H. Takayanagi. Toward the development of quantum media interface from optical to superconducting state: Report of recent progress

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Meeting (2011. Dec. 13-16) Toward the development of quantum media interface from optical to superconducting state: Report of recent progress H.

1 Meeting (2011. Dec ) Toward the development of quantum media interface from optical to superconducting state: Report of recent progress H. Takayanagi Tokyo University of Science, Tokyo, Japan NIMS, Tsukuba, Japan S. Kim, M. Kamio, H. Yabuki, B. Kaviraj, R. Ishiguro, E. Watanabe, D. Tsuya, K. Shibata, K. Hirakawa

2 Outline I. Research motivation II. single quantum dot (QD) coupled to SQUID III. QD with SIS SQUID IV. Preliminary experiment for optical excitation of QD spin

3 Superconducting qubit team From website of FIRST project

I. Research motivation Optical quantum bit 1.5 mm 0.

Spin in QD Superconducting Quantum bit Δ E~10GHz (~400 μev) Toward

quantum information from optical to superconducting state, our

dot (and ring) in hybrid semiconductorsuperconductor system.

memory) -Electronically coupled and embedded into active device -

4 I. Research motivation Optical quantum bit 1.5 mm ev Large mismatch on energy X Transport of quantum information Spin in QD Superconducting Quantum bit Δ E~10GHz (~400 μev) Toward the development of quantum media interface which can transfer quantum information from optical to superconducting state, our approach is based on the spin state of self-assembled InAs quantum dot (and ring) in hybrid semiconductorsuperconductor system. InAs Self-assembled quantums dot -Optically excited spin (spin memory) -Electronically coupled and embedded into active device - Strong spin orbit interaction - Schottky barrier free contact - Highly controllable electron system SA-QD SA-QR

5 Study for single spin/spin ensemble of electron(s) in InAs QD a. For single spin b. For spin ensemble QD Al/Au 70 nm Al/Au Laser QDs - + or or or SQUID with metal mask (SEM image from R. Ishiguro) QD-SQUID Al-SQUID 2011/12/26 5

II. QD-coupled to SQUID Quantum dot (QD) Superconducting quantum interference device (SQUID) QD-SQUID QD + = Highly controllable electron system Most sensitive detector for magnetic flux Applications

6 II. QD-coupled to SQUID Quantum dot (QD) Superconducting quantum interference device (SQUID) QD-SQUID QD + = Highly controllable electron system Most sensitive detector for magnetic flux Applications for future quantum information device. By employing InAs self-assembled quantum dot (SAQD) to QD- SQUID, we study the electrical transport properties of our device with two side-gates in order to study its potential for a quantum information device.

1. Electrode pad Laser lithography Deposition

QD-SQUID InAs dot growth by Hirakawa group (Univ.

23 mm 2 I SD (+) V SD(+) V SG1 SG2 EB lithography

(50Å/250Å) J1 Address mark (Dot size: ~200 nm) J2

RF sputter sg2 (or V J2 SD(-) chemical etching)

7 1. Electrode pad Laser lithography Deposition Lift-off <SQUID design on AFM image > Ti/Au(50Å/2500Å) 3. SQUID design on QD AFM mapping CAD design QD-SQUID InAs dot growth by Hirakawa group (Univ. of Tokyo) <Device configuration> <SEM image> Source Al D InAs SAQDs 2. Address mark SQUID loop area: 4.02x3.23 mm 2 I SD (+) V SD(+) V SG1 SG2 EB lithography Deposition lift-off J2 QD-SQUID Drain Ti/Pt (50Å/250Å) J1 Address mark (Dot size: ~200 nm) J2 J1 4.SQUID SG1 fabrication EB lithography V SG2 V RF sputter sg2 (or V J2 SD(-) chemical etching) Au Al I J1 Deposition SD(-) Al Ti/Al (50Å/1000Å) Al Au lift-off InAs SAQDs InAs QDs GaAs 200 nm AlGaAs 100 nm Si-GaAs 200 nm n+ GaAs substrate V

8 Current (na) I SD (na) I c (na) Supercurrent flow I-V 30 mk Tunable supercurrent 4 2 QD I c (a) (b) E off on V SD mv) 2 g 1. Observable supercurrent flow Highly transparent interface 2. Tunable supercurrent V by G <on/off tuning resonance energy tuned level by of gate QD voltage > V BG (V) (a) V BG Vsg2=0.1V Vsg2=-0.09 V BG Voltage (mv) (b)

