2011; 84(4):041302. DOI:10.1103/PhysRevB.84.041302 · 3.74 Impact Factor 
Article: Spin‐orbit interaction detection using Kondo effect in single self‐assembled InAs quantum dots
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ABSTRACT: We demonstrate the detection of spinorbit interaction and evaluation of spinorbit interaction energy using the Kondo effect in selfassembled InAs quantum dots with large tunnel coupling.AIP Conference Proceedings 01/2011; 1399:355. DOI:10.1063/1.3666400  [Show abstract] [Hide abstract]
ABSTRACT: We measure the nondissipative supercurrent in a single InAs selfassembled quantum dot (QD) coupled to superconducting leads. The QD occupation is both tuned by a backgate electrode and lateral sidegate. The geometry of the sidegate allows tuning of the QDlead tunnel coupling in a region of constant electron number with appropriate orbital state. Using the sidegate effect we study the competition between Kondo correlations and superconducting pairing on the QD, observing a decrease in the supercurrent when the Kondo temperature is reduced below the superconducting energy gap in qualitative agreement with theoretical predictions.Physical Review B 08/2010; 82(5):054512. DOI:10.1103/PhysRevB.82.054512 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The anisotropy of the spinorbit interaction (SOI) is studied for a single uncapped InAs selfassembled quantum dot holding just a few electrons. The SOI energy is evaluated from anticrossing or SOIinduced hybridization between the ground and excited states with opposite spins. The magnetic angular dependence of the SOI energy falls on an absolute cosine function for azimuthal rotation, and a cosinelike function for tilting rotation. Furthermore, the SOI energy is quenched for a specific magnetic field vector. The angular dependence of SOI is found to compare well with calculation of Rashba SOI in a twodimensional harmonic potential.Physical Review Letters 06/2010; 104(24):246801. DOI:10.1103/PhysRevLett.104.246801 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study transport in selfassembled InAs quantum dots contacted with one superconducting and one normalmetal electrode. Low bias transport is dominated by Andreev processes which are sensitive to local correlations such as electronelectron interaction and the Kondo effect. We identify that, for appropriate tunnel coupling with normal and superconducting leads, Andreev transport is enhanced by the Kondo effect and that the Kondo temperature is reduced relative to the normal state due to lack of lowenergy excitations with the superconducting lead.Physical Review B 03/2010; 81(12):121308R. DOI:10.1103/PhysRevB.81.121308 · 3.74 Impact Factor 
Article: Tunneling Spectroscopy of Andreev Energy Levels in a Quantum Dot Coupled to a Superconductor
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ABSTRACT: The coupling of a quantum dot with a BCStype superconducting reservoir results in an intriguing system where low energy physics is governed by the interplay of two distinct phases, singlet and doublet. In this Letter we show that the spectrum of Andreev energy levels, which capture the properties of the two phases, can be detected in transport measurements with a quantum dot strongly coupled to a superconducting lead and weakly coupled to a normal metal lead. We observe phase transitions between BCS singlet and degenerate magnetic doublet states when the quantum dot chemical potential is tuned with an electrostatic gate, in good qualitative agreement with numerical renormalization group calculations.Physical Review Letters 02/2010; 104(7):076805. DOI:10.1103/PhysRevLett.104.076805 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present measurements of proximity supercurrent in single self assembled InAs quantum dots contacted with superconducting Aluminium electrodes. The magnitude of the switching current displays strong evenodd parity as singletlike correlation impedes the proximity effect.AIP Conference Proceedings 01/2010; 1199:275. DOI:10.1063/1.3295406 
Article: Kondo Effect and Superconductivity in Single InAs Quantum Dots Contacted with Superconducting Leads
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ABSTRACT: We study the transport properties of a single InAs selfassembled quantum dot contacted with superconducting leads. The charging energy Ec of the quantum dot is much larger than the superconducting gap energy Delta. For the dot whose tunnel coupling Gamma to the lead is much larger than Delta but smaller than Ec, we observe enhancement of firstorder Andreev reflections by the Kondo effect. We find that the zerobias conductance measured for various Delta's and Kondo temperature TK's collapses onto a single curve with Delta/TK as the only relevant energy scale, providing experimental evidence for universal scaling in this system. On the other hand, for the dot with Gamma comparable to Ec we observe a supercurrent flowing through the dot, reflecting the charge fluctuation sufficiently greater that one.  [Show abstract] [Hide abstract]
ABSTRACT: We report a first direct observation of proximity supercurrent in a single InAs selfassembled quantum dot coupled to superconducting electrodes. We utilize large SAQDs with strong dotlead coupling allowing sufficient overlap of quantized state that charge fluctuation in the dot may exceed one electron. Although the charging energy is larger than the superconductivity gap, Δ, the voltage characteristic measured in the currentdriven fourterminal geometry signifies a supercurrent flow though the dot. The observed critical current is of the order of 10 pA, and much smaller than expected from theory. This current also shows the parity effect with the number of electrons in the dot, suggesting that the singlet correlation impedes the Cooper pair tunneling.Journal of Physics Conference Series 03/2009; 150(2):022032. DOI:10.1088/17426596/150/2/022032  [Show abstract] [Hide abstract]
ABSTRACT: Influence of spinorbit interaction (SOI) on the energy spectrum is studied for single uncapped InAs selfassembled quantum dots (SAQDs) holding just a few electrons. We measure Coulomb oscillations with magnetic field, and identify lifting of spin doublets in the low field range and transitions in the ground states with keeping the total angular momentum in the high field range. Both of these features are induced by large Zeeman splitting in the present SAQDs. For the ground state transitions, anticrossing due to the SOI hybridization between the ground and excited states with opposite spins is observed in the excitation spectra, and the SOI energy of about 0.1 meV is evaluated from the anticrossing size. This energy is comparable to that of previous report on InAs nanowires, and reproduced by simple calculation using the gfactor also derived in the excitation spectra.Journal of Physics Conference Series 03/2009; 150(2):022084. DOI:10.1088/17426596/150/2/022084
Publication Stats
185  Citations  
112.76  Total Impact Points  
Top Journals
Institutions

2013

RIKEN
 Advanced Device Laboratory
Вако, Saitama, Japan


2009–2012

The University of Tokyo
 Department of Applied Physics
Tōkyō, Japan