9 I c (na) I c (na) SQUID operation Critical current (I c ) oscillation as a function of external magnetic fields I c (F ext =F 0 )= I c1 +I c2 I c (F 1/2 )= I c1 I c F Magnetic ext /F field 0 (Guass) I c Measured period: Gauss [ Wb] F0 1.59[ Gauss] 2 A [ mm ] H 0 : the field needed to add a flux quantum F 0 =h/2e to the effective SQUID area. (In the limit of small self inductance) F I 2 2 ext 1/ 2 c( Fext ) [ Ic 1 Ic2 2Ic 1Ic2 cos 2 ] F0 Ic Ic( F0) Ic( F0 / 2) 2Ic2 Ic2 Ic / 2 I ( c1 Ic F0) Ic2 * Individual I c can be tuned by each side gate

10 V BG (V) V BG (V) Side-gate controlled junction behavior (a) V SG1 =0V 0.3 I c (na) (b) V SG1 =-0.2V I c (na) Junction I c maximum at zero field Josephson relation: Is I c sin( ) (c) V SG1 = -0.4V -0.3 I c (na) (d) phase shift I c (na) Junction - phase shift and a reversal of the sign of the supercurrent in a Josephson device I s I c sin( ) I c sin( ) F ext /F 0 F ext /F 0

11 I c (na) I c (na) I c (na) Which dot has pi junction transition? Individual I c profiles for each dot distinguished by analyzing interference properties of SQUID strong V SG1 : tune of the dot-lead coupling weak 2 (a) V SG1 =0V I c2 (b)v SG1 =-0.2V 2 2 (c) V SG1 =-0.4V V BG (V) I c V BG (V) 0 I c2 I c V BG (V) QD-SQUID Direct observation of negative supercurrent V SG1 J1 (I c1 ) J2 (I c2 ) V BG 11

junction behavior and spin state Spin flip tunneling in S-QD-S system Spin QD Magnetic on doublet 2 g Cooper pair singlet state [I. Kulik, Sov. Phys. JETP, 22, 841 (1966)] [L.N. Bulaevskii et al.

12 junction behavior and spin state Spin flip tunneling in S-QD-S system Spin QD Magnetic on doublet 2 g Cooper pair singlet state [I. Kulik, Sov. Phys. JETP, 22, 841 (1966)] [L.N. Bulaevskii et al., JETP Lett. (1977) ] [C. Benjamin et al., Eur. Phys. J. B, (2007)] During tunneling event, the spin-ordering of the Cooper pair is reversed. A reversal of the sign of the supercurrent junction transition by tuning coupling between QD and superconducting leads using side-gate due to transition from the Kondo singlet to magnetic doublet of the spin state of the InAs QD V G (weak) coupling (strong) on Kondo singlet V G ( junction) 2 g (0 junction)

13 Limitation of directly coupled QD-SQUID + spin state superconducting state + state - - state (a) junction behavior not sensitive to the spin direction results from the presence of electron spin. (b)the switching of magnetization by SQUID find flux change in SQUID loop

14 III. SIS-SQUID Quantum dot (QD) Superconducting quantum interference device (SQUID) QD + = Highly controllable electron system Most sensitive detector for magnetic flux For spin ensemble measurement Magnetic flux variation induced by the spin >flux sensitivity of SQUID F 4 (R. Ishiguro et al. in Takayanagi lab) F 0 10 ( )

15 (flux variation) 磁束 (Φ /Φ 0) (Slide from R. Ishiguro) スピンの数とループ磁束の変化の関係 ( 臨界電流の変化の関係 ) 距離 0.1μ m の平面に等方的にスピンが分布しているときのループの磁束半径 r の高さ d の円筒内に等方的に磁気モーメント m が分布しているときの上面での磁束 F m m0m B 2 02 r m 2 2 r d d N spin How many spins are necessary to detect with our SQUID? - About spins 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 磁束 (Φ /Φ 0) スピン数 Sensitivity limit of our SQUID (spin number) Considering QD density (25/µm 2 ), SQUID loop are should be more than 400 µm 2.(ex. 20x20 µm 2 )

16 (Slide from R. Ishiguro group) QD 基板上の SIS SQUID 金による SQUID のマスク InAs QDs substrate Al-SQUID Au metal mask 膜厚 :300/500A 酸化 :0.5Pa 10min 金マスク厚 : A 実験後の PL 測定よりドットのギャップは 1430nm±10nm 程度磁場応答あり

17 IV. Preliminary experiment for optical excitation of QD spin 1. Photoluminescence measurement 2. Transport measurement under the light irradiation Laser Laser QDs or SQUID with metal mask QD Al/Au 70 nm Al/Au (SEM image from R. Ishiguro) Al-SQUID For spin ensemble QD-Josephson junction For single spin

18 Optically generated spin state <Spin selection rules for QD exciton state> Circularly polarized photon conveys one unit of angular momentum for and for Produce excitons Total spin changes by J J e, z h, z 1 3 e, ( e, e ) J h, z ( h, h ) 2 2 J z e h e h 2 2 Only the transitions with (e, h ) for + and (e, h ) for - are optically active. [Heiss, et.al., PRB2007 ] 2011/12/26 18

19 [M. Kroutvar et. al., Nature, 432, 81, 2004]

20 Lens Which wavelength is sensitive to create exciton in QD? Check with Photoluminescence(PL) spectrum for Initial characterization of optically excited QD spin Sample structure Cryostat l exc =800 nm (Ti: Sapphire laser) Sample Detector QD substrate (in collaboration with Nomura-lab in Tsukuba Univ.) 1 µm

21 Intensity (arb. units) Intensity (arb. units) Intensity (arb. units) Intensity (arb.units) l exc =800 nm (Ti: Sapphire laser) (a) Large area (bulk) (b) Small area Energy (ev) Photoluminescence exc 1. 54eV Detector: MCT in FT- IR system Energy (ev) 30*30 mm 2 10*10 mm 2 3*3 mm /12/26 S. KIM (File: S meeting-kim.ppt) 21 Energy (ev) Broad PL spectrum Due to mixing of different dot size 30x30 mm 2 Detector: CCD PL data from Ishiguro-san Wavelength (nm) 30*30mm 2 10*10mm 2 3*3mm 2

22 <New QD substrate > GaAs 20 nm AlGaAs 50 nm SI-GaAs 30 nm SI-GaAs 50 nm P-type GaAs 300 nm GaAs substrate -Capping layer -Uniform small dots (~40 nm) -p-type GaAs

23 <QD from Hirakawa group> InAs QDs (dot size: 100~200 nm) GaAs (buffer layer) AlGaAs (barrier layer) 100 nm Si-GaAs (buffer layer) n+ GaAs substrate 200 nm 200 nm <New QD substrate > GaAs 20 nm AlGaAs 50 nm SI-GaAs 30 nm SI-GaAs 50 nm P-type GaAs 300 nm GaAs substrate -Capping layer -Uniform small dots (~40 nm) -p-type GaAs Configuration of our device for the spin (ensemble) detection using Al-SQUID Laser InAs QDs substrate Semitransparent layer Al-SQUID Gate electrode Au metal mask SQUID Ti/Au SiO nm GaAs 20 nm AlGaAs 50 nm (4/5 nm) SI-GaAs 30 nm SI-GaAs 50 nm P-type GaAs 300 nm GaAs substrate Au mask Au electrode Al 2 O 3 50 nm Schematic view of our device plan 1. We will do PL measurement with new substrate in order to determine the gate-voltage range for each charging state, along with the associated optical transition frequencies. 2.. Fabrication with new scheme of our SQUID device

24 FC connector (a) Front view Optical setup of TRITON <With Inoue-san and Tusumu-gun> Optical fiber Triton200 Cold finger Thermocoax filter (b) (a) Optical fiber P P PBS BS Holder for optics λ/4 λ/4 ~ Retarder Lens QD RC filter (LPF) magnet Laser 2011/12/26 24 Sample (b) Backside

25 Photocurrent measurement For the characterization of single dot, we tried photocurrent measurement of QD-Josephson junction. Spectrometer Cryostat (38 mk) I Light irradiation V QD Lock-in amp (I) 2011/12/26 To determine effective wavelength

=0 V Side gate Differential resistance mapping -5-10 -0.08-0.04 0.

26 Transport properties of QD Josephson junction I sd (na) InAs QD (~150 nm) Au Al Au Source Al (100 nm) Drain Al T=36 mk V sg =0 V Side gate Differential resistance mapping V sd (mv) I c = 3 na odd even odd even Open dot regime Δ 2Δ

27 Power (nw) V sd [mv] Under the light irradiation I c (na) Power (nw) PL int (A.U.) 30 Power dependence of I c 3 80 File: D50 and 25nW.obj Power I c l=1800 nm 2 60 D50 25 nw set light source voltage (Dial) <Light source: fixed at dial 50> Differential resistance mapping E E E E E E E E E E E E Wavelength (nw) (a) Large area (bulk) PL int (A.U.) Detector: MCT in FT- IR system (Data from Ishiguro-san) Wave Length [nm] 波長 (nm) Sample characteristics is change during irradiation Charge variation due to surface level?

28 Differential resistance at same power a. Dark b. Under the irradiation (power fixed at 25 nw ) Left : dial 0 Right : Power = 25 nw Laser を当てている場合当てていない場合で変化が見られない Next experiment -Power is not enough? Try with higher power. -Use of lock-in amp - Use small dot and weak coupling (few electron )

29 QD-SQUID with metal mask Au mask (200 nm) Ti/Al/Ti/Au Au SiO 2 (200 nm) Al InAs QD Laser Au mask Side gate (Au) Al 2 O 3 (50 nm) GaAs substrate Al Au Schematic diagram of our sample

30 Thank you for your attention!

Installation Manual WIND TRANSDUCER

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